Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Electrical signal parameter measurement system
Reexamination Certificate
1998-11-17
2002-05-28
Grimley, Arthur T. (Department: 2852)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Electrical signal parameter measurement system
C702S059000
Reexamination Certificate
active
06397156
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to protective relaying, and more particularly to a method and apparatus for use in connection with a protective relay or like device to accurately measure the impedance of a power system transmission line.
BACKGROUND OF THE INVENTION
In a power distribution system, electrical transmission lines and power generation equipment must be protected against faults and consequent short circuits. Otherwise, such faults and short circuits can cause a collapse of the system, equipment damage, and/or personal injury. Accordingly, and as shown in
FIG. 1
, a typical power system employs one or more protective relays to monitor impedance and other AC voltage and current characteristics on a protected transmission line, to sense faults and short circuits on such protected line, and to appropriately isolate such faults and short circuits from the remainder of the power system by tripping pre-positioned circuit breakers on such protected line.
As seen, a typical power system can be connected over hundreds of miles and include multiple power generators (generator S, generator R) at different locations. Transmission lines (the main horizontal lines in
Fig. 1
) distribute power from the generators to secondary lines or buses (the main vertical lines in Fig.
1
), and such buses eventually lead to power loads. Importantly, relays and circuit breakers are appropriately positioned to perform the isolating function described above.
A modern protective relay typically records voltage and current waveforms measured on a corresponding protected line, and employs a memory and microprocessor and/or digital signal processor (DSP) to process the recorded waveforms and to estimate impedance and voltage and current phasors based on such processed waveforms. As should be understood, a voltage or current phasor expresses the respective parameter in terms of its magnitude and phase angle. As used herein, the term ‘transmission line’ includes any type of electrical conductor, such as a high power conductor, feeder, transformer winding, etc. Based on the estimated impedance and voltage and current phasors, the protective relay can then decide whether to trip an associated relay, thereby isolating a portion of the power system.
In particular, and referring now to
FIG. 1A
, it is seen that a typical protective relay
10
samples voltage and current waveforms V
A
, V
B
, V
C
, I
A
, I
B
, I
C
from each phase (A-C) of a three phase line
12
. Of course, greater or lesser numbers of phases in a line may be sampled. The sampled waveforms are stored in a memory
14
and are then retrieved and appropriately operated on by a processor or DSP
16
to produce the aforementioned estimated impedances and phasors. Based thereon, the relay
10
may then decide that an associated circuit breaker
18
should be tripped to isolate a portion of the line
12
from a fault condition or from other detected phenomena, and issue such a command over a ‘TRIP’ output (‘TRIP
1
’ in
Fig. 1A
) that is received as an input to the circuit breaker
18
. The relay
10
may then reset the circuit breaker after the relay
10
senses that the fault has been cleared, or after otherwise being ordered to do so, by issuing such a command over a ‘RESET’ output (‘RESET’
1
in
FIG. 1A
) that is received as an input to the circuit breaker
18
.
Notably, the relay
10
may control several circuit breakers
18
(only one being shown in FIG.
1
A), hence the ‘TRIP
2
’ and ‘RESET
2
’ outputs. Additionally, the circuit breakers
18
may be set up to control one or more specific phases of the line
12
, rather than all of the phases of the line
12
. Owing to the relatively large distances over which a power system can extend, the distance between a relay
10
and one or more of its associated circuit breakers
18
can be substantial. As a result, the outputs from the relay
10
may be received by the circuit breaker(s)
18
by way of any reasonable transmission method, including hard wire line, radio transmission, optical link, satellite link, and the like.
As seen in
FIGS. 1 and 2
, transmission lines may oftentimes be series-compensated by series capacitance
20
that includes one or more capacitors or banks of capacitor installations (a representative series capacitor CAP is shown). Benefits obtained thereby include increased power transfer capability, improved system stability, reduced system losses, improved voltage regulation, and better power flow regulation. However, such installation of series capacitance introduces challenges to protection systems for both the series-compensated line and lines adjacent thereto.
In particular, series compensation elements installed within a power system introduce harmonics and non-linearities in such system. Particularly when using waveform-type algorithms (i.e., algorithms that rely on current and voltage waveforms to determine a parameter of interest) to estimate impedance and voltage and current phasors, several transient problems may cause very large errors. Such voltage and current phasors are employed in relaying applications, for example, to determine whether a fault is in a protected zone. It is imperative, then, that such phasor estimates be as accurate as possible in view of installed series capacitance. Examples of the aforementioned transient problems that may cause very large errors include:
DC Offset—In uncompensated and compensated power systems, a fault current waveform will contain an exponentially decaying DC offset component in addition to a fundamental frequency. The amount of the DC offset is dependent on the fault inception angle and system parameters such as network configuration, number and length of transmission lines, compensation percentage, power flow, generator and transformer impedances, etc. A variety of algorithms have been devised to compensate for DC offset. Some algorithms use a differentiation technique that eliminates the effect of the DC offset and ramp components in the fault current waveform. Mimic circuits and cosine filters have also been employed.
Sub-Synchronous Frequencies—On series-compensated lines, series capacitance introduces a sub-synchronous frequency which is dependent on capacitance value and various system values. When a fault occurs, the fault current waveform includes two sinusoids, one oscillating at the predetermined system frequency (50 Hz, 60 Hz, etc.), and the other at the system natural frequency (neglecting system resistance and load current). The system natural frequency is determined by the degree of compensation, the source impedance, and the distance to fault location, among other things. Accordingly, a higher system natural frequency occurs when a fault is closer to a respective relay. The higher frequency will not be as critical for close-in faults since a metal oxide varistor (MOV) associated with the series capacitance (shown in
FIG. 2
in parallel with the representative series capacitor (CAP)) will typically short the capacitance in such a situation. However, when a fault occurs farther out from a relay toward the end of a line, the lower system natural frequency will cause the aforementioned voltage and current phasor estimates to oscillate. Such oscillation affects the real and imaginary components of the phasor estimations, resulting in a ‘cloud’ effect. For most power systems, installed series capacitance results in a sub-synchronous harmonic component in the fault current waveform. The impact of high frequency components in the fault current waveform is usually reduced by low-pass filters in the relay.
MOV and Overload Protection Operation—Once a fault has occurred, a bypass breaker or bypass switch (SW) (shown in
FIG. 2
in parallel with the representative series capacitor) closes following operation of an overload protection system. Typically, and as seen, the breaker is controlled by a protective relay
10
via an appropriate BYPASS output (FIG.
1
A). Typically, and as shown in
FIG. 2
, bypassing the installed capacitance in actuality causes an inductance (L) to b
Bachmann Bernhard
Hart David G.
Hu Yi
Novosel Damir
Saha Murari M.
ABB Inc.
Charioui Mohamed
Grimley Arthur T.
Woodcock & Washburn LLP
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