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DC hybrid distribution networks
Vol. 1 No. 3 Aug. 2018 DOI: 10.14171/j.2096-5117.gei.2018.03.012
Global Energy Interconnection www.geidco.org Full-length article
Research on power electronic transformer applied in AC/DC hybrid distribution networks Yiqun Miao1, Jieying Song2, Haijun Liu2, Zhengang Lu2, Shufan Chen1, Chun Ding1, Tianzhi Cao3, Linhai Cai2, Yuzhong Gong 4 1. State Grid Shanghai Municipal Electric Power Company, Shanghai 200122, P.R. China 2. State Key Laboratory of Advanced Power Transmission Technology, Global Energy Interconnection
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Research Institute, Beijing 102209, P.R. China 3. State Grid Jibei Electric Power Company, Beijing 100053, P.R. China 4. Department of Electrical and Computer Engineering, University of Saskatchewan, Saskatoon, SK, Canada
Abstract: The AC/DC hybrid distribution network is one of the trends in distribution network development, which poses great challenges to the traditional distribution transformer. In this paper, a new topology suitable for AC/DC hybrid distribution network is put forward according to the demands of power grid, with advantages of accepting DG and DC loads, while clearing DC fault by blocking the clamping double sub-module (CDSM) of input stage. Then, this paper shows the typical structure of AC/DC distribution network that is hand in hand. Based on the new topology, this paper designs the control and modulation strategies of each stage, where the outer loop controller of input stage is emphasized for its twocontrol mode. At last, the rationality of new topology and the validity of control strategies are verified by the steady and dynamic state simulation. At the same time, the simulation results highlight the role of PET in energy regulation. Keywords: AC/DC hybrid distribution network, Power electronic transformer (PET), Clamping double sub-module (CDSM), Energy router.
1 Introduction The penetration of large-scale distributed generations (DGs) and the development of diversification of loads pose significant challenges for the traditional distribution Received: 15 March 2018/ Accepted: 12 April 2018/ Published: 25 August 2018 Yiqun Miao [email protected]
Chun Ding [email protected]
Jieying Song [email protected]
Tianzhi Cao [email protected]
Haijun Liu [email protected]
Linhai Cai [email protected]
Zhengang Lu [email protected]
Yuzhong Gong [email protected]
Shufan Chen [email protected] Open access under CC BY-NC-ND license. 396
network to fulfill reliability and efficiency requirements. AC/DC hybrid distribution networks have advantages in a lot of aspects, such as accommodating more DGs and reducing the investment of construction of power grid [1]. Therefore, it should be an essential part of the development of distribution networks in the future. With the development of renewable energies and more and more DC loads, traditional distribution transformers have been unable to meet the demands of flexibility, efficiency and power quality [2, 12]. Power electronic transformers (PET) are based on power electronic converters and high-frequency transformers (HFTs). Besides, the traditional functions of voltage transformation and electrical isolation can reduce volume and weight of core materials, compensate for reactive power, and ensure that the primary current and power factor can be controlled [3-4]. The outstanding advantages of a PET used in AC/DC hybrid distribution
Yiqun Miao et al. Research on power electronic transformer applied in AC/DC hybrid distribution networks
networks lie in its capability of controlling power flow between its AC and DC ports, which are helpful to accept sources loads and enhance the flexibility of distribution network. Therefore, it can be seen that the PET plays the role of energy router to decrease the number of converters, coordinate the distribution of power and improve the economy and reliability of power grid. Research on PET is currently focused on topologies and control strategies. On unbalanced-load handling capability, reference [5-6] made a comparison of two PET topologies based on H-bridge. However, DC bus cannot be led out from the H-bridge based PET structure, which is not convenient to accept DC loads as well as DGs. Reference [7] referred the topology of PET based on modular multilevel converter (MMC) and proposed a resonant PET used in power grid. MMC based PET structure adopts the halfbridge sub-modules without the capability of isolating DC fault. The topologies above are the hotspots in current studies, but none of them are totally suitable for AC/DC hybrid distribution networks. Consequently, the research on PET for AC/DC hybrid distribution networks is necessary. Traditional MMC has no DC fault isolation capability, under double pole DC short circuit fault, MMC would bear the AC short circuit current until the AC breaker opens [13-15]. Furthermore, in the existing of conventional DC distribution schemes, the DC breaker may be necessary to cut the DC fault current, which would increase total cost of AC/DC hybrid distribution networks. In this paper, a new PET topology with three stages is proposed, which can effectively cut off short circuit current shortly after fault.
