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Comparative study of semiconductor power losses between CSI-based STATCOM and VSI-based STATCOM, both used for unbalance compensation
Protection and Control of Modern Power Systems volume 5, Article number: 4 (2020)
In this paper, we present a detailed procedure to determine the semiconductor losses for both structures of a shunt STATCOM (Static Compensator), STATCOM based on Current Source Inverter (CSI) and STATCOM based on Voltage Source Inverter (VSI), both used for voltage unbalance compensation. As a first step, we study the VSI-based STATCOM and the CSI-based STATCOM used in high speed railway substations. Then we analyze the design and the sizing of the unbalance compensator in order to obtain an unbalance factor that does not exceed the limits imposed by the standards or by the energy provider. Following that, we compare the performances obtained with both structures VSI-STATCOM and CSI-STATCOM, after calculating the semiconductor power losses in the STATCOM converters. Finally, we validate our approach by simulation over real data of unbalance compensation caused by the new high-speed railway in Morocco. We use the tools MATLAB / Simulink/Simpowersys for performing our simulations.
The flexible alternative current transmission system (FACTS) is used in the power grid in order to improve the quality of energy. In particular, the static shunt compensators with voltage source inverters are installed in high-speed railway substations to compensate the unbalance [1,2,3].
The railway operator is mainly interested in reducing the compensator size, cost, and the converter losses; this is from where comes our idea to propose a replacement of the voltage source inverter structure by the current source inverter structure. To validate this proposition, we show the performances obtained by a CSI-based STATCOM used in the unbalance compensation. The rest of this paper is organized as follows:
In the second section, we give an overview of the principle of electric power supply system for high-speed railways (HSR). The third section presents the analysis of voltage unbalance generated by the railway substation. In the fourth section, we present the STATCOM structure used for unbalance compensation. While in the fifth section, we present our sizing methodology and our control strategy, for both STATCOM structure VSI and CSI. The sixth section, we show the detailed calculation of the semiconductor power losses in the converters. Then in the seventh section, we compare and discuss the obtained results for both structures (VSI-based STATCOM and CSI- based STATCOM) using data coming from railway operator in Morocco about the HSR substation.
Principle of electrification of high-speed railway
The HSR are electrified by a tow-phases system 2*25KV − 50 Hz (Fig. 1), where the single-phase transformer of the substation is connected between two phases of the HV power grid.
This system transmits the energy at 50 kV between the catenary and a complementary cable, called the “negative feeder”. A voltage of 25 kV between the catenary and the railis, is obtained by means of Mid-point autotransformer connected between the catenary, the rail, and the negative feeder. The first implementation of this solution was in 1981 on the HST South-East line in France .
Analysis of the unbalance caused by HSR
A three-phase system is balanced if the three currents or voltages have the same amplitude, the same frequency, as well as the difference between their phases is equal to 120°; otherwise the system is unbalanced. The HSR substations generate a significant unbalance in the high voltage lines with low short-circuit power (Scc), because they are more sensitive to the disruptions. Figure 2 shows the supply schema of a HSR substation by a three-phase line with line impedance (Zcc) .
The apparent power and the unbalance factor for HSR substations:
Compensation of the unbalance by STATCOM
Principle of the STATCOM
The static compensator (as shown in Fig. 3) is a three-phase sinusoidal AC current source that injects the negative sequence of currents to compensate the unbalance at the PCC point. This injection of current is made by the STATCOM which is based on power electronics converters [2,3,4, 5, 6].
For total compensation:
The apparent power of STATCOM used for unbalance total compensation is: .
For partial compensation:
In order to size the STATCOM, in first step, it is necessary to determine the percentage of the limit RMS value of the current negative component to be injected:
The apparent power of STATCOM used for unbalance partial compensation is given by:
Voltage source inverter based STATCOM (VSI-STATCOM)
A compensator STATCOM based on a VSI (see Fig. 4) is composed of several compensation cells coupled in parallel with the secondary of the three-phase transformer with Yy0 connection. Each cell contains an AC filter inductor (L), a VSI converter with sine-triangle PWM control, and a DC side voltage source made by a capacitor (Cdc). The VSI-STATCOM and the HSR substation (unbalanced load) are connected to the PCC bus
In general, the apparent power of the compensator (Sc) is equal to the number of cells (NVSI) multiplied by the power of each cell (Scel):
Current source inverter based STATCOM
A CSI-STATCOM (see Fig. 5) is composed of several compensation cells coupled in parallel to the secondary of the three-phase transformer with Yy0 connection. Each cell contains an AC LC filter, a CSI converter with sine-triangle PWM control, and a DC side current source realized by an inductor (Ldc). The CSI-STATCOM and the HSR substation (unbalanced load) are connected to the PCC bus.
