Suresh Main Project of Excitation

STUDY OF STATIC EXCITATION AND AVR DEPT OF EEE

CHAPTER -1

INTRODUCTION

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1. INTRODUCTION

The voltage regulator is intended for the excitation and control of generator

equipped with DC exciter employing generator field breaker. The excitation equipment of

the generator and it’s inter connections with the voltage regulator is shown in the black

diagram below.

The station auxiliary supply 415V, 50Hz 3-phase normally provides supply to the

thyristor sets. Arrangements in the AVR to work with two feeders of station auxiliary are

available. This will lead to change over of operation feeder. Once feeder-1 becomes

healthy there will be an automatic change over to feeder-1.The switching of feeder-1&2

will be by operating breaker Q3&Q4.

The main parts of the voltage regulator equipment are two closed loop control

system including a separate gate control set and thyristor set each. Control system-1 is

automatic channel controlling the generator voltage.Controlsystem-2 is the manual

channel that is a field current regulator. Field discharge normally is initiated on shut

down of the generator by a speed dependent of reverse power relay, or in fault situations

by the generator protection equipment. Field discharge commands to drive the thyristor

set to the maximum negative output.(inverter operation) via the gate control set in

operation .in addition to this a tripping command I given to the main exciter field breaker.

This field breaker connects a discharge resistor across the field of main exciter for

affective field suppression. On the generator field side, the breaker Q1 connects a

discharge resistor across generator field and disconnects field from the DC exciter.

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Channel- 1 (auto channel) is built as voltage regulator and is on doing normal operations.

In addition to the voltage regulator, which has a PID control algorithm, this auto channel

also contains various limiters and corrective control circuits to ensure the use and stable

operation of the synchronous machine unto its operating limits. This channel possesses a

gate control unit with a subsequent final stage to generate the firing pulses for the

thyristor converter. During normal operations (auto), the pulse final stage of the

automatic channel is active and transmits the firing pulses to the auto thyristor bridge

pulse transformer. Various monitoring functions of the auto channel initiate the automatic

switch over to channel-2 in case of a malfunction.

Channel-2 (manual) is built as a simple field current regulator with a PI control

algorithm. It serves awes aback up channel in case of a malfunction of the automatic

channel, and is also used for the commissioning and for special machine tests. This

channel consists number of special limiters to ensure that the synchronous machine will

always remain with in operating limits. For this reason, operation using the field current

regulator requires expert adjustment of the machines operating point and continuous

monitoring of the machine by the in operating staff. When ever the generators operating

limiters are exceeded, the safety devices respond immediately by the shutting down the

excitation and the generator.

The manual channel has its own gate control unit (the software for the field

current regulator is also implemented there in) and it own pulse final stage. During

normal operation (automatic), the output pulses from the final stage are blocked. Various

monitoring on the manual channel initiate an alarm in cease of malfunctioning while the

manual channel is on stand by. If the manual channel suffers a malfunctioning while it is

in operation the excitation is switched off (trip).

Both channels are equipped with tracking equipment so that the inactive channel

always generates the same control variable as the active channel during steady state

operation.

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This ensure that the manual channel will, in a switch over initiated by

malfunction, take over the operating point of the machine as is was prior to the problem;

the response of the tracking for the manual channel is set relatively slow.

The excitation system has an atones excitation monitoring. As one of its

functions, this equipment monitors for field currents that exceed acceptable maximum

limits. It initiates an emergency switch over through the manual channel whenever the

field current exceeds the present value. if even after such a switch over, the field current

does not drop back to the permissible level, the excitation is switched off. The excitation

monitoring checks these measuring inputs for discrepancy and plausibility. An alarm is

always initiated in case of malfunction. In certain cases, a switch over channel to is also

initiated.

Generally conventional method suffers from cooling and maintenance problems

associated with slip rings, brushes and commutators as alternator rating rise. The trend

towards modern excitation systems has been to decrease these problems by minimizing

the number of sliding contacts and brushes. This trend led to development of static

excitation.

A good number of protection devices are installed in the static excitation scheme

for an possible fault in the excitation system.

The advantages are:

1. Fast response characteristics needed in the modern power systems

2. Free from friction, wind age and commutation loss occurring in exciter

3. The excitation voltage is proportional to alternator speed; this improves the

overall system performance.

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1.1 Terms and Definitions on AVR and Excitation system

1. Excitation system: The equipment for providing field current to

synchronous machine. The equipment includes all power.

2. Regulated voltage: The voltage, which is held with in specified band or zone

during steady or gradually loads conditions with in specified range of load.

3. Band or Zone of regulated voltage is expressed as present of rated value of rated

voltage. (E.g. +3 present of rated Vt)

4. Exciter: The equipment providing field current for excitation of synchronous

machine.

5. Pilot exciter: The equipment providing for field current to the exciter field.

6. Automatic Voltage Regulator: A subsystem of excitation system for

regulating the terminal voltage of synchronous machine automatically.

7. Voltage regulator is a historic term modern term is synchronous machine regulator.

8. Synchronous machine regulator: The regulator that couples the output

variables of synchronous machines to the input of the exciter through a feedback

and feed forward control elements for controlling the synchronous machine output

variables

9. Rated field current: direct current in the field winding of the synchronous

machine operating at rated voltage, current, power factor.

10. Rated field voltage: Direct voltage required across the terminals of the field

winding of the synchronous machine under the rated continuous load conditions with

field winding at specified temperature (e.g. 75c).

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11. Excitation system nominal response: Rate increase of excitation output

voltage divided by rated voltage. Rate of increase of excitation system output voltage

determines from excitation system nominal response curve.

12. Exciter voltage response time: Time in seconds for exciter voltage to reach

95% of the under specified conditions.

13. Excitation system ceiling voltage: The maximum D.C voltage, which the

excitation system can supply to the generator field winding for a specified short time.

14. Field forcing the control function that rapidly forces the field current in the

synchronous machine in the positive or negative direction.

15. Voltage regulating adjuster: A device associated with the regulator by which

the adjustment in terminal voltage of synchronous generator can be made.

16. Limiter: It is an element in the excitation system that acts to limit variable under

the certain predetermined condition.

a. Under excitation limiter: prevents the voltage regulator from lowering

in the field current below specific limit.

b.Over –excitation limiter: Prevents the voltage from rising in field current

below specified limit.

C.Volts per hertz limiter: Acts through voltage regulator to limits v / f ratio

with in specified limits and takes corrective action to make v / f nominal.

17. Manual control: The control of terminal voltage of synchronous generator and

by operates action e.g. by adjusting field rheostat, controlling of firing the

thyristor controlled rectifier.

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18. De-excitation: Removal of excitation of main exciter or pilot exciter for example

by of opening field circuit and discharging the field by means of field discharge

circuit breaker.

19. Power system stabilizer: A group of elements in the excitation system that

supplement the voltage regulating function and provide additional regulating

function to improve the dynamic performance of the power system.

1.2 Exciter system’s Need

The aims of excitation system are:

1) To control the voltage so that operation is possible near to steady state stability limit.

2) To maintain voltage under system fault conditions to ensure rapid operation of

protective rear.

3) To facilitate sharing of reactive load between machines operating in parallel.

An indication of the rapidity of response of an exciter is given by the numerical

value of the average rate of rise from nominal slip ring voltage in the first half

second following the opening of the exciter armature circuit, expressed in terms of

nominal slip ring voltage. For steam turbo alternators response of 0.5 is adequate.

When a sudden change of the alternator load occur, demanding a change in its

excitation to give the same voltage under the new conditions, the automatic

regulating equipment must first operate on the exciter field, the exciter voltage

changes, and the alternator field current. The rapidity of response to a load change

thus depends on a chain of operations in, which there is a tendency to delay. Fluxes

cannot build up or decay in highly inductive field systems without an appreciable

laps of time. In the alternator itself the main flux has an appreciable time constant

and the change of flux with change of load is slow enough to allow the exciting

current to be adjusted by the exciter.

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1.3 AVR’S NEED

The automatic voltage regulator in the excitation system play a vital role for voltage

control, controlling reactive power supply, emf, voltage and power factor of the

generator, and also maintaining power system dynamic stability, and in protection of

alternators by imposing several limits on the generator variables.

Voltage regulator (synchronous machine regulator): It is defined as a regulator that

couples the output variables of synchronous machine to the input the exciter through a

feedback and feed forward control elements for controlling the synchronous variables

output.

The active mechanical power supplied by prime mover to the shaft is equal to active

power supplied by the generator to load plus loss in the generator. AVR does not change

the active power P of generator; nor does it change the frequency and the speed. However

the AVR influences the power angle & between the revolving the stator plus and

revolving rotor flux both locked up synchronously Ns.

