Protection current transformers

Iligh-voltage controllers which regulate prirnaiv input voltage to the rectifier and wiper transformer and house primary current-limiting protection, meters, and instrumentation are designed for local or remote operation. 

FIG. 29-8 Typical high-voltage ac motor starter illiistrating several protective schemes fuses, overload relays, ground-fault relays, and differential relays with the associated current transformer that act as fault-current sensors. In practice, the differential protection current transformers are located at the motor, hut the relays are part of the starter. 

When there is no dedicated transformer and these circuits are connected on the system bus directly a large inductor will be essential at the incoming of the static circuits, sufficient to absorb the trapped charge within the transformer and the interconnecting cables up to the converter unit. The size of the inductor can be calculated depending on the size (kVA) of the distribution transformer, its fault level and the characteristics of its current limiting protective device. An inductor sufficient to absorb //, L of the transformer and the cables may be provided at the incoming of the sialic circuits. 

Protective current transformers, for special purpose applications Part-4/1992 BS 7626/1991 ... 

Note For CTs this multiplying factor has been specified as 2.5 for all voltage systems, as in lEC 60044-1. See also Section 15.7 for metering and protection current transformers. 

Protective current transformers Requirements for transient performance 2705 parl-3/1992 2705 part-4/1992 BS 7626/1993 ... 

Protective current transformer for special purpose applications part-3/1992 part 4/1992 ... 

Protection of a domestic or an industrial single phase system Ground fault on an LT system Ground fault protection in hazardous areas. Ground leakage in an HT system Core-balanced current transformers (CBCTs). Ground fault (G/F) protection schemes... 

Figure 3-50 Methods of constant current limiting (a) discrete overcurrent limiting (constant-current limiting) (b) precision resistive current-sensing overcurrent protection (constant-current limiting) (c) use of a current transformer to sense ac current.

Relays have inputs from several current transformers (CTs) and the zone of protection is bounded by these CTs. While the CTs provide the ability to detect a fault inside the zone, circuit breakers (CBs) provide the ability to isolate the fault by disconnecting all of the power equipment within the zone. Thus, a zone bonndai y is usually defined by a CT and a CB. When the CT is part of the CB it becomes a natural zone boundaiy. When the CT is not an integral part of the CB, special attention must be paid to the fault detection and fault interruption logic. The CT still defines the zone of protection, but communication channels must be used to implement the tripping function. Figure 1 shows the zones of protection in a typical system. 

Differential relaying is the universal bus and transformer protection scheme. The inrush current associated with power transformers rec uires a special differential relay utilizing filters to provide harmonic restraint to differentiate between energizing current and fault current. 

The motor shaft is usually held in place by two insulated slide bearings equipped with ring lubrication. In addition, the bearings are cooled for example by an oil-water cooling system. The terminal box of the motor should be so dimensioned as to accommodate the current transformers for differential protection of the motor. To prevent the formation of condensation in the motor during standstill, a space heater is installed. 

Protective devices, usually relays or fuses, are installed at supply points in a power system to accurately detect and quantify a disruptive disturbance in the system. The variable most freqnently used for detection is the supply line current, and in most sitnations this is detected throngh the nse of current transformers. Occasionally direct acting devices are used e.g. fuses for voltages up to about... 

The star-point connection to earth is provided with a current transformer and a sensitive relay. The relay (51 G) is of a definite time delay or inverse time delay type so that it can be graded as back-up protection to earth fault relays at downstream feeders and consumers. 

Where a bus-section circuit breaker is used to divide the busbars (during abnormal operating conditions) each set of busbars is protected as a separate zone. Each zone consists of the incomers, the outgoing circuits and the bus-section circuit breaker(s). An accurate current transformer is connected in each line of each circuit. All the current leaving the zone must be balanced by current from the incomer circuits. A fault in the zone will be detected by the (87) relay. Rapid operation is required... 

In addition, the pulsing process requires sophisticated electronics which must prove durable over many years. They are likely to be less reliable than transformer rectifier systems providing straight DC for impressed current cathodic protection. 

A shielded torroidal current transformer can also be used as the phase/current pickup element. Such devices offer several advantages over the sample loop, including greater stability and refiabifity. Because they are located inside the tuning unit cabinet or house, TCTs are protected from wind, rain, ice, and vandalism. 

Outdoor high- aud medium-voltage substations usually employ oil-immersed circuit breakers as a system protective device. These devices are equipped with direct-acting internal current transformers and trip coils enclosed in a jacket filled with a mineral oil. The oil jacket provides rapid coohng for contacts which become heated due to arcing action created by each switching operation. The oil-Med jacket is a better heat dissipator than air, so the device can be smaller than one which is air-cooled. 

To cater for the supply company s needs, the standard describes the requirements for an incoming supply assembly (ISA). It has two compartments with lockable access - one for the incoming cable termination, service fuses, neutral link, current transformers and meters, and another for the consumer s main switchfuse or circuit breaker fitted with excess current protection and, where appropriate, earth leakage protection. 

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