DE102012010505A1 - Method for detecting unintended island formation in alternating current network, involves detecting unintended island formation based on variation of idle current at intervals and change in period length of alternating voltage - Google Patents

Method for detecting unintended island formation in alternating current network, involves detecting unintended island formation based on variation of idle current at intervals and change in period length of alternating voltage

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Publication number
DE102012010505A1
DE102012010505A1 DE201210010505 DE102012010505A DE102012010505A1 DE 102012010505 A1 DE102012010505 A1 DE 102012010505A1 DE 201210010505 DE201210010505 DE 201210010505 DE 102012010505 A DE102012010505 A DE 102012010505A DE 102012010505 A1 DE102012010505 A1 DE 102012010505A1
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Germany
Prior art keywords
change
period length
reactive current
islanding
voltage
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Ceased
Application number
DE201210010505
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German (de)
Inventor
Anmelder Gleich
Original Assignee
Klaus-Wilhelm Köln
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Application filed by Klaus-Wilhelm Köln filed Critical Klaus-Wilhelm Köln
Priority to DE201210010505 priority Critical patent/DE102012010505A1/en
Publication of DE102012010505A1 publication Critical patent/DE102012010505A1/en
Application status is Ceased legal-status Critical

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    • HELECTRICITY
    • H02GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02JCIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00Circuit arrangements for ac mains or ac distribution networks
    • H02J3/38Arrangements for parallely feeding a single network by two or more generators, converters or transformers
    • H02J3/381Dispersed generators
    • HELECTRICITY
    • H02GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02JCIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00Circuit arrangements for ac mains or ac distribution networks
    • H02J3/38Arrangements for parallely feeding a single network by two or more generators, converters or transformers
    • H02J2003/388Islanding, i.e. disconnection of local power supply from the network

Abstract

The uncontrolled islanding in AC or three-phase networks should be recognized and terminated by automatic shutdown of decentralized power sources, it is important to distinguish between an intentional and an unintentional islanding. Solution: In case of unintentional islanding, power generated and consumed happens to be in equilibrium at a certain voltage and frequency. A change in the reactive current equilibrium leads to a change of the frequency or the period length, whereas in a system stabilized by voltage sources this effect would not occur. This is used to detect unintentional islanding. Technically, the change in the reactive power balance is caused by the periodic change of a reactive current. With the aid of a microprocessor (4), the magnitude of the change in the period length, which occurs as a result of the change in the reactive current, is constantly measured. In a sudden increase of this size, the unintentional islanding is terminated by switching off the decentralized power source. Application: Safe connection of decentralized power sources to AC networks with inverters, automatically detecting in case of unintentional islanding and switching off the power source.

Description

  • The uncontrolled islanding in AC or three-phase networks should be recognized and terminated by automatic shutdown of decentralized power sources, it is important to distinguish between an intentional and an unintentional islanding. An intended islanding is the separation of a small part of a power grid from the main grid, being controlled and stabilized in the subnet by a suitable voltage source grid voltage and power frequency. When switching back to the main network, either synchronous conditions must be provided or the subnet must first be de-energized.
  • An unintended stand-alone grid arises when, in the event of a separation of a subnetwork, this subnetwork continues to be under voltage due to centralized generation plants, although a de-energized state is expected. This is possible if a manual shutdown of self-generation facilities is missed or unintentional reconnection occurs. In this case there is a risk for people working on the network who assume a voltage-free state. Therefore, there are automatic devices that shut down self-generating systems in case of grid disconnection and thus automatically terminate unintentional islanding. In such an unintentional islanding the affected subnet is not stabilized by a voltage source. Therefore, most of the time, voltage and frequency relays are tripped to detect overshooting or undershooting limit values. However, a state is possible in which voltage and frequency relays can not detect the unintentional islanding. This condition can arise when there is a random power balance between decentralized feed-in and consumption in a small subnetwork. In this case, the voltage and frequency may be within the tripping thresholds of frequency and voltage monitoring relays of decentralized power sources connected to this network. If in this state a separation from the main network, this islanding is not terminated by switching off the decentralized power sources by means of frequency and voltage relay and can continue until voltage or frequency by a random change in power balance far enough from the nominal values that the a tripping of voltage or frequency relay takes place. In this unintentional islanding, frequency and voltage are largely determined by an existing balance of real power and apparent power between generation and consumption.
  • Therefore, procedures are necessary, which recognize the unintentional islanding also in this special case and with which this state can be terminated automatically.
  • State of the art:
  • A number of methods are known in which, by changing this equilibrium, the value of frequency or voltage is shifted so far from the nominal value until limit values are exceeded or fallen below and a shutdown is triggered. There are two problems with these techniques: first, the effect of multiple devices may be mitigated if one part of the devices tries to reduce frequency or voltage and another part tries to increase frequency or voltage at the same time.
  • On the other hand, there may be network stability issues when many devices are trying to either increase or decrease voltage or frequency.
  • Non Islanding Inverter:
  • These are methods that are integrated into inverters, where an existing frequency change is detected and amplified until it leads to the triggering of the frequency-controlled shutdown. Also in this method, there may be problems with the network stability, because a occurring in the network frequency deviation can be further increased and thus an instability can be increased.

