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.

Voltage Transformer selection (VT selection)

Voltage Transformer selection (VT selection) is used in different protection  schemes where multiple VTs are installed and the requirement of VT secondary voltage to be used can change with the operation of dis-connectors.

In protection schemes, VTs are installed for three puposes:

1. Protection
2. Metering
3. Synchronization

Normally VTs are installed on busbars, and lines for EHV system. For voltages < 66kV, VTs are not installed in lines to reduce cost. In this case, Bus VTs is used for metering and protection of lines. As there will be no VT in line, synchronization is not possibe in this case. But in most of the case <66kV lines are radial type and synchronization is not required.

Double Bus Scheme: In the image below, having double bus scheme, only Bus VTs are used. Now, let us suppose we want to install Distance protection on feeder-1. As there is no VT in feeder, we have to use bus VT for protection purpose. But which bus VT? As the feeder-1 can be connected to Bus-1 or Bus-2 at any time and there may be cases when one of the bus is under shutdown (Bus voltage zero). So we require VT switching.

Now let us take one case of meshed system, where we have used VT in feeder, so that Protection and metering is done through Line VT. In this case, we have to check synchronization before closing of circuit breaker. We have to compare voltage of incoming feeder with existing bus voltage. But existing bus voltage of which bus? We need VT selection for synchronisation check, so that we may compare line VT voltage with selected bus voltage.

Bus selection is decided by position of bus side dis-connectors
. 
Case-1: If Bus-1 disconnector is closed, Bus-1 VT is selected for synchonization. 
Case-2: If Bus-2 disconnector is closed, Bus-2 VT is selected for synchronization. 
Case-3: If both disconnectors for Bus-1 & Bus-2 are closed, there is always priority for one of the bus, generally bus-1. Therefore, Bus-1 VT will be selected. Bus-2 VT can not be selected untill Bus-1 VT is deselected.

One and a half Breaker Scheme: Typical location of VTs is shown below. No VT is installed for transformer in this case. 

For protection and metering of Transformer, any one VT out of three VTs: Bus-1 VT, Bus-2 VT or Feeder VT may be selected depending on following:

Case-1: CB-4 Close, Bus-1 VT selected
Case-2: CB-4 Open, CB-5 & CB-6 close, Bus-2 VT selected
Case-3: CB-4 & CB-6 Open, CB-5 and Feeder disconnector closed, Feeder VT selected.

In this case, while closing CB, voltage on both side of CB is also required for synchronization checks. The voltages are also selected based on positon of CB / Dis-connectors.

Therefore, VT switching is required in feeders with position of Circuit breakers / dis-connectors for protection, metering and synchronization purposes.