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1.4 POWER SYSTEM STABILITY & VOLTAGE STABILITY

The automatic voltage regulator regulates the voltage and / or the flow of reactive

power during parallel operation from the synchronous machine by direct control of

the main exciter field current using thyristor converter. The supplied AVT output is to

be connected to the main exciter field in series with the rheostats.

The functions of automatic voltage regulator are:

1. To regulate the generator voltage

2. To regulate the effect of reactive and/ active current on the voltage(drops)

3. To limit volt per hertz.

4. To limit maximum field current

5. To limit inductive stator current

6. To limit capacitor stator current.

7. To limit the load angle

8. To stabilize the power system.

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10

CHAPTER -2

SYNCHRONOUS MACHINE

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2. SYNCHRONOUS MACHINE

Synchronous Generator

Consider an alternator connected to an infinite bus

An infinite bus means a large integrated power system whose frequency and voltage will

not change in case some changes are stipulated in the system input or the excitation

circuit of an alternator which is connected to the system. Our objective here is to study

the behavior of the generator (incoming) when a change in its steam input or change in its

excitation circuit is stipulated. It is to be noted that there is no signal bus in a large

interconnected system, which should be considered as infinite bus. Rather any bus could

be considered as an infinite bus and the whole existing system should be considered as an

infinite system with respect to the incoming alternator whose behavior is to be studied.

Or in large system we could consider one of the alternators whose behavior is to be

studied as a finite machine and the rest of the system as an infinite bus. In order to study

the effect of change in steam input and change in excitation on the performance of the

generator the following expressions are to be evaluated

I=E∟ (δ-90) Xd -V∟-90/Xd

Power P= [VI*] =EV sinδ/ Xd

Reactive power Q=Im[VI*]

= (EV cosδ-V2)/Xd

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If cosØ is the power factor of the alternator, the active and reactive power are also given

as

P=VI cosØ

Q=VI sinØ

Therefore

EV sinδ/Xd=VI cosØ

(EV cosδ-V²) Xd =VI sinØ

From the above equation it is clear that the alternator operates at unity P.F. If the reactive

power delivered by the alternator is zero i.e.

(EV cosδ-V²)/ Xd =0 or Ecosδ=V

The excitation of the alternator corresponding to this operation (unity power factor) is

known as normal excitation. The alternator delivers lagging VARS to the infinite bus if

(EV cosδ-V²)/ Xd>0 or

EV cosδ >V

And the alienator is said to over excited as cos δ<=1 and now E should be greater than

the value of E under unity power factor operation as V is constant for an infinite bus

system. Similarly, if the alternator absorbs lagging VARS from the system

EV cosδ <V and the alternator is said to be under-excited

Effect of change of excitation: Synchronous generator- suppose the generator is

delivering power p to the infinite bus when the bus voltage, frequency, power factor and

current are v, f, cosδ and 1 respectively. The reactive power delivered

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By the generator, its power angle and induced voltage are Q, δ and E respectively .Our

objective is to find out these values if excitation of the alternator is changes.

A) Excitation increased:

1) Initial power factor lagging

Esinδ/Xd=1cosφ

Since excitation is increased E increased and there is no change in active power.

1cosφ=constant and therefore

Esinδ=constant also

as E increased sinδ should decreased, therefore, δ decreases.

As δ decreases and E increases and so Q increases.

Again P²+Q² = (V1)²

Here P and V are constant,Q has increased, therefore I increases.

Also as 1 cosφ=constant (as P in constant) and I has increased so as cos Φ decreases.

Summarizing, the increase in excitation results in

P, f, V No change

Δ, cosφ Decreases

E, Q, 1 increases

2) Unity p.f (initial)

Result are exactly identical to when the p.f is lagging initially

3) p.f leading (initially)

for the same power P,δ decreases as E increases for increased excitation since it is an

under-excitation case.

(EV cosδ-V²)/Xd <0

and as E has increased and δ has decreased, the term EV cosδ/Xd becomes more positive

and Q less negative i.e. its magnitude decreases.

I decreases hence cos Φ increases as 1 cosφ = constant

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Summarizing

P,V,f no change 1,δ,|Q|

decreases E,cosΦ increases

B) Excitation decreases:

1) Power factor lagging

With decreases in excitation, the induced voltage E of the generator decreases and hence

for constant power , sinδ increases and δ, therefore, increases, Q decreases as

Q= (EV cosδ – V)/Xd > 0.

And E and cosδ decreases, therefore, Q decreases and I decreases.

Since I decreases, and as I cosΦ = constant, cosΦ increases.

Summarizing

P,V,f no change Q,I,E

decreases δ,cosΦ increases

similarly it can be justified that if power factor is leading initially

P,V,f no change E,cosΦ

decreases I,δ,|Q| increase

In fact all these result can be easily explained with the help of phasor diagram

Here E sinδ = constant and I cosΦ = constant i.e. with increase or decrease in E its

component normal to V will be constant i.e. its arrow head will lie along a line parallel to

and a at distance E sinδ. Similarly the arrow head of I will lie along a line whose loci is I

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cosΦ=constant.

From the phasor diagram

E1>E, I1>I, cosΦ,δ1 < δ

Hence Q1>Q,P,V and f remain unchanged

Incase the excitation is decreased, the same phasor diagram can be used with suffix I as

the initial operation condition.

C) Steam input increased: With steam input increases,V,f and E remaining constant,

δ increases as

EV sin δ/Xd = P1 as P1>P,δ1

1) initial p.f lag

Q = (EV cos δ-V²)/Xd >0

As cos δ decreases

Now Φ =tan-1(Q/P)

As Q has decreased and P has increased decreases and so p.f. improves. As δ has

increased and E1=E current increases

Summarizing

E,V,f no change

cos δ,I,P increase

Q decrease

2) Initially p.f leading

E,V,f no change

δ increases and hence

Q=(EVcosδ-V²)/Xd <0

Becomes more negative and the |Q| increases.

Since P and |Q| have increased, I increaeses, again, since P has increased

And there are relatively larger increases in Q, the p.f angle increases and hence p.f

decreases.

Summarizing

E, V, f no change

P, δ, I, |Q| increase

cosΦ decrease

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Similarly if the steam input is decreased its effect on the various parameters can be

studied. Figure below shows the effect of increases in steam input on the various

parameters for an initially lagg in p.f

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CHAPTER -3

TYPES OF EXCITATION SYSTEM

3. TYPES OF EXCITATION SYSTEM

1. Conventional DC17


The earliest AC turbine generator obtained their excitation supply from the power station

direct current distribution system. Each machine has a rheostat in series with its field

winding to permit adjustment of the terminal voltage and reactive load. This method was

suitable for machines, which needed small field power requirement also increased,

excitation power requirements also increased and became increasingly desirable for each

unit to be self excitation and thus the shaft driven DC exciter was introduced.

2. Static system

In order to maintain system stability it is necessary to have fast excitation system for

large synchronous machines which means the field current must be adjusted extremely

fast to changing operational conditions. Besides maintaining the field current and steady

state stability the excitation system is in this system, the ac power is tapped off from the

generator terminals stepped and rectified by fully controlled thyristor bridges and then

fed to the generator field there by controlling the generator voltage output. A high

controlled is achieved by using an inertial free control and power electronic system. Any

deviation in the generator terminal voltage is sensed by an error detector and causes the

voltage regulator to advance or retard the firing angle of the thyristors there by

controlling field excitation of the generator

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3. Brushless system:

Supply of high current by means of slip rings involves considerable operational problems

and it requires suitable design of slip rings and brush gear.

In brushless excitation systems diode rectifiers are mounted on the generator shaft and

their output is directly connected to the alternator. Thus eliminating brushes and slip

rings. This arrangement necessitates the use of rotating armature and stationary field

system for the main the AC exciter. The voltage regulator final stages take the form of

the main AC exciter. The voltage regulator final stages take the form of a thyristor bridge

controlling the field of the main AC exciter, which is fed from PMG on the same shaft.

The response ratio of brushless excitation is normally about 2. Required to extend the

stability limits. It because of these reasons the static excitation system is preferred to

conventional excitation systems.

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OPERATIONALAMPLIFIER:

The circuit schematic of an op-amp is a triangle. It has two input terminals and output

terminal. The terminal with a (-) sign is called inverting input terminal and the terminal

with (+) sign is called the non-inverting input terminal.

DIFFERENTIATOR:

One of the simplest of the op-amp circuits that contain capacitor is the differentiating

amplifier, or differentiator. As name suggests the circuit performance the mathematical

operation of differentiator that is the output waveform is the derivative of input

waveform.

Figure

INTEGRATOR:

A simple low pass RC circuit can also work as an integrator when time constant is very

large values of R and C. the output waveform for step, since wave and square wave.