U sa
2 A new topology of PET and the structure of AC/DC distribution network 2.1 A new topology of PET In order to meet the demands of AC/DC distribution networks, this paper puts forward a new PET topology, as it is shown in Fig. 1. The PET uses three-stage structure, including the clamping double sub-module (CDSM) of input stage, DC-DC converters of isolation stage and the DC/AC modules as well as DC/DC modules of output stage. Where Usa , Usb and Usc are phase voltages of 10 kV AC distribution network respectively, Larm is the inductance of each bridge leg and R is the clamping resistor on the side of medium voltage DC. The input stage is connected to the high voltage AC power network and the DC side of this stage accesses the medium voltage DC network. The DC-DC converters of isolation stage are connected in parallel with 750 V lowvoltage DC network. The DC/AC modules of output stage are used to supply power for the low voltage AC load, and the DC/DC modules are for the low voltage DC load. Fig. 2, 3 and 4 are respectively the topologies of CDSM, the DC / AC module of output stage, and the DC/DC module of output stage.
CDSM2
CDSM 2
CDSM 2
CDSM n
CDSM n
CDSM n
Larm
Larm
U dc1
C1
C2
HFT
Udc 2
CDSM 1
Hbridge
CDSM 1
Hbridge
CDSM1
L arm
Isolation stage
f 10 kV DC
Input stage
10kV AC
Then the control and modulation strategies of PET are designed in detail. At last, the rationality of the design and the function of PET in power flow regulation in AC/DC distribution networks are verified by simulation.
U dcL
C3
1
750 V DC
DC/AC R
U sb DC - DC converters
U dc
U sc Larm
Larm
Larm
DC/DC
Output stage
R
CDSM 1
CDSM 1
CDSM 1
CDSM 2
CDSM 2
CDSM 2 C2
CDSM n
k
Udc2
CDSM n
Hbridge
CDSM n
Hbridge
C1 U dc1
C3
f 10 kV DC
Fig. 1 PET topology applied in AC/DC hybrid distribution network 397
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Global Energy Interconnection
D31
A
T21 + −
T12
D12
Csm T0
D21
+ Csm
−
D0
D22
T22
D32
B
Fig. 2 The topology of CDSM
D3 T5
D1 T3
D5 T7
D7
L
750V DC
T1
T2
D2 T4
D4 T6
D6 T8
D8
n
a b c
C
750V DC
Fig. 3 The topology of DC/AC module of output stage
c11
c12
T1
T2
D1
D2
L
DC load
C
Udcload
Fig. 4 The topology of DC/DC module of output stage
Usa
Usb
Usc HFT
Module1
Module1
Module2
Module2
Module2
Modulen
Modulen
Modulen
Hbridge
Fig. 5 The topology of PET based on H-bridge 398
C2 Udc2 C3
Hbridge
Module1
Hbridge
D11
Hbridge
T11
The input stage of PET is used to convert the 10 kV AC voltage to a DC voltage. The structure of MMC based on CDSM is called C-MMC. Each CDSM is equivalent to two cascaded half-bridge modules (SM) during normal operation. The DC-DC converters of isolation stage including inverters, high frequency transformers and rectifiers are used to convert the DC voltage got from input stage to a 750 V DC voltage. The inverters of isolation stage are in series and the rectifiers are in parallel. The amount and type of converters of output stage are decided by the load at the side of low voltage. One or more DC/ AC modules and DC/DC modules can be connected to the 750 V DC bus. Reference to the grounding modes in the flexible DC transmission system, the clamping resistance grounding mode is applied in AC/DC distribution network as shown in Fig. 1. Compared with the topology of cascaded H-bridges shown in Fig. 5, the new topology shown in Fig. 1 cannot only reduce the quantity of high frequency transformers, but also improve the quality of the output voltage, while greatly satisfying the needs of AC/DC hybrid distribution networks. Take the example of 10 kV network mentioned in this paper, the following formula is true for the topology of cascaded H-bridges: m Ea = U dc (1) 2 If the maximum operating line voltage is 12 kV, m that represents voltage modulation ratio is 0.85, and the rated voltage of power unit (H-bridge) is 2 kV, then 6 cascaded units (only consider the H-bridge of input stage and DC-DC converter) are needed for each phase.
