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monitor any busbar residual voltage e.g. resulting from back e.m.f. generated by the induction motors
as they run down. The relay can act as a guard prior to re-connecting a supply from an alternative
source.
Where undervoltage relays are used on a system, the voltage elements should be set to a value below
that where a normal system voltage excursion can be expected. (See also section 2.5). Typically the
set values may be of the order of 65-80% of nominal for protection of the system or plant. For
confirmation that a monitored supply is 'dead' or that any residual voltage has reduced to a safe level,
typical set values should be of the order of 10-30% of nominal voltage.
3.2 Overvoltage Protection
Overvoltages may be caused for a number of different reasons. On generator sets for example, it may
be caused by defective operation of the voltage regulator, or, if there is a sudden loss of load due to
line tripping. Under this load rejection situation the generator set may overspeed causing a dangerous
voltage rise. This should be corrected by system regulating equipment such as tap changers and
AVR's, but if this equipment mal-functions then voltage levels may rise. High levels of overvoltages on
a system cannot be sustained for long periods because they can cause damage to the system
insulation and severely affect the life of the insulation. An overvoltage element with an appropriate DTL
time delay setting to allow for the normal system regulating equipment to operate can be used to
protect against this type of condition.
With a maximum of four overvoltage elements available, the Argus 8 relay can provide for a variety of
different applications. If the overvoltage condition is only small a relatively long DTL time delay can be
set on an element to clear the fault. If the overvoltage is more severe then another element, set at a
higher pickup level and with a faster DTL time, can be used to clear the fault more quickly.
Alternatively, elements can be set to provide alarm and tripping stages, with the alarm levels set lower
than the tripping stages.
Note - the use of instantaneous and wide ranging DTL settings allows a simple and secure grading
system to be applied to co-ordinate the network design, the regulating plant design and system plant
insulation withstand. The use of IDMTL protection is not recommended because of the difficulty of
choosing settings to ensure correct co-ordination and security of supply.
Generally, wherever voltage relays are employed, timing elements should be used to prevent operation
during transient disturbances. Also, overvoltage relays must be co-ordinated with other overvoltage
relays elsewhere on the system
3.3 Neutral Voltage Displacement Protection
The three phases of a balanced healthy system summate to zero. When a single-phase earth fault
occurs, the system balance is upset and a 'residual' voltage is produced. The residual voltage is
therefore a means of detecting earth fault conditions without any measurement of current. This may be
essential for high impedance earthed or insulated systems where it might not be viable to provide core
balance CT's on each feeder.
The residual voltage can be measured at the secondary terminals of a VT having an 'open delta'
secondary connection. The VT must be a standard five limb type or three single phase VT's can be
used. This is because the residual voltage is three times the zero sequence voltage and therefore zero
sequence flux has to flow in the core. For this to happen there must be a return path for the resultant
summated flux and this occurs on a five limb type because the outer limbs are unwound. Where three
single phase VT's are used, each phase unit has a core with a closed magnetic circuit. A three limb VT
is not suitable for this application because there is no magnetic path, through the core, which the zero
sequence flux can flow.
Another requirement for the VT is that the primary winding neutral has to be earthed. Without this, an
earth zero sequence exciting current cannot flow. Figures 4 and 7 are typical connection diagrams
showing the two versions of relay with a residual voltage connection to a five limb VT.
Internally the relay performs a zero sequence calculation, which is multiplied by 3 to derive the residual
voltage. (The actual residual voltage element setting is referred to as 3Vo). Note that the 'Vo'
instrument in the 'Instruments Display Menu' displays the zero sequence voltage regardless of whether
the residual voltage is directly measured or calculated internally. The actual system residual voltage is
three times the indicated Vo reading.
The output voltage of the open delta winding can contain triplen harmonics, with the 3
predominant. These triplen harmonics appear across the open delta winding even when there is no
earth fault on the network and no zero sequence voltage. The relay's main measuring algorithm being
©2011 Siemens Protection Devices Limited
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7SG11 Argus 8 Applications Guide
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