    US Pat. No. 7,432,618 B2 , Bill Taylor
  • This procedure can negatively influence the voltage quality in the network. A compromise has to be found between the reliability and speed of island detection and network perturbations.

    US Pat. No. 6,810,339 B2 Robert H. Wills
  • In the method described here, there are limitations in grid-connected PV inverters without memory, since the active power can not be increased by control measures, if the inverter was previously operated at the point of maximum power of the PV generator.
  • Impedance monitoring:
  • The network monitoring devices of UfE GmbH use a procedure according to DE 195 04 271 , In this case, a reactive current is generated by periodically connecting and disconnecting a capacitor, which leads in the network connection point to a time shift of the zero crossing of the mains voltage. From the size of the reactive current and the size of the time shift, the network impedance is determined. A sudden increase in network impedance, as occurs in uncontrolled islanding, is detected to trigger a shutdown.
  • This in DE 195 04 271 described and used in the devices mentioned method has been proven, but also has weaknesses. In this method, it may lead to unwanted false triggering when z. B. by harmonics or interference, the mains voltage is distorted. Also, the impedance jump to be detected is inversely proportional to the size of the uncontrolled island grid. The recognition of the uncontrolled islanding is thus the more uncertain, the larger the separated subnet.
  • The present invention is therefore based on the problem of reliably detecting an actual uncontrolled islanding and to distinguish it from a controlled islanding.
  • The invention can be used in essentially three ways:
    • 1. Devices, z. B Inverters, which control, by means of an appropriate control system, the current absorbed or supplied to the grid in order to produce a suitable change in a reactive current at intervals, and by measuring the mains frequency, the change in the period of time related to the change in the reactive current , detect and use an increase in the value of this change in the period length, based on the change in the reactive current, to detect an unintentional islanding and thus trigger the disconnection from the grid or the shutdown of the feed.
    • 2. Devices that use an independent device to cause a change in reactive current at intervals. This is possible for example with a circuit as shown in DE 195 04 271 is described. This is a capacitor ( 2 ) is switched on and off at intervals parallel to the network, thus achieving a corresponding change in a reactive current. The evaluation takes place via the measurement of the period length, ( 9 . 10 ) detects the change of the period length in relation to the change of the reactive current and uses an increase of this value to detect an unintentional islanding and to automatically actuate switches that have been switched off ( 5 ) of generating plants or other measures terminate the unintentional islanding.
    • 3. Inverters, where a controlled release of reactive current is possible, are controlled by an external device which supplies the control signals necessary for the process to change the reactive current via an interface to the inverter. The evaluation is carried out in the external device, which also controls the triggering of switches or controls the inverter via an interface, as required for the treatment of unintentional islanding. The evaluation takes place via the measurement of the period length, the change of the period length related to the change of the reactive current is detected and an increase of this value is used to detect an unintentional islanding.
  • According to the invention, the problem is solved essentially by the method described in claim 1.
  • QUOTES INCLUDE IN THE DESCRIPTION
  • This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.
  • Cited patent literature
    • US 7432618 B2 [0006]
    • US 6810339 B2 [0007]
    • DE 19504271 [0009, 0010, 0011, 0013]

