WO2008153419A2 - Anti-islanding system - Google Patents

Anti-islanding system Download PDF

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Publication number
WO2008153419A2
WO2008153419A2 PCT/NZ2008/000138 NZ2008000138W WO2008153419A2 WO 2008153419 A2 WO2008153419 A2 WO 2008153419A2 NZ 2008000138 W NZ2008000138 W NZ 2008000138W WO 2008153419 A2 WO2008153419 A2 WO 2008153419A2
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WO
WIPO (PCT)
Prior art keywords
generation unit
distributed generation
grid
voltage
utility
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Application number
PCT/NZ2008/000138
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French (fr)
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WO2008153419A3 (en
Inventor
Vinod Kumar
Josh Paul Baker
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Whisper Tech Limited
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Publication date
Application filed by Whisper Tech Limited filed Critical Whisper Tech Limited
Publication of WO2008153419A2 publication Critical patent/WO2008153419A2/en
Publication of WO2008153419A3 publication Critical patent/WO2008153419A3/en

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Classifications

    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
    • H02J3/388—Islanding, i.e. disconnection of local power supply from the network

Definitions

  • the invention relates generally to anti-islanding technology for distributed generators connected to a main utility power distribution grid.
  • An electric power distribution grid is energised primarily by generators owned and operated by utility companies. Independent generation sources such as photovoltaics, microturbines or micro combined heat and power units (mCHP) may also be connected to the g ⁇ d. These secondary power sources are referred to as distributed generators (DG) and are commonly operated at an individual household level. A household that operates a DG can either use the power generated (usually supplemented by the g ⁇ d) or feed power back into the grid. Islanding occurs when the utility power supply fails, leaving a branch of the grid primarily energised by one or more DGs, which can lead to dangerous conditions. Anti-islanding schemes are implemented to detect islanding and disconnect any DGs energising the branch.
  • Independent generation sources such as photovoltaics, microturbines or micro combined heat and power units (mCHP) may also be connected to the g ⁇ d.
  • mCHP micro combined heat and power units
  • These secondary power sources are referred to as distributed generators (DG
  • Anti-islanding schemes can be either active, which involves signal injection into die grid and monitoring the resulting signals, or passive, which involves monitoring the signals that are pre-existing on the g ⁇ d. Active schemes involve a trade-off between islanding detection time and distortion caused in the output current waveform, as faster detection generally causes more total harmonic distortion (THD).
  • TDD total harmonic distortion
  • the invention consists in a distributed generation unit for connection to an elect ⁇ c power distribution grid also energised by utility generation, comprising an anti-islanding system arranged to detect outage of utility generation from the g ⁇ d by reference to a harmonic or harmonics generated by the distributed generation unit, and to disconnect the distributed generation unit from the grid in the event of a utility outage.
  • the distributed generation unit is arranged to detect an increase in grid impedance seen by the distributed generation unit indicating outage of utility generation from the g ⁇ d, by reference to one or more harmonics generated by the distributed generation unit.
  • each distributed generation unit system generates a unique harmonic signature with its own harmonic content and modulated inter harmonics. It is possible to calculate the grid impedance or at least monitor for changes in grid impedance, from harmonic measurements taken at a point of common coupling between the utility supply and the distributed generation unit supply In the event of an outage of utility generation, the grid impedance increases
  • the distributed generation unit is arranged to measure the harmonic current or voltage or both at a point of common coupling between the distributed generation unit and die electric power distribution grid
  • the distributed generation unit is arranged to actuate disconnecting means to isolate the distributed generation unit from the electric power distribution g ⁇ d in the event that the impedance increases by a predetermined step change
  • the distributed generation unit comprises a voltage transducer arranged to provide a signal indicative of die voltage level at a point of common coupling between die distributed generation unit and die g ⁇ d More preferably, the voltage transducer is a voltage transformer
  • die distributed generation unit comprises a current transducer arranged to provide a signal indicative of the current level at the point of common coupling More preferably, the current transducer is a current transformer
  • the distributed generation unit comprises a filter connected to the voltage transducer and arranged to pass a predetermined harmonic frequency or frequencies
  • the distributed generation unit comprises a filter connected to current transducer and arranged to pass a predetermined harmonic frequency or frequencies
  • the distributed generation unit comprises a subsequent digital signal processing stage arranged to convert the output from a filter following each or both of the voltage and current transducers to frequency domain information and to monitor for an increase in grid impedance seen by the distributed generation unit indicating outage of utility generation
  • the distributed generation unit comprises a solenoid or power semi conductor switching device as said disconnecting means
  • the invention consists in a method for causing a distributed generation unit to disconnect from an electric power distribution grid energised by utility generation in the event of an outage of utility from the grid, comprising momtoimg foi changes in grid impedance by reference to a harmonic or harmonics generated b) the distributed generation unit, and causing the distributed generation unit to be disconnected from the grid in the event of a change in impedance indicating a utility outage
  • the method comprises monitoring to detect an increase in g ⁇ d impedance seen by the distributed generation unit indicating outage of utility generation from the grid, by reference to one or more harmonics generated by the distributed generation unit.
  • the method comprises measuring the harmonic current or voltage or both at a point of common coupling between the distributed generation unit and the grid.
