Busbar Protection

Busbar Arrangements & Protection

            In order to maintain system stability and minimise fault damage due to high fault levels, instantaneous tripping for busbar faults is necessary.

            Busbar protection scheme should be:

            a) Completely reliable

            b) Absolutely stable for heavy through faults

            c) Selective

            d) Accurate and fast operating

General Busbar Arrangements

a) Single Busbar Arrangement

           

           This has only single busbar to which all lines/Transformers/Generators, etc. are connected. In the event of the fault on the bus entire bus has to be deenergised and a major outage occurs.

b) Single Sectionalised Busbar Scheme

            In this, main bus is divided into two sections with a circuit breaker. One complete section can be taken out for maintenance or for breakdown works without distribution continuity of other section.

                                    

c)      Main & transfer busbar scheme: -

With this arrangement, any line breaker (one at a time) requiring maintenance can be transferred to transfer bus. The feeder protection thus gets transferred to trip bus couple breaker. On fault occurrence or maintenance, entire bus becomes de-energised.

d)      Double Bus arrangement: -

Flexibility of transferring any line to any of the buses. On fault occurrence or maintenance only one bus becomes dead, while other bus remains in service.

                                       

e)      Double bus and transfer bus arrangement:-

Combination of main and transfer bus and double bus arrangement.

f) Double bus and bypass isolator arrangement:

                                    

g) Double break bus system:

                                   

h) Breaker and half arrangement: - (One and half Breaker arrangement)

                               

Advantages:-

1)      It has 3 breakers for two connections.Each circuit is connected to a particular bus.

2)      No changeover of line from one bus to the other is required.

3)      This pairing is done such that one is a source and the other a load.

4)      For breaker maintenance of any line, the load gets transferred to the other bus.

5)      On occurrence of a bus fault or for maintenance all the interconnections will be on healthy bus.

6)       Even if both buses become dead, lines can still be in service through the tiebreakers.

Busbar Protection Scheme

1)         High impedance circulating current scheme

2)         Biased differential or low impedance circulating scheme.

S.No.	Details	High impedance circulating current relay	Low impedance biased differential relay
1.	Principle	The currents entering and leaving the busbar are compared continuously. It involves choosing of impedance high enough to stabilise the relay for heavy external faults.	It has differential and bias setting. The resultant bias is proportional to arithmetic sum of all currents, whereas the operating current is vector sum of all circuit currents.
2.	CTs	It requires all identical CT ratios	It can work  with CTs of unequal ratios also.
3.	Burden	Imposes comparatively high burden on CTs. Auxiliary CTs reduce the performance of scheme	Imposes less burden on CTs. Auxiliary CTs have no effect on performance of scheme
4.	CT Saturation	Operation of scheme even when CTs get saturated during internal faults	Operation of  scheme even when CTs get saturated during internal faults.
5.	Performance	Highly sensitive for internal faults and completely stable for external faults	Highly sensitive for internal faults and completely for external faults

High Impedance Busbar Protection:-

Relay Operating Current and Stabilising Resistor are to be set in high impedance scheme.  An Operating Current (I_op) of 10% or 20% of I_n can be set.

During through fault, the voltage developed across the relay is

            V = I_f (R_CT + 2 R_L)

Where I_f          = Fault current

            R_CT      = Internal resistance of CT

            R_L        = Cable resistance

Stabilising Resistor Rst can be computed as follows.

                           V                  VA burden of relay

Rst   =     ---      (-)        ----------------------

                           I_op                          I_op^**2

For some high impedance schemes, only Voltage Setting ‘V’ will be set.(The calculations  are similar to that of  Restricted Earth Fault relay setting for the Transformer protection).

Low Impedance Busbar Scheme:-

This relay operates on circulating current principle and differential current setting (  20% In) is adopted on the relay.  The bias setting is generally set by the relay manufacturer based on bus fault levels.

Busbar Protection

a) Check Feature: -

            To prevent incorrect tripping due to damage to wiring and equipment from extraneous sources, check relay is provided. This check relay is provided by duplication of primary protection using a second set of current transformers cores on all circuits other than bus section and bus couple units. The check system is arranged in a similar manner of the primary protection, but forms one zone only covering the whole of the busbars (in case of single sectionalized busbar or both the buses (in case of double busbar arrangement).

b) Supervision

            When a CT secondary winding or connections between CT and the relay circuit become open circuited, the relay may maloperate for load or through faults depending on the effective primary setting. This condition of an open circuit can be detected by using supervision (over voltage) relay, which is arranged to give alarm.

            The supervision must be time delayed to avoid a false alarm during genuine fault conditions, typically three seconds is adopted.

BREAKER FAILURE RELAY (LBB PROTECTION)

            Main protective schemes provided for line /transformer/generator are required to operate and clear the fault immediately, isolating the faulty section of the system. It is then important that the circuit breaker operates correctly, clearing the fault quickly by tripping. However there is a risk that breaker may not trip (either due to mechanical sluggishness or due to inability to interrupt heavy fault current). Then the fault gets cleared by backup relays at remote stations.

            Increasing power system complexity demands shorter fault clearing times. It is therefore necessary to provide breaker failure relay (also called “Local breaker backup relay” or “Stuck breaker protection”). This scheme will isolate the bus to which the stuck breaker is connected, faster. It comprises of O/L & E/L relays with a timer. The LBB relay is energised by trip command of main protection schemes and thus initiate master trip relay of the busbar protection scheme after elapsing of defined time. Then the entire breaker connected to the bus get tripped, thus isolating faulty element.

  

            Even if the busbar protection scheme is not available, the LBB scheme can be made use of by providing special trip circuits and trip relays similar to that of bus protection trip circuits for each line. A schematic for such application is indicated in figure.