https://ideology.atlassian.net/wiki/display/AP/High+Impedance%2C+Merz-Price%2C+Circulating+Current+Differential;jsessionid=86CC5D54F167A8A3B63B6423021DCF6E

High Impedance, Merz-Price, Circulating Current Differential

Differential relays use the principle that when there is no fault within the zone defined by the CT locations, the currents from each of the CTs are all balanced and hence circulate around the parallel connected CTs.  Although the relay is in parallel to the CTs, its relatively higher impedance than the CT paths and the balanced current flow on the secondary currents from all the CTs means there is minimal operating current through the relay and hence it should not operate.

High impedance differential protection effectively responds to a voltage across the relay connection, which in turn leads to sufficient operating current in the relay to operate.  Hence the relay setting may be in terms of voltage or the current sensitivity.

The figure below shows the simplified arrangement for no fault inside the zone of the bus bar protection scheme.  In this example the CT on the right hand side has saturated due to high current in one circuit but the scheme must remain stable and not operate as there is no internal fault. In this circumstance the saturated CT effectively is seen as a short circuit and has zero output voltage and hence does not produce any output current itself. The left hand CTs therefore have to produce sufficient voltage to drive the circulating current through the saturated CT on the right. Consequently there is a voltage profile on the secondary circuit from left to right resulting in a voltage appearing across the relay connection. Provided this produced relay voltage is less than the operating threshold of the relay, the scheme will be stable.

The relays may be inherently voltage setting based relays with the advantage of a natural high impedance with the resultant low operating current sensitivity typically less than 30milliamps on the secondary side.  Alternatively, current setting based relays with higher operating current can be used with external resistors to create the high impedance arrangement with the resistor calculated to give the required voltage threshold at the setting current.  Current setting based schemes have the possibility that the operating current can be set above the maximum load current of a single circuit in order to avoid mal-operation in the case of a CT open circuit which would otherwise cause operation due to the apparent differential current.

 

Principle of voltage setting to cater for saturated CT with no internal fault:

Hence the minimum CT knee point voltage is given by applying Ohm's Law

Vk ≥ If . 2. (Rct + Rl)

  Vk is the minimum knee point voltage If is maximum fault current (including for high Source Impedance Ratios)

Rct is the CT winding resistance

Rl is the loop impedance from the CT to the relay (i.e. twice the individual lead burden)

All CTs in a differential scheme must meet all four of the following criteria.  They must have:1. well-matched excitation characteristics,2. the same turns ratio,

3. low secondary winding impedance,

4. low excitation current.

When an “internal” fault occurs on the bus bar, the sum of the currents flowing in do not equal the sum of the currents flowing out.  This is reflected on the secondary circuit at the connection point of the relay.  Using Kirchhoff’s Law, it is clear that the differential current, representing the primary fault current, must flow through the relay path.  

The relay pick-up voltage must be less than half of the knee-point voltage of the current transformers to ensure reliable operation for internal faults as well as catering for the simplified analysis of the relay being located at the electrical midpoint of the CT wiring where the voltage across the relay is half the voltage developed by the CT when another CT is saturated. In some cases this results in CTs with several kV knee point voltages and hence non-linear resistors are required to limit the over-voltages that are experienced during internal faults to less than 2 kV peak, which is the standard insulation level used for secondary equipment and wiring.

Vs ≤ Vk / 2 Vs is the relay setting voltage Vk is the CT knee point voltage

The protection sensitivity corresponds to the sum of the magnetising currents of all parallel connected current transformers plus the relay current at the relay pick-up voltage given by the formula:

Io = Is + n.Ie

Io is the effective operating current sensitivity Is is the pick up current of the relay

n is the number of CT cores in parallel

Ie is the CT excitation current

Typically, as internal faults will result in CT saturation due to the high relay burden, the operating time for a high impedance differential relay must be less than 1 cycle prior to saturation making it a very effective protection system easily graded with remote line protections seeing into the substation.

Some applications have employed the use of a resistor in parallel to the high impedance relay circuit in order to desensitise the minimum fault current to operate the high impedance relays.  This may be useful to ensure that the minimum fault sensitivity is above the normal current of a single circuit to prevent mal operation due to an open circuit CT.  However the effect of this is to make the scheme a medium or low impedance scheme with the associated stability issues. In these cases, the secondary current flows during external faults with one CT saturated must be considered, i.e. the high impedance relay path is to aid in making all the circulating current flow through the saturated CT, rather than the relay.

One of the typical application difficulties of high impedance schemes is on complex bus bar arrangements, such as double bus arrangements, where the CTs connected to the protection zone must be changed to reflect different bus bar configurations. In these circumstances, the CT circuits must be reconnected dynamically as the isolators positions are changed. This is achieved by auxiliary contacts on the isolators which change the CT connections to different bus bar protection zones. This introduces a risk of maloperation of the auxiliary contacts leading to open circuit CTs with the risk of CT explosion and/or incorrect operation of the bus bar protection under healthy conditions. Given the high speed operation of bus bar protection relays, consideration must also be given to the intermediary arrangement where the CTs are connected to two zone simultaneously as the isolator changes position and the CT circuits move through a “make-before-break” sequence. Hence in these arrangements it is usual to also use some form of CT supervision to detect and guard against inadvertent CT open circuits.

Given the need to avoid maloperation of bus bar protection and the heavy reliance on CT performance and connections, complex substations with multiple bus bar protection zones often also employ a fixed check zone across the complete substation where the CTs do not need to be switched. This operates in conjunction with the individual zones in a “two out of two” tripping requirement to ensure there is a true internal fault prior to tripping the circuit breakers. However this adds the requirement for additional dedicated CT cores for each of the X and Y check schemes.