C2 Udc2 C3 C2 Udc2 C3 C2
Udc2 C3
3n
Yiqun Miao et al. Research on power electronic transformer applied in AC/DC hybrid distribution networks
10 kV AC main network 2 10 kVac
PV
10 kVdc
PET1
PET2
LOAD 2
DG
DC load
AC load
LOAD 1
As shown in Fig. 6, two terminals of 10 kV AC distribution networks are connected by PET and DC lines to constitute the hand in hand structure of the AC-DC hybrid distribution network. The low voltage port of PET can be connected with different types of AC or DC loads according to actual needs. The centralized photovoltaic connection into DC distribution network is realized by the DC/DC converter. The load carried by PET1 and PET2 are recorded as LOAD1 and LOAD2, respectively. In AC/DC hybrid distribution networks, PET can run in a variety of different modes according to the network structure, such as constant DC voltage control and constant active power control. At any time, at least one of the two PETs should be operated in the mode of DC voltage control to keep the DC voltage of network constant.
DC/DC
10 kVdc
DC load
2.2 The structure of AC/DC hybrid distribution networks
10 kV AC main network 1 10 kVac
AC load
Therefore, 18 high frequency transformers are needed in total for the H-bridge based PET structure. Whereas, for the topology shown in Fig. 1, 10 DC-DC converters are needed only. Consequently, the topology presented in this paper is helpful to improve the power density of PET and reduce the system cost. Besides, the topology of cascaded H-bridges is unable to lead out the medium voltage DC bus, but the new topology has the common DC bus which contributes to high-quality DC voltage output and adapts to the application environment in AC/DC hybrid distribution networks. Through the comparison above, it is not difficult to find that the C-MMC based PET structure is more suitable for AC/DC hybrid distribution networks. The fault of DC lines is one of the faults that should not be ignored in AC/DC distribution networks, so the converters with DC fault ride-through capability are the hot topics for scholars at present [8-10]. For the topology proposed in this paper, DC faults can be cleared by blocking the CDSMs of input stage, which is another advantage of the new PET. When a DC fault occurs, the system will quickly send a blocking signal according to the monitoring information, then CDSMs will be blocked. Cascaded module capacitors of each phase will provide a back electromotive force which is higher than AC line voltage, so the PET will be fully blocked and the fault arcs will not be resumed. Through this way, the complexity of the protection system will be reduced. Comparing to H bridge, the CDSM structure only uses one more IGBT to realize fault isolation, which can save the cost of the converter, but H bridge is more mature, so they both have advantages and disadvantages.
DG
Fig. 6 The structure of AC/DC distribution network
3 T he design of control and modulation strategies of new PETs The new topology proposed in this paper includes the input stage consisting of C-MMC, the DC-DC converters of isolation stage and the DC/AC modules as well as DC/ DC modules of output stage. In the following part of the paper, the control and modulation strategies design of each stage will be discussed separately.
3.1 The control and modulation strategies of input stage The CDSM of input stage is equivalent to two cascaded half-bridge modules during normal operation, so the control strategy can be designed in reference with MMC based on the half-bridge module. The control strategy of input stage uses the direct current control mode, and the controller adopts double closed-loop control. In the network shown in Fig. 6, PET enables the control DC voltage and AC power, so high voltage DC voltage control loop and AC power control loop should be designed for the outer loop. The switch of control mode can be realized by the instructions from the upper controller. The controller of input stage is shown in Fig. 7. Udcref
+ Udc
Pref
+ -
Kp1 + Ksi1 Kp2 + Ksi2
P
idref + id
−
Kp4 + Ksi4
Ud
+ - +
Vd
+ Uq
Vq
·L ·L
iq Qref
Kp3 + Ksi3
iqref
+ -
Kp5 + Ksi5
Q
Fig. 7 The structure of controller of input stage
The controller of outer loop receives the reference values from the system, namely U dcref ( Pref ) and Qref , then 399
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generates decoupled current references that are idref and iqref according to their own targets. Vd and Vq are the expected references of output voltage at the side of AC, which are got by the PI link and mathematical calculation. Finally, the three-phase modulation voltage of input stage can be got by dq/abc coordinate transformation from Vd and Vq. The input stage of PET proposed in this paper consists of 10 modules in each bridge arm, namely 5 CDSM modules, which belong to the multilevel situation, so the nearest level modulation (NLC) is adopted. The modulation strategy which can be realized easily can not only make the switching frequency and switching loss of power electronic devices lower, but also make the dynamic response faster.
