3.31
Date Code 20020903 SEL-587Z Instruction Manual
Relay Elements
Demand Ammetering
In general, as voltage V
C
across the capacitor in Figure 3.28 on page 3.30
cannot change instantaneously, the thermal demand ammeter response is not
immediate either for the increasing or decreasing applied instantaneous
current. The thermal demand ammeter response time is based on the demand
ammeter time constant setting DMTC. Note in Figure 3.27 on page 3.30 the
thermal demand ammeter response (bottom) is at 90 percent (0.9 per unit) of
full applied value (1.0 per unit) after a time period equal to setting DMTC =
15 minutes, referenced to when the step current input is first applied.
The SEL-587Z updates thermal demand values approximately every two
seconds.
Setting
Description
Demand Time Constant (DMTC)
Range: 5, 10, 15, 30, 60 minutes
The time constant setting defines the temperature rise of a device when a
current is flowing through the device.
Demand Threshold Settings (PDEMP, QDEMP, GDEMP)
Range: OFF, 0.50–16.00 A
The threshold settings determine the pick-up values for the phase, negative,
and residual demand threshold elements.
Setting Calculation
EXAMPLE 3.1 Setting the Relay (transformer)
The first step is to identify the thermal curve applicable to the device
in order to find the time constant. For this example, we use the IEEE
C57.91: 1995 standard for oil-immersed transformers. This standard
specifies the temperature parameters for a transformer operating at
full load at an ambient temperature of 30°C. An extract from the
standard is presented in Table 3.5 on page 3.32. The matching time
constant is two hours.
The DMTC setting is thus 2.0 • 60 = 120 minutes.
Ta b le 3.4 Tra n sfo r mer Deta i l
Operating Parameter Value
MVA 50 MVA
Voltage ratio 132/69 kV
CTR
HV
1000/5 (wye connected)
Cooling system 2 stage
Winding temperature rise 55°C
Permissible continuous overload <10%
To Next Page
Loading...
