Operation P54x/EN OP/La4
MiCOM P543, P544, P545 & P546
(OP) 5-125
OP
3.1.1 Loss of one or two phase voltages
The VTS feature within the relay operates on detection of negative phase sequence (nps)
voltage without the presence of negative phase sequence current. 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 nps current. The use of negative sequence quantities
ensures correct operation even where three-limb or ‘V’ connected VT’s are used.
Negative Sequence VTS Element:
The negative sequence thresholds used by the element are V2 = 10 V and I2 = 0.05 to 0.5 In
settable (defaulted to 0.05 In).
3.1.2 Loss of all three phase voltages under load conditions
Und
er the loss of all three phase voltages to the relay, there will be no negative 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 are fixed and will drop off at 10 V and pickup at 30 V.
The sensitivity of the superimposed current elements is fixed at 0.1 In.
3.1.3 Absence of three phase voltages upon line energization
If a VT
were inadvertently left isolated prior to line energization, 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
energization 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 energization.
The absence of measured voltage on all 3 phases on line energization can be as a result of
2 conditions. The first is a 3 phase VT failure and the second is a close up three phase fault.
The first condition would require blocking of the voltage dependent function and the second
would require tripping. To differentiate between these 2 conditions an overcurrent level
detector (VTS > Inhibit) is used which will prevent a VTS block from being issued if it
operates. This element should be set in excess of any non-fault based currents on line
energization (load, line charging current, transformer inrush current if applicable) but below
the level of current produced by a close up 3 phase fault. If the line is now closed where a 3
phase VT failure is present the overcurrent detector will not operate and a VTS block will be
applied. Closing onto a three phase fault will result in operation of the overcurrent detector
and prevent a VTS block being applied.
This logic will only be enabled during a live line condition (as indicated by the relays pole
dead logic) to prevent operation under dead system conditions i.e. where no voltage will be
present and the VTS > Inhibit overcurrent element will not be picked up.
Note: VTS > Inhibit logic is equally applicable for the situation where loss of
all three phase voltages occurs under load conditions. (Refer section
3.1.2).If the setting of VTS > Inhibit is le
ss than the load current and if
three phase VT fails during normal load, VTS block will not be applied.
Hence it is important that the VTS > Inhibit is always set above the
expected load current.
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