Fig

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CHAPTER -4

EXCITATION SYSTEM

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4. EXCITATION SYSTEM

EXCITATION SYSTEM

BASIC FUNCTION

To meet the excitation power requirements of the generator under all operating

conditions i.e. to provide the required DC current to the field of the generator.

OTHER FUNCTIONS

Voltage & Reactive power control.

Maintaining the field current & steady state operating point

Ensure that the capacity limits of the generator are not exceeded.

Enhancement of steady state stability.

Improving Transient stability after faults.

Quick recovery of voltage after faults.

Satisfactory parallel operation of the generator.

4.1 Features of Indirect Excitation System

The static AVR equipment manufactured at CED serves the purpose of marinating the

voltage on synchronous machine excited. By indirect excitation, it is meant systems

where an exciter machine is used. Static AVR consists of two channels, auto &manual.

Automatic channel is a closed loop system, which maintains the generator voltage to very

close accuracy. Manual is a closed loop system, which maintains exciter field voltage or

current constant.

Two types of static AVR’s are manufactured of CED . They type A & type B.

Some of the special features

Type A

a. Static AVR with different input sources.

b. Initial excitation build-up circuit.

c. Limiters.

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d. Series compounding.

Type B

a. Basic static AVR.

b. Static AVR with limiters.

c. Slip stabilization-optional features

Initial excitation build-up circuit: For all generators, which supply their

excitation power directly from generator terminals, a special excitation build-up circuit is

provided. The remanence voltage of the generator supplied by a transformer is brought to

auxiliary diodes through contacts of the contactor. The AC voltage is rectified and

impressed in to the field winding through a protecting resistance and miniature circuit

breaker. As soon as a remanence voltage, which is depending on the speed, is high

enough, the voltage at the generator is built-up. When about 60% of rated voltage is

attained the conductor, the coil of which is connected to the rectified second auxiliary

voltage interrupts the excitation build-up circuit. The control itself takes over and the

generator voltage according to frequency at the adjusted nominal value.

Basic advantages:

1. Long life and fast operation.

2. Absence of mechanical inertia and bouncing of contacts, high resistance to shock

and vibrations.

3. High noise immunity and surge proof.

4. Modular construction, alternating and expansion is easier on smaller side.

5. Interfacing with more elaborate control system.

Conclusion:

Static AVR for indirectly excited synchronous generators is an up to date

andinancially attractive system having certain specific advantages over other systems.

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4.2 Static Excitation System

In static excitation system, the ac power is tapped off from the generator terminal stepped

and rectified by fully controlled thyristor bridges and then fed to the generator field

thereby controlling the generator voltage output. A high controlled is achieved by using

an inertial free control and power electronic system. Any deviations in the generator

terminal voltage is sensed by an error detector and causes the voltage regulator to

advance or retard the firing angle of the thyristors thereby controlling the field excitation

of the alternator.

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The excitation of the generators is carried out by a THYRIPOL excitation system

obtaining the energy from the generator terminals. THYRIPOL is a static excitation

system for synchronous generators and synchronous condensers in substations. The

excitation of the synchronous of the machine is controlled direct by a thyristor converter

equipped with an electronic voltage regulator, i.e., the converter supplies the excitation

current to the rotor of the synchronous machine without any interposed rotary exciter

being required. The excitation transformer is connected to the generator bus bars tapped

off between the generator terminals and the generator circuit breaker.

The excitation system consists of the following equipments:

The excitation transformer

Channel 1 is housed in cubicle with

The rectifier bridge

The automatic voltage regulator (AVR)

The excitation current regulator(ECR)

Gating and control system

Channel 2 is housed in cubicle with

The rectifier bridge

The automatic voltage regulator (AVR)

The excitation current regulator(ECR) Gating and control system

The field over voltage protection is housed in cubicle

The field flashing equipment is mounted in cubicle the existing de-excitation

equipment consists the field circuit breaker and the deexcitation resistor.

The equipment is housed in standard cubicles. The rating of the excitation system is:

If normal = 1460A

Ifmax = 1606A

Ufnominal = 435V

Ufd = 696V

Maximum continuous current rating of the rectifier bridge of each channel is

1, 4 X If nominal = 2044 A short time (10seconds)

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The equipment is supplied with redundant service voltages. D.S. voltage of 220V is used.

The power supply for the internal electronics is galvanically isolated from the 220V

supply.

STATIC EXCITATION

MAIN COMPONENTS:

EXCITAION TRANSFORMER

THYRISTOR CUBICLES WITH DOUBLE COOLING FANS

AVR CONTROL CUBICLES

FIELD BREAKER & DISCHARGE RESISTOR

PRE- EXCITATION OR FIELD FLASHING UNIT

CROW-BAR MODULE

ROTOR TEMPARATURE MEASUREMENTS

ROTOR EARTH FAULT RELAY

It can be designed without and difficulty to provide high response ratio, which is required

by the system. The response ration of the order 3 to 5 can be achieved by this system.

This equipment controls the generator terminal voltage and hence the reactive load flow

by adjusting the excitation current. The rotating exciter is dispensed with and silicon

controlled rectifiers are used which directly feed the field of alternator.

The static excitation system consists of:

1) Excitation transformer

2) Converter bridge

3) Excitation starter (field flashing) and field discharge equipment (deexcitation)

4) Automatic Voltage Regulator and operational control circuits.

EXCITATON TRANSFORMER:

Maintenance free dry type with Epoxy cast insulation for high reliability

Three phase of 1275 KVA capacity each

HV winding is directly connected to the generator terminals at 11KV26


No load voltage on LV side is 705V

Current rating is 66.92/1314.50A

Forced air cooling with fans located on top of the panel

Instantaneous & time delay O/C protection

Winding temperature high protection (Alarm and trip at 115/130deg)

Bus-bar connections on LV side

Indication lamps / voltmeters are provided for supply healthiness on LV

side

Provision for measuring the LV voltages

The excitation power is taken from generator output and fed through the excitation

(rectifier) transformer, which steps down to the required voltage, for the SCR Bridge and

then through riled breaker to the generator field. The rectifier transformer used in the

static excitation system should have high reliability, as failure of this will cause shut

down of power station.

Dry type cast oil transformer is suitable for static excitation applications. The transformer

is selected such that it supplies excitation current at rated voltage continuously and is

capable of supplying ceiling current at the ceiling excitation for short time of 10 seconds.

The excitation system is supplied with a dry-type three phase transformer with the vector

Y d5.

The excitation transformer is connected directly to the 11kv generator. The secondary of

the transformer is connected to both rectifier cubicles via cables.

Specification of the three phase transformer : Rated power

1275KVA Primary voltage

11KV Frequency 50HZ

Vector group Yd5

Short circuit voltage > 6 %

Cooling AN

An earthed shield is situated between the primary and secondary windings of the

transformer in order protect the rectifiers from the transient over voltages in the 11KV

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mains. For the protection of the three phase transformer against overheating, a

temperature monitoring system is supplied. It contains three parts: two systems supervise

the voltage windings (alarm and trip), one system supervises the core (trip)

CONVERTER BRIDGE:

Two converter panels with open type forced ventilation with double

Fans in the same shaft with 100 % standby

Button type thyristers with aluminum alloy heat sinks

Two number of thyrister in series in a row

Tow rows per branch and two branches in parallel

High speed fuse and two arc protection for thyristers

Reactance is provided in series with the thyristor for limiting

Current peaks and for equal sharing of current between parallel paths

Neon indication lamp for each thyristor

Search coil for conduction monitoring

The SCR output stage consists of a suitable number of bridges connected in parallel.

Each thyristor bridge comprises of 6 thyristors, working as a six pulse full controlled

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bridge. Current carrying capacity of each bridge depends on the rating of the individual

thyristors. Thyristors are designed such that their junction temperature rise is well within

its specified rating. By changing the firing angle of the thyristors, variable out put is

obtained. Each bridge is controlled by one final pulse stage and is cooled by a fan.

These bridges are equipped with protection devices and failure of one bridge causes

alarm. If there is a failure of one or more thyrister bridge excitation current will be

limited to a pre-determined value lesser than the normal current. However, failure of the

Thired Bridge results in tripping and de-excitation of the generator.

The 3 phase A.C bridges are connected via cables to the excitation transformer. The D.C

output is connected via cables to the existing de-excitation system. Each of both cubicles

contains one Thyrister Bridge, the fuses and the auxiliary circuits for the thyristors. The

modules of AVR, ECR and the excitation equipment control system as well as the firing

circuits and the fuse monitoring devices are mounted into an electronic box near by both

the thyristor rectifier-bridges.

The firing pulses are generated in the trigger set abd amplified by the pulse amplifier. A

matched fuse protects each thyristor module.

A cooling fan cools each rectifier bridge. The fans are mounted on the top of the cubicles.

The ventilators are protected by motor circuit breakers.

The airflow monitoring devices are house in the ducts between the fans and the thyristors.

