Note: Descriptions are shown in the official language in which they were submitted.
CA 02470536 2004-06-09
A POWER LINE COM1VIUNICATION BASED TECIEINIQUE FOR DETECTING
ISLANDING CONDITIONS IN ELECTRIC POWER DISTRIBUTION SYSTEMS
S I. BACKGROUND OF THE INVENTION
Distributed generation (DG) has recently gained a lot of momentum in the power
industry due to market deregulation. Figure 1 shows a power system with DGs
connected.
During normal operation, the distribution system and the DGs provide the power
for the loads.
The total power from DGs is typically small in comparison with the total
feeder loads so the
distribution system often provides the missing power. If the feeder becomes
isolated from the
distribution system due to, for example, the opening of breaker A, the feeder
becomes a small
unregulated power system. The behavior of this system is unpredictable due to
the power
mismatch between the load and generation and the lack of controllers. This
operating condition
of DGs is called islanding. In another words, islanding occurs when a portion
of the distribution
system becomes electrically isolated from the remainder of W a power system,
yet continues to
be energized by distributed generatars. An important requirement for
distributed generation is
the capability of the generators to detect island conditions and trip the
generators accordingly.
Failure to trip islanded generators can lead to a number of problems to the
generator and the
connected loads. The current industry practice is to disccmnect all
distributed generators
immediately after the occurrence of islands. Typically, a distributed
generator should be
disconnected within 100 to 600 ms after loss of main supply according to
prevalent DG
interconnection standards.
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130k~ Transmission System
substation
~~1 Other feeders
25kv ~ Cttstorner load point
~ Breaker or fuse
B
DG
DG
Figure l : Typical distribution system with distributed generators.
To achieve such a goal, each distributed generator must be equipped with an
islanding
detection device. The common devices used for this purpose are the modified
versions of the
under/over voltage and underlover frequency relays. Representative examples of
such relays
are the Rate of Change of Frequency Relay (ROCOF) and the Vector Surge Relay
(VSR),
which is also known as vector shift or voltage jump relay. It is known if the
generation and load
have a large mismatch in a power system, the frequency of tile system will
change. In view of
the fact that the frequency is constant when the feeder is connected to the
distribution system, it
is possible to detect the islanding condition by checking the amount and rate
of frequency
change. The ROCOF and VSR relays are based on such principles. This is the
simplest
islanding detection technique. However, it cannot function properly or fast
enough if the
generation and load mismatch is small.
In order to overcome the above problem, a few active schemes that require a DG
to
inject small signals (or disturbances) to the system have been proposed. For
example, the
terminal voltage of the DG could be varied at a rate of O.lHz. Research has
shown that such
voltage variations could increase frequency change when the generator is
islanded. So the
frequency detection scheme can be improved by using this technique. This is an
expensive
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CA 02470536 2004-06-09
scheme and there is a lack of field experience. One of the main challenges
facing this scheme is
the interaction among the DGs that are introducing similar disturbances to the
system. At
present, there is no answer to this question yet.
In addition to the above local information based islanding detection schemes,
techniques that use telecommunication means to trip islanded DGs have been
used in industry.
With this 'transfer-trip' scheme, each DG is equipped with a cellular phone
like receiver.
Breaker A has a transmitter that sends a trip signal to the DG receivers if it
opens. With current
telecomm technologies, there is no major technical problem to do so. The
problem is the cost
and complexity. Firstly, it is expensive for areas that are not covered by
radio communications.
Secondly each breaker needs a transmitter and there could be several of them
between the DG
and the substation. Thirdly, some of breakers need to be reconfigured and
equipped with the
capability of interfacing with the signal transmitter. Fourthly, feeder
segments including their
DGs could be reconnected to a different system due to the practice of feeder
reconfiguration. In
this case, A DG signal receiver must have the capability to decide which
signal transmitters it
should listen to.
In summary, as more and more distributed generators are added to utility
systems, it is
highly desirable to have a reliable and low cost islanding detection
technique.
II. SUMMARY OF THE INVENTION
This invention provides a new type of the islanding detection method that
utilizes power
line as a signal earner. The proposed scheme is shown in Figure 2.
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130kV Transmission System
substation ~ ~----
Signal generator ,-. Awciliary inputs
25kV
~ 1
B
Signal
detector
SigrAal
detector
DG DG
Figure 2: Proposed islanding detection scheme.
