Chapter 10 - Voltage Protection Functions
Undervoltage conditions may occur on a power system for a variety of reasons, some of which are outlined below:
Undervoltage conditions can be related to increased loads, whereby the supply voltage will decrease in
magnitude. This situation would normally be rectified by voltage regulating equipment such as AVRs (Auto
Voltage Regulators) or On Load Tap Changers. However, failure of this equipment to bring the system
voltage back within permitted limits leaves the system with an undervoltage condition, which must be
If the regulating equipment is unsuccessful in restoring healthy system voltage, then tripping by means of
an undervoltage element is required.
Faults occurring on the power system result in a reduction in voltage of the faulty phases. The proportion by
which the voltage decreases is dependent on the type of fault, method of system earthing and its location.
Consequently, co-ordination with other voltage and current-based protection devices is essential in order to
achieve correct discrimination.
Complete loss of busbar voltage. This may occur due to fault conditions present on the incomer or busbar
itself, resulting in total isolation of the incoming power supply. For this condition, it may be necessary to
isolate each of the outgoing circuits, such that when supply voltage is restored, the load is not connected.
Therefore, the automatic tripping of a feeder on detection of complete loss of voltage may be required. This
can be achieved by a three-phase undervoltage element.
Where outgoing feeders from a busbar are supplying induction motor loads, excessive dips in the supply
may cause the connected motors to stall, and should be tripped for voltage reductions that last longer than
a pre-determined time.
UNDERVOLTAGE PROTECTION IMPLEMENTATION
Undervoltage Protection is implemented in the VOLT PROTECTION column of the relevant settings group. The
Undervoltage parameters are contained within the sub-heading UNDERVOLTAGE.
The product provides three stages of Undervoltage protection with independent time delay characteristics.
Stage 1 provides a choice of operate characteristics, where you can select between:
An IDMT characteristic
DT (Definite Time)
You set this using the V<1 Function setting.
The IDMT characteristic is defined by the following formula:
t = K/( M-1)
K = Time multiplier setting
t = Operating time in seconds
M = Measured voltage / IED setting voltage (V<(n) Voltage Set)
The undervoltage stages can be configured either as phase-to-neutral or phase-to-phase voltages in the V<
Measur't Mode cell.
There is no Timer Hold facility for Undervoltage.
Stages 2, 3 and 4 can have definite time characteristics only. This is set, for example, in the V<2 Status cell.
Outputs are available for single or three-phase conditions via the V< Operate Mode cell for each stage.