The origin of this article was the practical experience related to the installation of four phase shifting transformers, rated at 1,200 MVA each, at the Mikułowa substation. Phase shifting transformers with such rated power that have not been used in the NPS before. Yet they are operated in foreign power systems, e.g. in Germany – at Diele substation. The phase shifting transformers (PSTs) have been installed in two 400 kV Mikułowa – Hagenwerder international exchange lines to control the active power flow. The PSTs (Fig. 1) are practically symmetrical (sides: Source – S and Load – L) and each of them consists of two separate transformers: series and exciter.
Both windings of the exciter transformer are star-connected. Its primary winding is powered from the grid and the secondary winding interoperates with the on-load tap-changer. The series transformer’s secondary winding, which generates the booster voltage, is connected in series with the line, in which the voltage phase shift is controlled. The full line current flows also through the winding. The series transformer’s primary winding is, however, delta-connected (which provides a voltage shift of π/2) and is powered from the exciter transformer’s control winding .
The control of active power flow through the phase shifting transformers consists in changing the flow without changing the aggregate power output to the grid. The known dependency is used here, which determines the active power flow in a single inductive branch. It has the following form :
where: P – active power outgoing from the branch, Ui, Uj – at beginning and end of the branch, δ – load angle (difference of node voltage arguments at the beginning and end of the branch, δ = δi – δj).
The active power flow control with phase shifting transformers consists in changing the load angle by changing the tap position in the exciter transformer’s secondary winding. The phase shifting transformers at Mikułowa substation have 65 control steps (±32 taps), with which the value and direction of the active power flow in MIK-HAG (Mikułowa-Hagenwerder) line can be changed.
They are provided with two interoperable Reinhausen tap changers. The first on-load tap changer (OLTC) adjusts the tap positions in the range of 0... 32 and is located at the exciter transformer (Fig. 1). The second ARS (Advance Retard Switch) is responsible for widening the control scope from Advance – the direction of increasing active power import to Retard – increasing exports. The active power flow directions and values depending on the ARS and OLTC tap-changer positions are shown in Fig. 2.
To change the Advance/Retard direction, OLTC taps should be changed one by one in the direction of tap 0. The Advance/Retard control direction will change automatically upon OLTC transition from 0R to 1A or from 0A to 1R . Changing OLTC tap positions should be continued until the desired position is reached.
Fig. 1. Connections between the exciter and series transformers of the Siemens Weiz phase shifter in Mikułowa substation 
Fig. 2. Tap changer adjustment for active power flow control. Hence, the maximum active power import can be obtained in position 32A, and the maximum export in position 32R
2. Mikułowa node (MIK) specification
Mikułowa 400/220/110 kV substation is located near the Polish-German border in the vicinity of Turów Power Plant (ca. 30 km away). The substation is directly connected with two 400 kV lines (36 km) with Hagenwerder substation in Germany. In the line’s circuit labelled as HAG567, two PST1 and PST2 phase shifting transformers are installed in series. Similarly, two phase shifter PST3 and PST4 are installed in the line’s HAG568 circuit. The 400 kV switchgear is also connected with a single line to Czarna substation (Fig. 3).
Fig. 3. MIK control node topology
At present generators G2, G3, G4, G5, and G6 are connected to the Mikułowa 220 kV switchgear, and generator G1 is connected to the Turów Power Plant 110 kV switchgear.
At 400/220/110 kV Mikułowa substation AT1 and AT2 400/220 kV, 500 MVA autotransformers, and AT3 and AT4 220/110 kV, 160 MVA autotransformers are operated.
There are also two 500 MVA phase shifting transformers TD1 and TD2. TD1 and TD2 are located on the side of the lower 220 kV winding of autotransformers AT1 and AT2, respectively.
3. SSPF phase shifting transformers control system
The SSPF monitors the status of the 567 and 568 lines’ topologies and the SP coupler in the Mikułowa substation (Fig. 4) and matches its mode of operation to the current operating condition (switches, measurements from current and voltage transformers). Circuits for the current and voltage measurements and the switch status representation are output directly to the SSPF (Fig. 5). Likewise, the PST1–PST4 tap positions are controlled directly through the SSPF output contacts .
Fig. 4. Current and voltage transformers and switches, whose status is forwarded to SSPF
Fig. 5. SSPF hardware configuration
The SSPF is the source of the signals which allows for switching on the 567 and 568 lines and the coupler. These signals from SSPF are used by the SSiN system. For external technical reasons, no measurements and representations from the Hagenwerder substation side are available to the SSPF. For this reason, it was assumed that the HAG567 and HAG568 lines are permanently connected at the Hagenwerder 380 kV switchgear.