In the active channel (cubicle) fails a switch over to the other channel is caused. The

continuous follow-up control results in a switch over with very small changes in the

reactive load.

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Trigger set (used in each thyristor rectifier):

Unless a firing pulse is applied to a gate, a thyristor blocks the flow of current in both

directions. Current will not start to flow until a voltage of correct polarity is applied

(positive to the anode, negative to the cathode) and a pulse is transmitted to the gate

electrode. Once this current has exceeded a relatively low value- the bolding current- it

continuous to flow. The thyristor. Does not regain its forward blocking ability until the

current drops to zero and the voltage is negative. This may for example result from

commutation of the current to another thyristor in the converter connection whose applied

voltage is higher.

The trigger set produce the firing pulses required for firing the thyristor. The pulse

amplifiers of each thyristor amplify these pulses. The firing pulses are synchronized by

the transformed rectifier voltage.

Pulse generation:

Each output of the synchronization circuit is connected to the trigger set. This control unit

produces firing angles at the output with a duration of 12°0 electrical αo, αG , αw are

adjusted in the software of the digital firing.

The max firing angle is limited to 150°0 electrical.

Special precautions are taken to keep the delay angle within the limits which must be 30


adhered in order to ensure reliable communication, i.e. switching of the current from one

thyristor arm to the next.

Excitation start up and field discharge equipment:

PRE-EXCITATION (FIELD FLASHING):

Not possible to build-up voltage during starting with the residual voltage in the

field at nominal speed

3- PH 415 V AC supply is taken from the station supply

Stepped down, rectified and then fed to the field through a pre-excitation contactor

Pre- excitation contactor closes after FB closing

As soon as the voltage builds up to around 45 % , the main excitation takes over

Pre-excitation becomes off at 8 % of the nominal field current with a back- up timer

with 15 s

If the voltage is not reaching 50 % in 6 s while building up in Auto, change over takes

place to manual and there is a chance of Turbine tripping.

For the initial build up of the generator voltage, field flashing equipment is required.

The rating of this equipment depends on the no load excitation requirement and field time

constant of the generator. From the reliability point of view, provision for both AC and

DC field flashing is provided. The field breaker is selected such that it carries full load

excitation current continuously and also it breaks the maximum field current when the

three-phase short circuit occurs at generator terminals. The field discharge is normally of

non- linear type for medium and large machines i.e., voltage dependent resistor.

To protect the field winding of the generator against over voltages, an over voltage

protector along with a current limiting resistor is used to limit the over voltage across the

field winding. The OVP operates on the insulation break over principle. The voltage level

at which the OVP should operate is selected based on the insulation level of field winding

of the generator. 6 field flashing

31


Normally the field flashing device is supplied from the 415V station service. If that

supply fails, which is detected by U < really, the power supply is switched over to the

220V station battery.

Field Flashing fed byAC – station service :

The field flashing device is feed by the 415V, 3 phase station service and protected by

circuit breaker in addition to this the 415V are monitored by if the 415V fails during the

field flashing on sequence, switches the 220V station battery to the field flashing circuit.

After the command “Excitation On “the field flash sequence is started.

Field Flashing fed by 220V station battery:

The standby field flashing device is supplied from 220V station battery. Fuses protect the

feeder for the 220V DC input. A diode module is pleased in the positive line to avoid a

feed back from the excitation equipment into the station battery. The maximum field

flashing current is limited by resistors that are connected in series to the above mentioned

diode module.

If the voltage of the main rectifier exceeds the field flashing voltage the power supply

(i.e, main rectifier) with the higher potential takes over supplying of excitation current to

the rotor. The field flashing contactors are switched off after approximately 20% if no

load excitation current.

De-excitation:

Because the de-excitation system is a part of the generator protection, it must ensure the

de-excition of the synchronous generator, independent of the reaming excitation system.

Therefore a sure disconnection between the thyristor bridges (feeding power supply) and

the rotor is necessary. For this device the THYRIPOL Excitation System uses the

existing field circuit breaker, which disconnects the rectifier from the generator field.

An important assumption is to close in each case during de- excitation the field of the

generator; otherwise high over voltages will be caused. In our case the excitation energy

will be discharged over the de-excitation resistor. This resistor fixes the de-excitation

32


time and the voltage at the field and the rectifier thyristor. For redundancy the rectifier is

driven as inverted rectifier.

De-excitation via Inverted Rectifier Mode :

For the normal shut down of the unit, the machine will be de-excited via the inverter

mode of the rectifier; the field circuit breaker has not to be opened.

In this case the negative ceiling voltage is connected to the rotor, the rectifier therefore is

working in inverted rectifier mode (α w=150º el). The excitation current is reduced to

zero in short time.

The de-excitation time is in this case mainly dependent from the negative excitation

voltage.

About 10 seconds after the starting of the de-excitation, the impose of the firing

sets will be blocked and the field circuit breaker could be opened. Then NC-contact will

be closed. If the de-excitation is not finished up to the point, the de-excitation will be

finished over the de-excitation resistor de-excitation. This ensures a correct de-excitation

in case of failures in the de-excitation equipment. In the following the two ways of de-

excitation are described. A restart of the excitation system is blocked for the next 20

seconds.

33


De-excitation via “TRIP” signal:

If the de-excitation of the generator is initiated by an external TRIP signal, the field

circuit breaker disconnects the rectifier from the generator rotor.

The excitation system is off.

Control Electronics:

Regulator is the heart of the system. This regulates the generator voltage by controlling

the firing pulses to the thyristors.

a) Error detector & Amplifier:

The generator terminal voltage is stepped down by a three-phase power transformer and

fed to the AVR. The AC input thus obtained is rectified, filtered and compared against a

highly stabilized reference value and difference is amplified in different stages of

amplification. The AVR is designed with highly stable elements so that variations in

ambient temperature does not cause any drift or change in the output level. Three current

transformers sensing the output current of the generator feed proportional current across

variable resistors in the AVR. The voltage thus obtained across the resistors can be added

vertically either for compounding or for transformer drop compensation.

b) Grid control unit:

The output of the AVR is fed to a grid control unit, it gets its synchronous AC reference

through a filter circuit and generates a double pulse spaced 60^o elec. Apart whose

position depends on the AVR, i.e., the pulse position varies continuously as a function of

control voltage. The two relays are provided, by energizing, which the pulses can be

either blocked completely or shifted to inverter mode of operation.

C) Pulse amplifier:

The pulse output of the grid control unit is amplified further at intermediate stage

amplification. This is known as a pulse intermediate stage, the unit has a DC power

supply, which operates from a three phase 380V supply and delivers +5V, -15V, +5V and

a source stabilized voltage V. a built in relay is provided which can be used for blocking 34


the six pulse channels. In a two-channel system, the change over is affected by

energizing/ de-energizing the relay.

d) Pulse final stage:

This unit receives input pulses from the pulse amplifier and transmits them through pulse

transformers through the gates of the thyristors. A built in power supply provides the

required DC supply to the final pulse amplifier. Each thyristor bridge has its own pulse

stage. Therefore, even if a thyristor bridge fails with its final pulse stage, the remaining

thyristor bridges can continue to cater to full load requirement of the machine/ and

thereby ensure (n-1) operation.

e) Manual control channel:

A separate manual control channel is provided where the controlling DC signal is taken

from the stabilized DC voltage through a motor operated potentiometer, i.e., the signal is

fed to a separate grid control unit whose output pulses after being amplified at an

intermediate stage can be fed to the final pulse stage. When one channel is working,

greeting the required pulses, the other remains blocked.

Therefore, blocking or releasing the pulses of the corresponding intermediate stage

effects a change over from auto to manual channel or vice-versa. A pulse supervision unit

detects spurious pulses or loss of pulses at the pulse bus bar and transfers control from

auto channel to manual channel.

f) Follow up unit:

To ensure a smooth switch over from a auto to manual control, it is necessary that the

position of pulses on the both channels should be identical a comparison unit defects any

difference in position of pulses and with a help of follow up unit activates the motor

operated potentiometer on the manual channel to turn in to a direction so as to eliminate

the difference, however, while transferring control from manual to auto made any

difference in control levels can be visually checked on a balanced meter and adjusted to

obtain null before change over.

35


g) Limit controllers:

When a generator is running in parallel with the power network, it is essential to maintain

it in synchronism without exceeding the rating of the machine and also without the

protection system tripping. The automatic voltage regulator by itself cannot ensure this. It

is necessary to supplement the basic voltage regulators by limiters to limit over the

excitation and under excitation. Limiters don’t replace the protection system but only

prevent the protection system from tripping unnecessarily under extreme transient

conditions.