3.2 The control and modulation strategies of isolation stage The H-bridge structure is used at both primary and secondary sides of the high frequency transformer so as to make power flow in both directions. Generally, there are two methods for the control of isolation stage. One is duty cycle control, and the other is to convert the DC voltage from input stage to high-frequency square wave with a duty cycle of 50% via open-loop control. In order to simplify the control system and reduce the influence of harmonics, the latter method is adopted as the control strategy of isolation stage. First, the DC voltage from input stage modulates into a high-frequency square wave through the primary H-bridge, and after the coupling of high frequency transformer, the DC voltage can be output through the rectification of secondary H-bridge. It is worth mentioning that the control signals of the primary and secondary H-bridges are identical, thus solving the synchronization problem in the voltage conversion.
3.3 The control and modulation strategies of output stage 3.3.1 DC/AC module Three-phase four-wire structure is applied in the DC/ AC module to make it adapt to unbalanced load. Taking into account that the n-phase arm voltage has nothing to do with load current, a, b, c three-phase can be used as a single-phase inverter control alone. In order to supply high quality output voltage for load, the control strategy adopts double closed-loop control. The outer voltage loop adopts proportional resonance (PR) control. Compared with proportional integral (PI) control, PR control can realize zero steady-state error. As for the inner current loop, the PI control is adopted. The structure of the controller is shown in Fig. 8. 400
Urref
+ −
Kp1 +
Kr1 s s 2 +ω 02
Ur
+
−
K p2 + Ksi2
SPWM
Driving signal of aǃbǃc
iLr 0
SPWM
Driving signal of n
Fig. 8 The structure of controller of DC/AC module
In Fig. 8, Urref is the reference value of phase voltage, Ur is the three-phase AC voltage of low voltage side, and iLr is the current of inductor, where r = a, b, c. Finally, the phase voltage reference value output of the controller will modulate the driving signal of the three-phase IGBT. In addition, sinusoidal pulse width modulation (SPWM) is used in the DC/AC inverter, which is widely used and easy to implement. 3.3.2 DC/DC module DC/DC module uses a Buck-Boost circuit. Generally, there are two methods for the control of Buck-Boost circuit. One is voltage mode control and the other is current mode control. However, the voltage mode control, that is a single closed-loop control system with output voltage as feedback, has several disadvantages, such as the low response speed and the poor system stability. In comparison, the current mode control is a double closed-loop control system by adding the current loop with the output current as feedback so that the system will be more stable, the dynamic characteristics will be better, and the response will be faster. Therefore, the DC/DC module of output stage adopts the current mode control. Besides, DC/DC module uses the independent PWM mode which is widely used at present. It ensures that T1/D1 and T2/D2 will not act at the same time in order to reduce the converter switching loss effectively [11].