- EpsP – dead zone of active power setpoint Pzad
- DeadTime – time [s], for which SSPF’s automatic operation is stopped
- RatioDif – permissible tap ratio difference between lines
- UnderVLock value – undervoltage lock value
- OverVLock value – overvoltage lock value
- OvercurrentLock value – overcurrent lock value.
The system performs control tasks based on the control parameters set locally from the SSPF terminal or the substation’s SSIN computer system terminal, or remotely from the regional/ national power dispatch level using the DYSTER system. The SSPF terminal in the station serves to locally enter the control setpoints, to control, and graphically render the SSPF control process performance. The presentation of the results in the simplified SSPF functional diagram (SSPF control screen – Fig. 6) consists in the display of the actual voltages, power, tap numbers, ARS positions, setpoints, messages and signalling. Detailed readings of the measurement from transformers in individual bays are shown on the measurement screen (Fig. 7).
Fig. 6. SSPF control screen
Fig. 7. Screen of all available SSPF measurements
The SSPF operation is remotely controlled and supervised from the regional/national power dispatch level through the DYSTER system terminal with the same SSPF handling functionality as that of the local terminal in the substation.
The connection of the substation’s SSPF system with SSiN is redundant and enables the transmission of the control process related data and the retrieval of SSiN setpoint data for the control process. The SSNN provides the communication with the regional/national power dispatch level.
4. SSPF operation description
The SSPF system is designed for automatic management of the Mikułowa substation operation in the following areas:
- maintaining a setpoint of active power flow through the substation’s phase shifting transformers – criterion P
- control of the phase shifting transformers tap positions (setting the PSTs tap number setpoint and/or tap position control up/down) – criterion Z.
No asymmetric PST operation (1-0, 2-0 and 2-1) is permitted in the SSPF automatic adjustment mode. The criterion P (according to active power setpoint Pzad) is the basic condition of automatic operation. With criterion P, the control system (SSPF) task is to maintain the active power setpoint Pzad for the MIK node, and to maintain the appropriate flows in 400 kV MIK-HAG lines, e.g. to avoid circular power flows.
No PST operation with various automatic (with SSPF) and manual (without SSPF) control modes is allowed. When this happens, then PF, which is in automatic mode, will be automatically excluded from it by SSPF. Example: Both PSTs are set for automatic operation in each line; if then one of them is switched to manual operation, then the other PSTs automatic operation will be blocked.
The SSPF detects undesired operating conditions of the phase shifters operating in HAG 567 and HAG 568 lines. Upon detection of such a condition, the SSPF generates the warning signals shown below (Fig. 7):
1. Unsymmetrical operation of HAG 567/568 lines – notice of asymmetry (1–0, 2–0, 2–1) operation of HAG 567 and HAG 568 lines
2. Unsymmetrical configuration of HAG 567 line – notice of asymmetry of switched-off HAG 567 line with respect to switched-on HAG 568 line
3. Unsymmetrical configuration of HAG 568 line – notice of asymmetry of switched-off HAG 568 line with respect to switched-on HAG 567 line
4. RatioDif overrun for HAG567/568 – notice of aggregate mismatch of taps between the lines
5. RatioDif overrun for PF1-PF2 – notice of mismatch of the taps of PF1 and PF2 phase shifters (in HAG 567 line)
6. RatioDif overrun for PF3-PF4 – notice of mismatch of the taps of PF3 and PF4 phase shifters (in HAG 568 line).
SSPF operation rules under criterion Z
The SSPF enables a change of the PSTs OLTC tap positions by setting the tap number or moving the tap up/tap down arrows. The OLTC tap number setpoints range from –32 to +32 (from 32 Retard to 32 Advance).
The SSPF (for all PSTs) performs sequential control simultaneously in pairs: PST1 and PST3 and PST2 and PST4.
1. With one, either HAG 567 or 568, line in operation, when taps are changed by setting a tap number in a phase shifting transformer in a line, the SSPF transfers the tap position setpoint also to the other PST in the line.
2. With both HAG 567 and 568 lines in operation, when taps are changed by setting a tap number for a PST in one of line, the SSPF transfers the setpoint also to all other PSTs in the both line circuits.
3. With the tap up/tap down arrows, taps are changed on each PST individually. Any tap number change so affected also changes the setpoint of the PST.
