Page 345 - Handbook of Electrical Engineering

332 HANDBOOK OF ELECTRICAL ENGINEERING

winding. Instantaneous protection is applied to respond to major three-phase faults at or near to the
primary terminals e.g. in the main terminal box or chamber. It should not be used to detect major
faults in the secondary winding or its downstream circuit. The settings for the primary instantaneous
protection can therefore be chosen to be relatively high. However, the choice may be influenced
by the upstream source of fault current e.g. the number of generators, another transformer, a utility
connection, as explained in sub-section 12.5.2.2.
The situation for the secondary circuit is different. The purpose of instantaneous protection
is to detect major faults at or near to the secondary terminals and at the downstream switchgear
e.g. busbar fault. This protection must also be coordinated with the instantaneous protection settings
of downstream circuits e.g. static loads, motors. The settings chosen are much less sensitive to the
upstream source characteristics than those of the primary protection, because of the inclusion of the
leakage impedance of the transformer in the faulted circuit.

12.4.3 Characteristics of the Upstream Source
Where the upstream source is another transformer, or a utility connection, the calculation of the
three-phase fault current is straightforward and it will not usually vary significantly with the operating
configuration of the upstream network.
If the upstream source is one or more generators then the situation is more complicated,
especially for the transformer primary protection. When a major fault is applied near to generators
they respond in a complicated manner due to the sub-transient and transient dynamics of their
windings and to the dynamic response of their voltage regulators. The response from their windings
is also modified by the impedance connected between the generator terminals and the point where
the fault is applied. The sub-transient and transient direct-axis time constants, governing the decay
of fault current, change with the amount of impedance added to the fault circuit. As this impedance
increases from zero to a large value, the time constants change from their short-circuit values to
their open-circuit values, see 7.2.11 and 20.3.2. The inclusion of the impedance reduces the fault
current, which is more significant when only one generator is operating. The decrement of fault
current can be plotted on the coordination graphs for the various operating situations. In the example
of sub-section 11.9 and 12.1 there are four or more generators and therefore the two main situations
to consider are four generators running and only one generator running.

12.5 FEEDER CABLE PROTECTION
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.

12.5.1 Overcurrent Protection
Overcurrent protection is usually provided by a (51) relay, which has separate elements for each
2
phase. The overcurrent curve should be chosen with a margin below the I t characteristic of the