VT supervision

Introduction: For protection relays, analogue inputs are connected from Current Transformers (CT) and Voltage Transformers (VT). VT is a voltage source, any short circuit in wiring will cause heavy current to flow in VT winding. This may result in failure of VT. Therefore, Connection from VT is always with fuse or MCB to take care of short circuit / overload.

Now it is evident that in case of fuse failure, relay will not get any voltage from VT. Some of the protections may see this as abnormal condition in system and may cause unwanted trip or may not trip in actual fault. For example:

1. Distance relays measure Impedance from Voltage and current (Z = V/I). If there is no voltage (V=0), Z will also be Zero. This will cause trip of distance protection.
2. Directional Overcurrent relay derives direction by comparing angle of voltage with current. If there is no voltage, it can not measure its angle. Therefore, it will not operate in case of actual fault.

Absence of voltage or lower voltage to relay may be due to one of the following reason:

1. Actual fault on primary side of power system, which has caused voltage to dip. One disturbance record for this condition with B-N fault is given below. We can see presence of VN (3V0) with significant IN (3I0).
   
   
   
2. No actual fault on primary side, only secondary fuse fail. One disturbance record for this condition with C-phase fuse failure is given below. We can see presence of VN (3V0) without any significant IN (3I0).
   

It is important for protection relays to differentiate between above two conditions and declare VT fuse fail only when there is no fault in primary side. VT fuse fail condition leads to blocking of certain protection functions and generation of alarm to operator.

Method of detection: VT fuse fail may be classified in two types:

1. Single phase VT fuse fail, when fuse is used in VT secondary circuit and it has blown due to short circuit.
2. Three phase VT fuse fail, when MCB is used in VT secondary circuit and it has tripped. There may be another case when VT selection is used and VT selection relay has failed to operate.  

Single phase VT fuse fail: Its detection is easy and reliable. In case of actual single phase fault, voltage will decrease and current will increase for that phase, ie. there will be zero sequence current and zero sequence voltage present in system. 

However, in case of VT fuse fail, currents will remain same and only voltage will be decreased. Most of the relays detect it by presence of Zero sequence voltage without presence of Zero sequence current in system. In some case Negative sequence voltage and Negative sequence current is used for detection.

Three phase VT fuse fail: It is a little unreliable and works to some extent. Due to absence of all three phases there will not be any zero sequence or negative sequence voltage in the system. 

It can be detected by change in voltages without any change in currents by comparing with previous cycles values (ΔV and ΔI). But in case three phase fuse fail condition is persisting, and current goes below a certail level, due to load variation. Relay may detect it as dead line condition (no voltage and no current). This will reset three phase VT fuse fail condition. Now sudden rise of current can cause trip of relay. Therefore, three phase fuse fail is a little unreliable. 

Or, relay can detect MCB trip through auxilliary contact of MCB, if MCB is used in secondary circuit.

Add-on stabalization technique in Bus Bar protection

 Introduction: In earlier post we have discussed about Bus bar differential protection.

For Low impdance bus bar protections, different manuafacturer use different algorithm to avoid mal-operation of relay during CT saturation.

Add-on stabalization technique: One of the method is use of Add-on stabalization technique for detecting internal / external faults. Add-on stabalization function adds additional current to biasing current.

Phase comparison technique in Bus Bar protection

Introduction: In earlier post we have discussed about Bus bar differential protection.

For Low impdance bus bar protections, different manuafacturer use different algorithm to avoid mal-operation of relay during CT saturation. 

Phase comparison technique: One of the method is use of Phase comparison technique for detecting internal / external faults. Phase comparison function compares the phase-angles of the fundamental components of all the feeder currents.

During a through-fault, at least one of the currents is ~180° out of phase with the others. In case of CT saturation its magnitude may be lower but relay can compare its angle to distinguish internal fault from external fault. 

During internal fault of bus bar, currents from all feeders will flow towars busbar. This means, phase angle of all currents will be nearly same.

If the phase-angles of all the feeder currents of a protection zone lie within a band of ~74° (typical value), the phase comparison function decides that there is an internal fault.

