Relay Element Settings 2-33
SEL-300G Instruction Manual Date Code 20020206
Set the Zone 2 time delay equal to 0.5 to 0.6 seconds or according to the generator
manufacturer’s recommendations.
seconds 0.540Z2D =
In this case, the 40DIR setting is hidden.
Loss-of-field protection typically trips the main generator breaker and the field breaker. When
applying loss-of-field protection with a negative Zone 2 offset, you can use the time-delay Zone 1
and Zone 2 Relay Word bits, 40Z1T and 40Z2T, directly in the generator breaker and field
breaker tripping SEL
OGIC Control Equations. See Section 4: SELOGIC
®
Control Equations of
this instruction manual for more details and an example.
Loss-of-Field Protection Using a Positive Offset Zone 2
When setting Zone 2 with a positive offset, set the Zone 1 diameter:
Ω
2
Xd'
Xd•1.140Z1P
−
+=
Set the Zone 1 offset equal to one-half the generator transient reactance, Xd', in secondary ohms.
Ω
−
=
2
'Xd
1XD40
Traditionally, the Zone 1 delay for this type of scheme is 0.25 seconds.
second 0.2540Z1D =
Set the Zone 2 diameter using the direct axis reactance and the step-up transformer reactance.
Ω XTXd•1.140Z2P +=
Set the Zone 2 offset using the reactance of the generator step-up transformer, XT.
Ω XT40XD2 =
Traditionally, the Zone 2 delay for this type of scheme is 1 second.
second 1.040Z2D =
A shorter delay traditionally is applied if the Zone 2 element picks up at the same time that an
undervoltage condition is detected. This logic is discussed below.
In this case, the 40DIR setting is required. Set 40DIR equal to –20° or the arccosine of the
minimum rated power factor, whichever is smaller.
When applying loss-of-field protection with a positive Zone 2 offset, you can use the
time-delayed Zone 1 Relay Word bit, 40Z1T, and the long-time-delayed Zone 2 Relay Word bit,
40Z1T, directly in the generator breaker and field breaker tripping SEL
OGIC Control Equations.
The traditional application of this scheme provides accelerated (0.25 second) Zone 2 tripping in
the event of an undervoltage condition occurring during the loss-of-field. To achieve this
accelerated tripping, it is necessary to use a SEL
OGIC Control Equation variable and a positive-
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