The AVR also has a built in frequency dependent circuit so that when the machine is

running below the rated frequency, the regulated voltage should be proportional to

frequency. With the help of a potentiometer in the AVR, the circuit can be made to

respond proportionally to voltage above a certain frequency. The range of adjustment of

this cut-off frequency lies between 40 and 60HZ. The static excitation system is

equipped with three limiters, witch act in conjunction with AVR

1. Rotor current limiter:

This avoids the thermal overloading of the rotor winding and is provided to protect the

generator rotor against excessively long duration over loads. This Ceiling excitation is

limited to a predetermined limit and is allowed to flow for a time, which is dependent

upon the rate of rise of field current before being limited to the thermal unit value.

2. Rotor angle limiter:

The unit comprehends the rotor and a DC signal proportional to the load or rotor angle by

means of a simple analog circuit. When the rotor angle exceeds the limit settable with the

reference potentiometer the excitation is increased immediately to reduce the load angle

to the limit value. The rotor angle limiter takes over control de-coupling the output of the

AVR

3. Stator current limiter:

36


This avoid thermal over loading of the stator windings. Stator current limiter is provided

to protect the generator against long duration of large stator currents. For excessive

inductive currents it acts over the AVR after a certain time lag and decreases the

excitation current

Slip stabilizing units:

The slip stabilizing unit is used for the suppression of rotor oscillation of the alternator

through the additional influence of excitation. The slip as well as the acceleration signals

needed for the stabilization is derived from the active power delivered by the alternator.

Both the signals, which are correspondingly amplified and summed up, influence the

excitation of the synchronous machine through AVR in a manner as to suppress the rotor

oscillations.

4.3 Power supplies

220VDC is used for fan control,FB control, indication lamps, local/remote

controls

+/_24 V DC is used for pulse generation, transducers, relays, alarms

+/_15 V DC is used for all electronic cards

240 V AC UPS is used for rotor E/F relay and rotor temperature measuring

circuits

DC/DC converter with automatic input breaker and current limit for protection

against over- loads and short circuits

Protection against input polarity reversal

ERSPA/ERSNA power supply cards

ECMPA/ECMNA crow-bar intervention cards for initiating a thyristor for

maximum and minimum values of 24/15V

Suppression of pulse during intervention

The voltage regulating equipment needs an AC supply 380V,3phase for its power supply

units, which is derived from the secondary side of the rectifier transformer through an

37


auxiliary transformer. The voltage is reduced to different levels required for the power

packs by means of multi winding transformers.

A separate transformer supplies the synchronous voltage 3*380V for the filter circuit of

each channel and the voltage relay. During testing and pre-commissioning activities when

the generator voltage is not available, the step down transformer is used for testing

purpose with the help of a regulator test/service switch.

The supply for the thyristor bridge fan taken from an independent transformer, which gets

its input supply from the secondary of the excitation transformer. The control &

protection relays need 48V & 24V DC which are delivered from the station battery by

means of DC/DC converters, which are internally protected against overload.

Power supply for internal electric circuit:

To assure a high level of redundancy of power supply for the regulation and controlling

of excitation, a multiple redundant power supply is designed.

The redundancy results out of two power supply, fed from the two independent inputs

from the 220V station batteries, and one power supply fed from the excitation

transformer via the internal adaptation transformer. By this, different power supplies a

high availability has been realized.

A fault in the feeding or in the power supplies will be announced by the under voltage

monitoring devices of the internal power transducers.

The internal voltage level is 24V DC. All power supplies units are connected in parallel

by a diode module, which also decouples these different units.

The +24V level is feeding the following part:

Electronic modules channel 1

Electronic modules of channels 2

The internal signal level 2L+

The supply for the temperature monitoring device of the excitation transformer

The supply for the operator panels

38


4.4Faults and Protection in Excitation System

EXCITATION FAULTS

30F RELAY ACTUATION(CLASS-A TRIP):

Failure of both thyristor cooling fans

Failure of power supply to protection channel

Thyristor Over-load & Crow-bar Over-load Instantaneous

maximum current protection Excitation

transformer O/C and temperature protection Failure of

either thyristors or fuses in two branches of the same phase Failure of

changeover from Auto to Manual

30 GATUATION (CLASS- B TRIP):

Failure of Auto channel power supply in case Manual follow-up switch is in off

position

Failure of Manual channel power supply

Rotor earth fault relay OPERATION

PROTECTION ON AC SIDE:

Protection against High frequency over-voltages coming from the Excitation

transformer primary

Protection against voltage oscillations due to change over

PROTECTION ON DC SIDE:

Field breaker along with Discharge resistor

Crow-bar (Thyristor operated over-voltage protection)

Thiryties (Non-linear resistance)

1. over voltage Protection: 39


The rotor, as well as the thyristors, must be protected against over voltage as it may be

caused by switching operations, slipping of the rotor or atmospherically disturbances.

According to these reasons the over voltage protection is placed in parallel to the

thyristors and the rotor. The over voltage protection, called “crow bar system”, consist of

two anti parallel thyristors and a firing stage. The most important part of the firing stage

is a break over diode (BOD), which is responsible for the value at which the over voltage

thyristors will be fired. The value is fixed to at 100 V below the calculated peak inverse

voltage of the thyristor (PIV).

2. Protection against Negative Over voltage:

Anti parallel to the thyristor rectifier an independent thyristor is mounted. If negative

over voltage occurs, caused by a current flowing against the rectifier direction or a failure

of all thyristor fuses, that crow bar thyristor is fired. After firing of the thyristor, the

current is flowing over the existing deexcitation resistor and the series connected over

current relay. The auxiliary contact of this relay erects a Trip signal. The field circuit

breaker switches off and its normally closed contact short-circuits the crow bar thyristor.

3. Protection against Positive Over voltage:

Parallel to the thyristor rectifier an independent thyristor is mounted. In case of positive

over voltage, caused by atmospherically disturbances or by slipping of the rotor, that

thyristor will be fired by the firing stage.

40

40


CHAPTER -5

AUTOMATIC VOLTAGE

REGULATOR

5. AUTOMATIC VOLTAGE REGULATOR

41


Maintaining the Terminal voltage at the set reference [A U T O]

Field current of the Generator is matched extremely fast to the varying operating

conditions

Actual value of the Generator terminal voltage is sensed

Compared with the set reference value and a control signal is derived Control

signal is used to change the angle of the firing pulses Output voltage of

the bride is changed by advancing or retarding the firing angle of the thyristor

Maintaining the Excitation current at the set reference [M A N U A L]

ANALOG VERSION:

In general meet the requirements of speed and adaptability

Pulses are generated in Gate control units provided with discrete

Difficult to incorporate automatic diagnostic features like self monitoring & fault

detection

Number of cards & modules required for configuring a system with complex control

features are many

Lack of operational flexibility

DIGITAL VERSION:

Microprocessor based DVR with a Central processing unit connected to

A serial data bus for inputs and outputs

A control bus

DIGITAL VOLTAGE REGULATOR [D V R]

42


Self monitoring & diagnostic features

Drift free parameter set values for long term stability Ease of setting & measuring of the stored parameters and variables using local

Micro terminal

Settings are in digital form and hence stable behavior even with distorted synchronous

voltage Smaller

number & less varieties of different electronic cards and modules Easy adaptation

to Customers individual requirements Communication link with

other system. Adequate security to prevent

unauthorized operation Very fast response (< 20 ms)

to network disturbances High noise immunity &

Better reliability Precision Rotor temperature

measurement

Electronic regulation and control:

Each of the cubicle contains all modules of electronic regulation and control and

Is built of plug-in moguls of the system SIMADYN-D mounted in one electronic

Box nearby the thyristor rectifier .A detailed description of the modules is enclosed.

Each of the cubic accommodates all electronic gating and control devices.

These includes the voltage and current transducers, the references setters of the AVR and

ECR(both setters being suitable for remote adjustment),the voltage regulator, the field

current controller, the thyristor firing circuits, the current limiting controller and the

additional control devices. All open and closed loop functions are realized in the software

Designs:

43


The digital voltage regulator is for the execution and closed loop control of generators

with static excitation systems. Open loop and closed loop control of the excitations

system are implemented in the T400 processor modules. The T400 modules are located in

the SIMOREG-DG –MASTER unit. This digital processor module is part of the

SIMADYN-D family and contains a 32-bit microprocessor with I/Os. The software of

digital open loop and closed loop control is the D7-ES programming language. The open

loop control comprises evaluation and interlocking of command from the control room

and other part of plant on the input side, and essentially the formation and evaluation

status signals and faults alarms on the output side. Signal exchange between the

excitation system and the higher level I&C is implemented by means of hardwired

connection via the serial communication interface S-32.from here, data transfer is

performed to the T400 processor module via fibro elastic connection to the serial

communication board SCB1which is also integrated in the electronic box of the imoreg-

dc-master. The SCB1 is connected to the T400 via the rear SIDE BUS.