  • the method comprises actuating disconnecting means to isolate the distributed generation unit from the electric power distribution g ⁇ d in the event that the impedance increases by a predetermined step change.
  • the method comprises filtering the output of a voltage transducer providing a signal indicative of the voltage level at a point of common coupling between the distributed generation unit and the grid.
  • the voltage transducer is a voltage transformer.
  • the method comprises filtering the output of a current transducer providing a signal indicative of the current level at the point of common coupling. More preferably, the current transducer is a current transformer Preferably, the method comprises subsequendy digitally processing the harmonic frequency or frequencies to frequenc) domain information and monitoring for an increase in g ⁇ d impedance seen by the distributed generation unit indicating outage of utility generation from the g ⁇ d
  • the disconnecting means is a solenoid or power semi-conductor switching device.
  • the drawing is a schematic diagram of the anti-islanding system of the invention.
  • a distributed generator (DG) 100 for connection to an electric power distribution grid 102 which is also energised by one or more utility company generators 104
  • the electric power distribution grid 102 supplies power to an electric load 106 such as households.
  • the DG 100 shown is a micro combined heat and power unit but can be a photovoltaic, microturbine or any other type of distributed generator.
  • the DG 100 comprises an anti-islanding system generally indicated by 200.
  • the DG 100 is connected to the grid by a conducting busbar (not shown in diagram).
  • the anti-islanding system 200 provides two instrument transducers, a voltage transducer (VT) 202 and a current transducer (CT) 204.
  • VT voltage transducer
  • CT current transducer
  • the VT 202 provides a signal indicative of the voltage level at a point of common coupling (PCC).
  • the VT 202 is also provided to isolate the sensitive hardware of the anti- islanding system 200 from the potential high voltages and high power on the conducting busbar.
  • the VT 202 is a voltage transformer. More preferably, the VT 202 is a step-down transformer Even more preferably, the VT steps down the voltage on the primary side to a lower voltage on the secondary side such that the secondary side voltage is suitable to be used or measured by the anti-islanding system 200.
  • any suitable voltage transducer may be used.
  • the CT 204 provides a signal indicative of the current level at the PCC
  • the CT 204 is provided to isolate the sensitive hardware of the anti-islanding system 200 from the potential high currents and high power on the conducting busbar.
  • the CT 204 is a current transformer. More preferably, the CT 204 is constructed by passing the conducting busbar (which would act as the primary side) through a insulated toroidal core, around which is wound many turns of a continuous piece of wire (which would be the secondary side)
  • any suitable current transducer may be used.
  • a first filter 206 may be connected to the secondary winding of the VT 202.
  • the filter 206 is a harmonic filter.
  • the filter 206 may be any suitable filter such as any combination of active or passive, linear or non-linear, digital or analogue, finite impulse response (FIR) or infinite impulse response (HR), and discrete or continuous.
  • FIR finite impulse response
  • HR infinite impulse response
  • An example of a suitable filter is a digital, FIR filter with an analogue to digital converter (ADC) on the input and a digital to analogue converter (DAC) on the output.
  • ADC analogue to digital converter
  • DAC digital to analogue converter
  • other filter configurations may be used.
  • the filter 206 is a bandpass filter configured to isolate a particular harmonic frequency
  • the filter may isolate the third harmonic which is 150 Hz in a 50 Hz system, and attenuate all other frequencies.
  • the filter 206 may isolate any harmonic or other frequency as needed
  • the filter 206 may comprise a plurality of subfilters 206a, 206b, 206c etc connected in parallel. Each one of these subfilters is configured to isolate a certain harmonic or other frequency.
  • the output of the filter 206 may be an analogue signal 208 indicative of the amplitude and phase of the voltage of each frequency being measured
  • An amplifier 210 may be provided on the output of the filter 206.
  • the amplifier 210 may be an active component amplifier for example a transistor amplifier, or a pre-package amphfier for example an operational amplifier, or any other type of amplifier
  • the amplifier 210 may amplify the analogue signal 208 output from the filter 206 by some predetermined gain or variable gain
  • a second filter 212 is connected to the secondary winding of the CT 204
  • the filter 212 is a harmonic filter
  • the filter 212 may be any suitable filter such as any combination of active or passive, linear or non-hnear, digital or analogue, finite impulse response or infinite impulse response, and discrete or continuous
  • the filter 212 is a bandpass filter configured to isolate a particular harmonic frequency, and attenuate all other frequencies
  • the filter 212 may isolate any harmonic or other frequency
  • the filter 212 may comprise a plurality of subfilters 212a, 212b, 212c etc connected in parallel Each one of these subfilters is configured to isolate a certain harmonic or other frequency
  • the output of the filter 212 may be an analogue signal 214 indicative of the amplitude and phase of the current of each frequency being measured
  • an amplifier 216 may be provided on the output of the filter 212
  • the amplifier 216 may be an active component amplifier for example a transistor amplifier, or a prepackaged amplifier for example an operational amplifier, or any other type of amplifier
  • the amplifier 216 may amplify the analogue signal 214
  • System impedance may then be calculated in the digital signal processing stage 306 and this value may be used by a decision algorithm 308.
  • the decision algorithm compares the current value of system impedance with past values of system impedance. If there is a step change in system impedance greater than some predetermined threshold a signal 310 is sent to the DG 100 that instructs it to turn off or disconnect from the electric power distribution grid 102 or both.
  • a solenoid or power semiconductor device such as a power MOSFET may be used to disconnect the DG 100 from the electric power distribution gird 102, however this may be done in any suitable manner. If the step change is not greater than the predetermined threshold no signal 310 is sent and the system works normally, constantly monitoring the system impedance.