The invention includes two devices: a signal generator associated with a
substation and
a signal detector associated with a DG. The signal generator may be connected
to the
substation bus and the signal detector may be connected at the terminal of a
given DG. The
signal generator broadcasts a signal to all distribution feeders with a preset
protocol
continuously. If the signal detector of the DG does not sense the signal
(caused by the opening
of any breakers between the substation and the DG) for certain duration such
as 200rns, it is
considered as an island condition and the DG can be tripped immediately. If
the substation bus
loses power, which is another islanding condition, the signal generator also
loses power and
stops broadcasting so that downstream DGs will also trip. Furthermore, the
signal generator has
several auxiliary inputs. Any one of the inputs can stop the broadcast,
resulting in tripping all
DGs in the system. This feature is particularly useful when transmission
system operators need
to trip the DGs. It is also useful if a transmission system island is formed.
Since the
transmission system is well equipped with telecommunication means, it can send
a stop signal
to the signal generator.
This invention is fundamentally different from the published schemes. It
combines the
positive features of the telecomm-based and the local detection based schemes.
Power line is
used as a telecomm medium. Another important advantage is that the scheme can
be tested
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without actually breaking up the distribution feeders. The main tests could be
done by simply
stopping the signal generator. The signal detectors should detect zero signals
in this case.
III. DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION
There are many methods and apparatus available or possible as communications
systems for transmitting signals through power lines and for receiving such
signals. The
present invention may use any such method or apparatus which is compatible
with the power
distribution system with which it is associated. Preferably the signal
generator and the signal
detector of the present invention provide a fast response, are reliable and
cause minimum or no
interference with power line communication schemes that may already exist on a
distribution
system.
In a preferred embodiment, the present invention is implemented by injecting
voltage
signals to specific cycles of the substation voltage. An example of the
voltage waveform that
contains the signal is shown in Figure 3. In this waveform, every second cycle
contains a small
distortion that is an indication of the existence of a signal. Depending on
the requirement of
response time and cost of signal generation, one can also let every third
cycle or every fourth
cycle to contain the distortion. The objective of the signal detector is to 1)
detect the presence
of distortion at certain cycles of the voltage waveform and 2) determine if
the distorted cycles
follow a pre-established pattern such as one distorted cycle for every two
cycles or for every
three cycles.
Figure 3: Sample waveform that contains signals for islanding detection.
(signal pattern 010101010101)
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The distortion signal is preferably injected into all three phase-to-ground
voltages at the
substation. It could also be injected into three phase-to-phase voltages. The
preferred
embodiment is to inject the signal to phase-to-ground voltages. Furthermore,
the signal voltage
is preferably injected at the instant that is far away from the distortion
signals injected by other
power line communication devices. The preferred embodiment is at the zero-
crossing instant
when the voltage is rising from a negative value to a positive value. In this
case, it has been
assumed that the other power line communication device {such as automatic
meter reading
AMR device) sends signals around the zero-crossing instant when the voltage is
going from a
positive value to a negative value. This situation is shown in Figure 4.
Figure 4: Sample waveform that contains signal for islanding detection and
signal for other
power line communication application.
The methods to generate and to detect the above described signal are presented
next. A
method to resolve the interference between the DG signal and AMR signal is
also explained.
A. Signal Generator
The first component of the present Invention is a signal generator. As
previously
indicated, the signal generator may be comprised of any suitable method,
system, device or
apparatus which is capable of generating a signal in a power line.
In the preferred embodiment the signal is generated by short-circuiting the
substation
voltage through a transformer impedance and a thyristor. Figure 3 shows the
structure of the
signal generator. In the preferred embodiment the signal generator includes or
is associated
with the following components/features as numbered in the figure:
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CA 02470536 2004-06-09
9
7
' '1
, y ,
, ,
,
, ~ , 2 3
,
, y ,
, g ,
J--~~ Voltage AMR
AMR Transducer Detector
a b c
,____ ___ __ _____,
Gating
' Controller A~liar5' inputs
,
'_____ ____ _____ _____'
-. ~
Figure 5 Architecture of the signal generator
1. Step-down transformer: In the preferred embodiment, a step-down transformer
transforms
the substation voltage (for example, 25kV or 14.4kV) to a reduced level (say
480V) for
thyristor operation. The transformer also behaves an impedance to limit the
thyristor current
and to reduce the amount of distortion introduced to the substation voltage.
Common power
transformer can be used. Other similar power line communication devices such
as AMR
devices may also use this transformer. The impedance of the transformer may be
selected
according to the following equations:
Xr - Uu~~sin&-ks)
3spGks
where
CA 02470536 2004-06-09
XT = Reactance of the signal transformer measured at the feeder primary side
(Ohm)
UN = Supply system rated line to line voltage (V)
sPG = The supply system single phase to ground short circuit capacity (VA)
s =Thyristor firing angle ahead of the zero crossing point (typically
30°)
kS = ~P Relative strength of the signal to be detected. Typical value is 3% to
5%.
