Breaker Failure Protection

Intruduction: Breaker Failure Protection (BFP) is also known as BFR (Breaker Failure Relay) or LBB (Local Breaker Backup) protection.

The idea behind BFP is that in case of any protection operation the Circuit Breaker (CB) should interrupt the fault current within specified time (Normally < 60ms, depending on voltage level). There may be cases where CB is unable to interrupt the current doe to any of the following causes:

1. CB tripping circuit problems
2. CB operating mechanism problems
3. CB interrupter problems

In the first two cases, the interrupter is not at all operated. Therefore there is no risk of prolonged arc inside interrupter, Only risk is continuous feeding of fault current may lead to disturbance in grid operation.

In the third case, due to prolonged arcing inside interrupter, temperature may go high and it may explode due to high pressure buid-up. Normally porcelain enclosures are used for SF6 filled interrupters. Failure may lead to cracking of porceline into pieces, which are dangerous for men working the vicinity of CB. Further SF6 gas will be released to atmosphere, which is not good for environment. In addition, risk of disturbance in grid operation is already there.

Working Principle: The BFP protection is activated when tripping command is issued to CB. If after a pre-set time (typical 200ms), current is still flowing through CB, BFP will operate. Operation of BFP will isolate the concerned CB from all sides by tripping all the CBs connecting to it. For example in the image shown below, following will be the isolation logic:

BFP operated for	CB to be opened
CB-1	CB-2, CB-4, CB-11
CB-2	CB-1, CB-3, CB-11, CB-21
CB-3	CB-2, CB-6, CB-21

There are two methods used for operation of BFP:

1. Current level detection: If current through CT is above set value (typical value 100A) after pre-set time, relay interprets that CB has failed to interrupt the current.
2. CB status from auxiliary contacts: If CB status as per auxiliary contacts (52a) is still closed, relay interprets that CB has failed to open.

First method is more reliable and works for all the three cases mentioned in introduction, therefore used in most of the cases. Only drawback is there must be some significant amount of current flowing for operation of BFP.

Second method does not work for third case (CB interrupter problems). It is used where lower level of current, which is not significant of operation of relay, is also dangerous for health of equipment / person working in the vicinity,

The working principle can be understood from image below:

Restricted Earth Fault (REF) protection

REF protection is Based on Kirchhoff's current law : 

Current flowing into a node (or a junction) must be equal to current flowing out of it. Or in other words vector sum of currents in and out from a node is zero

REF protection is a basic protection used in many equipment in Power system like transformers, reactors, generators etc.

For REF protection is applied to windings having electrical couplings. Magnetic coupling is ignored for this purpose. For example we may consider following single phase two winding transformer:

Here both windings are electrically isolated and magnetically coupled. current entering from primary side I11 shall be same as current leaving primary side I12.  Similarly, current entering from primary side I21 shall be same as current leaving primary side I22. Therefore

I11 + I12 = 0
I21 + I22 = 0

In this example we have used two REF relays, one for primary and other for secondary side. In some cases, in transformers having small transformation ratios, single REF can be used for cost saving. In that case 

I11 + I21 + I12 + I22 = 0

Setting calculation: The simplified circuit is shown as below:

In normal condition I11 and I12 will be equal and opposite to each other, these will cancel each other and there will be no current flow through relay coil.

The typical setting for transformers is ~15% of full load current. For example transformer has full load current of 400A and CT ratio is 500/1A. CT ratio  should be same for all CTs connected to REF relay. The current setting for REF relay shall be 400 x 15% = 60A primary (0.12A secondary).  

In some relays the setting is entered in Volts. Voltage is calculated by multiplying setting current with stabilizing resistor value. Say stabilizing resistor is  1000 ohm, the voltage setting will be 120V.

Value of stabilizing resistor: CT saturation sometimes occur during high fault currents, we need stable relay operation during CT saturation. Stabilization resistor is used for avoiding relay operation during CT saturation during through faults.  Its value is calculated on maximum through fault current of the protected equipment. 

Let us assume CT2 (Neutral side CT) saturates during through fault. It will not generate any output and will act as resistor as per its secondary winding resistance.

In this case let us assume percentage impedance of transformer to be 10%. Maximum through fault current will be 400A/10% = 4000A Primary (8A Secondary). If CT secondary resistance is 6 ohm and lead resistance is 4 ohm, Voltage developed in case of maximum fault current will be 8A x (4 + 6) = 80V. Now our requirement is that for this voltage relay should not operate. In other words stabilizing resistor should restrict the relay current below set value. 

Now keeping 150% safety margin the current produced by 120V throgh relay coil shall be 0.12A. For this requirement stabalizing resistor shold be 120V / 0.12A =  1000 ohm. In the above case of CT saturation, Voltage developed was 80V, the current through relay coil will be 80V / 1000 ohm = 0.08A. This is much below the set value of 0.12A. Therefore relay will not operate during this CT saturation, which meets our requirement.

Varistor:  Stablizing resistor is in series to relay circuit, it may have values upto 2 K ohm. During fault condition, there may be high voltage across the CT terminals due to higher value of relay circuit. To protect equipment (lCT, cables relays etc) and persons working in relay panels varistor is used to limit the voltage below ~300V.

Three Phase transformers: For three phase transformers the example is as below:

z

REF protection will not operate on phase to phase fault, as the vector sum of IR + IY + IB + IN shall be zero in fault case also. 

Similarly, REF protection can be used for 3-phase auto transformers as shown below:

Generally the relay is set to operate for Earth Fault current of ~15% of rated winding current.