power line disturbance.pdf

VTC 20131

Advanced Electrical Services

Chapter 5 : Power Line Disurbances

Learning outcome

� suggest solutions to improve power line disturbances in the electrical systems

Learning contents� Common power electronics related power line disturbances,

Voltage spikes, Chopped voltage waveform, Harmonics, Electromagnetic Interference.

� Harmonics Filter

Conclusion

Common Power Disturbances

Voltage SagsOne of the most common power frequency disturbances is voltage sag. By

definition, voltage sag is an event that can last from half of a cycle to several

seconds.

Voltage Sags due to motor starting or Arc furnace.

Voltage sags typically are due to starting on large loads, such as an electric

motor or an arc furnace. Induction motors draw starting currents ranging

between 600 and 800% of their nominal full load currents.

The current starts at the high value and tapers off to the normal running

current in about 2 to 8 sec.

Figure below contains the waveform of the starting current of a 50-hp

induction motor with a rated full-load current of 60 A at 460 VAC.

Conclusion

Common Power Disturbances

Waveform of Starting current

Voltage sag due to starting of induction motorVoltage sag due to starting of induction motorVoltage sag due to starting of induction motorVoltage sag due to starting of induction motor

� The voltage experienced by a load fed from the same bus as the motor is starting:

� Vsag = Zm/(Zs+Zm)� Zs=V2/Ssc,, Zm=V2/KSmotor

� Where K= ratio between the starting current and the nominal current

� Smotor = a motor of rated power

� Ssc = Short-circuit power

~

M

Zs

Other loads

Example : Suppose that a 5MVA motor is started from a 100MVA,11kV

supply. The starting current is six times the nominal current. Please

calculate the voltage at the busbar during motor starting.

Vsag = Ssc/ (Ssc+KSmotor

)

� Vsag = 100MVA/ (100MVA+6X5MVA) X100%

� Vsag = 77%.

� Hence, the voltage at the bus during motor starting = 77% x

11kV =8.47kV.

Operational measures to minimize voltage disturbanceOperational measures to minimize voltage disturbanceOperational measures to minimize voltage disturbanceOperational measures to minimize voltage disturbance

� The starting of large motor causes disturbances to locally connected loads and to the motor itself. One profound effect of motor starting is voltage dip.

� A number of schemes of reducing voltage dip during motor starting, that is � Reduction of terminal voltage by using an auto-transformer.� Reduce of terminal voltage by using a star-delta switch.� By using the Variable Frequency Drive (VFD) to control the motor.

Mitigation Measures Mitigation Measures Mitigation Measures Mitigation Measures On Customer side:On Customer side:On Customer side:On Customer side:

� Specify equipment with ride through capacity� Properly adjust the settings of the protection for an

equipment.� Install equipment protection device

� Constant voltage transformer� Voltage Dip Proof Inverter� contactor hold-in circuit

� Use a UPS for sensitive/essential load

Case Study: Chiller

Precaution

Problem

Voltage Dip

Improper time delay setting of U/V Relay cannot over

ride the effect of Voltage Dip and causes tripping.

U/V

Relay

Chiller

Voltage Dip

Choose a suitable time delay to over ride the effect of

Voltage Dip.

U/V

Relay

Chiller

Voltage dip mitigation solutionsVoltage dip mitigation solutionsVoltage dip mitigation solutionsVoltage dip mitigation solutions

� Ferroresonant transformer (Constant-voltage transformers)� Voltage regulator� UPS� Voltage dip proofing inverter � Coil lock Voltage Dip

Proofing Inverter

(DPI)

$6000/kVA

Constant Voltage

Transformer

(CVT)

$5000/kVA(UPS)

$7000/kVA

Coil Lock ($600/unit)

VOLTAGE DIP PROOF INVERTER (DPI)VOLTAGE DIP PROOF INVERTER (DPI)VOLTAGE DIP PROOF INVERTER (DPI)VOLTAGE DIP PROOF INVERTER (DPI)

� Provides a voltage ride through solution for process controls that suffer from voltage interruptions (including sags) less than 3 seconds in duration

� No batteries; therefore, no replacement and maintenance costs or hazardous waste.

� More reliable; fast (<700µS) transfer, off-line system develops little heat & fails to safety.

� Able to withstand high inrush currents; no need to oversize as with UPS’s & CVT’s.

