Page 223 - Offshore Electrical Engineering Manual

210 CHAPTER 1 Alternator Faults and Protection Devices

FIELD FAULTS AND ASYNCHRONOUS OPERATION
If the generator field current fails and the generator is running as the sole supplier
of power on the installation, the set will trip on undervoltage, causing a blackout
until the emergency generator starts. However, if the generator is running in paral-
lel with a second machine, it would continue to generate power as an induction
generator, whilst demanding a heavy reactive power flow from the machine in par-
allel with it. Both machines will tend to heat up, and in some cases, it may appear
as if the healthy machine is the offender. A field failure relay of the mho imped-
ance type is normally used to protect generators from this condition. The generator
reactance for a machine where the excitation has failed is not fixed but describes a
circular locus, and so the relay characteristic should be set to enclose this locus as
fully as possible.

PACKAGE CONTROL AND SUPERVISION, MASTER TRIP AND
LOCKOUT RELAYS
Individual relay protection elements are usually provided without any form of
lockout facility, although some means of electronically or mechanically (i.e., flag)
recording relay operation is often included. Where a switchboard is fed by a num-
ber of generators, the operational logic of the system must be carefully studied to
ensure every mode of operation possible is reliable and safe. It is usual to provide a
master trip relay for each generator, which will operate from a signal derived from
any of the fault-sensing relays and then lock the generator out of service. Without
the master trip relay, indication of the fault on the control panel could well be lost,
and there would be nothing to prevent an operator attempting to restart the machine
before the fault is cleared. If failure of one generator leads to automatic starting
of another, it is important that the control logic differentiates between generator
failure and uncleared switchboard faults, in order to avoid the second generator
circuit breaker closing on to the same (switchboard) fault. The generator circuit
breaker tripping coil and its circuit should be monitored by a trip supervision relay,
which provides an alarm on failure of coil continuity or loss of tripping supply.
Because of the risk of generator, switchboard or transformer fires due to sustained
fault conditions if a generator circuit breaker fails to trip, and their possibly serious
consequences offshore, it is advisable to fit (to main generator circuit breakers at
least) trip supervision relays which continuously monitor both tripping supply and
trip circuit continuity.
For fault calculations see PART 4 Chapter 6.
For relay discrimination topics see PART 4 Chapter 7.