4. The SSPF prevents setting a tap number for a change larger than by one tap step – such a larger change would require subsequent setting of setpoints by one higher than the current tap position.
SSPF operation rules under criterion P
The SSPF system allows to change the actual active power flow in HAG567 and HAG568 lines by pre-setting its setpoint value Pzad. The power setpoints range from –1,170 MW to +1,170 MW. A power setpoint is assigned to each of HAG567 and HAG568 lines (Fig. 6). When the lines are operated in parallel, entering a new setpoint for either line assigns it to the other line too.
The actual power flow through a line (PST) may be different from the setpoint by the SSPF system’s deadband, which is epsP +/–30 MW. For example, for power setpoint 1,170 MW the resulting power flow through the shifter will range from 1,140 MW to 1,200 MW. To change the active power flow direction, the power setpoint should be entered with the appropriate sign, “+” for imported power and “-” for exported power, respectively.
The SSPF changes the tap positions by one at a time towards the set setpoint. The Advance/Retard control directions toggle automatically upon a tap change from 0R to 1A or 0A to 1R by the OLTC. Then the SSPF keeps on changing the OLTC taps until the power setpoint is reached.
The SSPF (for all PSTs) performs sequential control simultaneously in pairs: PST1 and PST3, and PST2 and PST4, changing their position by one at a time.
5. Results of phase shifter performance tests
The active power flow control with phase shifting transformers was tested on May 16, 2016 as part of the SSPF acceptance tests. Four PST1–PST4 were operated during the tests. Both HAG567 and HAG568 were operated in parallel – connected to the R400 kV Mikułowa and R380 kV Hagenwerder switchgear.
The first test consisted in a change of PSTs taps from 0 to 20 under criterion Z. The test is shown in Fig. 8. The resulting aggregate (HAG567 and HAG568) ranges of changes in the active and reactive power flows were 700 MW and 80 MVAr, respectively. The resulting voltage change at the R400 kV Mikułowa switchgear was 6.5 kV.
Fig. 8. Criterion Z – tap position changes from 0 to 20
The next test consisted in changing the criterion from Z to P, and setting power setpoint Pzad = 500 MW for each line. This enforced the reverse change of the PST taps from position 20 (Fig. 9). Once the expected active power flow had been reached (with the accuracy up to dead band ±30 MW), the setpoint Pzad was changed to 400 MW. This was to force the PST tap position change to the initial zero. When the PST zero tap had been reached, the setpoint Pzad was changed to 450 MW to stop the control under criterion P.
Fig. 9. Criterion P – tap position changes from 20 to 0
Then the control under criterion Z was tested, with negative tap positions (ARS switch position = Retard) from 0 to –10. The resulting aggregate (HAG567 and HAG568) ranges of changes in the active and reactive power flows were ∆P = 400 MW and ∆Q = 30 MVAr, respectively (Fig. 10). The resulting voltage increase at the R400 kV Mikułowa switchgear was ∆U = 3 kV.
Fig. 10. Criterion Z – tap position changes from 0 to –10
The last test consisted in enforcing the upward change of PST taps when operated under criterion P (Fig. 11) during PST operation. For this purpose, the new active power flow setpoint Pzad = 350 MW was set for both lines.
Fig. 11. Criterion P – tap position changes from –8 to –2
Once the pre-set active power flow setpoint had been reached (with the accuracy up to dead band ±30 MW), the Pzad setpoint was changed to 400 MW. To conclude the control under criterion P, Pzad = 440 MW was set. Finally, during the test, the range of PF tap numbers change from –8 to –2 was accomplished.
The installation of four PSTs in the international exchange lines allows efficient control of the active power flow between the grids managed by the Polish grid operator (PSE SA) and the German grid operator (50Hertz). The SSPF system implemented by the Institute of Power Engineering is an effective tool for the automatic control of the active power flow. In addition, the SSPF monitors the operating status of all PSTs and counteracts the uncontrolled switching operations of HAG567 and HAG568 lines on the Mikułowa substation side.
The completed tests allowed verifying the estimated control capacity of the PSTs. Under the test conditions, the average change by. ca. 40 MW/tap and 0.35 kV/tap was achieved. In addition, it has been observed that with an active power flow change also the reactive power flow changes by ca. 10%.
The actual conditions and grid constraints did not allow to test the PST control capacity over the full tap adjustment range (from –32 to 32).
The test results and the 50Hertz operator’s experience with similar phase shifters in Diele substation are sufficient to adopt linear extrapolation of the obtained characteristics for the remaining PST control range.