Power System Protection

Electrical equipment has to be designed and constructed in such a way as to withstand the foreseeable loading during its lifetime under normal and emergency conditions. It is therefore necessary to install devices for the protection of equipment which limit the effects of unforeseeable faults and loading on the equipment and protect it against cascading damage. These protection devices must be capable of differentiating between normal and disturbed operating conditions and they must operate reliably to isolate the damaged or endangered equipment as soon as possible from the power supply.

Protective devices are to fulfil the following conditions:

• Selectivity – protective devices shall switch off only that equipment affected by the system fault or impermissible loading condition, the non-faulted equipment shall remain in operation.
• Sensitivity – protective devices must be able to distinguish clearly between normal and impermissible operating conditions or faults. Permissible high loading of equipment during emergency operation and small short-circuit currents are to be handled in a different way.
• Speed – protective devices are to switch off the faulted equipment from the power supply as soon as possible in order to limit the effects of the short circuit or impermissible loading.
• Security – protective devices with all their associated components such as transducers, cable connections, wiring and trip circuits must operate safely and reliably.

General Structure of Protective Systems

Protective systems consists of several components whose reliable and coordinated operation ensures the intended function of the total system. The figure below shows the principle structure of a protective system as an illustration of distance or impedance protection.

Distance protection requires the measurement of current and voltage normally through current and voltage transformers or transducers. For adjustment of the phase-angle or adaptation of the transformation ratio, auxiliary transducers are necessary sometimes. The connection of the secondary side of the current and voltage transformers to the protection device is carried out with cables, and in modern protection devices also with glass-fiber cables. For the operation of the protective device, an auxiliary power supply is needed, which must be secure and reliable in supply, usually from a battery powered directly from the power system.

Trip circuits between the protection device and the circuit-breaker or load disconnecting switch consists of wiring (cables), auxiliary relays and circuits and trip coil. Auxiliary power is also needed for the operation of the breaker or switch, usually available directly from the current transducer, from a separate AC voltage source or from a capacitor release unit.

Protective systems supervising the operation of circuit breakers directly or by an auxiliary relay are called primary trip-relays or primary relays. They are used today only in MV systems. Their demerits include: difficulty of maintenance checking during operation, lack of adaptation of different currents and voltages to the desired measuring system, coupling between the high voltage circuit and secondary technology, increased thermal and dynamic stress and the protective device.

The arrangement, number and connection of current and voltage transformers are determined by the need of protection. For the case of unidirectional overcurrent protection, only current transformers are required, whereas for directional overcurrent protection, or impedance protection, additional voltage transformers are required.

For overcurrent protection in systems with isolated neutral, current transformers are required in two phases only, as the single-phase earth fault cannot be detected by the overcurrent protection due to the small current; only two-phase and three-phase short-circuits will be detected by the overcurrent protection. Three current transformers are needed for protection of HV lines with single-phase auto-reclosing as single-phase faults in any of the three-phases need to be detected by the protection. If faults currents through earth are to be detected, current transformers are required in three phases in Holmgreen arrangement or a separate current transformer is needed in the neutral.

Differential protection schemes require current transformers at the sending and receiving ends of the equipment (line or transformer) to be protected.

Voltage transformers are required in protection schemes, if either, the current direction, the power-flow, the impedance, the voltage or the power frequency is to be measured. For measurement of the line to earth voltage, line to earth voltage transformers are also needed, whereas in medium voltage systems, two voltage transformers are used to measure line to line voltage (V-arrangement).

The third line-to-line voltage can be determined by special arrangement of the secondary side or can be calculated from the two measured line to line voltages. The line to line earth voltages can be determined correctly only in the case of equally loaded transformers. The voltage of the neutral can be measured by a separate voltage transformer provided in the neutral or can be obtained from the open secondary circuit of a delta winding of three-single voltage transformers.

Protection Equipment

 Selection of the protection scheme and the protection device for equipment depends on the types of faults, the type of power system and the handling of the neutral, the voltage level, the type of the load to be supplied and the required selectivity.

Protection of Lines (Overhead Lines and Cables)

Typically the protection of lines (overhead lines and cables) is implemented as short-circuit protection, which is not suitable for protection against overload. The selection of the protection device depends on the kind and the mode of the operation of the power system e.g. radial, meshed system, etc.

Overcurrent Protection

Radial power systems can easily be protected with independent maximum current time protection (UMZ) or with overcurrent relays having inverse time characteristic. The selectivity of the protection is obtained by progressive grading of the trip time in the individual substations such that the trip time increases toward the feeding substations.