UPG
2. Voltage transducers: In the preferred embodiment, voltage transducers are
used 1) to
provide reference information to time the thyristor gating operation and 2) to
determine the
activity of the AMR devices. Common PT can be used in this invention.
3. AMR Detector: If another similar power line communication device such as an
AMR
device is present in the distribution system, an AMR detector may be provided
in order to
determine the activities of the AMR device. If the AMR detector finds that the
AMR device
starts to use one of the available channels (such as phase-A to ground
voltage) for
communication, the AMR detector may send a signal to the SCR gate controller
to disable
the use of that channel by the signal generator. As a result, the channel may
be freed up for
AMR use. Similarly, a previously unused channel may be activated for signal
broadcasting.
In the preferred embodiment of the present . invention as described herein, it
has been
assumed that the AMR device will not use all channels simultaneously.
4. SCR gating controller: In the preferred embodiment, a controller may
perform several
functions. Firstly, it may establish the pattern of signal to broadcast, such
as one distortion
for 2 or 3 cycles. Secondly, the controller may trigger the thyristor to
conduct at cc degrees
before the zero-crossing of the voltage waveform when signal injection is
needed for the
cycle. Thirdly, it may decide which channel to disable/enable signal
broadcasting based on
the information provided by the AMR detector. Fourthly, the controller is
preferably
equipped with several auxiliary inputs and is preferably configured such that
any one of the
auxiliary inputs can disable the triggering operation. When this happens, the
signal
generator will stop broadcasting signals.
_g_
CA 02470536 2004-06-09
5. SCR module: In the preferred embodiment, an SCR module behaves as a switch
to connect
the transformer secondary to ground, leading to a momentary short circuit of
the
transformer secondary. The short circuit introduces a voltage dip to the
substation or
transformer primary voltage. This voltage dip is the distortion needed to
represent the
presence of a signal. Conventional SCR module and driving circuit may be
employed in the
present invention.
6. AMR Device: An AMR device may be used for automatic meter reading or for
other power
line communication applications. The AMR device is not part of the present
invention.
Depending on the power distribution system, one or more AMR devices may or may
not
exist.
7. Substation Transformer: A substation transformer steps down a transmission
voltage to a
distribution voltage. This component is a component of a typical substation.
8. Substation Feeder Bus Bar: A substation feeder bus bar is the sending end
of the
distribution feeders. This component is a component of a typical substation.
9. Distribution Feeders: A plurality of distribution feeders typically
distribute electrical
power from the substation. The distribution feeders are a component of a
typical
substation.
Figure 6, for example, shows an example of short-circuiting of one phase for
every two
cycles, both the voltage waveform and SCR gating signal are plotted. All the
three phases are
preferably short circuited in a similar way. The voltage waveform is
consequently distorted by
the short-circuiting. The distortion can propagate through power lines to the
signal detector at
DG locations. The voltage waveform measured at a downstream location is shown
in Figure 7,
from which the presence of the distortion signal can be clearly observed. The
distortion signal
itself is plotted separately in the figure.
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Voltag
Gatm~
4
3
2
t
n I
p -J
1
2
-4~ V V V v V v v v
L i i 1 i i t
~0 0.02 0 04 0.06 0.08 0.1 0.12 0.14
Time (s)
- Figure 6: The voltage wavefonn and signal generator gating signal
6 7 8 9 10 11 12 13
Time (s) x 10~
S
Figure 7: Voltage waveform at a downstream location and the distortion signal.
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B. Sighal Detector
The second component of the present invention is the signal detector. As
previously
indicated, the signal detector may be comprised of any method, system, device
or apparatus
which is capable of detecting a signal in a power line and which is compatible
with the signal
generator.
In the preferred embodiment the signal detector is installed at the location
in the
distribution system where a distributed generator is connected. It preferably
senses the three
phase voltage at the generator terminal and determines the presence of the
distortion signal and
its repetition pattern. If the signal is not present, the signal pattern is
not consistent with the pre-
established rule, or the waveform frequency deviates significantly from power
frequency (60Hz
or SOHz), the signal detector will send a signal to trip the distributed
generator. Accordingly, a
signal detector preferably works like a relay and in the ;preferred embodiment
the signal
detector executes the function of signal detection in a microprocessor-based
relay. The signal
detection function therefore may become an added function to the
microprocessor-based relays
used in many distribution generator stations.
