Relay Models For Protection Studies: July 2003
Relay Models For Protection Studies: July 2003
Relay Models For Protection Studies: July 2003
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was due to relay false tripping. These blackouts have cost the
Abstract-- Power system protection relays have changed signifi- economy of the west USA approximately two billions US
cantly during the last ten years. Processor based designs are now dollars, including all aspects of interconnected infrastructures
highly reliable integrated protection, control and monitoring and even the environment. A large-scale blackout is a rare
systems that significantly outperform the suite of relays and con- event but, when it occurs, the impact on the system and na-
trol equipment from the electromechanical and static era. Tech- tion’s economy could be catastrophic [7].
nological advances and utility restructuring ensures protection
Software modelling of relays is therefore essential for the
remains an immensely challenging field, especially for young IT
literate electrical power engineers. reliable operation of a modern power system. It not only helps
Modelling and simulation can help engineers better under- us to evaluate the protection system performance during faults
stand how a new or proposed relay reacts during a fault or other and other conditions but also generates useful data that can be
non-fault disturbances. Such knowledge can help a manufacturer used in the event of a real fault to understand what happened.
improve the quality of the design and hopefully correct any It also ensures better protection planning and a more reliable
weaknesses before the relay is applied to an actual network. design strategy.
Similarly, utilities can use a simulator to investigate whether a
particular relay is suitable for their network and perhaps if one II. PROTECTION RELAY MODELLING
has mal-operated what caused the problem.
The paper describes various software relay models (developed Microprocessor relays are capable of generating mountains
using FORTRAN IN PSCAD/EMTDC) and discusses how they can of data. During a widespread blackout that involves numerous
assist a protection study. Numerous types of faults were applied cascading trips, the quantity of data can be overwhelming.
to the simulated networks to verify the robustness and functional The data often incorporates irrelevant, unclear and inaccurate
accuracy of the model. The results from these tests and the steady fragments of information and can severely affect the decision
state operating characteristics of each relay model are presented. making process [1]. Assuming all the protection devices oper-
ate correctly, and the data is communicated without error, any
Index Terms—Relay Modelling, FORTRAN, PSCAD/EMTDC fault situation results in a particular symptom pattern (states of
relays and breakers). By correctly recognizing this pattern,
I. INTRODUCTION hidden knowledge about the faulted network can be identified
This equation (3) holds for all values of current above or Figure 1 displays a simple simulation result showing how
below the pickup threshold. When the integration is equal or the ‘virtual disc’ is plotted against time. The relay model is
greater than one, a trip command is generated. Any constant subjected to a constant fault current for a period of time that is
magnitude of current below the pickup threshold will never slightly less than the operating time. It can be seen clearly in
trigger the relay to trip. The reason is because the value of t(I) the diagram that how the disc increments to the boundary of
will be negative and the integral equation will never equal one operation and then decrements after the transient fault current
[6]. subsides below the pickup threshold.
Breaker opens
I3
I1
I3< I2
I1
X
Disc resets
Fault injected
Relay trips as disc
reaches 100%
2) External Fault
Filtering, FFT, Relay For an external fault PICKUPa increased while DIFFa de-
Relay Algorithms Calculations creased. Therefore, no trip signals are initiated.