External binary signals (5L+)

Under voltage monitoring relays supervise the internal voltages 1L+2L+.if the voltage of

running channel is missing,a switchover to the standby channels results.

Operator Control and Operating Models:

All functions described in this section for operator control. Interlocking, openloop, closed

loop control are implemented in the software of the T-400 processor module (see

software documentation) of each channel.

Operator control from the control room/ unit control board (UCB)

Open loop control is performed from the control room / UCB and is working to both

channels.

Each of both channels consists of

- one AVR

- one ECR

44


Operation is possible both in automatic mode (generator voltage regulator) and in

manual mode (excitation current regulation) from the control room.

The following control options can be implemented from the control room/ UCB:

- Excitation on/off

- Automatic ON (AVR ON)

- Manual ON (ECR ON)

- Voltage set point higher / lower in automatic mode

- Field current set point higher / lower in manual mode

- Switch-over channel 1/channel 2

In addition to various status signal, e.g. “excitation is on /off, different Alarms”, the

reference value of field current and the actual value field current are given to the control

room via isolating amplifiers as 4 to 20mA analogue signals.

LOCAL CONTROL:

For maintenance and commissioning or for failure detection the local control unit OP1S

are implemented, which are mounted in the front door of each cubical (+CHN01/ 02).

Also different analogue actual values will be retrieved.

All alarms will be stored in the message processor system and will be indicated in clear

text at the display of the parameterization module is possible after selecting/ reading of

all message and elimination of the faults in position “local control” the following control

functions are possible:

Adjusting of the reference value.

Excitation on/ off.

Switch over manual mode / AVR mode.

The reference value can be modified by raise/ lower.

“Excitation off” is possible when the generator is not connected to the grid. If the

excitation system does not react to the “on” or “off” command please puse first the other

button and then once more the correct button. E.g. if the excitation is off and can not be

45


switched on, first press the “o” – button and after this the “1” – button.

Switch over from ECR to AVR is only possible, when the actual of the generator voltage

is in the range of the reference value setter.

Emergency Switch Over to Channel 2:

Automatic switchover from channel 1 to channel 2 and vice versa (emergency

switchover) is implemented on:

1. actual value detection error

2. MCB trip for voltage actual value detection

3. Ceiling voltage monitoring

4. MCB trip for channel-specific signal supply voltage

5. Fault in the trigger set

6. Trip synchronizing voltage

7. Fault UR_DETECTION

8. fan draft

Emergency switch over to Manual mode

Automatic switchover from automatic to channel emergency switchover after a failure –

according item is implemented on:

1. Actual value detection error

2. MCB trip foe voltage actual value detection

3. Ceiling voltage monitoring

46


CHAPTER-6

PERFORMANCE AND

CHARACTERISTICS OF STATIC

EXCITATION EQUIPMENT

6. Performance and characteristics of static excitation

Equipment47


The study state and transient behaviors of a synchronous machine coupled to an infinite

system must be matched to the desired operating conditions by suitable section of control

functions in the entire excitation system.

The basic requirement of a closed loop excitation control is to hold the terminal voltage

of a generator at a pre-determined value independent of the change in loading condition

in addition to this the excitation system as to contribute the following functions also.

a) Maintenance of stable operation of a machine under study state transient dynamic

conditions.

b) Satisfactory operation with other machines connected in parallel.

c) Effective utilization of machine capabilities without exceeding machine operation

limits.

CEILING VOLTAGE:

It is the maximum voltage that can be impressed on the field under specified conditions.

Ceiling voltage ultimately determines how fast the field current can be change. For

normal disturbance, ceiling condition prevails for a few cycles to either increase or

decrease the excitation until the system returns to steady operating state. For static

excitation, the ceiling voltage ranges from 1.6 to 2.0 times the rated one, this is

considered to be adequate for a fast system response.

48


Exciter ceiling voltage as a function of response ratio for a high initial response excitation

system

RESPONSE:

Response is defined as the rate of increase of the excitation system output voltage seen

from the excitation voltage-system response curve. The stating point for evaluating the

rate of change shall be the initial rated value. This is a rough measure of how fast the

exciter output circuit voltage will rise with in a specified time, when the excitation

control is adjusted in the maximum increase direction. Response ratio defined by ASA, is

the numerical value, which is obtained when the excitation system response in volts per

seconds. Measured over first 0.5 sec. this applies only for 0.5 sec as shown. Area

abd=Area acd, by approximation (see figure)

It is being recognized for sometimes that the modern fast excitation systems may

reach ceiling in 0.1 sec or less and as such the earlier definition is not applicable. There is

called high initial response excitation system.

SYSTEM STASTE ACCURACY:

It is the degree of correspondence between the control variable and the ideal value under

the specified steady state conditions. The accuracy of the excitation system for changing

the field parameters to keep the generator terminal voltage at a fixed level depends on its

static gain and time constants. By choosing higher static gain for the system, the steady

state error can be minimized appreciably and there by improving the steady state

accuracy with in + or – 0.5 % this can be reduced for the proper integral control.

OTHER SPECIFICATIONS:

Excitation system performance could be judges by the exciter voltage Vs time

characteristics in response to a step change in generated voltage. (See figure below)

The factors to be studied for optimum performance are: 1)

Overshoot 2)

Rise time 3)

49


Setting time 4)

Damping ratio

For ideal performance, it should have one over shoot and one under shoot with a quicker

rise time to have a smaller steady state error.

TRANSIENT AND DYNAMIC STABILITY LIMIT:

The success of excitation control lies upon the extent of meeting the requirement of

capability of the machine and there by giving the dynamic performance of the system. A

power is a constant voltage system and is made because of system component of design

and close voltage control. Fast excitation helps during disturbances and contributes to the

system stability allowing the require transfer of power event during the disturbance due

to smaller time constants in the excitation control loop, it is assumed that quick control

efforts could be achieved through this.

In transient stability the machine is subjected to several disturbances for a short time.

This results in dip in the machine terminal voltage and power transfer. Taking one

machine connected to infinite bus, the equation for power transfer can be written as

P = ((V1 x V) / x) X si

Where Vt = machine internal voltage

50


V = infinite bus voltage

X = interconnected reactance

δ = load angle

From the equation if Vt is reduced ‘P’ is reduced by corresponding amount. For

maintaining the power transfer ‘p’ the excitation should be fast acting enough to boost of

the field to ceiling and there by holding the terminal voltage Vt at the desired value. Thus

it is advantageous to have higher speed and ceiling value in excitation control circuitry.

The fault is removed the reactance ‘X’ suddenly changes there by causing unbalance

conditions due to high power swings which in turn needs fast corrective action through a

excitation system to bring the machine to normal operating conditions.

Modern fast and high response excitation system helps in two ways by reducing the

severity of the machines first swing during transient disturbances and also ensuring that

the subsequent swings are smaller than the first one. Thus it helps in increasing the

transient stability limit. With a typical static excitation system, ceiling level can be

achieved with in 20 ms due to which it offers an improved transient stability limits.

Following a disturbance the group of machines operating in the control group experience

smaller oscillations. Moreover the oscillating control group of machines reacts with each

other reinforcing these oscillations. Here the change in excitation may not result in stable

operation because by the time corrective action being taken by the excitation system the

excitation system the oscillating system changes casing separate excitation requirements

to be need. Through faster excitation system avoid this problem to certain extent power

system stabilizers as mentioned earlier or employed along with the AVR to damp out the

subsequent smaller swings in the system. This stabilizer gain in adjusted a value

depending on the negative damping of the system and the other network parameters.

Power system stabilizer helps to damp out inner area oscillation explained above and also

local machine system oscillations.

EFFECT OF EXCITATION SYSTEM ON TRANSIENT STABILITY:

51


Since the transient stability problems deals with the performance of power system when

subjected to sudden disturbances, sometimes leading to loss of synchronism, it is

worthwhile to study the behavior during the first swing as the period is of very short

duration. The major factors influence outcomes are the machine behavior and the power

network dynamic relations. For this is assumed that the mechanical power supplied by

the prim mover remains constant during the disturbance. Therefore the effect of

excitation control on this type of transient depends on its ability to help the generator to

maintain its output power in the above period.

The main factors that affect the performance during sever transients are

1) The disturbing influence of impact: this includes type of disturbance, its location

and duration.

2) The ability of the transient system to maintain synchronizing forces during the

transients.

3) Turbine and generator parameters.

These factors mainly affect the first swing transient. The system parameters influencing

these factors are

1) Synchronous machine parameters. Of these, the most important are

A) The inertia constant.

B) The direct axis transient reactance.

C) The direct axis open circuit time constant.

D) The ability of the excitation system to hold the flux level of the

synchronous machine and increase the output during transients.