4 Simulation verification In order to verify the rationality of the PET topology proposed in this paper, the rationality of the control strategies and the power flow regulation of PET in the AC/DC distribution network, in this section, we will build the simulation model in PSCAD/EMTDC of the structure of AC/DC distribution network shown in Fig. 6. System parameters and PET main parameters are shown in the following tables. Table 1 System parameters Rated Voltage
Rated Capacity
10 kV AC
1.5 MVA
±10 kV DC
1.5 MW
750V DC
1.5 MW
Yiqun Miao et al. Research on power electronic transformer applied in AC/DC hybrid distribution networks
5
The capacitance value of sub-module (Csm)
650 μF
The inductance value of bridge arm (Larm)
8 mH
The number of DC-DC converters (k)
10
The capacitance value of primary side (C1)
650 μF
The capacitance value of secondary side C2/C3
420 μF
The model of the AC/DC distribution network includes two PETs and one DC/DC PV boost converter, which are key components to the hybrid AC/DC distribution systems. In this system, PET1 works in DC voltage control mode and PET2 works in power control mode, which means PET1 works like a slack bus and PET2 works like a PQ bus. PET1 and PET2 can exchange real power so as to completely use the PV energy supplied by the DC/DC PV boost converter. In the simulation model, the output of PV is 1 MW, LOAD1 is 800 kW, and LOAD2 is 1 MW. The rationality of the control strategies can be verified by the dynamic characteristics of PETs. Initially, PET1 works in the mode of constant DC voltage control, while PET2 works in the mode of constant active power control and the power reference is 1 MW. After the system reaches steady state, the switches at the ±10 kV lines are open, so PET2 switches to the mode of constant DC voltage control. The process above can be shown in Fig. 9-12. As this can be seen from the simulation results above, in the dynamic process of the system switching to another state, the power flow of the system can be regulated by PET, which costs tens of milliseconds. At the same time,
DC voltage 12.5
PET110kV negative voltage
PET110kV positive voltage
PET210kV negative voltage
PET210kV positive voltage
PET1750V⭥
PET2750V⭥
kV
The number of CDSM modules(n)
–12.5
12.5 kV
5.774 kV
–12.5
0.900 kV
The voltage of input stage (Usa /Usb /Usc)
0.500 0.00
0.10
0.060 0.040 0.020 0.000 –0.020 –0.040 –0.060 0.0150 0.0100 0.0050 0.0000 –0.0050 –0.0100 –0.0150 0.060
0.50
0.60
PET210kV positive current
PET210kV negative current
PV positive current
PV negative current
0.10
0.20
0.30
0.40
0.50
0.60
Fig. 11 The simulation result of DC current
DCAC : Graphs
Pac2
1.20 1.00 0.80 0.60 0.40 0.20 0.00 –0.20
Qac2 y
MW MW
0.40
–0.060
System power flow Qac1
Pac1
0.30
DC current PET110kV positive current PET110kVnegative current
0.00
0.250 0.200 0.150 0.100 0.050 0.000 –0.050
0.20
Fig. 10 The simulation result of DC voltage
kA
Values
kA
Parameters
the DC voltage remains almost constant and the voltage of AC load is not affected. Therefore, the rationality of the topology and the strategies presented in this paper are fully validated.
kA
Table 2 The main parameters of pet
PET110kV DC power flow
PET210kV DC power flow
PV10kV DC power flow
MW
y
1.25
–1.25 0.00
0.10
0.20
0.30
0.40
0.50
Fig. 9 The distribution of system power flow
0.60
0.40 0.30 0.20 0.10 0.00 –0.10 –0.20 –0.30 –0.40 1.00 0.75 0.50 0.25 0.00 –0.25 –0.50 –0.75 –1.00 0.00
Uan
Ubn
Ucn
Iaload
Ibload
Icload
0.10
0.20
0.30
0.40
0.50
0.60
Fig. 12 The simulation result of AC load 401
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Global Energy Interconnection
5 Conclusion In this paper, a PET topology suitable for AC/DC distribution networks is proposed from its particularity and the general demands. Its advantage is the ability to accept DG and DC loads. In addition, it can clear DC faults by blocking the switching devices. The new PET consists of three stages, where the input stage adopts the structure of C-MMC, the isolation stage includes DC-DC converters with HFTs, and the output stage is used for AC loads and DC loads. Then, the control and modulation strategies are designed in detail. It is worth noting that the outer loop of input stage can switch between constant DC voltage control and constant active power control. Finally, the simulation model is built in PSCAD/EMTDC and the dynamic and steady state simulations are carried out, which fully verifies the rationality of new topology and the validity of control strategies. However, it is not hard to find that the loss of new PET is slightly large, so it also needs to be studied further.
Acknowledgements This work was supported by National Key Research and Development Program of China (2016YFB0900500, 2017YFB0903100); the State Grid Science and Technology Project (SGRI-DL-F1-51-011).