VOLTAGE DIP PROOF INVERTER (DPI)

� To determine the size of a DPI for an application it is necessary to have data as below:

� Load voltage� Load current� Load power factor� Uptime required

(Constant-voltage transformers)

� CVTs are especially attractive for constant, low power device.� CVTs are excited high on their saturation curves, thereby providing an output

voltage which is not significantly affected by input voltage variations.� As the loading increased, the corresponding ride-through capability is reduced and

when the CVTs is overloaded, the voltage will collapse to zero. � Typically, you would want a factor of 2.5 times the measured load.

Coil lockCoil lockCoil lockCoil lock

� Literally any device that is controlled by energizing an ac coil, when powered through a Coil-Lock unit will continue to operate as intended through these power disturbances.

� Not only does the Coil-Lock protect the coil circuits from voltage sags, but also has the ability to protect coils from lightning and switching induced transients just like a surge suppressor.

� The figure shows the typical unprotected relay or contactor's capability to ride through voltage sags down by 15% from its nominally rated voltage (yellow area in the figure).

� The same relay or contactor, when protected by the Coil-Lock, will continue to keep operating with voltages sags down by 75% from its nominally rated voltage (green area in the figure).

� For voltage sags down by greater than 75%, the Coil-Lock de-energizers the relay or contactor coil to maintain the safety function for emergency stop circuits (red area in the figure).

Coil lock

Conclusion

Static Uninterruptible Power Source System

Static uninterruptible power sources (UPSs) are devices that maintain power

to the loads during loss of normal power for a duration that is a function of

the individual UPS system.

All UPS units have an input rectifier to convert the AC voltage into DC

voltage, a battery system to provide power to loads during loss of normal

power, and an inverter which converts the DC voltage of the battery to an AC

voltage suitable for the load being supplied.

Static UPS systems may be broadly classified into offline and online

units.

Conclusion

Static Uninterruptible Power Source System

In the offline units, the loads are normally supplied from the primary electrical

source directly.

The primary electrical source may be utility power or an in-house generator. If

the primary power source fails, the power to the loads is switched to the

batteries and the inverter. The switching is accomplished within half of a cycle

in most UPS units, thereby allowing critical loads to continue to receive power.

During power transfer from the normal power to the batteries, the loads might

be subjected to transients. Once the loads are transferred to the batteries,

the length of time for which the loads would continue to receive power

depends on the capacity of the batteries and the amount of load.

UPS units usually can supply power for 15 to 30 min, at which time the

batteries become depleted to a level insufficient to supply the loads, and the

UPS unit shuts down.

Uninterruptible Power Source System (offline UPS)Uninterruptible Power Source System (offline UPS)Uninterruptible Power Source System (offline UPS)Uninterruptible Power Source System (offline UPS)

ConclusionStatic Uninterruptible Power Source System ( Online UPS)

In online UPS units, normal power is rectified into DC power and in turn

inverted to AC power to supply the loads. The loads are continuously

supplied from the DC bus even during times when the normal power is

available. A battery system is also connected to the DC bus of the UPS unit

and kept charged from the normal source.

When normal power fails, the DC bus is supplied from the battery system. No

actual power transfer occurs during this time, as the batteries are already

connected to the DC bus. Online units can be equipped with options such as

manual and static bypass switches to circumvent the UPS and supply power

to the loads directly from the normal source or an alternate source such as a

standby generator.

Static Uninterruptible Power Source System ( Online UPS)Static Uninterruptible Power Source System ( Online UPS)Static Uninterruptible Power Source System ( Online UPS)Static Uninterruptible Power Source System ( Online UPS)

The online UPS is the most advanced and most costly UPS. The inverter is

continuously providing clean power from the battery, and the computer

equipment is never receiving power directly from the AC outlet. However,

online units contain cooling fans, which do make noise and may require some

location planning for the home user or small office.

Harmonics Harmonics Harmonics Harmonics

What is harmonics ?

Multiples of the fundamental frequency of any

periodical waveform are called Harmonics.

50Hz (Fundamental Frequency)

+

+

+

150Hz (Third Harmonic)

250Hz (Fifth Harmonic)

Some load equipment does not draw a sinusoidal current

from a perfectly sinusoidal voltage source!

Harmonic distortion

Harmonics vs Transients� Harmonics are associated with the continuing operation

of load. Harmonics occur in the steady state and are integer multiples of the fundamental frequency.