2) The transmission system impedance’s under normal, faulted, and post fault

conditions. Here the flexibility of switching out faulted section is important such that

the large transfer admittance between synchronous machines is maintained when fault

is cleared.

3) The protective relaying scheme equipment. The object is to defect the fault and

isolate the fault sections quickly with minimum disruption.

During transients initiated by a fault, the armature reaction as the tendency to reduce the

flux linkage. Hence the type of excitation must be so chosen as to have a fast speed of

response and a high sealing voltage voltage as an aid to transient stability. With the help

of faster boosting up of the excitation, the internal machine flux can be off-stetted and

52


consequently the machine out-power may be increased during the first swing this results

in the reduction of accelerating power and there by effects improvements of transient

performance of the system.

The subject is not dealt details in greater details as a performance are to be evaluated on a

case to bases after obtaining the transfer functions of each elements inside the static

excitation equipment and the interconnected network.

53


CHAPTER-7

LIMITERS IN STATIC

EXCITATION SYSTEM

7. LIMITERS IN STATIC EXCITATION SYSTEM

Limit controllers:

With ever increasing size of generating units today, more stringent requirements have to

be met by excitation systems. Today, it is proven beyond doubt, that static excitation

assures stable operation both under dynamic and transient conditions. Generators running

parallel with the power network even under extreme conditions must remain in

synchronism without the maximum load limit on it being exceeded and without the

protective relays operating. An automatic voltage regulator AVR alone cannot ensure

this. Optimum utilization of the generator can be ensured only if the basic AVR is

54


influenced by additional signals to limit the under excitation and over-excitation of the

machine. Thus, limit controllers working in conjunction with the AVR ensure.

A) Optimum utilization of the machine.

B) Security of parallel operation etc.

Limit controllers simplify the job of the operating-staff and stable operation close to the

limiting values. With limit controllers in service errors and fault in the regulator led only

to the limit value control and not to disconnection.

It has to be understood that limit controllers however are not meant replace the protection

system but they are only indented to prevent the protection system from operating under

extreme transient conditions.

Parameters for limitations:

Limiters, whenever they intervene, influence the voltage regulator suitable to bring about

a corresponding change in the excitation. The following are the parameters which are to

be limited.

1) stator current under condition of over excitation and under excitation

2) rotor current

3) rotor angle or the lo

Mechanism of limiter intervention:

During over-excitation, the rotor current and stator current limiters intervene to bring

about a reduction in excitation. On the other hand during under-excitation, limitation of

rotor angle and stator current influences to increase the excitation. Rotor and stator

current limiters must be designed to intervene after a certain delay so as to permit

temporary over/ceiling excitation. Limiters do not impair the control behavior of the

AVR as over-excited condition can exist in the event of load surge of because of short-

lived faults in the power supply network. The AVR reacts to a distant fault (say 3-phase

short circuit) and commands ceiling excitation to be applied, thereby increasing the

synchronizing torque of the machine and prevents it from losing synchronism.

55


However, if the short circuit persists ands has not been cleared by system protection after

a set time; delayed rotor current limiter comes in to operation preventing the generator

and the excitation equipment from being subjected to thermal over load. An identical

situation prevails during sudden connection o load to the system. The AVR enables short-

time ceiling excitation to increase the excitation to prevail so as to obtain lower settling

time.

In under-excited mode, the rotor angle limiter and stator current limiter must intervene

instantaneously to increases the excitation to prevent the further increment in the rotor

angle.

In the under excitation mode, stator current limiter is essentially used with multiple-pole

synchronous condensers which run at suitable level of excitation to increase the

capacitive absorption capability of the machine. Power diagram of the generator and

range of influence of limit controllers.

The operational limits of the synchronous machine are shown in the power circle

diagram. The application and range of influence of the limiters depends on the conditions

in the installation and the generator data. The possible zone of intervention of the limiter

is marked in the power Chanel.

DIFFERENT TYPES OF LIMITERS

Rotor angle limiter:

Line AB represents the range of influence of the rotor angle limiter, the maximum angle

of which has been taken as 85°. Although stable operation can be ensured even beyond

85° with the fast acting load angle limiter in the action and achieve greater possible

reactive power absorption capability, the load angle is limited for practical purposes to

85° because of the following considerations.56


1. In the event of short circuit in the systems, the generators may accelerate owing to the

abrupt partial removal of the electrical load and as the turbine governor can not act fast,

the rotor angle increases and the angle can be become so large relative to the system

vector that the machine may fall out of the step.

2. The excitation system (AVR) switches over to the manual mode in the event of

internal faults in the auto-mode. Changeover to manual mode signifies constant excitation

and hence a stable operation up to a maximum angle of 90º electrical only possible.

The rotor angle limiter limits the load angle of the machine to an acceptable present

value. The load angle is the electrical angle between the voltage vector of the system and

vector of the machine voltage ‘e’ figure below.

Uv system voltage Φ power factor

e Generator voltage Xq quadrature axis reactance

I generator stator current δ rotor angle

The system vector is derived from the voltage vector of the generator Uv by adding to it

the voltage drop in reactance’s external to the machine. This takes in to account the

transformers and transmission lines between the generator and the system load center.

The rotor voltage is simulated adding the inductive voltage drop in the machine Ixe drop

(reactance drop in the transmission line, transformers etc.) from the generator terminal 57


voltage. The phase angle between ‘e’ and Un is converted in to a proportional D.C.

voltage. The actual value is compared with an adjustable reference and fed to the input of

an operational amplifier. In case the angle exceeds the set value and the output signal

immediately takes over the control of the thyristor network to build of the generator air

gap flux fast enough to avoid slipping. It stands to reason that the output of the limiter

acts directly over AVR output to avoid any loss of time due to filter time constants in the

AVR. Figure below explains the operation of automatic voltage regulator in conjunction

with rotor angle limit.

The AVR may drive the field or the thyristtor network in to over load for one or more of

the following reasons:

a) Faulty handling

b) System voltage reduction

c) Loss of sensing voltage to AVR and

d) Failure within the controller.

The excitation limiter must prevent this over load from persisting. On the other hand,

during dynamic disturbances in the system the excitation should not be reduced at once

but ceiling excitation should be possible for a limited time.58


The limiter can be operated in three different modes as explained below to cater the

above requirements.

1. Simple mode: in this mode the excitation current is limited to a present maximum

value. The limiter intervenes with a time delay which is proportional to the

magnitude of the over load. And which the limiter in operation, the current is

limited steadily to the rated value.

2. Dips steeply for any reason, the ceiling excitation limit are validated again. The

ceiling excitation current helps in the increasing the short circuit current in the

fault zone and hence aids selective tripping of the faulted section.

3. Tching mode: in the switching mode the excitation is limited to the thermal of

rated current value. Only in case of sharp dip in the machine voltage, the ceiling

limit is enabled momentarily. The limit switches back to the rated value after the

set time.

Stator current limiter:

The machine is over-excited or under-excited. The excitation current is to be suitably

reduced to limit the inductive stator current and is increased to limit the inductive stator

current and is increased to limit the capacitive current. The rotor angle limiter provides a

more definite protection in preventing the machine from falling out of step. Capacitive

stator current limitation comes in to play only with synchronous condensers which are to

some extent negatively excited generators it prevents excessive leading MAVR loading

corresponding to any given MW load.

The generator stator current is converted in to polarized dc +ve or –ve, depending upon

whether the machine is over-excited or under-excited. This voltage forms the actual value

for the controllers which process each of the bipolar signals independently. One of these

controllers compares the capacitive stator current against its reference and acts directly on

the regulator via a decoupling diode to increase the excitation. The action of second

controller that limits the inductive stator current is delayed by means of an integrator

before it influences the control input of the AVR so as to reduce the excitation. The time

log offered is perfectly acceptable as for as stator over-heating is concerned because the

59


integrator time constant is set one order less than the stator terminal time constant. Figure

below shows an AVR operation in conjunction with a stator current limiter.

Power factor regulator:

The possible applications to which the power factor regulator can be employed include:

a) Power factor regulation of a synchronous generator.

b) Power factor regulation of a synchronous motor.

c) Power factor regulation of lines leading from an installation comprising a

synchronous motor, of synchronous generator and various other consumers.

Power factor regulation of synchronous generator

In case of synchronous generator, the p.f. regulator does not influence the excitation

directly. This is because; with a short circuit in the mains no ceiling excitation can occur.

On the contrary, because of the inductive nature of the short circuit current, the generator

is de-excited by the power factor regulator as a result. The machine commences to slip

and large voltages are including in the rotor. Hence, in synchronous generator power

factor regulators operate by influencing the reference setting of the AVR. It should be

noted that this mode of influence is limited to a percentage of the range to which the

reference of the AVR is limited.

60


The power factor regulator influences the AVR reference through an intermediate stage

that has adjustable dead band so that regulation to corresponding precision can be

achieved. AN advantage of this mode of operation is that the stabilization of the regulator

circuit poses no special problem, as the gains chosen are normally low.