References [1] Zhang L, Tang W, Liang J et al (2016) A medium voltage hybrid AC/DC distribution network and its economic evaluation. In: 12th IET International Conference on AC and DC Power Transmission (ACDC 2016), Beijing, pp: 1-6 [2] Wei Z, Yu T, Wang Z et al (2016) Hierarchical voltage control strategy based on typical scenes in AC/DC hybrid distribution network with operation of multiport flexible DC looped network. In: 2016 IEEE PES Asia-Pacific Power and Energy Engineering Conference (APPEEC), Xi’an, pp: 2220-2224 [3] López M, Briz F, Saeed M et al (2016) Comparative analysis of modular multiport power electronic transformer topologies," In: 2016 IEEE Energy Conversion Congress and Exposition (ECCE), Milwaukee, WI, pp: 1-8 [4] Tatcho P, Li H, Jiang Y et al (2013) A novel hierarchical section protection based on the solid-state transformer for the future renewable electric energy delivery and management (FREEDM) System. IEEE Transactions on Smart Grid, 4(2):1096-1104 [5] Wang X, Liu J, Ouyang S et al (2015) Research on unbalancedload correction capability of two power electronic transformer topologies. IEEE Transactions on Power Electronics, 30(6):3044-3056
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[6] Wang X, Ouyang S, Liu J et al (2013) Comparison on unbalanced-load handling capability of two power electronic transformer topologies. In: 2013 IEEE Energy Conversion Congress and Exposition (ECCE), Denver, CO, pp: 5266-5272 [7] Wang Z, Ouyang J, Zhang J et al (2014) Resonant power electronic transformer for power grid. In: 2014 IEEE Energy Conversion Congress and Exposition (ECCE), pp: 4531-4536 [8] Xue Y, Xu Z, Tu Q (2013) Modulation and control for a new hybrid cascaded multilevel converter with DC blocking capability. In: 2013 IEEE Power & Energy Society General Meeting, Vancouver, BC, pp: 1-1 [9] Xue Y, Xu Z (2013) DC fault ride-through mechanism and improved topology scheme of C-MMC. Proceedings of the CSEE, 33(21):63-71 [10] Merlin MMC, Green TC, Mitcheson PD et al (2010) A new hybrid multi-level Voltage-Source converter with DC fault blocking capability. In: 9th IET International Conference on AC and DC Power Transmission (ACDC 2010), London, pp: 1-5 [11] Yao C, Ruan X, Cao W et al (2013) An input voltage feedforward control strategy for two-switch Buck-Boost DC-DC converters. Proceedings of CSEE, 33(21):36-45 [12] Sun G, Qi C, Han B et al (2016) Study on key technologies of AC/DC hybrid distribution network planning and operation. Distribution & Utilization, 33(8):7-17 [13] Huang R, Cheng L, Li H (2015) Study on key technologies of AC/DC hybrid active distribution network. Electric Power Construction, 36(1):46-51 [14] Zhang Y (2017) Flexible power electronics substation: “The Top Technology to Realize AC/DC Transformation”. State Grid News, 2017-02-21 [15] Zhai C, Yang X, Tang X (2015) Application of real-time digital simulator RTDS in power system. In: 2015 Annual Meeting of the National Smart Grid User Side Energy Management, pp: 57-64
Biographies Yiqun Miao received his Ph.D. degree from Zhejiang University, China, in 2012. He is currently working in State Grid Shanghai Municipal Electric Power Company. His research interests include HVDC & FACTS, and integration of electric vehicles into power systems. Jieying Song, engineer, received her master degree from North China Electric Power University in 2012. Her research interests include power electronic transformer technology and its application in power system, and HVDC & FACTS.
Yiqun Miao et al. Research on power electronic transformer applied in AC/DC hybrid distribution networks
Zhengang Lu, senior engineer, He received his bachelor degree in electrical engineering from Xi’an Jiaotong University, China, in 2007. He received his master degree from CEPRI in 2010. His research interests include power electronic transformer technology and its application in power system, and HVDC & FACTS. Shufan Chen received his bachelor degree from North China Electric Power University, Beijing China, in 2011. He is currently working in State Grid Shanghai Municipal Electric Power Company. His research interests include HVDC & FACTS and power system analysis. Haijun Liu received his bachelor degree in electrical engineering from Beijing Jiaotong University, China in 2007. He received his master degree in power electronics from North China Electric Power University, Beijing, China in 2010. His research interests include power electronic transformer technology and its applications. Chun Ding, assistant engineer, electrical test senior technician. He graduated from Tongji University majoring in Electrical Engineering and Automation. He is currently working in State Grid Shanghai Municipal Electric Power Company. His research interests include HVDC & FACTS, and power system analysis.
