Areva MiCOM P441, Техническое руководство

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Application Notes P44x/EN AP/E33
MiCOM P441/P442 & P444 Page 113/220
There are three main aspects to consider regarding the failure of the VT supply. These are
defined below:
1. Loss of one or two phase voltages
2. Loss of all three phase voltages under load conditions
3. Absence of three phase voltages upon line energisation
4.2.4 Loss of One or Two Phase Voltages
The VTS feature within the relay operates on detection of residual voltage without the
presence of zero and negative phase sequence current, and earth fault current (
Σ
I
ph). This
gives operation for the loss of one or two phase voltages. Stability of the VTS function is
assured during system fault conditions, by the presence of I0 and/or I2 current. Also, VTS
operation is blocked (and distance element unblocked) when any phase current exceeds 2.5
x In.
Zero Sequence VTS Element:
The thresholds used by the element are:
•
Fixed operate threshold: V
N
≥
0.75 x Vn;
•
Blocking current thresholds, I
0
= I
2
= 0 to 1 x In; settable (defaulted to
0.05In),
and Iph = 2.5 x In.
4.2.5 Loss of All Three Phase Voltages Under Load Conditions
Under the loss of all three phase voltages to the relay, there will be no zero phase sequence
quantities present to operate the VTS function. However, under such circumstances, a
collapse of the three phase voltages will occur. If this is detected without a corresponding
change in any of the phase current signals (which would be indicative of a fault), then a VTS
condition will be raised. In practice, the relay detects the presence of superimposed current
signals, which are changes in the current applied to the relay. These signals are generated
by comparison of the present value of the current with that exactly one cycle previously.
Under normal load conditions, the value of superimposed current should therefore be zero.
Under a fault condition a superimposed current signal will be generated which will prevent
operation of the VTS.
The phase voltage level detectors is settable (default value is adjusted at 30V / setting
range : min:10V to Max:70V).
The sensitivity of the superimposed current elements is settable and default value is
adjusted at 0.1In (setting range : 0,01In to 5In).
4.2.6 Absence of Three Phase Voltages Upon Line Energisation
If a VT were inadvertently left isolated prior to line energisation, incorrect operation of voltage
dependent elements could result. The previous VTS element detected three phase VT
failure by absence of all 3 phase voltages with no corresponding change in current. On line
energisation there will, however, be a change in current (as a result of load or line charging
current for example). An alternative method of detecting 3 phase VT failure is therefore
required on line energisation: in that case the SOTF logic is applied.