19-10-2012, 05:47 PM
Relay Element Performance During Power System Frequency Excursions
Relay Element Performance.pdf (Size: 1 MB / Downloads: 36)
Abstract
Many voltage and current protection elements in
microprocessor relays use the fundamental frequency component
of current and voltage. Distance relays also calculate apparent
fault impedance at the power system frequency. Most
microprocessor relays track system frequency to calculate the
current, voltage, and impedance quantities. When the integrity of
the power system is in jeopardy, system frequency can experience
a large and rapid excursion because of generation and load imbalances
from system separations into regional islands. Such
separations occurr in major system events such as the U.S.-
Canadian blackout of August 14, 2003, that affected 50 million
people in eight U.S. states and two Canadian provinces. Relay
elements in such a disturbed system state must perform reliably
to prevent any protective element misoperations from aggravating
the disturbance and causing widespread outages. Relay element
performance during a system frequency excursion depends
on factors including the system frequency rate of change, the
tracking rate and tracking limit of the relay, and the element
type. This paper reviews overcurrent, distance, and current differential
element designs. It then examines the performance of
these elements during frequency excursions.
INTRODUCTION
Microprocessor relays use numerical algorithms to calculate
phasors from voltage and current inputs based on either
the power system nominal frequency or the actual measured
frequency. Relays use voltage and current phasors to construct
different protection elements such as overcurrent, current differential,
and distance elements. Different filters help relays
reduce the impact of noises on phasor calculation and improve
overall protection element accuracy.
The power system frequency, although generally stable,
seldom remains at the nominal value. Major system disturbances
such as load shedding or generation shutdowns can
cause great imbalance between load and generation, and the
system frequency can experience a sudden significant change.
Protective relays should be stable during a system frequency
excursion to prevent misoperations that can further degrade
system stability.
Frequency Measurement
The first step of frequency tracking is to measure power
system frequency. Different frequency measurement algorithms
exist [2]. The most common frequency measurement
method is to measure the time elapsed between the zero crossings
of the input signals. The inverse of this time is twice the
system frequency. The following input quantities are available
for this purpose:
• Single-phase voltage
• A combination of three-phase voltages such as alpha
component of voltages: vα = va – [(vb + vc)/2]
• Single-phase current
• Combination of all three-phase currents such as alpha
component of currents: iα = ia – [(ib + ic)/2]
Voltages are good choices for the zero-crossing measurement
because they have high magnitudes and minimum harmonic
contents. For current-only relays, we must use currents
with proper filtering and smoothing for zero-crossing measurement.
The advantage of using a combination of all threephase
quantities is that we can still obtain the measurement
during loss of one or two phases, such as in a single-pole open
condition. A zero crossing is only available every half cycle,
so the frequency measurement using a single-phase input cannot
be faster than a half cycle. Measuring zero-crossings on all
three phases simultaneously accelerates the frequency measurement
to as fast as every one-sixth of a cycle.
Differential Protection Element
Current differential protection is popular for transmission
line, transformer, reactors, and motor protection. It operates on
a simple principle that the current going into an apparatus
must equal the current leaving the device when there is no
fault.
Fig. 18 shows a current differential protection scheme for a
two-terminal device. A relay measures currents at both ends of
the apparatus. A phasor sum of all currents provides an operating
current (IOP) for the current differential protection. There
are different ways to set up the restraining current for the differential
element [3]. Equation (17) shows the most popular
restraining current (IRT), which is an average of two current
magnitudes for two-terminal devices.
CONCLUSION
A mismatch between the relay sampling frequency and input
frequency causes phase and magnitude oscillating errors in
phasor calculation. The amount of error is proportional to the
frequency difference and depends also on the type of filters in
use for phasor extraction.
When a relay employs a frequency tracking scheme to
adapt sampling frequency to that of the input frequency, the
relay cannot track to an arbitrarily high frequency because of
limited processing power. The relay also introduces delays in
the frequency measurement to ensure accuracy and delays in
tracking to stabilize the sampling frequency. When the system
frequency slew rate is constant, the frequency measuring and
tracking delay introduce a fixed steady-state frequency difference.
The slower the frequency tracking, the larger the frequency
difference.
Different protection elements behave differently during a
system frequency excursion. The accuracy of magnitude protection
elements such as overcurrent elements relates directly
to the magnitude oscillation of a phasor during a frequency
excursion. The amount of overcurrent overreach is proportional
to the difference between the input and relay tracking
frequencies. This difference depends on the input frequency
slew rate and the speed of the frequency tracking algorithm.