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  • Engineering & Computer Science (AREA)
  • Power Engineering (AREA)
  • Supply And Distribution Of Alternating Current (AREA)
  • Stand-By Power Supply Arrangements (AREA)

Abstract

A distributed generation unit for connection to an electric power distribution grid also energised by utility company generation, comprising an anti-islanding system arranged to detect outage of utility generation from the grid by reference to a harmonic or harmonics generated by the distributed generation unit, and to disconnect the distributed generation unit from the grid in the event of a utility outage.

Description

"ANTI-ISLANDING SYSTEM"
FIELD OF THE INVENTION
The invention relates generally to anti-islanding technology for distributed generators connected to a main utility power distribution grid.
BACKGROUND
An electric power distribution grid is energised primarily by generators owned and operated by utility companies. Independent generation sources such as photovoltaics, microturbines or micro combined heat and power units (mCHP) may also be connected to the gπd. These secondary power sources are referred to as distributed generators (DG) and are commonly operated at an individual household level. A household that operates a DG can either use the power generated (usually supplemented by the gπd) or feed power back into the grid. Islanding occurs when the utility power supply fails, leaving a branch of the grid primarily energised by one or more DGs, which can lead to dangerous conditions. Anti-islanding schemes are implemented to detect islanding and disconnect any DGs energising the branch.
Anti-islanding schemes can be either active, which involves signal injection into die grid and monitoring the resulting signals, or passive, which involves monitoring the signals that are pre-existing on the gπd. Active schemes involve a trade-off between islanding detection time and distortion caused in the output current waveform, as faster detection generally causes more total harmonic distortion (THD).
It is an object of the present invention to reduce these drawbacks or at least offer the public an alternative choice.
SUMMARY OF THE INVENTION
In broad terms in a first aspect the invention consists in a distributed generation unit for connection to an electπc power distribution grid also energised by utility generation, comprising an anti-islanding system arranged to detect outage of utility generation from the gπd by reference to a harmonic or harmonics generated by the distributed generation unit, and to disconnect the distributed generation unit from the grid in the event of a utility outage.
Preferably, the distributed generation unit is arranged to detect an increase in grid impedance seen by the distributed generation unit indicating outage of utility generation from the gπd, by reference to one or more harmonics generated by the distributed generation unit.
Typically, each distributed generation unit system generates a unique harmonic signature with its own harmonic content and modulated inter harmonics. It is possible to calculate the grid impedance or at least monitor for changes in grid impedance, from harmonic measurements taken at a point of common coupling between the utility supply and the distributed generation unit supply In the event of an outage of utility generation, the grid impedance increases Preferably, the distributed generation unit is arranged to measure the harmonic current or voltage or both at a point of common coupling between the distributed generation unit and die electric power distribution grid
Preferably, the distributed generation unit is arranged to actuate disconnecting means to isolate the distributed generation unit from the electric power distribution gπd in the event that the impedance increases by a predetermined step change
Preferably, the distributed generation unit comprises a voltage transducer arranged to provide a signal indicative of die voltage level at a point of common coupling between die distributed generation unit and die gπd More preferably, the voltage transducer is a voltage transformer Preferably, die distributed generation unit comprises a current transducer arranged to provide a signal indicative of the current level at the point of common coupling More preferably, the current transducer is a current transformer
Preferably, the distributed generation unit comprises a filter connected to the voltage transducer and arranged to pass a predetermined harmonic frequency or frequencies Preferably, the distributed generation unit comprises a filter connected to current transducer and arranged to pass a predetermined harmonic frequency or frequencies
Preferably, the distributed generation unit comprises a subsequent digital signal processing stage arranged to convert the output from a filter following each or both of the voltage and current transducers to frequency domain information and to monitor for an increase in grid impedance seen by the distributed generation unit indicating outage of utility generation
