BUS BAR (1).pdf

29 August 2012 1

Bus Bar Protection

Manoj Barsaiyan

Presentation Outline

Introduction Bus arrangements Bus protection techniques

INTRODUCTION: Importance of Busbars

Busbars are the most important component in a power network.

They can be open busbars in an outdoor switch yard, or inside a metal clad cubicle restricted within a limited enclosure with minimum phase-to-phase and phase-to ground clearances.

They can be insulated as well as open

They form an electrical node where many circuits come together, feeding in and sending out power

Bus bars : Taken for Granted

Bus bars are frequently left without protection because:

Low susceptibility to faults especially metal clad switchgear

The bus bars and switchgear have a high degree of reliability, to the point of being regarded as intrinsically safe

Rely on system back-up protection

Problems with accidental operation greater than infrequent bus bar faults

However, Bus bar faults do occur

Introduction: The requirements for good protection Speed

Limit damage at fault point Limit effect on fault stability

Selectivity Trip only the faulted equipment Important for busbars divided into zones

Stability Not to operate for faults outside the zone Most important for busbars

Stability must be guaranteed Reasons for loss of stability

Interruption of CT circuits imbalance Accidental operation during testing

Tripping can be arranged two-out-of-two Zone and check relays.

Bus Bar Protection Requirements High bus fault currents due to large number of circuits

connected:

CT saturation often becomes a problem as CTs may not be sufficiently rated for worst fault condition case

large dynamic forces associated with bus faults require fast clearing times in order to reduce equipment damage

False trip by bus protection may create serious problems:

service interruption to a large number of circuits (distribution and sub-transmission voltage levels)

system-wide stability problems (transmission voltage levels)

With both dependability and security important, preference is always given to security

1 2 3 n-1 n

ZONE 1

- - - -

Distribution and lower transmission voltage

levels

No operating flexibility

Fault on the bus trips all circuit breakers

Bus arrangements : Single bus - single breaker

ZONE 1ZONE 2

Distribution and lower transmission voltage

levels

Limited operating flexibility

Bus arrangements : Multiple bus sections - single breaker with bus tie

ZONE 1

MAIN BUS

TRANFER BUS

Increased operating flexibility

A bus fault requires tripping all breakers

Transfer bus for breaker maintenance

Bus arrangements : Main and transfer buses

ZONE 1

ZONE 2

Very high operating flexibility

Transfer bus for breaker maintenance

Double bus single breaker with transfer bus

ZONE 1

ZONE 2

High operating flexibility

Line protection covers bus section between two CTs

Fault on a bus does not disturb the power to circuits

Double bus - double breaker

ZONE 1

ZONE 2

Used on higher voltage levels

More operating flexibility

Requires more breakers

Middle bus sections covered by line or other

equipment protection

Breaker-and-a-half bus

Higher voltage levels

High operating flexibility with minimum breakers

Separate bus protection not required at line

positions

B1 B2

TB1

L1 L2

L3 L4

TB1

Ring bus

TYPES OF PROTECTION SYSTEM

Frame-earth protection :

Used mainly for smaller busbar protection schemes at distribution voltages and for metalclad busbars.

Differential protection :

Used for switchgears and EHV switchyards

Frame-earth protection for Switchgears

Bus Protection Techniques

Overcurrent (unrestrained or unbiased) differential

Overcurrent percent (restrained or biased) differential

High-impedance bus differential schemes

Low-impedance bus differential schemes

Overcurrent (unrestrained) Differential

Overcurrent (unrestrained) Differential

Differential signal formed by summation of all currents feeding the bus

CT ratio matching may be required

On external faults, saturated CTs yield spurious differential current

Time delay used to cope with CT saturation

Instantaneous differential OC function useful on integrated microprocessor-based relays

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BUS BAR PROTECTION AND LBB PROTECTION

BUS BAR PROTECTION CIRCULATING CURRENT DIFFERENTIAL PRINCIPLE

CURRENT DISTRIBUTION EXTERNAL FAULT

CURRENT DISTRIBUTION INTERNAL FAULT


BUS BAR PRIOTECTION A. HIGH IMPEDANCE TYPE

Relay Branch made High Impedance to limit differential

current due to unequal CT saturation on external fault. Requires exclusive CT core of identical ratio and rating. Simple in design and execution.

