Alstom MiCOM P846, Technical Manual

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Operation P846/EN OP/D
MiCOM P846
(OP) 5-13
OP
ope )
nega r
fault conditions, by the presence of nps current. The use of negative sequence quantities
2.3
such
re fixed and will drop off at 10V and pickup at 30V. The
rrent elements is fixed at 0.1In.
2.3.1
d result. The previous VTS element detected three phase VT failure
ges with no corresponding change in current. On line
er, be a change in current (as a result of load or line charging
should be set in excess of any non-fault based currents on line
f 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
Ι
> Inhibit overcurrent
element will not be picked up.
Required to drive the VTS logic are a number of dedicated level detectors:
•
IA>, IB>, IC> level detectors operate in less than 20 ms and their settings should be
greater than load current. This setting is specified as VTS I > Inhibit.
•
I2> level detector operates on negative sequence current and has a user setting VTS
I2> Inhibit
•
Δ IA>, Δ IB>, Δ IC> level detectors are operating on superimposed phase currents and
have a fixed setting of 10% nominal
2.2.1 Loss of one or two phase voltages
The VTS feature within the relay rates on detection of negative phase sequence (nps
voltage without the presence of tive phase sequence current. This gives operation fo
the loss of one or two phase voltages. Stability of the VTS function is assured during system
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 = 10V and I2 = 0.05 to 0.5In
settable (defaulted to 0.05In).
Loss of all three phase voltages under load conditions
Under 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
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 a
sensitivity of the superimposed cu
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 coul
by absence of all 3 phase volta
energization there will, howev
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
Ι
> Inhibit) is used which will prevent a VTS block from being issued if it
operates. This element
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
phases 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 o