GE P444 User Manual

Application Notes
P44x/EN AP/Hb
6
MiCOM P40 Agile P442, P444
(AP) 5-
137
5.8.1 Voltage transformer supervision (VTS) – Main VT for minZ measurement
5.8.1.1 VTS description
The voltage transformer supervision (VTS) feature is used to detect failure of the analog ac
voltage inputs to the relay. This may be caused by internal voltage transformer faults,
overloading, or faults on the interconnecting wiring to relays. This usually results in one or
more VT fuses blowing. Following a failure of the ac voltage input there would be a
misrepresentation of the phase voltages on the power system, as measured by the relay,
which may result in maloperation of the distance element.
The VTS logic in the relay is designed to detect the voltage failure (with internal thresholds or
external opto input), and automatically adjust the configuration of protection elements (the
distance element is blocked but may be unblocked by I1, I2 or I0 conditions in the event of a
fault during VTS conditions) whose stability would otherwise be compromised (Distance,
DEF, Weak infeed, Directionnal phase current & all directional elements used in the internal
logic).
A settable time-delayed (VTS Time Delay) alarm output is also available (min 1s to max
20s). This alarm is instantaneous if an opto inputs is energised by an external fuse blowing
signal (for instance from a micro circuit breaker contact). This external information is secure,
and will instantaneously block the distance function and the functions using directional
elements.
In the absence of load, the time-delay covers the duration of the Dead time-delay 1 of the
autoreclose cycle which could be detected as a 1-pole VT failure.
Where a miniature circuit breaker (MCB) is used to protect the voltage transformer ac output
circuits, it is common to use MCB auxiliary contacts to indicate a three phase output
disconnection. As described previously, it is possible for the VTS logic to operate correctly
without this input. However, this facility has been provided for compatibility with the current
practices of various utilities. Energising an opto-isolated input assigned to “MCB Open” on
the relay will therefore provide the necessary block.
Fuse failure conditions are confirmed instantaneously if the opto input receiving the fuse
blowing signal is energised and assigned in the PSL, or after elapse of the VTS Time delay
in case of 1-, 2- or 3-phase Fuse Failure.
Confirmed Fuse Failure blocks all protection functions which use the voltage measurement
(Distance, Weak infeed, Directional overcurrent,etc.). The directional overcurrent element
may be blocked or set to become non-directional with a dedicated time-delay ('I>1 Time
Delay VTS' or 'IN>1 Time Delay VTS' setting).
An unconfirmed Fuse Failure will be a detection of an internal fuse failure before the time-
delay has expired. In that case a fault can be detected by the I2>,I0>,I1>, ∆I> criteria and will
force the unblocking functions:
• Distance Protection
• DEF Protection
• Weak-infeed Protection
• I> Directional
• U>, U<
5.8.1.2 Loss of one, two or three voltages (‘VTS I2 & I0 Inh’ and ‘Detect 3P’)
The three main aspects to consider regarding the failure of the VT supply are:
• Loss of one or two phase voltages
• Loss of all three phase voltages under load conditions
• Absence of three phase voltages upon line energisation
5.8.1.2.1 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
Iph). This

GE P444 Specifications

General

Brand	GE
Model	P444
Category	Protection Device
Language	English