Page 264 - Offshore Electrical Engineering Manual

CHAPTER

Protection and 7

Discrimination

INTRODUCTION
The worked example below is a continuation of the example in PART 4 Chapter 5
where Generator and Switchboard fault currents were manually calculated. Note that
PART 4 Chapter 6 gives some guidance on calculation standards and commercial
software available.

WORKED EXAMPLE (CONTINUED): RELAY SETTING OF
TYPICAL MV PLATFORM SCHEME
The full load current of the largest motor on the system is 600 A. The starting current
of this motor is 1300 A. (Note that this is with an autotransformer starter on 80% tap,
and gives a starting time with this particular motor of 10 s). The maximum momen-
tary fault current is:

17.7/(0.195 × 3 × 6.6 = 7.94 kA (rms).
The steady state fault current from manufacturer’s test data is 3.54 kA (after
0.25 s).
Let us select in this instance a CEE ITG 7231 relay for three-phase duty, using
a range of 0.7–2.0/n. A CT ratio of 2000/1 A is selected, with 15VA5P10 CTs. The
relay settings must coordinate with the following:

1. The total standing load of 200 A on the switchboard that the generator is sup-
plying, plus the starting current taken by the 600 A motor. Therefore the load
current is 200 + 1300 = 1500 A for 10 s.
2. From simulation studies or voltage dip calculations, the maximum transient load
for the maximum voltage dip of 80% (which must not be exceeded whilst this
motor is being started) is obtained. This gives a load of 2010 A for an initially
fully loaded generator.
3. The 600 A motor thermal overload relay. This is set to operate at 3.4 kA
after 10 s.

Therefore the minimum relay setting current required is I = 2010/2000 or approx-
r
imately 1 A.
To coordinate with point 2 above, the operating time should be approxi-
mately 10.0 + 0.5 = 10.5 s at 3.4 kA. The required plug setting multiple (PSM) is

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