Figure imgf000003_0001
Preferably, the distributed generation unit comprises a solenoid or power semi conductor switching device as said disconnecting means
In broad terms in a second aspect the invention consists in a method for causing a distributed generation unit to disconnect from an electric power distribution grid energised by utility generation in the event of an outage of utility from the grid, comprising momtoimg foi changes in grid impedance by reference to a harmonic or harmonics generated b) the distributed generation unit, and causing the distributed generation unit to be disconnected from the grid in the event of a change in impedance indicating a utility outage Preferably, the method comprises monitoring to detect an increase in gπd impedance seen by the distributed generation unit indicating outage of utility generation from the grid, by reference to one or more harmonics generated by the distributed generation unit.
Preferably, the method comprises measuring the harmonic current or voltage or both at a point of common coupling between the distributed generation unit and the grid.
Preferably, the method comprises actuating disconnecting means to isolate the distributed generation unit from the electric power distribution gπd in the event that the impedance increases by a predetermined step change.
Preferably, the method comprises filtering the output of a voltage transducer providing a signal indicative of the voltage level at a point of common coupling between the distributed generation unit and the grid. More preferably, the voltage transducer is a voltage transformer.
Preferably, the method comprises filtering the output of a current transducer providing a signal indicative of the current level at the point of common coupling. More preferably, the current transducer is a current transformer Preferably, the method comprises subsequendy digitally processing the harmonic frequency or frequencies to frequenc) domain information and monitoring for an increase in gπd impedance seen by the distributed generation unit indicating outage of utility generation from the gπd
Preferably, the disconnecting means is a solenoid or power semi-conductor switching device.
The term "comprising" as used in this specification and claims means "consisting at least in part of. When interpreting each statement in this specification and claims that includes die term "compπsing", features other than that or those prefaced by the term may also be present. Related terms such as "comprise" and "compπses" are to be interpreted in the same manner.
The invention consists in the foregoing and also envisages constructions of which the following gives examples only
DETAILED DESCRIPTION OF PREFERRED EMBODIMENT A preferred embodiment of the invention is descπbed with reference to the accompanying drawing and by way of example only.
The drawing is a schematic diagram of the anti-islanding system of the invention. Provided is a distributed generator (DG) 100 for connection to an electric power distribution grid 102 which is also energised by one or more utility company generators 104 The electric power distribution grid 102 supplies power to an electric load 106 such as households. The DG 100 shown is a micro combined heat and power unit but can be a photovoltaic, microturbine or any other type of distributed generator. The DG 100 comprises an anti-islanding system generally indicated by 200. The DG 100 is connected to the grid by a conducting busbar (not shown in diagram). The anti-islanding system 200 provides two instrument transducers, a voltage transducer (VT) 202 and a current transducer (CT) 204.
The VT 202 provides a signal indicative of the voltage level at a point of common coupling (PCC). The VT 202 is also provided to isolate the sensitive hardware of the anti- islanding system 200 from the potential high voltages and high power on the conducting busbar. Preferably, the VT 202 is a voltage transformer. More preferably, the VT 202 is a step-down transformer Even more preferably, the VT steps down the voltage on the primary side to a lower voltage on the secondary side such that the secondary side voltage is suitable to be used or measured by the anti-islanding system 200. However, any suitable voltage transducer may be used.
Similarly, the CT 204 provides a signal indicative of the current level at the PCC The CT 204 is provided to isolate the sensitive hardware of the anti-islanding system 200 from the potential high currents and high power on the conducting busbar. Preferably, the CT 204 is a current transformer. More preferably, the CT 204 is constructed by passing the conducting busbar (which would act as the primary side) through a insulated toroidal core, around which is wound many turns of a continuous piece of wire (which would be the secondary side) However, any suitable current transducer may be used.
