DIRECTIONAL COMPARISON BUS RELAYING

Occasionally, it is desirable to add bus protection to an older substation where additional current transformers and control cable are too costly to install. In this instance, the existing current transformer circuits used for line relaying can also be used to provide protection for bus faults.

A directional comparison bus protection system compares the direction of current flow in each circuit connected to the bus. If the currents in all the circuits flow into the bus, a bus fault exists.

If the current in one or more circuits is flowing away from the bus, an external fault exists. Fault detectors are used to initiate the system. This system can be used for either or both phase and ground bus fault protection.

The basic system requires directional relays, fault detectors, and a timer. Directional relays are used on each circuit connected to the bus. Fault detectors are used to indicate a fault within the vicinity of the bus.

Phase fault detectors are instantaneous overcurrent relays connected to a bus tie breaker or to one or more of the circuit connections. Ground fault detectors are instantaneous overcurrent relays connected in power transformer neutrals or to one or more of the circuit connections.

A timer is required to permit contact coordination of the directional relays. The contacts of the directional relays are connected in series with the timer contact to initiate a trip signal.

Current transformer saturation is usually not a problem when comparing direction of current rather than comparing magnitudes of current as is done in conventional bus relaying systems. The current transformers in each circuit do not require the same ratio and can be used for other forms of relaying and metering.

In some cases, the directional elements of relays used for normal circuit connection relaying can also be used for this bus protection system. The system is rather complex and requires rigorous maintenance due to the number of relay contacts.

The timer must be set for at least four cycles to assure contact coordination. Contact bounce must be avoided in the directional relays and in the overcurrent fault detector relays.

Contact bounce should be checked when high-magnitude fault currents are likely.

The relays and relay settings must be reviewed when system changes are made near the protected bus to make certain that selectivity between initiating and blocking relays exists. There is presently available a directional comparison system based on directional comparison of the current in all circuits connected to the bus in relation to the sum of all the currents.

This system employs solid-state relays. Intermediate current transformers are used, and main current transformer ratios and characteristics need not be the same on all circuits.

GROUNDING TRANSFORMER DIFFERENTIAL RELAYING

Configuration
To establish a grounded system, a grounding transformer is frequently tapped on the low-side leads of the supply transformer and is thereby included in the transformer differential zone.


Problem
Zero sequence current supplied by the grounding transformer may cause differential relay operation during an external ground fault.

Solution
Since external ground faults cause zero sequence current to flow in the CT secondary circuits, a zero sequence filter is provided for the low-side differential Y-connected transformers. This filter is composed of three auxiliary CTs and can be formed in several ways.

The simplest form is to connect the primaries in Y and the secondaries in Δ. In Figure below, the ratio of the auxiliary CTs is not critical, but a 5:5 ratio is suggested.

Grounding transformer in differential zone (external fault condition shown)

The alternative filter connection in Figure 16 requires a 1:3 ratio for the auxiliary CTs. The primaries are connected in Y and the junction or sum of the primaries is wired to the secondaries connected in series.

Thus the secondaries carry three times the primary current. Both of these connections present relatively high magnetizing impedance to all but zero sequence current.

However, modern differential relays are of even lower burden than the usual auxiliary CTs. Thus, the common point of the relay connections should not be connected to the common point of the Y-connected transformers (a connection that is necessary without the zero sequence filter).

Only the filter neutral should be connected to the CT common point. The figure also shows the primary current and CT secondary current for an external ground fault.

The zero sequence filter prevents a relay imbalance. A 1:1 overall voltage ratio is assumed in the figure above with 1 PU fault current flowing.


An application example using an inverse relay has relay pickup at 30% of transformer rating and a time of 1.7 s at 300% of setting. The pickup depends on the effective contribution of the equivalent tertiary of the case.

The transformer manufacturer should be consulted.

DIFFERENTIAL PROTECTION OF SINGLE PHASE TRANSFORMER IN A THREE PHASE BANK WITH SPARE TRANSFORMER

With the increase in use of single-phase transformers in three-phase banks with spare transformers, the question frequently arises of how to best include the spare in a transformer differential scheme.

Differential relay connections are dependent to some extent on transformer connections, location of CTs, and whether the spare power transformer will be energized all the time.

If the differential zone extends to circuit breakers on both sides of a transformer, then changing the CT secondary circuits is not required to place the spare transformer in service.

However, if the spare transformer is to remain energized all the time, consideration should be given on how to protect the spare when not in use.


When the transformer bank differential is used to protect the spare transformer, the result is not always as sensitive to protection of the spare as of the transformers in service.

If bushing CTs are used on both sides of the transformer bank, a separate relay for the spare could be used to ensure that it is put in service rapidly.

This is true whether or not it will remain energized. It will provide an energized spare with adequate protection. To connect a differential relay for three-phase and single-phase transformers.

The most difficult situation to handle is that in which CTs on one side of a transformer bank are located in a circuit breaker, and those on the other side are in the transformer. Unfortunately, this is a common occurrence.

In such cases, CT secondary circuits have to be switched or rewired to place the spare power transformer in service.

With any of the above combinations of transformer connections, it is possible to switch or rewire the CT secondaries.

However, switching CT secondary circuits is not recommended as a good practice without a thorough analysis of the switching device and the risks of an open CT connection during the switching or as a result of a defective switching contact.

TESTING OF BUS DIFFERENTIAL RELAYING

The testing of bus differential relays and associated systems requires special care and accuracy on the part of the tester. The consequences of an inadvertent trip, due to tester error or of a false trip due to incorrect or incomplete test practices, are usually severe in that numerous circuits can be affected.

Practices have been developed which considerably ease the problems facing the relay tester and improve quality and thoroughness. Specific practices are as follows.

Test Switches
Many utilities provide a test switch in each output circuit and in the coil of the lockout relay or other auxiliary devices used to control multiple breakers. These test switches are used to ensure that no inadvertent trip occurs when removing the relay from service or returning it to service.

Test switches are also of significant value when performing trip tests in that the breakers can be tripped and closed one at a time to verify the integrity of the relay output circuitry. Covers are usually provided which cannot be put in place until the test switches are returned to their normal position.

Permanent Test Facilities
Permanently installed test equipment has been designed to check some types of differential relay circuits while the power circuit is in operation. These facilities use indicating meters to monitor pre selected points in the relay circuit by means of a test selector switch. These types of test systems are described in the manufacturers’ literature.

Verification of Relay Input Sources
Many problems with bus differential relay systems can be attributed to improper connections of current transformers and associated circuitry. Consequently, it is important to verify the ratio and polarity of the current transformers and the connections to the relays.

Procedures for performing the above tests are well known throughout the industry. However, if not properly performed, failure to trip or false trips will result.