Fundamentals + Principles of unit protection [Industrial Electrical Power Systems]

Protective relay systems

The basic function of protection is to detect faults and to clear them as soon as possible. It’s also important that in the process the minimum amount of equipment should be disconnected. The ability of the protection (i.e. relays and circuit breakers) to accomplish the latter requirement is referred to as 'selectivity'. Speed and selectivity may be considered technically as figures of merit for a protection scheme. In general; however greater the speed and/or selectivity, the greater is the cost.

Hence, the degree of speed or selectivity in any scheme is not purely a technical matter, it’s also an economic one.

Main or unit protection

The graded overcurrent systems described earlier don’t meet the protection requirements of a power system. As seen in Section 10, the grading is not possible to be achieved in long and thin networks and also it can be noticed that grading of settings may lead to longer tripping times closer to the sources, which are not always desired. These problems have given way to the concept of 'unit protection' where the circuits are divided into discrete sections without reference to the other sections.

Ideally, to realize complete selectivity of protection, the power system is divided into discrete zones. Each zone is provided with relays and circuit breakers to allow for the detection and isolation of its own internal faults.

This ideal selective zoning. The protection used in this manner -- essentially for internal faults in a particular zone - is referred to as main or unit protection.

Back-up protection

It’s necessary to provide additional protection to ensure isolation of the fault when the main protection fails to function correctly. This additional protection is referred to as 'back-up' protection. For example, referring to the above figure, assume that a fault has occurred on the feeder and that the breaker at A fails to open. To clear this fault, the circuits which are able to feed current to the fault through the stuck breaker A must be opened. The fault is outside the zones of the main protection and can only be cleared by the separate back-up protection. Back-up protection must be time delayed to allow for the selective isolation of the fault by the main or unit protection.

Busbar protection; Feeder protection; Transformer protection; Motor protection

++++ Overall schematic indicating busbar, feeder, transformer and motor protection.

Methods of obtaining selectivity

The most positive and effective method of obtaining selectivity is the use of differential protection. For less important installations, selectivity may be obtained, at the expense of speed of operation, with time-graded protection.

The principle of unit protection was initially established by Merz and Price who were the creators of the fundamental differential protection scheme. These systems basically employ the direction of current rather than their actual values, protecting a particular zone by means of detecting the circulating currents through pilot wires and relays. The basic principles of these well-known forms of protection will now be considered.

Differential protection

Differential protection, as its name implies, compares the currents entering and leaving the protected zone and operates when the differential between these currents exceeds a pre-determined magnitude. This type of protection can be divided into two types, namely balanced current and balanced voltage.

Balanced circulating current system

The principle is shown. The CTs are connected in series and the secondary current circulates between them. The relay is connected across the midpoint thus the voltage across the relay is theoretically nil, therefore no current through the relay and hence no operation for any faults outside the protected zone. Similarly under normal conditions the currents, leaving zone A and B are equal, making the relay to be inactive by the current balance.

Under internal fault conditions (i.e. between the CTs at end A and B) relay operates.

This is basically due to the direction of current reversing at end B making the fault current to flow from B to A instead of the normal A to B condition in the earlier figure.

Relay End A; End B Fault

++++ Balanced circulating current system, external fault (stable)

Relay End A; End B Fault

++++ Balanced circulating current system, internal fault (operate)

The current transformers are assumed identical and are assumed to share the burden equally between the two ends. However, it’s not always possible to have identical CTs and to have the relay at a location equidistant from the two end CTs. It’s a normal practice to add a resistor in series with the relay to balance the unbalance created by the unequal nature of burden between the two end circuits. This resistor is named as 'stabilizing resistance'.

Balanced voltage system

As the name implies, it’s necessary to create a balanced voltage across the relays in end A and end B under healthy and out-of-zone fault conditions. In this arrangement, the CTs are connected to oppose each other. Voltages produced by the secondary currents are equal and opposite; thus no currents flow in the pilots or relays, hence stable on through-fault conditions. Under internal fault conditions relays will operate.

Relay B, Relay A, End A, End B Fault

++++ Balanced voltage system - external fault (stable)

Relay B, Relay A, End A, End B, Fault

++++ Balanced voltage system, internal fault (operate)

The balanced or circulating current systems are invariably used for generator, transformer and switchgear main protection where it’s convenient to readily access the midpoint of the pilots. This is because both sets of CTs are mounted in the same substation and a single relay is used to detect the fault condition within the protected zone.

On the other hand, balanced voltage systems are used mainly on feeder protection where the CTs are mounted in different substations, which are some distance apart. As there are two relays involved, one at each end, they can each be mounted in their respective substation.

Although similar, the various forms of differential protection differ considerably in detail. The differences are concerned with the precautions taken to ensure stability - i.e. to ensure that the protection does not operate incorrectly for a through fault.

