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Chapter 7) 7SR158 Applications Guide
Section 2: Protection Functions
2.1
Under/Over Voltage Protection (27/59)
Each under and over voltage element has a variable hysteresis setting which allows the user to adjust the pick-
up/drop-off ratio of the element. Where for example the '27/59-n Hysteresis' setting is set at 2% for over-voltage
operation then a pick-up setting of 100V would have a drop-off value of 98V.
When using the variable hysteresis, care has to be taken to ensure that with under-voltage elements, the reset
level of the element is not set to a value higher than that at which the system rated voltage is expected to operate.
Conversely, the level of hysteresis set for an over-voltage element should not be set below that at which the
system rated voltage is expected to run.
2.1.1
Under-Voltage
During normal system operating conditions regulating equipment such as transformer On Load Tap Changers
(OLTC) and generator Automatic Voltage Regulators (AVR) ensure that the system runs within acceptable
voltage limits. Power system under-voltages on may occur due to system faults, increase in system loading or a
non-energized power system e.g. loss of an incoming transformer.
Undervoltage/DTL elements can be used to detect abnormal undervoltage conditions due to system overloads.
Binary outputs can be configured to trip non-essential loads - returning the system back to its normal operating
levels. This 'load shedding' should be initiated via time delay elements so avoiding operation during transient
disturbances. An undervoltage scheme (or a combined under frequency/under voltage scheme) can provide
faster tripping of non-essential loads than under-frequency load shedding so minimising the possibility of system
instability.
Where a transformer is supplying 3-phase motors a significant voltage drop e.g. to below 80% may cause the
motors to stall. An undervoltage element can be set to trip motor circuits when the voltage falls below a preset
value so that on restoration of supply an overload is not caused by the simultaneous starting of all the motors. A
time delay is required to ensure voltage dips due to remote system faults do not result in an unnecessary
disconnection of motors.
To confirm presence/loss of supply, the voltage elements should be set to values safely above/below that where a
normal system voltage excursion can be expected. The switchgear/plant design should be considered. The 'Dead'
level may be very near to the 'live' level or may be significantly below it. The variable hysteresis setting allows the
relay to be used with all types of switchgear.
2.1.2
Over-Voltage
System over-voltages can damage component insulation. Excessive voltage may occur for sudden loss of load, a
tap changer run-away condition, generator AVR equipment malfunctions or reactive compensation control
malfunctions.
System regulating equipment such as transformer tap changers and generator AVRs may correct the overvoltage
– unless this equipment mal-functions. The overvoltage/DTL elements can be used to protect against damage
caused by system overvoltages.
If the overvoltage condition is small a relatively long DTL time delay can be used. If the overvoltage is more
severe then another element, set at a higher pickup level and with a shorter DTL can be used to isolate the circuit
more quickly. Alternatively, elements can be set to provide alarm and tripping stages, with the alarm levels set
lower than the tripping stages.
The use of DTL settings allows a grading system to be applied to co-ordinate the network design, the regulating
plant design, system plant insulation withstand and with other overvoltage relays elsewhere on the system. The
DTL also prevents operation during transient disturbances.
The use of IDMTL protection is not recommended because of the difficulty of choosing settings to ensure correct
co-ordination and security of supply.
Chapter 7) Page 6 of 24
©2014 Siemens Protection Devices Limited
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