Data processing: generic control systems or specific application – Specific application – apparatus or process – Electrical power generation or distribution system
Reexamination Certificate
1998-11-17
2002-01-01
Grant, William (Department: 2121)
Data processing: generic control systems or specific application
Specific application, apparatus or process
Electrical power generation or distribution system
C700S286000, C361S016000, C361S080000
Reexamination Certificate
active
06336059
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to protective relaying in a power distribution system, and more particularly to a method for use in connection with a protective relay or like device to provide an accurate reach measurement method.
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 system conditions 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 arc appropriately positioned to perform the isolating function described above.
A modern protective relay typically measures and records voltage and current waveforms on a corresponding protected line, and employs a microprocessor and/or digital signal processor (DSP) to process the recorded waveforms. As used herein, the term ‘transmission line’ includes any type of electrical conductor, such as high power conductors, feeders, etc. Based on the processed waveforms, 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 estimated impedances and phasors. As should be understood, such impedances and phasors are employed to determine whether a fault condition exists, and if so to estimate fault location.
Based on the estimated impedances and phasors, then, the relay
10
may 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 are oftentimes series-compensated by series capacitance in the form of 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.
Typically, and as best seen in
FIG. 2
, installed series capacitance includes a metal oxide varistor (MOV) or other non-linear protection device in parallel with the series capacitance (CAP), which limits the voltage across the capacitance in a pre-defined maimer. Additionally, a bypass breaker or bypass switch (SW) is installed in parallel with the series capacitance, which closes at some point following operation of the MOV. Typically, and as seen, the breaker is controlled by a protective relay
10
via an appropriate BYPASS output (FIG.
1
A). Conduction through the MOV and the closing of such breaker introduce transients in the system as the impedance seen by the protective relay is altered. The quick response of the MOV, the breaker, and the spark gap (SG) installed in parallel with the series capacitance removes or reduces the capacitance and limits the impact of the transient.
Protection of a power distribution system with one or more series compensated lines is considered to be one of the most difficult tasks both for relay designers and utility engineers. A protective relay should be designed to have a high level of security and dependability. A utility engineer should be able to set the protection properly. However, protection settings depend on prevailing system conditions and system configuration, and both may change significantly if series capacitors are present in the system. In particular, such changes result from the fact that series compensation elements installed within a power system introduce harmonics and non-linearities in such system, arising from the aforementioned MOV, bypass switch, spark gap, and other elements. A protective relay or like device must therefore have an accurate reach-measure scheme to take proper action, especially in view of the changes resulting from installed series capacitance and its related elements.
SUMMARY OF THE INVENTION
In the present invention, an accurate reach-measurement scheme improves the reach-measurement of impedance relays and the fault location estimation using local information only. The improvement is accomplished by numerically solving the ordinary differential equation that describes the series installed capacitance installation. The scheme is simple and accurate and requires only local voltage and current at the bus. Furthermore, the scheme easily adapts to different series installed capacitance installations and operation of the installed capacitance protection, and is independent of surrounding power system elements. Existing numerical relays can easily incorporate the new reach-measurement scheme in their protection functions so that such improvement is achieved on a minimal cost basis.
In particular, in the present invention, a reach-measurement method is used in connection with a series-compensated line of a power system. The series-compensated line includes an installed series capacitance having a bus side and a line side, and a non-linear protection device parallel to the installed series capacitance. The series-compensated line has a line current, a bus side voltage, and a line side voltage. The series capacitance and the non-linear protection device have a capacitance voltage thereacross equal to the bus side voltage minus the line side voltage.
In the method, a number (n) of line current samples are measured, where such samples are representative of values of a line current waveform at successive instants of time on the series-compensated line. Capacitance voltage values are computed based on the measured line current samples in accordance with an equation which t
Bachmann Bernhard
Hart David G.
Hu Yi
Novosel Damir
Saha Murari M.
ABB Power T&D Company Inc.
Grant William
Rao Sheela
Woodcock Washburn Kurtz Mackiewicz & Norris
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