Bias

The spill current in the differential relay due to the various sources of errors is dependent on the magnitude of the through current. Hence it’s necessary to consider the setting of the differential relay to be more than or proportional to the worst spill current likely to occur under through-fault conditions. Because of the wide range of fault current magnitudes, it’s not always satisfactory to make the relay insensitive to lower-spill current values. This problem had been overcome by adjusting the operating level of the relay according to the total amount of fault current. This was done originally by providing a restraining winding or electromagnet which carries the total fault current while an operating electromagnet was allowed to carry only the differential current. This principle of bias is applied to circulating current protection to ensure proper operation under all fault conditions.

If the two zone boundary currents are I1 and I2, then Operating quantity: K1 (I1 - I2)

Biasing quantity: K2 (I1 + I2)

Suitable choice of constants K1 and K2 ensures stability for external fault currents despite measurement errors, while still ensuring stability under internal fault conditions.

Machine differential protection

The balanced circulating current principle is normally used. The bias feature is introduced to ensure stability despite possible small differences in the performance of the two nominally balanced sets of current transformers.

The sensitivity of this protection is normally of the order of 10%, which means that the protection will operate when the differential current is greater than 10% of the normal full load. Without bias, for a through-fault current of ten times full load, the protection would operate if the 'spill' or differential current exceeded 10% of full load or 1% of the through-fault current. To avoid the necessity of matching current transformers to this degree of accuracy the protection is biased with through current.

Transformer differential protection

A typical transformer differential protection system also adopts the circulating current principle. The first point to notice is that the CTs on one side are connected in delta whilst they are connected in star on the other. This has been done for two reasons:

1. To correct for the phase-shift through the transformer in order to obtain co-phasal currents at the relay.

2. To prevent the relay from operating incorrectly for an external ground fault on the side of the power transformer where the windings are connected in star with the neutral grounded.

Through-current bias is necessary on these relays not only for the inherent unbalances of the CTs but also to take care of any voltage tappings on the transformer provided by the tap-changer. For example, a transformer having a nominal ratio of 132/40 kV having a tap change range of +15 to -5% on the 40 kV side would have the CT ratios selected to be balanced at the midtap, namely 132/42 kV. The above is discussed in more detail.

13.7 Switchgear differential protection

In switchgear differential protection, all the currents entering and leaving the protected zone are added and if the resultant is zero then the busbars are healthy. However, if the current exceeds the chosen setting, the protection will operate and trip all associated circuit breakers.

The stability of this type of protection is obviously of vital importance since an incorrect operation could result For example in the shutdown of a power station. On account of the large number of circuits involved, all carrying different currents, stability is also a more difficult problem than with machine or transformer differential protection.

A number of different schemes are used for this protection, normally referred to as 'bus zone protection'. The schemes differ mainly in the principle adopted to obtain stability and these are discussed.

Feeder pilot-wire protection

Pilot-wire protection is similar to differential protection in that it normally compares the current entering the circuit at the one end with the current leaving at the other end. Its field of application is the protection of power cables and short transmission lines. For these circuits the distance between the current transformers at the two ends of the protected zone is too great for the circulating current differential protection of the type described previously for machines and transformers, etc. The pilot-wire provides the communicating channel for conveying the information relative to conditions at the one end of the feeder to protective relays at the other end of the feeder and vice versa.

These relays or groups of relays, at the two ends are able to make a comparison between local and remote conditions and thus determine if there is an internal fault. Each relay normally trips only its associated circuit breaker. There are many different types of pilot wire protection schemes, but the most commonly used are of the opposing voltage type.

Time taken to clear faults

With the inherently selective forms of protection, apart from ensuring that the relays don’t operate incorrectly due to initial transients, no time delay is necessary. Operating times for the protection, excluding the breaker tripping/clearing time are generally of the following order: Machine differential - few cycles Transformer differential - 10 cycles Switchgear (busbar) differential - 4 cycles Feeder differential - few cycles These operating times are practically independent of the magnitude of fault current.

Recommended unit protection systems

• Cable feeders: Pilot wire differential

• Transformer: HV-balanced (restricted) ground fault

- HV high-set instantaneous overcurrent (low transient over-reach)

- LV-restricted ground fault

- Buchholz

• Busbars: Medium-/low-impedance schemes for strategic busbars (including busbars operating on closed rings)

- Busbar blocking schemes for radial networks

• Unit protection: Should be used where possible throughout the network to remove the inverse time. Relays (IDMT) from the front line

• IDMT: Must be retained as back-up only to cover for a failure of the main protection.

Advantages of unit protection

Fast and selective

Unit protection is fast and selective. It will only trip the faulty item of plant, thereby ensuring the elimination of any network disruptions.

Easy to set

Unit protection is easy to set and once installed very rarely requires changing, as it’s independent of whatever happens elsewhere on the system.

No time constraints

Time constraints imposed by the supply authorities don’t become a major problem anymore. They only need consideration when setting up the back-up inverse time (IDMT).

Maximum operating flexibility

The system can be operated in any switching configuration without fear of a loss of discrimination.

Better continuity of supply

In many applications rings can be run closed, so that switching would not be necessary to restore loads resulting in better continuity of supply.

Future expansion relatively easy

Any future expansion that may require another in-feed point can be handled with relative ease without any change to the existing protection.