Typical grading time in approximately 300 ms when electronic or digital protective devices and modern circuit breakers are installed. The increase of the trip time can be avoided by reverse interlocking of the UMZ protection. This is achieved by interlocking the tripping command of each circuit breaker of each station with the status of the overcurrent relay in the next (downstream) substation.

Trip times in each substation can be realized in the range 300 – 500 ms, reserve protection is given by both concepts by the upstream protective devices.

In radial and meshed systems in the medium voltage range, the individual feeders are often equipped only with load break switches and only the feeding transformer with a circuit breaker.

In case of a short circuit in any feeder, the circuit breaker opens first and afterwards the load break switch related to the faulted feeder is operated. The circuit breaker is reclosed after an interval of ~500 ms. thus all faults are switched off selectively in the same interval. If several load break switches are installed, one following another, impedance protection for the circuit breaker and a time delay for the individual load break switches are required.

Overcurrent protection of the UMZ type can also be used for the connection of the public system and industrial power systems having own generation.

In case of faults in the public system, the circuit breaker is to be switched off immediately or with small delay, in order not to endanger reliable supply of the industrial system. In the case of the faults, in the industrial system, however, the circuit breaker is to be operated with time delay, in order to ensure time for the disconnection of the fault by the protection device of the industrial system.

Overcurrent protection of the UMZ type has a number of disadvantages, such as increase of trip-time with increasing current, the lack of selectivity in meshed systems and the tendency to malfunction in power systems having multilateral infeed. Thus, UMZ protection is a typical overcurrent protection employed in urban MV systems, which are operated as ring-main or radial systems.

Distance (Impedance) Protection

Distance protection or impedance protection device use current and voltage measurements at the location of the protective device to determine the location of the fault by assessment of the measured impedance. The measured impedance is compared with the impedance of the line, including existing current-limiting reactors or blocking reactors of carrier frequency signaling. If the measured impedance is below adjustable limit values, the associated circuit breaker is released immediately or with a time delay. Each line section is switched off selectively by the next distance protection device in shortest time. The remote backup function is given by distance protection devices in the next line section; local backup is realized by other independent protection. Distance protection devices are used for both primary protection of lines and as backup protection of busbars and transformers.

Ground Fault Protection

Ground fault protection is used in MV and HV transmission systems (for instance with nominal voltages 10 KV to 132 KV) having ground fault compensation or isolated neutral. Since ground faults (single-phase to earth) do not cause short circuit currents, an immediate disconnection of the fault is not necessary. The key task of ground fault protection therefore is the detection and signaling of the fault.

Generally, the detection of a ground fault is realized by means of the delta-winding of a potential transformer or by measurement of the voltage in the neutral. The setting of the protection must be done in such way that the displacement voltage in the neutral under normal operating conditions does not lead to a ground-fault signaling. If transient ground faults are not to be signaled, the signal emission has to be delayed. The direction of the ground fault with regard to the location of the protection device is determined from the earth fault current and the displacement voltage by determination of the active energy flow direction by watt-metric devices. By assessing the indications of several ground fault protection devices, the fault location can be found.

Protection of Transformers

Transformers are one of the most important items in power systems, having long repair times, non-self-healing insulation with voltage stress from internal and outside overvoltages and larger thermal time constants in case of overloading. The protection scheme of transformers must therefore take into account the special importance of the transformers for a reliable supply of the consumer load. In addition to the measurement of current and voltage, other parameters such as oil, temperature, flow rate of oil and gas concentration of the insulating oil are measured for protection purposes.

We look at some of the protection measures for transformers as discussed in the following section:

Differential Protection

Differential protection of transformers is arranged as longitudinal comparison as outlined in the figures below:

The connection of the protection device to the current transformers must be done in such a way that comparison of the currents can be carried out with respect to r.m.s. value and phase angle.

Inrush currents occurring during energizing of transformers, act as differential current for the differential protection. To avoid malfunction of the differential protection during energizing, rush stabilization of the differential protection is required, which analyses the 100 Hz component in the inrush current.

Differential protection of the transformer is only significant if all windings of the transformer are connected to the power system by means of circuit breakers. Therefore, differential protection of transformers is rarely used for transformers with rated power up to 2 MVA, generally with larger ratings in the range above 10 MW. The differential protection represents a primary protection and offers no reserve protection.