For proper operation, it is necessary to exclude feeders conducting very little or no current from the comparison to prevent noise generated by them.

A minimum current is therefore determined below which a feeder is excluded from the phase comparison. Typical settings are 0.8 IN for the phase currents and 0.25 IN for the neutral current.

Tripping only takes place if the differential current and the stability factor (Slope) are both above their pick-up settings and the phase difference between the currents is less than setting.

Bus bar differential protection

Introduction: Bus bar is important element in a power system due to following reasons:

1. Fault current is higher for bus bar faults, as every feeder will contribute in fault current and bus bar impedance is very low.
2. Bus bar fault may lead to outage of many feeders, depending on bus bar scheme. Mal-operation in case of out of zone fault will also cause un-necessary outage. Therefore, generally trip decision is taken after confirmation by two different components in bus bar protection.
3. Non-operation of busbar protection in case of actual fault is very dangerous for equipment, persons working in system and power system itself. It will lead to delayed fault clearance and impact larger area. To avoid this redundant bus bar protection schemes are used for important installations.
4. Bus bar has to compare currents of all feeders connected to it, there is requirement of all CTs having similar characteristics to avoid mal-operations in case of through faults.
5. Being smaller sections, faults are considered rare for bus bars. As bus bar protection cost is higher, generally bus bar protection is not used for smaller systems.

Principle of operation: Differential protection works on the principle of Kirchhoff's current law, which states - "The current flowing into a node (or a junction) must be equal to the current flowing out of it" or equivalently "The vector sum of currents in a network of conductors meeting at a point is zero."

Bus bar can be considered as node, where all feeders are connected. The vector sum of all these currents shall be zero. 

For understanding we may consider one bus bar with two feeders.

Case-1: For out of zone fault, current through relay will be I1 - I2, which is zero. 

Case-2: For in zone fault, current through relay will be I1 + I2, which will have higher value depending on source behind Feeder-1 & Feeder-2.

The protection shown in this example will have following issues:

1. It may trip for out of zone faults due to CT. Ratio error during heavy current
2. Unstable due to saturation of CT magnetic circuit during heavy current

For avoiding this we have to increase stability of bus bar protection for these conditions. One solution would be to add one stabalization resistor in series of differential relay. It is similar to Transformer REF relay 

In this case impedance of bus bar protection is high due to series resistor, therefore protection is called high impedance differential protection. As all CTs are to be connected directly in parallel, all the CTs should have same CT ratio.

Second solution is to measure through fault current for each feeder and use this current for biasing element. It is similar to Transformer differential protection.

In this case impedance of bus bar protection is low, therefore protection is called low impedance differential protection. As every CT is connected directly to differential relay, relay has values of every individual feeder current. Relay calculates Differential and Biasing currents based on internal algorithm and settings adopted. 

I_Differential  = |I₁ + I₂ + ……. I_n|

I_Bias = |I₁| + |I₂| + ……. |I_n|

Stability factor k = I_Differential / I_Bias

There is a minimum value of Differential current, below which relay will never operate. Above this differential current, operation is decided by ratio of Differential current to Biasing current. For bus bar protection fault current levels are high, and CT saturation is expected to cause error in CT secondary currents. Different manufacturers use different techniques to take care CT saturation, some are discussed below:

1. Phase comparison
2. Add-on stabalization

Bus bar protection becomes more complicated when CT switching is possible, line in Double busbar scheme. 

Now, relay has to check which feeders are connected to bus bar through status of disconnectors. There may be discripancy in status due to problem of an auxilliary switch or when performing bus shifting in charged condition. Bus bar relay may see differential current due to time mismatch between actual position of disconnector and auxilliary switch of disconnector.

Additional Check zone is used in this case. Check zone calculates vector sum of all feeders in and out of a station, considering all bus bars as one bus bar, whithout any CT switching.

For double bus bar, Tripping is issued as below:

• Bus Bar-1 : BB Zone-1 operated and Check Zone operated
• Bus Bar-2 : BB Zone-2 operated and Check Zone operated