In the preferred embodiment the algorithm for signal detection is based on the
principle
of detecting the presence of the distortion signal. In the preferred
embodiment the distortion
signal has a pattern shown in Figure 8. The distortion signal can be obtained
by digitally
subtracting two consecutive cycles of the measured voltage waveform since the
signal is
present at most in one of the cycles. The difference between an undistorted
cycle and a
distorted cycle is the distortion signal. Due to changes and disturbances in
power systems, the
result of subtraction between two consecutive cycles, called differential
signal, can have
various forms. One of the main goals of the signal detector is to determine if
the differential
signal indeed represents a genuine distortion signal. Details of the algorithm
for signal
detection of the preferred embodiment of the invention are summarized below:
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Figure 8: Typical form of the distortion signal.
1) Extract N consecutive cycles by zero-crossing detection at the undistorted
edge. N is the
number of cycles determined by the signal generator pattern (see 6) of the
signal generator
description). For example, if short circuit is conducted for every three
cycles, N=3. Zero-
crossing detection is performed at the edge which is not used by signal
generator. For
example, if the signal generator conducts short circuit at positive-to-
negative transition of a
voltage cycle, negative-to-positive edge will be detected to extract the
consecutive N
cycles, s(1), s(2),..., s(l~. The following steps assume N=2 as example.
2) Find the waveform distortion d--s(k}-s(k+1) as shown in Figure 9. If s(k)
and s(k+1) have
different size due to frequency drift, the shorter one will be zero-padded to
ensure they have
same length. Partition d into two zones:
~ Zone 1: the portion from zero degree to 150 degrees of d
~ Zone 2: the rest of the cycle of d, from 151 to 360 degrees
3) Code the distortion by checking the RMS and peak absolute values of the
zones of d. The
output of coding is a two-bit description of s(k) and s(k+1). An undistorted
cycle will be
defined as "0" and a distorted will be defined as "1". The following rules
will be applied in
decoding:
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Distortion signal
(not to scale) ~~.~ '
CA 02470536 2004-06-09
~ If peak absolute values of zone l and zone 2 are lower than a peak-
threshold, and
RMS values of zone 1 and zone 2 are lower than a RMS-threshold, then both s(k)
and s(k+1) are undistorted. d is coded as 00.
~ If the peak absolute value and RMS value of zone 1 are lower than the
thresholds
respectively, and the peak absolute value and RMS value of zone 2 are higher
than
the thresholds respectively, one cycle of s(k) and s(k+1) is distorted. One
sub-zone
in zone 2 is extracted to determine which cycle is the distorted one. The sub-
zone is
defined as 151 degrees to 180 degrees of d. If peak value in the sub-zone is
negative, s(k) is distorted and s(k+1) is undistorted. d is coded as 10. If
peak value
in the sub-zone is positive, s(k) is undistorted and s(k+1) is distorted. d is
coded as
O1.
~ For all of other combinations, d is coded as 11, which indicates abnormal
condition.
Figure 10 and Figure 11 shows the thresholds, RMS trends and the waveform of
(d~ for
islanded and normal operating conditions.
4) The code should be 10 or Ol always if DG is connected to the system. If DG
is islanded, the
code should always be 00. Therefore, the DG islanding detection and tripping
will use the
following rules:
If the code is 00 continuously for M cycles, the DCa will be tripped. M is a
threshold
set according to DG protection standard.
~ If the code is 11 continuously for M cycles, the DG will not be tripped but
a
warning will be given.
5) For each cycle extracted at step 1), voltage frequency wall be calculated
by counting the
points of sampling between two zero-crossings. If the frequency is higher than
60.SHz or
lower than 59.SHz, DG will be tripped immediately. That is, frequency
deviation will
overrule the islanding detection algorithm to trip the DG.
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6) Three phase-to-ground voltages are checked individually, as long as one
phase shows the
DG is connected to system (return Ol or 10 code), DG will not be tripped. In
another word,
only when all of the three phases indicate DG is islanded, tripping can be
enabled.
7) If the signal generator conducts short circuit less often than every two
cycles, that is, N>2 in
Step 1, a N-bit word will be used to describe the N cycles (one sampling). If
DG is
islanded, each bit should be 0. If not islanded, one and only one bit of the
word is 1, the rest
bits should be 0. For example, if N=3, 000 is a word for islanding. 001, 010
or 100 is a
word for normal operation. Note only one word out of the three is possible for
normal
operation, that is, the code word for the current sampling must be the same as
that of the
previous sampling. For example, if 001 is detected for the previous sampling,
current
sampling must give 001. If 010 is detected instead, a connmunication error
warning will be
given.