Advantages Simple

Low cost

Different ratios of CTs can be used with the help of Aux CT

High operating time All CTs must have equal ratio Low sensitivity Degree of CT saturation level to be very low

Disadvantages

Complications of Bus Bar Differential

All paralleled CTs must have the same ratio to ensure that all secondary currents are compared on the same base as the primary currents.

Protection should perform identically under all conditions, including external faults with heavy through current.


The reality is that all conventional iron-core current transformers, regardless of ratio and accuracy class, are susceptible to saturation.

This causes a difference current that appears to the differential relay as an internal fault.

Bus differential relays, regardless of the design, must differentiate between true internal bus faults, and false differential currents caused by CT saturation for a fault outside the bus differential zone of protection.


Objective of protections scheme is to provide secure operation for external faults with CT saturation and still provide fast operation for internal bus faults. There are two most common techniques:

Low-impedance bus differential, and

High-impedance bus differential

High Impedance Differential Overvoltage element

operates on voltage developed across resistor connected in secondary circuit

Operating signal created by connecting all CT secondaries in parallel

CTs must all have the same ratio

Must have dedicated CTs

Cannot easily be applied to reconfigurable buses and offers no advanced functionality

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High Impedance relay

R

The relay branch is made high impedance either by using a voltage operated high impedance relay or by connecting an external series resistor (Stabilizing resistor) in case of current operated differential relay

This type of protection requires special class PS CTs (with low turns ration errors) of identical ratio and ratings on all circuits

Exclusive CT cores are required for high impedance schemes which cannot share common CT cores with other protections

High Impedance Differential Protection

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Setting Criteria for High Impedance Relays

Assuming CT-B completely saturated while CT-A fully active, the maximum voltage that can appear across the relay branch during through fault condition is

Vs = (IF /n) (RCT + 2RL) volts

The differential relay should be set above this voltage to ensure stability on through fault

A B

RCT

RL

VS

32


In case of over-current type of differential relays, the above voltage setting can be achieved by adding a series stabilizing resistor, such that the current is limited to less than differential pickup current

Relay branch impedance Vs/Is, where Is = setting current of differential relay

If relay burden at setting current "Is" = VA

Relay impedance Rr = VA / (Is)2

The external stabilizing resistor setting

Rst = Total relay branch impedance - Relay impedance

= [Vs / Is] [VA / (Is)2] 33



BUS BAR PROTECTION (87) HIGH IMPEDANCE SCHEME- Typical Setting Workout.

Max. Through fault Current (If) = 20,000A (Prim) / 20A(SEC) Vs = 20 (5 + 1) = 120V Stabilising Resistor (RST) = 120/0.2 1.0/0.2 = 600 - 25 = 575 ,

(assuming relay VA burden = 1 VA)


BUS BAR PROTECTION HIGH IMPEDANCE SCHEME Typical Setting Workout. Primary Fault Current setting (assuming number of circuits

connected to Bus Bar, N=10) = CT Ratio (Is + N. Im) = 1000 (0.20 + 10 x 0.03) = 500A.


BUS BAR PROTECTION (87) HIGH IMPEDANCE SCHEME Need For Metrosils.

Peak Voltage across CT Secondary for Max. Internal Fault = 2 2 [Vk x (Vp Vk)] Vk = 240V, Vp = 20 (575 + 25) = 12000V Peak Voltage = 22 [(240 x (12000-240)] = 5300V


BUS BAR PROTECTION HIGH IMPEDANCE SCHEME Need For Metrosils. If Peak voltage exceeds 3000V threatening secondary

insulation, Metrosils (Non-linear resistors) are used to limit voltage to safe value (within 1000V) on internal fault.

Metrosil offers high resistance at lower voltage levels which

drastically reduces at higher voltages, thereby quenching the voltage across relay branch.

Typical Metrosil Equation V = C x I ( C = 900 , = 0.25)

Advantages

It avoids the problem of unequal CT performances.

It uses high impedance voltage relay.

Highly sensitive and fast.

Disadvantages

All CTs must have equal Turns ratio

All CTs must have high Knee point voltage preferably twice the nominal value

The CT must be exclusively used for differential protection.