A first filter 206 may be connected to the secondary winding of the VT 202. Preferably, the filter 206 is a harmonic filter. The filter 206 may be any suitable filter such as any combination of active or passive, linear or non-linear, digital or analogue, finite impulse response (FIR) or infinite impulse response (HR), and discrete or continuous. An example of a suitable filter is a digital, FIR filter with an analogue to digital converter (ADC) on the input and a digital to analogue converter (DAC) on the output. However, other filter configurations may be used. Preferably, the filter 206 is a bandpass filter configured to isolate a particular harmonic frequency For example, the filter may isolate the third harmonic which is 150 Hz in a 50 Hz system, and attenuate all other frequencies. The filter 206 may isolate any harmonic or other frequency as needed The filter 206 may comprise a plurality of subfilters 206a, 206b, 206c etc connected in parallel. Each one of these subfilters is configured to isolate a certain harmonic or other frequency. The output of the filter 206 may be an analogue signal 208 indicative of the amplitude and phase of the voltage of each frequency being measured
An amplifier 210 may be provided on the output of the filter 206. The amplifier 210 may be an active component amplifier for example a transistor amplifier, or a pre-package amphfier for example an operational amplifier, or any other type of amplifier The amplifier 210 may amplify the analogue signal 208 output from the filter 206 by some predetermined gain or variable gain
Similarly, a second filter 212 is connected to the secondary winding of the CT 204
Preferably, the filter 212 is a harmonic filter The filter 212 may be any suitable filter such as any combination of active or passive, linear or non-hnear, digital or analogue, finite impulse response or infinite impulse response, and discrete or continuous Preferably, the filter 212 is a bandpass filter configured to isolate a particular harmonic frequency, and attenuate all other frequencies The filter 212 may isolate any harmonic or other frequency The filter 212 may comprise a plurality of subfilters 212a, 212b, 212c etc connected in parallel Each one of these subfilters is configured to isolate a certain harmonic or other frequency The output of the filter 212 may be an analogue signal 214 indicative of the amplitude and phase of the current of each frequency being measured Similarly, an amplifier 216 may be provided on the output of the filter 212 The amplifier 216 may be an active component amplifier for example a transistor amplifier, or a prepackaged amplifier for example an operational amplifier, or any other type of amplifier The amplifier 216 may amplify the analogue signal 214 output from the filter 212 by some predetermined gain or variable gain The anti-islanding system 200 comprises a digital signal processing (DSP) stage 300 A digital signal processor 302 may be used Preferably, the digital signal processor 302 is a specialised DSP chip but may be some other microprocessor or pre-packaged component, or may be implemented in hardware or software or both The digital signal processor 302 may comprise an analogue to digital converter (ADC) 304 which takes the amplified signal representations of the voltage 208 and current 214 and converts them into a digital representation signal suitable for processing The digital signal processor may also comprise a digital signal processing stage 306 which receives the output signal from the ADC 304 and coverts it into the frequency domain via a discrete Fourier transform (DFT) algorithm The impedance of the system at any given frequency and time may then be calculated with the formula
where Z is the complex impedance of the system, V is the complex voltage of the system and / is the complex cuirent of the system derived from the DFT applied to both the digital representations of the current and voltage signals. System impedance may then be calculated in the digital signal processing stage 306 and this value may be used by a decision algorithm 308. The decision algorithm compares the current value of system impedance with past values of system impedance. If there is a step change in system impedance greater than some predetermined threshold a signal 310 is sent to the DG 100 that instructs it to turn off or disconnect from the electric power distribution grid 102 or both. A solenoid or power semiconductor device such as a power MOSFET may be used to disconnect the DG 100 from the electric power distribution gird 102, however this may be done in any suitable manner. If the step change is not greater than the predetermined threshold no signal 310 is sent and the system works normally, constantly monitoring the system impedance.
The foregoing description of the invention includes preferred forms thereof. Modifications may be made thereto without departing from the scope of the invention as defined by the accompanying claims.