In general, the apparent power of compensator (Sc.) is equal to the number of cells (NCSI) multiplied by the power of each cell (Scel):
Sizing and control strategy of STATCOM
Sizing of VSI-STATCOM cell
The power circuit of the VSI-STATCOM cell is composed of a six bidirectional current switch (IGBT with anti-parallel diode), a capacitive storage source in a DC side inverter, and a three-phase inductor filter connected between the AC side inverter (a’,b’,c’) and the transformer secondary (a,b,c). The IGBT switch is controlled by the PWM signal (see Fig. 6).
The voltage and current supported by IGBT are given by the flowing expressions :
The VSI_STATCOM cell apparent power is given by:
Sizing of the CSI-STATCOM cell
The power circuit of the CSI-STATCOM cell is composed of six unidirectional current switches (only IGBT or IGBT with anti-parallel diode and diode in series), an inductor storage source in the DC side inverter, and a three-phase L’C filter connected between the AC side inverter (a’,b’,c’) and the transformer secondary (a,b,c). The IGBT switches are controlled by the PWM signal (see Fig. 7).
The DC inductor and the L’C filter are given by the flowing expressions: .
The voltage and current supported by IGBT are given by the flowing expressions: .
The CSI-STATCOM cell apparent power is given by:
The fundamental component of the AC side current (IO) is given by: .
Control of VSI_STATCOM cell
The shunt STATCOM must inject the negative sequence of the current consumed by the single-phase load (High speed railway substation) in the transformer’s secondary. According to equations of Table 1, the block diagram for the set-point generation of the injected currents is shown in Fig. 8 [9, 13].
The VSI-STATCOM cell is controlled by two loops: The first loop controls the injected current (Iinj) in the d-q plan (park transformation) with PI controller RI(p) (see Fig. 9), the second loop regulates the DC bus voltage (Vdc) around its reference value with a proportional delay controller Ru(p) (see Fig. 10). The generation of three-phase PWM control signals from the corrected sinusoidal modulating signals (sine) is shown in Fig. 11.
The transfer functions of controllers used in the control part are given by :
The transformation matrices (Park & Clark) are given by:
Control of CSI-STATCOM cell
The CSI-STATCOM cell is controlled by two loops: The first loop controls the injected current (Iinj) in the d-q plan (park transformation) with PID controller RI1(p) (see Fig. 12); the second loop regulates the DC bus current (IDC) around its reference value with a proportional delay controller RI2(p) (Fig. 13). We use the same block diagrams (Figs. 8 and 11) for the injection of set-point currents and the generation of three-phases PWM signals,
The transfer functions of controllers used in the control part are given by :
The transfer functions of the physical system for CSI-STATCOM in dq plan are given by :
Calculation of power losses in semiconductors
Analytical expressions of power losses in semiconductors
The IGBT module is a bidirectional current switching device. It is composed of a bipolar transistor with an insulated gate and an anti-parallel diode which allows bidirectional current (VSI cell case) (Fig. 14 (a)). The IGBT combines the advantages of a bipolar transistor (high voltage, high intensity) and a MOSFET (fast switching).
The diode module added in series with the IGBT module ensures the unidirectional current circulation (CSI cell case) (Fig. 14 (b)).
Serial diode (for CSI):
Power losses calculation for VSI cell
If we take a single-phase AC side of the VSI cell as an example, the both power switches (IGBT module) in the arm (TOP and BOTTOM), switches in a complementary way, and they have the same losses. The Fig. 16 shows the waveform of the current in the IGBT transistor and its anti-parallel diode.
The modulating signal and the injected current for one phase in the VSI cell are expressed as follows:
With a’ is a phase reference.
The collector current of IGBT is:
The IGBT transistor is conducting in the interval [φ;φ + π], while the antiparallel diode is conducting in the interval [φ + π; φ + 2π]. Therefore, the expressions of the peak, RMS, and AVG values of the IGBT currents and the ant parallel diode current are  (see Table 2):
Power losses calculation for the CSI cell
Application and simulation (case study)
The simulation has been performed with the real parameters of the IGBT module of the manufacturer ABB (VCE.r = 4.5KV / IC.r = 1.2KA, ref.: 5SNA1200G450300) and the diode module of the manufacturer SEMIKRON (VRRM.r = 2.2KV / IFAV.r = 1.37KA, ref.: SKKE 1200/22 H4). The Table 4 presents the characteristics of these semiconductors.