� Transient are associated with changes in the system such as switching a capacitor bank. The natural frequencies have no relation to the system fundamental frequency.

� Harmonic distortion is caused by non-linear devices in the power system.

� When both the positive and negative half-cycles of a waveform have identical shapes, the fourier series contains only odd harmonics.

� In fact, the presence of even harmonics is often a clue that there is something wrong – either with the load equipment or with the transducer used to make the measurement. There are notable exceptions to this such as half-wave rectifiers and arc furnaces when the arc is random.

Harmonic Distortion

What is harmonic ?What is harmonic ?What is harmonic ?What is harmonic ?

� For power networks, 50 Hz (60 Hz) is thefundamental frequency and 150 Hz (180 Hz), 250 Hz (300 Hz) etc. are higher order harmonics viz. 3rd & 5th

� Odd Harmonics (5th, 7th… ..)� Even Harmonics (2nd , 4th … .)� Triplen (3rd, 9th , 15th ..)� Inter harmonics (2.5th => 125 Hz)

Triplen Harmonics� Triplen harmonics are the odd multiples of the third harmonic

(h=3,9,15,21……)� Two typical problems are overloading the neutral and telephone interfence.� Third-harmonics components are three times the third-harmonic-phase

currents because they naturally coincide in phase and time.� In the wye-delta transformer, the triplen harmonics current remain trapped in

the delta side when the system is operated in balanced load condition.� Measuring the current on the delta side of a transformer will not show the

triplens and not give a true idea of the heating the transformer is being subject to.

� Although fed by a delta-delta connection, the third harmonics show up in the large magnitudes in the line current when the furnace is operating in an imbalanced state.

Harmonics classificationHarmonics classificationHarmonics classificationHarmonics classification

Order Group Effects

n = 1 Fundamental active power

n= 6K+1, k=1,2 + sequence heating

n= 6K-1, K=1,2 - sequence heating &

motor

problems

n = 3n 0 sequence heating &

neutral

problems

Harmonic frequency and SequencesHarmonic frequency and SequencesHarmonic frequency and SequencesHarmonic frequency and Sequences

Harmonic order

Freq(Hz)) Sequence

Fundamental 50 +

3rd 150 0

5th 250 -

7th 350 +

9th 450 0

11th 550 -

13th 650 +

*note: Zero sequence may add (not cancel) in a common

wire and result in very high neutral currents

Sequences Phasor

Positive Seq. Negative Seq. Zero Seq.

R

YB

R

Y B

Source of Harmonics

Load Side

VL(t)

Harmonic Currents flowing

through the system impedance

results in Harmonic VoltageDistortion at load

Normal loadVS(t)

Supply

Side

Cable

Dirty Load

Harmonic

current

Voltage vs Current Distortion

• Nonlinear loads appear to be sources of harmonics current and injecting the harmonic currents into the power system.

• For nearly all analyses, it is sufficient to treat these harmonic-producing loads simply as current sources.

• Voltage distortion is the result of distorted current passing through the linear, series impedance of the power delivery system. This results in voltage harmonics appearing at the load bus.

• The amount of voltage distortion depends on the impedance and the current.

Where do the harmonics come from?Where do the harmonics come from?Where do the harmonics come from?Where do the harmonics come from?

� All magnetisation non-linearities� Transformer operating near saturation

� Higher voltage� No load current

� Inrush current� Arc Furnaces, ...� Power electronics, converters, drives...

� Rectifiers� Inverters� Cycloconverters

Where do the harmonics come from?Where do the harmonics come from?Where do the harmonics come from?Where do the harmonics come from?

� Arc welders� Uninterruptible power supplies (UPS)� Fluorescent lighting systems

All non linear loads(current response is not the same as the voltage source).

