24-11-2012, 06:20 PM
Power Quality and Digital Protection Relays
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Abstract.
Power Utilities need to satisfy the demands of
supplying high quality of power to their consumers efficiently
and effectively, while maintaining investments and generating
profit. The use of digital multifunction relays and their proper
integration into power system is an important element of this
process. However, the performance of digital relays depends on
many factors and needs to be carefully evaluated. In the first
part of the paper the main advantages and limitations related to
digital relays are outlined. The key standards related to power
quality are introduced and their relevance to protective relaying
is indicated. Finally, the influence of power quality on
protection relays is assessed through several simulation
examples using the software tool Mathcad.
Introduction
Today's advanced industrial processes are often sensitive
to power supply interruptions and voltage distortions. On
the other hand, the growing use of non-linear loads
generates interference and contributes to the increased
levels of voltage contamination in power networks. This
situation can impact normal operation of other customers
connected to those networks. Thus, in recent years the
sensitivity towards power quality issues has been
increasing mainly as a result of the demands posed by
many consumers.
Power quality can be defined by four fundamental
parameters: frequency, amplitude, shape and symmetry.
It is quantified by evaluating these parameters and
comparing them with the normative limits. Depending on
which of these parameters fall outside the prescribed
limits, there will be different phenomena affecting power
quality supplied to customers.
Protective Relays
Protection relay is a device which by means of measuring
power system quantities (currents and voltages) and
processing them through its internal logic, has the
capacity to control the operation of a circuit breaker. The
internal logic allows the relay to initiate a tripping
sequence when anomalous conditions arise within the
power system.
With regard to their construction three types of relays can
be distinguished: electromechanical, static and digital. In
electromechanical relays, the actuating force is created by
electromagnetic interaction. Static relays are based on
analogue electronic components such as diodes,
transistors, capacitors, etc. whereas numerical relays have
their logic implemented in software and microprocessor
technology. Some relay designs combine the static and
digital technology. This paper concentrates on the
analysis of digital relays.
Influence of Power Quality on Protection
Relays
The influence of distorted waveforms on protective
relays is not well documented because of the large
variety of measuring principles employed in different
relays. For example: electromechanical relays tend to
respond to the fundamental frequency component of the
distorted waveform. However, this may vary
considerably among different manufacturers and
electromechanical relay designs. Initially, static relays
were overly sensitive to high frequency components but
continuous model design revisions corrected these
deficiencies. With the advent of microprocessor based
numerical relays, filtering techniques were developed to
accommodate a wide variety of phenomena. When we
analyse the influence of wave quality on the performance
of digital relays,
Results
In order to demonstrate these phenomena, several
examples have been analysed by simulation, using the
software tool Mathcad.
For the purposes of the impact of frequency variation, the
behaviour of a distance digital relay subjected to a singlephase
fault is analysed. Different wave frequencies are
considered while the digital relay maintains constant
sampling frequency of 1.66 ms (i.e. 12 samples per 20 ms
cycle).
In order to illustrate harmonic distortion, an incorrect
operation of a distance digital relay is presented. The
relay calculates the RMS value with the Mean Square
method instead with the Fourier Transform, and as a
result, the relay produces an incorrect trip.
Conclusions
This paper analyses different power quality phenomena
and their influence on digital protection relays. Digital
relays are the last generation relays. Based on
microprocessor and software, these devices are applied
mainly in transmission system and generator unit
protection but their applications grow rapidly with the
advancements in microprocessor and communication
industries.
The last part of the paper analyses how two phenomena
such as frequency variation and harmonic distortion
influence digital protection relays. In the first case, the
solution is to adapt relay sampling frequency to the actual
power system frequency. In the case of harmonic
distortion, the Fourier Transform filters harmonics, and
therefore it is recommended to use it to calculate the
RMS value.