Advertisement
2.5 Voltage Element Hysteresis
Each under and over voltage element has a variable hysteresis setting which allows the user to alter
the pick-up / drop-off ratio (or drop-off / pick-up ratio) of the element. Note that the NPS, NVD and
frequency elements have fixed levels of hysteresis which are not adjustable.
When using the variable hysteresis, care has to be taken to ensure that with undervoltage 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. The system rated voltage will have a tolerance of typically +/- 6% and so the
upper level of the hysteresis must be below the lower limit of the tolerance, otherwise the element
might not reset in practice. Conversely, the level of hysteresis set for an overvoltage element should
not be set below that at which the system rated voltage is expected to run.
Typical values for the amount of hysteresis applied to a voltage element are < 5%. When setting the
hysteresis level the user has to be aware that if the amount of hysteresis is set too low e.g. 1%, then
for large frequency excursions and low values of voltage element setting, the element might become
unstable and 'chatter'. This will produce nuisance alarms / tripping and generate large numbers of
stored event records. A minimum recommended level is 2% for this reason.
2.6 Trip Circuit Supervision
Argus relay can be used to supervise trip circuits while the associated circuit breaker (CB) is either
open or closed. A low value of dc current, derived from the auxiliary supply, is passed through the
entire trip circuit to monitor the trip coil, its auxiliary switch, the CB secondary isolating contacts and the
relevant wiring. If the current flow ceases, the energised status input drops off and if it is user-
programmed to operate one of the output relays, this relay will close an output contact to signal trip
circuit failure. In addition, the LCD display on the Argus relay will indicate 'Trip Circuit Fail'.
To avoid giving spurious alarm messages while the CB is operating, or protection device on adjacent
supply circuits is operating, the status input should be programmed to have a 500ms drop-off delay.
The Electricity Association H6 Scheme is shown in Figure 1.
3 Application of Functions
3.1 Undervoltage Protection
Undervoltages are reasonably frequent events on power systems and can occur for a number of
different reasons. Faults on the system can cause the phase voltages to be depressed, the actual
voltage drop being dependent upon a number of factors including the fault type and system earthing
etc. During system earth fault conditions, the undervoltage protection is not generally required to
operate and thus connecting the relay in the phase-phase configuration will make it less susceptible to
single-phase voltage depressions.
Another cause of undervoltage is an increase in system loading, which should be corrected by system
regulating equipment such as tap-changers and AVR's. However, if this equipment is defective then an
undesirable situation will occur which will require an undervoltage relay to trip non-essential loads to
correct for this voltage excursion and to bring it back to its nominal level. This tripping should happen
after an appropriate time delay has expired. Generally, wherever voltage relays are employed, timing
elements should be used to prevent operation during transient disturbances.
If the system is supplying 3-phase induction motors or variable frequency thyristor drives,
undervoltages can have the following effect. Voltage depressions down to approximately 80% of rated
voltage cause the load current to increase, possibly resulting in a larger voltage depression due to the
supply source impedance. Below 80% the current drawn is proportional to the voltage and an induction
motor is likely to stall. The current drawn is then dependent on the drive design e.g. thyristor drives
include current limitation. An undervoltage element can be set to trip out a motor circuit when the
voltage falls below a preset value, selected based on the motor drive and system design parameters,
and after a preset time delay. The time delay is required to ensure voltage dips due to remote system
faults do not result in an unnecessary trip.
If the system supply to a group of motors is lost, undervoltage protection can be applied to ensure that
each of the motor circuit breakers or contactors are tripped so that on restoration of the main supply, it
is not overloaded by the simultaneous starting of all the motors. A 3-phase undervoltage relay may be
used for this task of tripping a feeder for the detection of a complete loss of voltage. Also, where a
supply to induction motors is lost, the undervoltage relay can be used to detect the loss of supply or to
©2010 Siemens Protection Devices Limited
7SG11 Argus 8 Applications Guide
Chapter 5 Page 4 of 16
Advertisement
Chapters