Figure below gives the block diagram of p.f. Regulation of synchronous

generator.

Power factor regulation of a synchronous motor:

Principally a synchronous motor behaves identical to a synchronous generator, regarding

generation and consumption of synchronous torque and stability. In synchronous motor,

care must be taken so that the driving torque does not fall below a critical level because

of low excitation; an AVR in conjunction with a power factor regulator ensures this. The

AVR responds to voltage dips and enable the synchronizing torque of the machine to be

increased by forcing ceiling excitation. The mode of p.f. regulation however is the same

as adopted in case of synchronous generator. The power regulator can also be used as a

limiter without delay, acting directly upon the AVR output so that with a set power factor

61


no under excitation can occur. However, it becomes an absolute necessity to use a rotor

current limiter on the over excited side.

The above mode of p.f. regulation is rarely resorted so as it becomes a problem to

stabilize the loop is the p.f. regulator is used without a delay in conjunction with fast

acting AVR.

P.F. Regulator Operated in the Limiter Zone:

Two stage rotor angle limiter

While the rotor angle limiters described earlier introduce a definite limit on the rotor

angle it imposes a restriction on the utilization of a machine capacity in the leading zone.

As a general rule, practical angle set have a 10% margin over the safe maximum δ

recommended by the machine manufacturers. If that be so as a portion of the under

excited region in the power capability diagram remains unutilized this is because of the

linear characteristics of the rotor angle limiter and non-linear machine power capability

diagram.

This limitation of rotor angle limiter can be over come with usage of two stage rotor

angle limiter. A view at the power Chart would be set high at low loads and then

modified suitably to a lower value at higher loads so as to operate within the stability

limits. Its is only necessary to predetermine the changeover point after which the load

angle reference set on the rotor angle limiter should be influenced to reduce the δ

reference value. The changeover point can be determined by measurement of active and

reactive stator currents.

The unit a basically consists of 3 modules namely;

1. Active current measuring unit.

2. Reactive current measuring unit.

3. Under excitation regulator

Which act in conjunction with the rotor angle limiter?

The active & reactive current measured values from active & reactive current measuring

units is summed up and compared against a reference value in one channel in under

excitation limiting regulator. The active current measured in the measuring unit is

compared independently against a reference value in a second channel. As the active and

reactive determine the changeover point, output of channel 1 overrides the rotor angle

62


limiter signal once the summated active and reactive current exceeds the reference value.

Output of channel 2 influence the reference as soon as the active current measured value

exceeds active current reference value.

Figure below shows the block diagram of two stage rotor angle limiter.

1. Active current measuring unit.

2. Reactive current measuring unit

3. Under excitation regulator.

(N-2) Current Limiter

Static excitation system for large machines invariably consist of 3 or 4 thyristor bridges

operating in parallel cater to the excitation requirements of the generator. Failure of one

out of the total number of bridges does not impose any restriction on the field current

owing to redundancy. However the current limiter unit introduces a limit on the

excitation current on the occurrence of failure of yet another thyristor bridge. The unit

cop raises a relay and the reference value is led through the relay contacts to the rotor

current limiter output stage. Intervention of (N-2) limiter introduces a definite limit on the

excitation current and application of ceiling excitation is prevented as the thyristor

bridges are not designed to meet ceiling excitation requirement under two bridge failure

conditions.

Manual limiter:

Static excitation system designed for medium and large synchronous machines generally

have two independent channels namely the auto channel & the manual channel. While

63


auto-mode is normal mode of operation, operation is manual mode could at time be

inevitable.

In the auto mode, the machine is adequately protected against risks of outage by limiter

but in the event of operation in manual mode the machine is guaranteed only by the

protection system. Hence with view to increase the safety and reliability of operation in

manual channel, a manual limiter unit is essential

The unit basically consists of:

1. (N-2) bridge current limitation unit for auto channel.

2. (N-2) bridge current limitation unit for the manual channel.

3. Rotor angle limiter influence circuit in the manual channel.

4. If, limiter for manual channel.

(N-2) limiter (Auto channel):

This is identical to the (N-2) limiter described. The limiter derives its power

supply from supply “A”.

(N-2) limiter (Manual channel):

The output of manual reference potentiometer (Manual list) is led through a buffer stage.

The output of the buffer stage is limited to a preset value under (N-2) bridge failure

conditions. LED indications are provided to indicate the presence of limit condition.

Rotor Angle Limiter Influence:

The rotor angle limiter unit is carryover for both auto manual mode of operation. On

change over to manual mode operation the rotor angle limiter output is led to a buffer

stage through the contact of a relay which pulls in as soon as the system changes over to

manual.

The manual control signal is also led to the input of the same buffer stage but the

impedances of the two inputs are so adjusted that rotor angle limiter in service the output

to rotor angle limiter over rides manual control signal. With rotor angle limiter not in 64


service output of rotor angle limiter follows the manual use by approximately 5V. The

unit derives its power supply from supply manual channel supply, ‘supply M’.

ROTOR CURRENT LIMITER FOR MANUAL CHANNEL:

While operating in the manual channel, there can be over loading on the machine

continuously. This is owing to the reason that the protection relays are set corresponding

to ceiling excitation. Hence protection would not protect the machine if the machine over

loaded continuously beyond its over-load capacity. This would result in rotor over

heating and tripping of the unit on over temperature. If limiter circuit provides an

adjustable limit and is switched in to circuit wherever the unit changes over to manual

mode. This if limit is adjustable and can be suitable set to limit the manual Ust

corresponding to full load excitation requirement of the machine.

(N-2) Limiter (Manual Channel):

The rotor current limiter together with (N-2) limiter (Auto Channel) provides a definite

protection by limiting the field current to predetermined limit under (N-2) bridge failure

conditions. However, to avoid accidental over loading if operated in manual channel

under two bridge failure conditions, it is necessary to limit the channel Ust. It is for this

reason that (N-2) limiter for the manual channel is essential.

Rotor Angle Limiter Influence:

In order to protect the machine against under excitation because of an operational error

and prevent tripping of the unit, it is essential to limit the under excited operation in the

manual mode. It is with this view that the rotor angle unit arranged to influence the

manual Ust suitably

SPECIAL FEATURES:

Use of modular elements

Easy access & replacement of converter modules, Fuses, Control modules etc.

open cycle forced ventilation with independent double cooling fans

Two independent channels auto and manual65


Follow-up circuit for automatic transfer from auto to manual in case of faults

in auto

Independent power supplies for auto and manual

Search coils for continuous monitoring of conduction of thyristors

Indication lamps for thyristors healthiness

Limiters for auto channel. Thyristor operated Over-voltage protection

Independent protection circuit

MERITS & DE-MERITS:

Fast response &no moving parts

No inherent time delay

Rapid field suppression by inversion and discharge resistance

Better dynamic performance due to negligible time constant

Direct measurement of field voltage and current

Higher capacity field breaker is required

Separate initial voltage build-up circuit is required

Higher rating thyristors are required

Regular maintenance and periodic dressing of slip-ring

Replacement of carbon brushes, inventory of carbon brushes

CONCLUSION

While a large portion of this article has dealt with existing standard provisions in the

excitation system, two stage rotor angle limiter and manual limiter are the new

provisions. The former has been incorporated is same SEE’s based on customer’s

requirements but the latter is still to find place as a regular feature. It is proposed to

introduce manual limiter as a regular feature of SEE in order to ensure more reliable and

safe manual operation. However, two-stage rotor angle limiter will only be provided, if

consumer specifications call for the same.

The latest regulator and limitations functions contained in the excitation system comply

with the strangest operating requirements in terms of safety, governing quality and 66


availability. The primary task of the AVR in the synchronous machine is to maintain the

terminal voltage of the generator at constant level and guarantee reliable machine

operations in all operating points. The governing functions of the AVR include voltage

control; reactive power control, power factor control and the controlling the functions

linked to the AVR include excitation current limitations, stator current limitation, and

rotor displacement angle limitation. The different variants for generating excitation

current include Auxiliary exciter, Permanent magnet generator, Field circuit transformer

and Auxillary minding in the main machine.

REFERENCE BOOKS

Electrical Power System BY C.L.WADHAWA

Power System Design & analysis BY B.R GUPTHA

Switch Gear and protection BY SUNIL S. RAO

Electrical & electronic measurement

& Instrumentation BY A.K SAWHNTY

67


Modern power system analysis BY D.P. KOTHARI

I.J. NAGRATH

Electrical machines BY P.S.BIMBRA

Electrical machines BY D.P. KOTHARI

I.J. NAGRATH

Power electronics BY P.S.BIMBRA

Power electronics BY M.D.SINGH

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Suresh Main Project of Excitation