Feeders Cable

Have a separate emergency electrical supply and feeder cable ... 

Important loads can be connected to redundant feeder circuit breakers, one on each side of the double-ended switchgear, by connecting the load sides of the circuit breakers together with a feeder cable. This approach should be used with caution. Another approach is to connect the redundant feeder circuit breakers to a time-delayed automatic transfer switch. 

The conductor size and length of the motor feeder cable need to be chosen carefully and the following points should be considered -... 

A good rule-of-thumb guide is to assume that between 15% and 25% extra capacity will be required. Hence the chosen rating will be 115% to 125% of the best-known estimate at the early design stage. This requirement also applies to power transformers and their main cables or overhead power lines, and to outgoing feeder cables to auxiliary switchboards and motor control centres. It does not usually apply to individual motor consumers, see Chapter 1. 

Figure 8.4. Protection of an induction motor and its feeder cable.

When calculating the volt-drop in low voltage motor feeder cables it is necessary to consider three factors in particular -... 

For high voltage cables, and low voltage feeder cables between switchboards, the disconnection time can be reduced from 5 seconds to not less than 0.2 seconds. The time of 0.2 seconds is determined from the total clearance time of a circuit breaker protected by a fast acting overcurrent relay. For high voltage motor circuits in which the short-circuit protection is provided... 

Table 9.33. Volt-drop in worked example the motor feeder cable for the ...

LV motors can occasionally be ignored when HV switchboard faults are being calculated but this will depend npon circumstances, e.g. the number of intermediate voltages exist in the system, whether there are many small motors or a few large motors, the average route length of motor and transformer feeder cables. On offshore platforms it is advisable to seriously consider the LV network. LV motor control centres will be inflnenced by their motor loads, and the effect of motor contribution will mainly be determined by the fnse, contractor and circuit breaker configurations. 

The type of feeder cables described in this section are those between switchboards within an oil industry site, rather than those between a utility power plant and an oil industry site. These feeders may be described as primary feeders as opposed to secondary feeders downstream in the system. Feeders from a utility power plant or a transmission network have protective relaying systems that are more sophisticated than those described herein, e.g. multi-zone distance protection, admittance relays, carrier protection schemes. 

Two basic requirements apply to feeder cables, firstly to protect the cable from overcurrents, which may be related to the connected load, and secondly to detect faults along the length of the cable. 

Short circuits that do not involve earth, and which are within the length of the cable, can be detected by setting the instantaneous elements of the overcnrrent relays to a value of current calculated at the receiving end of the cable that flows into a zero-impedance fault. Customarily this fault is a three-phase fault for which the calculations are straightforward. If the fault is beyond the cable for example in a consumer then the fault current will be less and shonld be cleared by the consnmer protective device. The feeder cable relays will then act as a back np to the consumer relays. 

If the feeder cable is protected by fuses then these shonld be chosen to rapidly clear an internal line-to-line or three-phase fault. They should be supplemented with a (51) relay to provide overcnrrent protection. 

Note For small motors, e.g. 22 kW and below, the earth loop impedance inclnding the feeder cable armouring may be too high. When this is the situation a risk of electric shock exists dnring a short circnit at or near to the motor. To reduce the exposure to the risk it is necessary to nse a 51 N or a 50 N core balance current transformer and relay at the motor control centre. The choice of a 50 N is preferred subject to the contactor being properly coordinated with its upstream fuses. 

Instantaneons overcnrrent protection wonld detect short circnits in the load and along its feeder cable. It wonld nsnally be practical to set the instantaneons elements of a monlded case circnit breaker to their lowest valne e.g. five times the nominal cnrrent. If the protection is provided by a set of fnses then the fnsing factor wonld be marginally above nnity, the nearest fnse rating above the load cnrrent would be chosen. The protection must fully cover the Pt capacity of the feeder cable. 

The aerials and feeder cables should be located in a safe area as close as possible to the radio room. 

These are the parallel elements of the load in per-unit at the system base, f) Motor feeder cable. Convert to system base. 

Now add the feeder cable impedance in series to obtain the total series impedance between the MCC and the motor. Call this total impedance Zmnic-... 

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