We choose one substation among others of the new Moroccan HSR as a study case (see Fig. 17). This substation has the following characteristics (see Table 5) . The unbalance (see Fig. 18) caused by this substation exceeds the limit of 2% imposed by the standard NF EN50160 . Soit is interesting to install an unbalance compensator in this substation.
Sizing of the unbalance compensators substation (PK100)
In this subsection, we calculate the compensator power with a total unbalance compensation for each type of the converter (VSI and CSI). The calculation results are presented in the Table 6.
Table 7 shows the values of the parameters calculated for the power circuit of each cell (VSI and CSI):
Table 8 depicts the values of the controller parameters used for control of each cell (VSI and CSI):
Simulation of unbalance compensation
Calculating of power losses in full load
The Table 9 presents the conduction and switching power losses for each structure of STATCOM (VSI and CSI):
If considering the active power losses in the AC side filters, and in the DC side storage circuit, the Table 10 presents the global power losses for STATCOM:
Simulation of power losses in dynamic power variation
Figure 21 shows the dynamic evolution of the global power losses according to the variation of the power consumed by the substation during railway traffic.
Discussion and evaluation of the results
In sizing terms, the calculation results in Table 10 show that the unbalance compensator with a voltage source inverter (VSI) needs up to 24 converter cells, but the one with a current source inverter (CSI) needs only up to 9 converter cells. This is the advantage of STATCOM with CSI cells; which is the congestion in the railway substation, and the reduced costs.
In terms of power losses and efficiency, the calculation results in (Table 10) show that, the compensator based on CSI (4.75 MW, 92.56%) is more optimal than the one based on VSI (14.52 MW, 80.51%), in steady state with full load condition. In addition, Fig. 21 demonstrates that the STATCOM with CSI cells has less loss than the one with VSI in dynamic daily railway traffic. Thus, we conclude that the unbalance compensation with CSI cells is an optimal solution for railway operator.
According to the sizing procedure described in this paper, the use of STATCOM compensator is much more required when daily railway traffic reaches a certain value, especially in rush hours. This compensator implies a big investment for the railway operator. For this reason, the designer engineering office tries to choose the optimal solution in terms of cost and efficiency. The obtained results show that the VSI-STATCOM has lower efficiency then the CSI-STATCOM (80,51% VS 92.65%) with full load (rated conditions). The CSI-STATCOM presents a reduced number of converter cells compared to the VSI-STATCOM which reduces the size and the cost of the compensator while keeping the unbalance factor lower than the limit of 2% imposed by the standard. We conclude so that the STATCOM based on CSI is an optimal solution in high power compensation.
The experimental validation for this comparative study makes the objective of a second paper.
a, b, c phase indexes
+, −positive and negative sequence indexes
ScCompensator apparent power
SccShort-circuit apparent power
SssApparent power consumed by substation
Sc.totalApparent power for total compensation
Sc.partialApparent power for partial compensation
Scellconverter cell apparent power
V HVline-ground rated voltages
VpccHV line-ground voltages in PCC
UHV line-line RMS voltage
VDC.refDC voltage set-point for VSI
ΔVDCvoltage ripple in DC side for VSI
VCEIGBT collector-emitter voltage
VFDiode forward voltage
VLine-ground in filter capacity for CSI
Isw and Vswcurrent and voltage of the semiconductor switch
ITCurrent consumed by train
iReturn current to substation
IssCurrent consumed by substation
IHV line phase current
Iinjinjected current by compensator
ICIGBT collector current
ΔIinjinjected current ripple in AC side inverter for VSI
Tivvoltage unbalance factor
NVSInumber of voltage source inverter cell
NCSInumber of current source inverter cell
ω0Natural frequency of RLC filter for CSI
ωrpower grid pulsation
ZccLine impedance in PCC
rondynamic resistance diode
Availability of data and materials
Data sharing not applicable to this article as no datasets were generated or analyzed during the current study.
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The authors would like to thank the anonymous reviewers for their helpful and constructive comments that greatly contributed to improving the final version of the paper. They would also like to thank the Editors for their generous comments and support during the review process.
The work is not supported by any funding agency. This is the authors own research work.
The authors declare that they have no competing interests.
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Benslimane, A., Bouchnaif, J., Essoufi, M. et al. Comparative study of semiconductor power losses between CSI-based STATCOM and VSI-based STATCOM, both used for unbalance compensation. Prot Control Mod Power Syst 5, 4 (2020). https://doi.org/10.1186/s41601-019-0150-4
- Power losses
- Unbalance compensation