05-05-2014, 10:53 AM
Reducing the Effects of Short Circuit Faults on Sensitive Loads in Distribution Systems
Effects of Short Circuit.pdf (Size: 374.5 KB / Downloads: 89)
INTRODUCTION
The electric utilities industry is going through significant changes related to the reduced
tolerance of major customers to deviations from the nominal parameters of electric power. Most
of the power quality events that result in failure of different types of equipment involved in a
manufacturing or other processes are caused by short circuit faults. Microprocessor based
multifunctional Intelligent Electronic Devices (IED) are becoming the standard protection,
control, monitoring and recording equipment in new or existing substations. They provide
increased capabilities to detect short circuit or other abnormal conditions and reduce the total
fault clearing time, thus reducing the effect of the fault on sensitive equipment.
The sensitivity of many industrial facilities to variations in system parameters [1] results in
increased requirements for improved quality of power supplied by the utility. A new approach to
distribution protection will help avoid costly interruptions of manufacturing or other processes
when a short circuit fault occurs in the distribution system.
The paper discusses the effects of different short circuit faults on the voltage profile across the
distribution system. The behavior of typical distribution feeder protection or substation
protection systems is analyzed from the perspective of the definitions of voltage related power
quality events.
VOLTAGE VARIATION POWER QUALITY EVENTS
The most common power quality events that affect sensitive customers and may result in shut
down of manufacturing process and significant losses are in the group of voltage variations. The
definitions of such events vary in the details, but essentially they are not very far from the ones
from the IEC standards.
A voltage dip is a temporary reduction of the voltage at a point in the electrical system below a
threshold. Dips (or sags in North America) are classified as events by the duration and voltage
level.
EFFECTS OF SHORT CIRCUIT FAULTS
The improvement of power quality during short circuit faults can be achieved in several
different ways. Like any other problem that has to be solved, we need first to understand the
nature of the problem and it’s effect on sensitive users. The most common short circuit faults in
the system – single-phase to ground faults – are characterized by the fact that they introduce a
voltage sag in the faulted phase, and at the same time they result in a voltage swell in the two
healthy phases. This is clearly seen in Figure 1 that shows the recorded waveform and the
voltage phasor diagram for a single-phase to ground fault.
Distribution Bus Protection.
The selection of protection equipment for bus faults until recently was based on the requirements
for stability of the power system. Because of that the protection of buses in the case of short
circuit faults at the transmission level is usually provided by high or low impedance bus
differential relays. Since they require the installation and maintenance of additional equipment,
in most cases the distribution bus protection has been done by the backup time delayed
overcurrent protection of the transformers.
The increased understanding of the effects of longer fault clearing times on sensitive industrial
equipment results in a change in the philosophy on distribution bus protection. In many cases it
is now based on the exchange of signals between the feeder relays and the transformer protection
relays in order to provide faster clearing of bus faults.
CONCLUSIONS
Modern multifunctional protection and monitoring or recording IEDs allow the optimization of
the protection and controls schemes that lead to significant reduction in the duration of voltage
sags or swells.
Combining an inverse-time overcurrent protection element with definite time delayed and
instantaneous elements results in an overall characteristic that optimizes the overcurrent function
of the relay.
Use of negative sequence overcurrent elements for faster detection of phase-to-phase faults
further improves the performance of a distribution feeder relay.