Data processing: generic control systems or specific application – Specific application – apparatus or process – Electrical power generation or distribution system
Reexamination Certificate
1998-03-10
2001-02-06
Sheikh, Ayaz R. (Department: 2781)
Data processing: generic control systems or specific application
Specific application, apparatus or process
Electrical power generation or distribution system
C700S292000, C700S294000, C700S297000, C700S298000, C361S086000, C361S096000
Reexamination Certificate
active
06185482
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to digital protection systems which employ frequency tracking and phasor estimation for fault detection. More particularly, the invention relates to a novel method and system for fault detection when input voltages that are typically used as source values in frequency tracking techniques become unavailable.
BACKGROUND OF THE INVENTION
Digital protection systems are used, for example, in power systems to monitor the voltage and current signals provided by electrical generators. Such digital protection systems may be embodied in a generator protection unit (GPU). A GPU estimates voltage and current phasors based upon the voltage and current signals and from those phasors detect various fault conditions requiring system shut down. The phasors are also used for metering functions performed by remote users and for other power system control functions. Abnormal conditions may additionally be detected through relatively large changes in the generator's operating frequency. Accordingly, an adaptive technique for tracking the generator frequency is important for both generating accurate voltage and current phasors and for monitoring fault conditions in the generator by detecting a significant change in the generator frequency.
FIG. 1
shows GPU
20
and generator
10
. Voltage signals output from generator
10
(V
out
) are sensed by voltage sensor
12
. Potential transformers, resistive dividers, or the like, may be employed as voltage sensors. The sensed voltage signals are output from voltage sensor
12
to GPU
20
. It should be understood that generator
10
produces three-phase power, and accordingly, the voltage signals output from generator
10
may include V
A
, V
B
, and V
C
, or alternatively may include line-to-line voltages V
AB
, V
BC
, and V
CA
.
Current sensor
14
such as current transformers, may be used to sense current signals output from generator
10
(I
out
) to both GPU
20
and to the power system (not shown). If, for example, the current output to GPU
20
and to the power system are not equal then a fault condition may exist. The sensed currents, which may include each line current I
A
, I
B
, and I
C
, is provided as input to the GPU
20
.
GPU
20
includes A/D converter
15
, digital signal processor (DSP)
16
, microprocessor
17
, and external interface
18
. A/D converter
15
samples the sensed voltage and current signals and provides voltage and current samples to DSP
16
. DSP
16
may generate a voltage phasor each sampling interval by utilizing a discrete fourier transform (DFT). A DFT and system for implementing the transform are discussed in commonly assigned co-pending U.S. patent application Ser. No. 08/574,357, filed Dec. 18, 1995, entitled “System and Method for Phasor Estimation and Frequency Tracking in Digital Protection Systems,” the contents of which are hereby incorporated by reference. The operating frequency of generator
10
can be tracked based on the generated phasors. Phasor data and frequency estimates are output from DSP
16
to microprocessor
17
.
Microprocessor
17
uses the phasor and frequency data to detect faults in the power system. If a fault or malfunction is detected, microprocessor
17
outputs a signal through external interface
18
to a circuit breaker (not shown) causing the circuit breaker to open its current carrying contacts so that the system is effectively shut down.
Thus, a fault protection system as described above receives sensed voltage signals, and from these signals generates phasor and frequency data which is subsequently used to detect faults in the power system. As noted, typically such systems have three phase voltage inputs. If any one of the three signals are available, a fault detection system as described above could operate. However, if none of the input voltage signals are available, the protection system cannot operate. Thus, systems and methods of fault protection analogous to those described above present the problem of providing fault protection when the input sensed voltage signals are not available. Furthermore, because generators operate over a wide frequency range, particularly during startup or shutdown, any solution to the problem of loss of input voltage must also be capable of operating over a wide frequency range.
SUMMARY OF THE INVENTION
The present invention fulfills these needs by providing a method and apparatus for detecting faults in the system when input voltages become unavailable.
In a preferred embodiment of the invention, there is provided a method of over current backup for a system having a current signal. The method comprises the following steps: sampling the current signal to produce a plurality of current samples; generating a plurality of root mean square (rms) current values from the plurality of current samples; generating a current estimate from the plurality of rms current values; and activating a fault protection mechanism if the current estimate is above a predetermined threshold. The step of sampling the current signal at a predetermined fixed rate to produce a plurality of current samples, comprises the following steps: obtaining a sample current measurement; determining whether frequency tracking is available; if frequency tracking is available, employing the frequency tracking; if frequency tracking is not available, determining whether a sufficient number of sample current measurements have been obtained to calculate an rms current value; and if a sufficient number of current measurements has not been obtained, obtaining an additional current measurement. A sufficient number of current measurements may be 32.
Each of the plurality of rms current values, c
rms
, is estimated according to the following relationship:
c
r
m
s
=
1
N
∑
k
=
0
N
-
1
f
k
2
where f
k
is the current sample at a particular instant and N is the number of samples per cycle of input current.
Each successive rms current value of the plurality of rms current values is calculated using eight current samples which were collected after the previous rms current value was calculated. The number of samples per cycle of input, N, may be 32. The current estimate may be the average of the plurality of rms current values. In one embodiment, the current estimate is the average of four consecutive rms current values. The threshold value is about 70% of the rated value of the system. The fault protection mechanism comprises opening a circuit breaker.
According to another aspect of the invention, there is disclosed a method for over current backup protection in a generator protection unit (GPU), wherein the generator outputs a plurality of current signals to the GPU. The method comprises the following steps: sampling the current signal to produce a plurality of current samples; generating a plurality of root mean square (rms) current values from the plurality of current samples; generating a current estimate from the plurality of rms current values; and activating a fault protection mechanism if the current estimate is above a predetermined threshold. The plurality of rms current values, c
rms
, is estimated according to the following relationship:
c
r
m
s
=
1
N
∑
k
=
0
N
-
1
f
k
2
where f
k
is the current sample at a particular instant and N is the number of samples per cycle of input current. The current estimate is the average of the plurality of rms current values.
According to another aspect of the invention, there is disclosed an apparatus for over current backup protection comprising the following items: a current sensor for sensing the current being transmitted; an A/D converter operably coupled to the current sensor for sampling the sensed voltage at a fixed sampling interval; a digital signal processor (DSP) operably coupled to the A/D converter for generating rms current values from the sensed voltage; and a microprocessor operably coupled to the DSP, having code therein for calculating a current estimate from
Egolf Michael
Hart David G.
Stoupis James D.
ABB Power T&D Company Inc.
Jean Frantz Blanchard
Sheikh Ayaz R.
Woodcock Washburn Kurtz Mackiewicz & Norris LLP
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