Overcurrent Protection, Distance Protection, Ground Fault Protection

 Generally, overcurrent protection is used for the protection of transformers if protection is not realized by fuses, as exist with distribution transformers. Overcurrent protection is a backup protection for the differential protection and also a backup protection for the overcurrent protection of the line feeders fed from the transformer and the associated busbar.

Monitoring of the temperature of the oil and the transformer tank can likewise be regarded as overcurrent protection. Normally the temperature measurement doesn’t release the circuit breakers but is only signalized, since other causes than short circuits can lead to the rise in temperature, which can then be reduced for instance, by suitable countermeasures such as reducing the transformer load.

The operation time can become quite high due to the coordination of the grading time with the overcurrent protection of the lines. If high grading time and as a result long time delays in operation of the overcurrent protection are to be avoided, distance protection must be replaced as the backup protection concept, which depending on the type of fault can operate on the busbar side and on the transformer side at different times, thus also in primary time.

Buchholz Protection

Buchholz protection is an important primary protection for transformers. Buchholz protection is a mechanical relay used for oil-immersed transformers with rated power of 630 KVA and above. Buchholz protection is also used for auxiliary transformers in substations and power stations, for installations in explosive environments and for transformers in mining installations.

Buchholz protection is released by the oil current flow between the transformer tank and the oil expansion tank or radiators as well as by the oil level. It works as a warning device in case of slow oil loss, for example by leakage or during gasification in case of faults with low fault currents.

With sudden oil loss or with strong gas bubble formation or large oil flow rates for instance, following faults with high currents, the Buchholz protection releases the circuit breakers on the HV and LV sides of the transformer. If an on-load tap-changer is installed with separate oil insulated housing, a separate Buchholz protection is needed for the tap-changer. The time-delay of the Buchholz protection is in the range 20 – 60 ms. The Buchholz protection responds only to faults inside the transformer tank and represents no backup protection for other protective devices.

Protection of Busbars

The protection of busbars needs special care because of the special importance of this equipment for a reliable power supply. The loss of a busbar following a busbar fault can result in subsequent loss of lines and transformers connected to the busbar.

Current as a Basis for Busbar Protection

Simple busbar arrangements with one feed and outgoing feeders without the possibility of back feed can be protected using overcurrent (UMZ) protection devices. In case of a busbar fault, the excitation of the UMZ protection in the feeding transformer is active and all other protection devices are inactive; all circuit breakers are tripped in primary time. In case of a fault on an outgoing feeder, the UMZ protection devices are all in active status; the busbar protection is blocked, since the faulted line is to be switched off by the assigned protection of the faulted feeder in primary time. The UMZ protection of the feeding transformer works as backup protection. This type of busbar protection does not represent an independent protection device of the equipment; the operating time is comparatively long at ~100 ms. Its merit lies in the simple and inexpensive realization.

Single busbars with several infeed can also be protected with a high impedance protection. The secondary currents of the current transformers of all feeders are connected in parallel to a high impedance resistance, the voltage at the resistance serving as tripping criterion. Since the currents are added with the correct phase angle, busbar faults lead to high voltages and feeder faults lead to low voltages. Operating time is in the range up to 60 ms.

If independent busbar protection with short operation time is required, differential busbar protection is to be employed. This arrangement is adequate for protection of busbars with arbitrary numbers of feeders. The switching status of the substation i.e. which feeders are connected to each busbar is modelled by an auxiliary circuit (switching image) e.g. by connection of auxiliary contacts of the circuit breakers and isolating switches into the protective device. False representation of the switching can result inaccurate operation of the busbar protection in case of normal operation or can result in unnecessary operation in case of faults on the feeders due to the “preloading” of the protection.

Monitoring of the current sum of all the feeders is an appropriate countermeasure. Special considerations are necessary if the busbar protection operates only on those circuit breakers which carry partial short circuits currents in the direction of the busbar or when the transformers with isolated neutral for example, in power station feeders; are connected. Operation time of the protection is in the range up to 20 ms. With the combination of differential protection and high impedance protection, the operating time can be reduced to less than 10 ms.

An additional structure for busbar protection is based on the comparison of the phase angles of the currents of all feeders with the phase angle of the total current sum. A switching image is also needed for this scheme.

Related: Types of Busbar Arrangements in Grid Stations and Substations

Protection of other Equipment

In addition to the protection of lines, transformers and busbars, other equipment such as capacitor banks, resonance and earthing reactors, short circuit limiting devices, motors and generators have to be equipped with protective devices as well. Considerations of security, sensitivity, selectivity, and the importance of the equipment must lead to the right design and to the selection of a protection concept.

You can also read: Types of Insulators used in Power Systems