Tianzhi Cao received his bachelor degree in electrical engineering from North China Electric Power University, Baoding, China in 2003. He received his master degree in electrical engineering from North China Electric Power University, Beijing, China in 2008. Since April 2008, he has been an electrical engineer at the North China Electric Power Research Institute, Beijing. His research interests include power electronic transformer technology. Linhai Cai received his master degree in electrical engineering from North China Electric Power University, Beijing, China. He is currently working in Global Energy Interconnection Research Institute, Beijing. He is a senior engineer. His research interest includes flexible AC transmission technology. Yuzhong Gong (S'13-M’16) received the bachelor and Ph.D. degrees in electrical engineering from Zhejiang University of Technology and Zhejiang University, Hangzhou, China, in 2010 and 2015, respectively. He is currently a Postdoctoral Fellow in the Department of Electrical and Computer Engineering, University of Saskatchewan, Saskatoon, SK, Canada. His current research interests include the optimization of power system operation and renewable energy integration. (Editor Chenyang Liu)
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Global Energy Interconnection www.geidco.org Full-length article
Research on power electronic transformer applied in AC/DC hybrid distribution networks Yiqun Miao1, Jieying Song2, Haijun Liu2, Zhengang Lu2, Shufan Chen1, Chun Ding1, Tianzhi Cao3, Linhai Cai2, Yuzhong Gong 4 1. State Grid Shanghai Municipal Electric Power Company, Shanghai 200122, P.R. China 2. State Key Laboratory of Advanced Power Transmission Technology, Global Energy Interconnection
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Research Institute, Beijing 102209, P.R. China 3. State Grid Jibei Electric Power Company, Beijing 100053, P.R. China 4. Department of Electrical and Computer Engineering, University of Saskatchewan, Saskatoon, SK, Canada
Abstract: The AC/DC hybrid distribution network is one of the trends in distribution network development, which poses great challenges to the traditional distribution transformer. In this paper, a new topology suitable for AC/DC hybrid distribution network is put forward according to the demands of power grid, with advantages of accepting DG and DC loads, while clearing DC fault by blocking the clamping double sub-module (CDSM) of input stage. Then, this paper shows the typical structure of AC/DC distribution network that is hand in hand. Based on the new topology, this paper designs the control and modulation strategies of each stage, where the outer loop controller of input stage is emphasized for its twocontrol mode. At last, the rationality of new topology and the validity of control strategies are verified by the steady and dynamic state simulation. At the same time, the simulation results highlight the role of PET in energy regulation. Keywords: AC/DC hybrid distribution network, Power electronic transformer (PET), Clamping double sub-module (CDSM), Energy router.
1 Introduction The penetration of large-scale distributed generations (DGs) and the development of diversification of loads pose significant challenges for the traditional distribution Received: 15 March 2018/ Accepted: 12 April 2018/ Published: 25 August 2018 Yiqun Miao [email protected]
Chun Ding [email protected]
Jieying Song [email protected]
Tianzhi Cao [email protected]
Haijun Liu [email protected]
Linhai Cai [email protected]
Zhengang Lu [email protected]
Yuzhong Gong [email protected]
Shufan Chen [email protected] Open access under CC BY-NC-ND license. 396
network to fulfill reliability and efficiency requirements. AC/DC hybrid distribution networks have advantages in a lot of aspects, such as accommodating more DGs and reducing the investment of construction of power grid [1]. Therefore, it should be an essential part of the development of distribution networks in the future. With the development of renewable energies and more and more DC loads, traditional distribution transformers have been unable to meet the demands of flexibility, efficiency and power quality [2, 12]. Power electronic transformers (PET) are based on power electronic converters and high-frequency transformers (HFTs). Besides, the traditional functions of voltage transformation and electrical isolation can reduce volume and weight of core materials, compensate for reactive power, and ensure that the primary current and power factor can be controlled [3-4]. The outstanding advantages of a PET used in AC/DC hybrid distribution