Load Mode Funda. Current THD-F (%) Dominating Harmonics

Computer with monitor On 0.54A 110% 3rd 58%

Laser printer Print 0.34A 113% 3rd 55%

Idle 0.11 160% 3rd 52%

Fax machine Send 0.16A 120% 3rd 87%

Print 3.74A 6% 3rd 5%

Idle 0.11A 98% 3rd 54%

Photocopier Copy 5.56A 26% 3rd 20%

Idle 0.35A 106% 5th 42%

UPS #1 Server 40A 35% 3rd 25%

UPS #2 PC 4.3A 130% 3rd 89%

Magnetic ballast w/cap On 0.21A 30% 3rd 18%

Electronic ballast #1 On 0.19A 34% 3rd 26%

Electronic ballast #2 On 0.23A 10% 3rd 9%

Sodium Lamp On 0.24A 64% 7th 44%

Compact florescent lamp On 0.1A 136% 3rd 49%

Fan coil On 8.5A 5% 5th 4.8%

Lift Run 39A 36% 5th 28%

Examples of Harmonic load - Commercial

Typical Harmonic Sources in Office

Switching-mode power supply:

THD 77%, 3rd 65%

Magnetic ballast

THD 22% , 3rd 20%

Typical Harmonic Sources in Office

DC drive :

THD 36%, 5th harmonics 33%

Typical Harmonic Sources from Industries

Adjustable Speed Drive -ASDs (HVAC system):

THD 45%, 5th 40%


PWM drive (no choke):

THD 131%, 5th 83%, 7th 78%


Single-phase power supplies

� A distinctive characteristic of switch-mode power supplies is a very high third-harmonics content in the current.

� Since third-harmonics current components are additive in the neutral of a three phase system, the increasing application of switch-mode power supplier causes concern for overloading of neutral conductors and transformer heating.


Three-phase power converters –D.C Drives

� Three phase power converters differ from single phase converter mainly because they do not generate third-harmonics current.

� The two largest harmonic currents for the six pulse rectifier are the 5th and the 7th.

� A 12-pulse rectifier can eliminate about 90% of the 5th and the 7th harmonics.


� The harmonic current distortion in adjustable-speed drives is not constant.

� For example, Speed increase, percentage of harmonics also increase.

Impact of operating condition

Problems created by harmonicsProblems created by harmonicsProblems created by harmonicsProblems created by harmonics

� Excessive heating of deviceHarmonic Distortion -> Increase of RMSPower loss = R . I1

2 + ∑ In2*R -> heating

� Nuisance tripping of circuit breaker� Increase of RMS ® Thermally� Increase of peak ® Magnetically

� Blown fuses

� Motor problemsAdditional losses in windings & iron(RMS increase & skin effect)

� Perturbing torques on shaft(negative sequences harmonics)


� Damage to electronic sensitive equipments� Electronic communications interferences� Excessive neutral current

(mainly zero-sequence harmonics)� Excessive harmonic current may lead to overheating (or even

burning) of network components


� Erratic operation of control and protection relays� Faulty reading of kWh meters� Capacitor problems

� Decrease of impedance with frequency� Resonance problems

� Capacitor overload� Capacitor problems

Due to its lower impedance, capacitors are even more susceptible to higher orderharmonics. If not designed for harmonic duty, a capacitor may fail pretty soon


Harmonic Impact on transformer� There are three effects that result in increased transformer

heating when load current includes harmonic components.� The increased total rms current results in increased conductor

losses.� Eddy-current losses. This component of transformer losses

increase with the square of the frequency of the current causing the eddy currents.

� Core losses

Harmonic Impacts on motors� Motors can be significantly impacted by the harmonic voltage

distortion. Harmonic voltage distortion at the motor terminals is translated into harmonic flux within motor.

� Decreased effeciency, along with heating, vibration and high-pitched noises are symptoms of harmonic voltage distortation.

Solutions to harmonic problemsSolutions to harmonic problemsSolutions to harmonic problemsSolutions to harmonic problems

• Work with the equipment manufacturer and utilities

• Specify equipment with low harmonics emission

• Properly design of passive filters

• Install active filters

Solutions to harmonic problemsSolutions to harmonic problemsSolutions to harmonic problemsSolutions to harmonic problems

� Active harmonic filters� Filtering principle: cancellation of harmonics by equal and opposite

harmonic generation by an active filter device

Active Filter

Active

Filter

Active filtersActive filtersActive filtersActive filters

Active filters are relatively new types of devices for eliminating harmonics. They are based on sophisticated power electronics and are much more expensive than passive filters. However, they have the distinct advantage that they do not resonate with the system.

Active filtersActive filtersActive filtersActive filtersThe basic idea is to replace the portion of the since wave that is missing in the current in a nonlinear load.An electronic control monitors the line voltage and/or current, switching the power electronics very precisely to track the load current or voltage and force it to be sinusoidal.

