Phase shifting transformer
A phase shifter transformer , also called cross-regulating transformer , is a special power transformer which, in the field of electrical AC networks , serves to specifically control the electrical load flow in parallel lines. These transformers are used in high-voltage networks such as the 220 kV or 380 kV voltage level. The apparent power is up to 1,500 MVA .
General
In contrast to the usual application of transformers , the conversion of alternating voltages to different voltage levels , these transformers serve as phase shifters in order to specifically influence the power through an electrical line, for example an overhead line . If several lines are routed on different routes between two switchgears or substations , a phase-shifting transformer can be used to determine how the power is divided. This is particularly important if the existing lines are assigned to different voltage levels, have significantly different transport capacities or overhead lines are combined with underground cables .
construction
As shown in the diagram on the right, a phase shifting transformer consists of a series transformer, similar to a current transformer , and an excitation transformer ("shunt transformer"), which can be used to set a specific phase shift using a tap changer .
With the three-phase alternating current that is usually used, a series transformer and a variable transformer are available for each outer conductor . The AC voltage is supplied on the left via the connections L1, L2 and L3. Via the variable transformer, a voltage is tapped per phase via a switch, which is offset by 90 ° from the phase-to-phase voltage to earth and, via the series transformer shown on the right, leads to a phase-shifted voltage at L1 ', L2' and L3 ' by means of vector addition . This type of load flow influencing is also known as cross compensation , in contrast to the longitudinal compensation by means of chokes or capacitor banks , as found in the static reactive power compensation (SVC). Since the load flow through the phase shifter transformer can take place in both directions, "input" or "output" can in principle be selected as desired.
The setting range of the phase-shifted voltage differs depending on the type. It is typically in the range of ± 10 ° and can be up to 30 ° in special designs. With real phase shifting transformers (there are several circuit variants) additional components are used, e.g. B. an Advanced Retard Switch (ARS) on the series transformer to reverse the sign of the phase position.
The phase shifter transformer creates an additional load flow in the mesh (loop), which in the simplest case is formed by two parallel lines, which is superimposed on the external load flow through the two lines. Depending on the set phase shift, this leads to a reduction or increase in the load flow in the individual lines. If the external power flow is reversed, the phase position must be inverted while maintaining the power distribution.
application
The single-line diagram shown on the right shows the distribution of the load flows between two lines with different settings on the phase-shifting transformer. The power fed in at the generator and the power withdrawn are the same in both cases, for example 100 MVA in each case, regardless of the losses . These are exemplary numerical values; the shifted active power, measured in MW , at a given phase angle depends on the electrical properties of the phase shifter and the transmission system.
The power flows and thus the currents in the two lines can now be adjusted using the phase shifting transformer. In the left part of the picture, the phase angle is selected so that the same power of 50 MW is transmitted over both lines. In the right part of the figure, the phase angle at the transformer has been changed, which means that 73 MW are transported on one line and 27 MW on the other. The sum corresponds to the total output of 100 MW each time.
In real systems, additional losses, not shown here, occur due to the thermal losses of the phase shifting transformer and additional line losses on the line with the greater power flow, which in this case reduces the power that can be drawn compared to the power fed in. In addition, in real energy supply networks there are usually not only two parallel lines between two substations, as in this simplified example, but the meshing in an interconnected network results in further reciprocal influences on the load flows.
Installed systems
Installed systems are located in the 400 kV network between the Netherlands and Belgium , on the border between Germany and the Netherlands, in the German network in the Diele substation and in the Austrian Power Grid (APG) 220 kV network in the Ternitz substation , Transformer station Ernsthofen and transformer station Tauern as well as in the 110 kV network in the transformer station Hagermarsch , where the submarine / underground cable connection of the offshore wind farm alpha-ventus is fed into the overhead line network.
On January 17, 2017, the Czech transmission system operator ČEPS put 2 phase shifting transformers into operation in the Hradec substation . Two more transformers are to follow by the middle of the year. Each of the two transformers weighs 300 tons. The total costs for this are around € 75 million. The installation of the phase shifting transformers was necessary to better regulate and control the flow of green electricity from Germany. The German transmission system operator 50 Hertz put two phase-shifting transformers into operation in the Röhrsdorf substation in January 2018. The Röhrsdorf and Hradec substations are connected via two 400 kV lines. When there is a high level of electricity production in Germany, there are always unplanned flows of electricity that place heavy loads on the Czech network and endanger the safe operation of the network .
Web links
- Technical data of a phase shifting transformer ( Memento from January 21, 2005 in the Internet Archive )
literature
- Rene Flosdorff, Günther Hilgarth: Electrical energy distribution . Teubner, 2003, ISBN 3-519-26424-2 .
Individual evidence
- ↑ Phase shifting transformers in APG , ew - magazine for the energy industry, volume 106, 2007, issue 11
- ↑ a b Czech Republic puts a barrier against German green electricity into operation. www.finanzen.net, January 17, 2017, accessed January 20, 2017 .
- ↑ a b Czech Republic puts first phase shifter into operation. www.energate-messenger.de, January 17, 2017, accessed on January 20, 2017 .
- ↑ Electricity flows to the Czech Republic have improved. www.zfk.de, January 19, 2018, accessed on January 22, 2018 .