0.15
i
Zane 1 ; ~ Zone 2
i
i
0 1 ...._._____ _______.__ ...___.... __. ~ . ... .._......._ _._.._._..
.._..._.__ ........__ ......._..
:i
i
in
i .e
,
:e
, ~e
n
:n
O.DS .._.._._... ......._._ .___...... ... ;. ..... ,~______._._.. ......._..
....._.... ._......_ ._....._..
,n
i
i o
~iSuh- '~ _
,i
~iZona ;'~
p .__ ._._. ...._. _._ ,... _.___
i
n :,
a
i "
a
-0.05 ._____.__.. _____...._ __.__..... _._ ;...__._ ,~. _.. _. ..._...._.
........__ .._....._. ......_._.
,,
,
o ,,
i "
i ,,
i
-0.1
0 O.OD2 0.004 0.006 0.008 0.01 0.012 0.014 0.016 0.018
Times)
Figure 9 Difference between distorted and normal cycles
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CA 02470536 2004-06-09
0.12
0.1
0.08
0.06
0.04
0.02
0 10 20 30 4f7 50 60
Sample
Figure 11. Distortion energy level check for each zone
-15
Tima(s)
Figure 10 Distortion peak value check :for each zone
CA 02470536 2004-06-09
In summary, the present invention provides a reliable, easy to use and
economical
method, system and apparatus for detecting an interruption i:n an electrical
power distribution
system. The invention is particularly suited for detecting islanding
conditions in electrical
power distribution systems.
In one aspect, the invention may comprise the steps of transmitting at least
one signal
through at least a portion of a power distribution system, detecting an
interruption in the receipt
of the signal, and causing an action to be taken as a result o~f detecting the
interruption in the
receipt of the signal. Preferably the signal is transmitted over a period of
time, either as a
continuous signal or signals or as a repeating signal or signals, so that
interruptions in the
receipt of the signal can be detected over the period of time. The signal may
be comprised of a
continuous single signal, a repeating single signal, or mar be comprised of a
plurality of
identical or different signals which are associated with each ether in a
predetermined pattern or
relationship and which are either transmitted continuously or repeatedly.
Preferably the signal or signals are transmitted by a signal generator, which
signal
generator may be comprised of any method, system, device or apparatus which is
capable of
transmitting the signal or signals through the power distribution system.
Preferably the detection of the interruption of receipt of the signal or
signals is
performed by a signal detector, which signal detector may be comprised of any
method,
system, device or apparatus which is capable of detecting the interruption of
the receipt of the
signal or signals and which is compatible with the signal or signals being
generated.
The detection of the interruption of receipt of the signal or signals may
reflect an
interruption in the transmission of the signal or signals or may reflect an
interruption in the
receipt of the signal or signals.
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Preferably the power distribution system includes a distributed generation
(DG) system
and preferably the detection of the interruption of receipt of the signal or
signals is associated
with the distributed generation system.
More preferably the detection of the interruption o:f receipt of the signal or
signals
provides an indication of islanding conditions in a DG system associated with
the power
distribution system. Preferably the action which is caused to be taken as a
result of the
detection of an interruption in the receipt of the signal or signals is to
"trip" the DG system.
The signal or signals may be comprised of a distortion of the electrical power
waveform
or may be comprised of a separate signal which is independent of or
superimposed upon the
electrical power waveform. A signal comprised of a distortion of the
electrical power
waveform may be comprised of an alteration of the shape, frequency or
amplitude of the
waveform. The signal may be continuous, may be applied to every cycle of the
electrical
power waveform, or may be applied only to selected cycles of the electrical
power waveform.
In a preferred embodiment, the signal or signals are comprised of a distortion
signal
which distorts the shape of a selected portion of the electrical power
waveform when the
distortion signal is superimposed upon the electrical power waveform. In the
preferred
embodiment, the distortion signal is applied to selected portions of selected
cycles of the
electrical power waveform.
Where the signal or signals is not continuous, the signal or signals are
preferably
transmitted on a repeating basis with a frequency which is high enough to
provide a suitable
"response time" for causing the action to be taken when an intemiption in the
receipt of the
signal or signals is detected.
Where the invention is utilised to detect islanding conditions in distributed
generation
(DG) systems, the frequency of transmission of the repeating signal or signals
should be such
that the system will facilitate "tripping" of the DG system within about 100
to about 600
milliseconds from the time of an occurrence of the islanding conditions. In
the preferred
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CA 02470536 2004-06-09
embodiment, where the electrical power waveform has a :frequency of about 60
hertz, the
distortion signal is preferably applied to every second, third or fourth cycle
of the electrical
power waveform.
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