Claims (6)

  1. Method and device for detecting unintentional islanding in alternating current networks, characterized by the following features: - electric variables are determined and evaluated, - in the AC network, a reactive current is changed in size, in an uncontrolled islanding this change leads to a change in the period length of AC voltage ( 8th ), while in a network controlled by voltage sources the period length remains unchanged. An abrupt increase in the value of the period length change in response to the change in reactive current is therefore used to detect the onset of unintentional islanding. It is the change of the period length with one or more time measurements ( 9 . 10 ), - the change of the reactive current occurs at intervals. - In periods of every 0.1-5 s, a change of a reactive current is generated at intervals or at least once, measured the change in the resulting period length of the voltage and detected by an increase in this value, an unintentional islanding,
  2. A method according to claim 1, characterized in that the reactive current change by a parallel switching on and off a capacity ( 2 ) or inductance to the AC mains is generated.
  3. A method according to claim 1, characterized in that the reactive current change is generated by appropriate control of an inverter in the AC mains or three-phase network.
  4. A method according to claim 1 and 2, characterized in that the duty cycle of the capacitance or inductance is substantially one or more voltage periods.
  5. A method according to claim 1-3, characterized in that the intervals in which the reactive current change is generated, form an irregular pattern that is not to be expected in the power grid, so by the evaluation of the frequency changes based on only this pattern other disturbances and mutual Influencing can be suppressed.
  6. A method according to claim 1-4, characterized in that for measuring the period length several time measurements per period are performed, each starting and ending when passing different voltage levels. ( 9 . 10 )
DE201210010505 2012-05-26 2012-05-26 Method for detecting unintended island formation in alternating current network, involves detecting unintended island formation based on variation of idle current at intervals and change in period length of alternating voltage Ceased DE102012010505A1 (en)

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DE201210010505 DE102012010505A1 (en) 2012-05-26 2012-05-26 Method for detecting unintended island formation in alternating current network, involves detecting unintended island formation based on variation of idle current at intervals and change in period length of alternating voltage

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DE201210010505 DE102012010505A1 (en) 2012-05-26 2012-05-26 Method for detecting unintended island formation in alternating current network, involves detecting unintended island formation based on variation of idle current at intervals and change in period length of alternating voltage

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DE102012010505A1 true DE102012010505A1 (en) 2013-11-28

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Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE19504271C1 (en) 1994-09-29 1996-02-08 Koeln Klaus Wilhelm Impedance measuring system for AS network
US6810339B2 (en) 1997-11-24 2004-10-26 Plug Power, Inc. Anti-islanding method and apparatus for distributed power generation
US7432618B2 (en) 2005-04-07 2008-10-07 Pv Powered, Inc. Inverter anti-islanding method
US20110115301A1 (en) * 2009-11-13 2011-05-19 Vijay Bhavaraju Method and area electric power system detecting islanding by employing controlled reactive power injection by a number of inverters

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE19504271C1 (en) 1994-09-29 1996-02-08 Koeln Klaus Wilhelm Impedance measuring system for AS network
US6810339B2 (en) 1997-11-24 2004-10-26 Plug Power, Inc. Anti-islanding method and apparatus for distributed power generation
US7432618B2 (en) 2005-04-07 2008-10-07 Pv Powered, Inc. Inverter anti-islanding method
US20110115301A1 (en) * 2009-11-13 2011-05-19 Vijay Bhavaraju Method and area electric power system detecting islanding by employing controlled reactive power injection by a number of inverters

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
BOWER, Ward ; ROPP, Michael: Evaluation of islanding detection methods for photovoltaic utility-interactive power systems. In: Report IEA PVPS T05-09: 2002, 01.03.2003, 1-59. *

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Effective date: 20131126