Low-voltage ride through
The terms English Low Voltage Ride Through ( LVRT ), English Under-Voltage Ride Through ( UVRT ) or English fault ride through ( FRT ) ( dt. : Driving through undervoltage or traversing fault conditions ) describe in electrical networks , a minimum tolerance of electrical Generating units (GU) such as generators against brief voltage drops. In the event of a voltage drop, an EZE must not disconnect itself from the grid immediately, but must continue to work for a certain period of time. Short-term voltage dips can occur, for example, as a result of network faults such as B. short circuits , earth faults or lightning strikes . The FRT capability of decentralized generation plants is intended to prevent a large-scale loss of generation capacity through a domino effect of many smaller generation plants in the event of errors in the transmission network.
history
In the past, smaller decentralized generation systems (e.g. wind turbines , photovoltaic systems , small hydropower plants , combined heat and power plants, etc.) had to disconnect from the power grid immediately, even in the event of brief drops in the grid voltage. The energy supply was primarily ensured by larger power plants independent of smaller decentralized generation plants.
However, the widespread use and the increased share of the cumulative output of decentralized generation systems has led to the fact that this originally required immediate separation from the electrical network can lead to a destabilization of network operation. It can lead to a large-scale power failure if too much generator output is switched off at the same time and the system balance of generation and consumption can no longer be balanced in good time. This risk from extensive voltage drops, which are triggered, for example, by a short circuit in the transmission network, was z. B. thematized in 2005 by the dena network study I.
In Germany , therefore, the medium-voltage directive of the Federal Association of Energy and Water Management (BDEW) required from 2009 that decentralized generation systems (DEA) support the system stability of the electrical network by being able to "run through" voltage drops of up to several seconds in order to immediately after the Error condition to be fed back normally. In addition, the System Service Ordinance was issued in 2009, which gave existing systems a financial incentive to retrofit FRT capability.
In Austria, the static and dynamic network support z. B. required by TOR D4, item 7.1.2, for the medium-voltage network .
technical basics
For the electrical energy supply, network stability and security of supply are two very important aspects of the quality of supply . Due to the limited possibility of storing electrical energy, it is necessary in the transmission and distribution network to keep the power balance balanced. Parameters such as grid frequency as a global indicator and grid voltage as a local indicator in the distribution network indicate an excess or deficit of electrical power. Due to the strong expansion of decentralized electricity generation from renewable energy sources (e.g. wind and sun), these, like large power plants (e.g. coal-fired power plants , nuclear power plants ), must contribute to maintaining network stability.
If there is a fault in the transmission network , there is, among other things, an extensive voltage drop, the so-called voltage funnel. In larger interconnected networks and depending on the network topology, this voltage drop can extend over several hundred kilometers. So that decentralized generation plants in this area do not all shut down almost at the same time and thus suddenly a significant proportion of the generation output is missing, they must have a certain minimum robustness against voltage drops and feed power into the grid as usual after the error state has ended.
Usually, in the area of fully automatic network protection, as with distance protection, a maximum error clearance time of 150 ms is assumed. After the error has been cleared by automatically switching off the faulty network element, the effective value of the electrical voltage does not immediately rise back to the nominal value. This settling process can take up to a few seconds due to inductive effects . EZE therefore not only have to withstand an error of up to 150 ms in length, but also a longer voltage dip - but then at a reduced level. The FRT requirements, which are often shown as time-voltage curves, vary depending on the voltage level and network operator.
In addition, there is a requirement for decentralized generating plants based on converters to be able to feed in an inductive or capacitive reactive current . Rotating synchronous machines, as in large power plants, automatically react to a voltage drop with a supporting, sub-transient equalizing current due to the magnetic fields in the rotor and stator . Network operators can activate the fast reactive current control in the converter control so that power electronic DEA also contribute to the triggering of the network protection devices and also mitigate the voltage drop.
Norms
Examples of the underlying standards are:
- IEC TS 62786: 2017 Distributed energy resources connection with the grid
- IEC 61400-21
- IEC 61000-4-11 (EMC)
- IEC 61000-4-34
Verification
With the so-called Under-Voltage Ride Through Test (UVRT-T), the behavior of generating units in the event of voltage drops is theoretically and practically investigated and documented in the test setup in the high-voltage test field. Here, a voltage drop is ( English voltage dip simulated) at the terminals of the test unit, which switch off the EZE for this voltage drop without the Vorfehlerleistung further has to provide. To simulate the network fault, either a network simulator is used - apparent powers of up to a few 100 kVA are common - or for larger systems such as B. Wind turbines, a so-called FRT container is used, which contains an inductive voltage divider . In addition to the main unit, such as a synchronous machine or converter, the auxiliary units (fan, cooling, control, protection system, etc.) must also withstand the use of voltage without damage.
A type test is usually carried out for devices and EZE. Generating plants that pass the test can be certified. Network operators can request certification of the entire system from several GUs before the generating system is connected.
Certification
Certification of the generating plant as a whole or of individual devices confirms the ability to provide dynamic grid support and conformity with all requirements of the technical connection conditions. For both unit certification and system certification, a simulation model is created and calculated with which the behavior of the generating system or device (e.g. an inverter ) can be simulated in the event of a fault. The certificate is issued by accredited certification bodies who compare the test results of the testing institutes with the requirements of the grid connection conditions.
Individual evidence
- ↑ Decentralized generating plants or generating units are connected to a medium or low voltage network and are located a short distance from source to sink.
- ↑ A generating plant (GP) consists of one or more generating units (GU) as well as auxiliary plants and facilities for the grid connection. The EZE is the smallest inseparable installation for electrical energy generation. (VDE-AR-N 4105)
- ↑ a b Energy planning for the grid integration of wind energy in Germany on land and offshore by 2020 ( memento from October 17, 2017 in the Internet Archive ), 2005
- ↑ Text of the system service regulation
- ↑ Main section D4: Parallel operation of generating plants with distribution networks.
- ↑ a b J. Dirksen: Low-Voltage Ride Through , DEWI Magazine No. 43, August 2013, pp. 56-60
- ^ Clause 4.5 Immunity to disturbances