Percent Differential

Percent characteristic used to cope with CT saturation and other errors

Restraining signal can be formed in a number of ways

No dedicated CTs needed

Used for protection of re-configurable buses possible

5187

Low Impedance Differential (Distributed)

Data Acquisition Units (DAUs) installed in bays

Central Processing Unit (CPU) processes all data from DAUs

Communications between DAUs and CPU over fiber using proprietary protocol

Sampling synchronisation between DAUs is required

Perceived less reliable (more hardware needed)

Difficult to apply in retrofit applications

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DAU

52

DAU

52

DAU

CU

copper

fiber

Low Impedance Differential (Centralized)

All currents applied to a single central processor

No communications, external sampling synchronisation necessary

Perceived more reliable (less hardware needed)

Well suited to both new and retrofit applications.

52 52 52

CU

copper


BUS BAR PROTECTION HIGH IMPEDANCE SCHEME -CT SUPERVISION (95)

Required to detect CT open circuit on load to prevent maloperation. Connected for alarm & short CT bus wires on operation. Time delayed to avoid operation on internal / external faults (3Sec) Voltage relay with sensitive setting (2 -14V) is used. Normally set to detect primary unbalance of 25A or 10% of least loaded

circuit. Also should not operate with normal unbalance on load.

Isolators Reliable Isolator Closed signals are needed for the

Dynamic Bus Replica

In simple applications, a single normally closed contact may be sufficient

For maximum safety: Both N.O. and N.C. contacts should be used

Isolator Alarm should be established and non-valid combinations (open-open, closed-closed) should be sorted out

Switching operations should be inhibited until bus image is recognized with 100% accuracy

Optionally block 87B operation from Isolator Alarm

Each isolator position signal decides: Whether or not the associated current is to be included in the

differential calculations

Whether or not the associated breaker is to be tripped

Isolator Typical Open/Closed Connections

29 August 2012 53

LBB PROTECTION

In EHV substations, reliability of fault detection is enhanced by providing duplicated protections (either main 1/main 2 or main and backup protection)

The D.C. sources for protection are also duplicated for better redundancy, circuit breakers are provided with duplicated trip coils

All these measures, improve the reliability of fault detection and isolation, however, the possibility of mechanical failures of the switchgear or interrupter flash over cannot be covered by these means

A failure of the breaker therefore, result, in spite of correct operation of the protection and energisation of trip coils

LBB Protection

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LBB Protection

HasProtectionOperated?

FaultCleared?

Yes

No

Yes

No

NormalOperation

StartBreakerFailure

Protection

Wait forFault

Clearance

ResetBreakerFailure

Protection

TripBack-up

Breaker(s)

TripMain

Breaker(s)

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This situation can be corrected by providing local breaker backup (LBB) or breaker fail protection

LLB protection comes into operation only if, the breaker fails to trip, following energisation of its trip coil, through the circuit trip relays

The main component of LBB protection is a current check relay initiated by the circuit trip relays and a follower timer

The current check relay, on initiation, checks the presence of the current in the faulted circuit and if it persists beyond a preset time, proceeds to trip all other circuits connected to the Bus bar to which the stuck breaker is connected, thereby, ensuring local isolation

Tripping of remote breaker is also initiated through a separate carrier channel, in case of line breakers to arrest infeeds from remote end

LBB Protection

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Phase A AC signals wired

here, current status

monitored here

Phase B AC signals wired


monitored here

Phase C AC signals wired


monitored here

Breaker Fail Op command

generated here and send to

trip appropriate breakers

Trip

Trip Trip

Example Architecture Breaker Failure Tripping Trip

Typical setting range for the current check relay and follower timer

A more sensitive setting is generally adopted for Generator application in view of the fact, that a stuck breaker situation for certain abnormal conditions like motoring, may involve very low current infeeds

LBB Protection

Application Current

Range

check relay

Recommended

Setting

Follower

Range

Timer

Recommended

Setting

Generator Circuit 5-80% 5% 0.1-1sec 0.2 sec

All other circuits (TFRs

/Lines /Bus Coupler

etc.)

20-30% 20% 0.1-1sec 0.2 secs

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LBB Protection

TIME- CHART Fault occurs

Normal Clearing Time Current Detector

Dropout Time Normal

Clearing Protective

Time

~30ms

Breaker Interrupting

Time

~60ms

THANK YOU

29 August 2012 63

Documents

BUS BAR (1).pdf