Claims

1. A distributed generation unit for connection to an electric power distribution grid also energised by utility generation, comprising an anti-islanding system arranged to detect outage of utility generation from the grid by reference to a harmonic or harmonics generated by the distributed generation unit, and to disconnect the distributed generation unit from the grid in the event of a utility outage.
2. A distributed generation unit according to claim 1 arranged to detect an increase in grid impedance seen by the distributed generation unit indicating outage of utility generation from the grid, by reference to one or more harmonics generated by the distributed generation unit
3. A distributed generation unit according to either of claims 1 or 2 arranged to measure die harmonic current or voltage or both at a point of common coupling between the distributed generation unit and the gπd.
4. A distributed generation unit according to any one of claims 1 to 3 arranged to actuate disconnecting means to isolate the distributed generation unit from the electric power distribution grid in the event that the impedance increases by a predetermined step change.
5. A distributed generation unit according to any one of claims 1 to 4 comprising a voltage transducer arranged to provide a signal indicative of the voltage level at a point of common coupling between the distributed generation unit and the god.
6. A distributed generation unit according to claim 5 wherein the voltage transducer is a voltage transformer.
7. A distributed generation unit according to any one of claims 1 to 6 comprising a current transducer arranged to provide a signal indicative of the current level at the point of common coupling.
8 A distributed generation unit according to claim 7 wherein the curient transducer is a current transformer.
9 A distributed generation unit according to either of claims 5 or 6 comprising a filter connected to the voltage transducer and arranged to pass a predetermined harmonic frequency or frequencies
10 A distributed generation unit according to either of claims 7 or 8 comprising a filter connected to current transducer and arranged to pass a predetermined harmonic frequency or frequencies
11 A distributed generation unit according to either claim 9 or claim 10 comprising a subsequent digital signal processing stage arranged to convert the output from a filter following each or both of the voltage and current transducers to frequency domain information and to monitor for an increase in grid impedance seen by the distributed generation unit indicating outage of utility generation from the gπd
12 A distributed generation unit according to claim 4 comprising a solenoid or power semi-conductor switching device as said disconnecting means
13 A method for causing a distributed generation unit to disconnect from an electric power distribution grid energised by utility generation in the event of an outage of utility from the gπd, comprising monitoring for changes in gπd impedance by reference to a harmonic or harmonics generated by the distributed generation unit, and causing the distributed generation unit to be disconnected from the grid in the event of a change in impedance indicating a utility outage
14 A method according to claim 13 compπsing monitoring to detect an increase in grid impedance seen by the distnbuted generation unit indicating outage of utility generation from the gnd, by reference to one or more harmonics generated by the distributed generation unit
15 A method according to either of claims 13 or 14 compnsing measuring the harmonic current or voltage or both at a point of common coupling between the distributed generation
Figure imgf000009_0001
16 A method according to any one of claims 13 to 14 comprising actuating disconnecting means to isolate the distributed generation unit from the electric power distribution gπd in the event that the impedance increases by a predetermined step change
17. A method according to any one of claims 13 to 16 comprising filtering the output of a voltage transducer providing a signal indicative of the voltage level at a point of common coupling between the distributed generation unit and the grid.
18. A method according to claim 17 wherein the voltage transducer is a voltage transformer.
19. A method according to any one of claims 13 to 18 comprising filtering the output of a current transducer providing a signal indicative of the current level at the point of common coupling.
20. A method according to claim 19 wherein the current transducer is a current transformer.
21 A method according to any one of claims 17 to 20 comprising subsequendy digitally processing the harmonic frequency or frequencies to frequency domain information and monitoring for an increase m grid impedance seen by the distributed generation unit indicating outage of utility generation from the gπd.
22 A distributed generation unit according to claim 16 wherein the disconnecting means is a solenoid or power semi-conductor switching device.
PCT/NZ2008/000138 2007-06-11 2008-06-11 Anti-islanding system WO2008153419A2 (en)

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WO2018112481A1 (en) * 2016-12-16 2018-06-21 Ideal Power Inc. Anti-islanding systems and methods using harmonics injected in a rotation opposite the natural rotation
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