Principles for Controlling HarmonicsPrinciples for Controlling HarmonicsPrinciples for Controlling HarmonicsPrinciples for Controlling Harmonics

Passive filters

� Passive filters are made of inductance, capacitance, and resistance elements

� They are employed either to shunt the harmonic current off the line or to block their flow between parts of the system by tuning the elements to create a resonance at a selected harmonic frequency.

Example 2: Refer to the equivalent circuit as below.� System line-to-line voltage=VLL=13.8KV� The offending harmonic current ih of the fifth order, and it has been

measured to be 10A.� The size of the capacitor bank is 800kVar. The determination of filter

components should be based on the maximum system voltage 13.8kV.

~VLL

L

C

HarmonicSource

Shunt Filter

� The capacitive reactance can be determined by using the formula Xch= V2/(Qh)

� Where h=harmonic order� Xch=capacitive reactance at harmonic order h� Capacitive reactance at the fundamental frequency (60HZ) :

Xc1=13.8kV2/800kVar=238.05 ohm� C = 1/(2πf Xc) = 1/(2π60 238.05) =11.14uF � Capacitive reactance at the fifth harmonic (300HZ) :

Xc5=Xc1/5 = 47.61 ohm

� For the filter to be tuned at the fifth harmonic, the capacitive reactance Xc5 should be equal to the inductive reactance (XL5), thus� Xc5=XL5 =47.61 ohm

� The reactance of the reactor at the fundamental frequency is given by� XL1=XL5/5 = 9.522 ohmL = 9.522/ 2πf = 0.025H

� The thermal load on the reactor. The filter reactor must be capable of carrying the total rms current, fundamental plus harmonics without overheating.

� The peak voltage across the capacitor.

� To determine the amount of current that the filter will have to handle,Ic1=VLL/(Xc1-XL1) =13800/(238.05 -9.522)=60.386AThe total rms current through the reactor is IL=√(I1)2+(I5)2 = √(60.386)2+(10)2 =61.2A

� The worst-case condition will be assumed for the peak voltage across the capacitor: the fundamental and harmonic components are in phase with each other; therefore, the peak voltage across the capacitor is the arithmetic sum of the peak fundamental and the peak fifth harmonic voltage.

� The fundamental component of the voltage across the capacitor is

� Vc1=Xc1Ic1=(238.05)(60.386) =14373.9V� Vc5=Xc5Ic5=(47.61)(10)=476.1V� The peak voltage across the capacitor is � Vp=√2 (Vc1+Vc5) = √2 (14373.9+476.1)� Based on applicable capacitor standards, capacitors should

be capable of withstanding 120% of rated voltage.� Thus, the capacitor should be capable of withstanding

1.2Vp=1.2x(14850) = 17820V.

Total Harmonic Distortion� The formula for Total Harmonic Distortion (THD) is:-

( )

%100...

%100%1

24

23

22

1

2

2

×+++

=×=

∑∞

=

I

III

I

I

THDh

h

� THD can provide a good idea of how much extra heat will be realized when a distorted voltage is applied across a resistive load.

� However, it is not a good indicator of the voltage stress within capacitor because that is related to the peak value of the voltage waveform.

Total Harmonic Distortion

THD =

Σh max

h=2V

h

2

V1

For Voltage

Rms h max

h=1V

h

2Σ= V1 x 1+ THD2

=

Type of Distortion Type of Abnormal Load Operational Limit

Harmonic Current

Distortion

Other Non-linear

Equipment with size ‘I’ in

Ampere

1.At 380V/220V

Total odd harmonic distortion:

I < 30A 20%

30A<=I<300A 15%

300A<=I<600A 12%

600A<=I<1500A 8%

I >= 1500A 5%

total even harmonic distortion:

25% of the odd harmonic

limits

Existing CLP Supply Rules – Current

Existing CLP Supply Rules (Voltage)

Type of Distortion Type of Abnormal Load Operational Limit

Harmonic Voltage

Distortion

Electric arc furnace •At 132kV or above

odd harmonic distortion 1%

total harmonic distortion 1.5%

•At 66kV or 33kV


total harmonic distortion 3%

•At 11kV


total harmonic distortion 4%

Maximum allowable THD� In case of motor circuits using VSDs, group compensation at

the sub-main panel or MCC is allowed, provided that the maximum allowable 5th harmonic current distortion at the VSD input terminals during operation within the variable speed range is < 35%.

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