Yiqun Miao et al. Research on power electronic transformer applied in AC/DC hybrid distribution networks
networks lie in its capability of controlling power flow between its AC and DC ports, which are helpful to accept sources loads and enhance the flexibility of distribution network. Therefore, it can be seen that the PET plays the role of energy router to decrease the number of converters, coordinate the distribution of power and improve the economy and reliability of power grid. Research on PET is currently focused on topologies and control strategies. On unbalanced-load handling capability, reference [5-6] made a comparison of two PET topologies based on H-bridge. However, DC bus cannot be led out from the H-bridge based PET structure, which is not convenient to accept DC loads as well as DGs. Reference [7] referred the topology of PET based on modular multilevel converter (MMC) and proposed a resonant PET used in power grid. MMC based PET structure adopts the halfbridge sub-modules without the capability of isolating DC fault. The topologies above are the hotspots in current studies, but none of them are totally suitable for AC/DC hybrid distribution networks. Consequently, the research on PET for AC/DC hybrid distribution networks is necessary. Traditional MMC has no DC fault isolation capability, under double pole DC short circuit fault, MMC would bear the AC short circuit current until the AC breaker opens [13-15]. Furthermore, in the existing of conventional DC distribution schemes, the DC breaker may be necessary to cut the DC fault current, which would increase total cost of AC/DC hybrid distribution networks. In this paper, a new PET topology with three stages is proposed, which can effectively cut off short circuit current shortly after fault.
U sa
2 A new topology of PET and the structure of AC/DC distribution network 2.1 A new topology of PET In order to meet the demands of AC/DC distribution networks, this paper puts forward a new PET topology, as it is shown in Fig. 1. The PET uses three-stage structure, including the clamping double sub-module (CDSM) of input stage, DC-DC converters of isolation stage and the DC/AC modules as well as DC/DC modules of output stage. Where Usa , Usb and Usc are phase voltages of 10 kV AC distribution network respectively, Larm is the inductance of each bridge leg and R is the clamping resistor on the side of medium voltage DC. The input stage is connected to the high voltage AC power network and the DC side of this stage accesses the medium voltage DC network. The DC-DC converters of isolation stage are connected in parallel with 750 V lowvoltage DC network. The DC/AC modules of output stage are used to supply power for the low voltage AC load, and the DC/DC modules are for the low voltage DC load. Fig. 2, 3 and 4 are respectively the topologies of CDSM, the DC / AC module of output stage, and the DC/DC module of output stage.
CDSM2
CDSM 2
CDSM 2
CDSM n
CDSM n
CDSM n
Larm
Larm
U dc1
C1
C2
HFT
Udc 2
CDSM 1
Hbridge
CDSM 1
Hbridge
CDSM1
L arm
Isolation stage
f 10 kV DC
Input stage
10kV AC
Then the control and modulation strategies of PET are designed in detail. At last, the rationality of the design and the function of PET in power flow regulation in AC/DC distribution networks are verified by simulation.
U dcL
C3
1
750 V DC
DC/AC R
U sb DC - DC converters
U dc
U sc Larm
Larm
Larm
DC/DC
Output stage
R
CDSM 1
CDSM 1
CDSM 1
CDSM 2
CDSM 2
CDSM 2 C2
CDSM n
k
Udc2
CDSM n
Hbridge
CDSM n
Hbridge
C1 U dc1
C3
f 10 kV DC
Fig. 1 PET topology applied in AC/DC hybrid distribution network 397
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D31
A
T21 + −
T12
D12
Csm T0
D21
+ Csm
−
D0
D22
T22
D32
B
Fig. 2 The topology of CDSM
D3 T5
D1 T3
D5 T7
D7
L
750V DC
T1
T2
D2 T4
D4 T6
D6 T8
D8
n
a b c
C
750V DC
Fig. 3 The topology of DC/AC module of output stage
c11
c12
T1
T2
D1
D2
L
DC load
C
Udcload
Fig. 4 The topology of DC/DC module of output stage
Usa
Usb
Usc HFT
Module1
Module1
Module2
Module2
Module2
Modulen
Modulen
Modulen
Hbridge
Fig. 5 The topology of PET based on H-bridge 398
C2 Udc2 C3
Hbridge
Module1
Hbridge
D11
Hbridge
T11
The input stage of PET is used to convert the 10 kV AC voltage to a DC voltage. The structure of MMC based on CDSM is called C-MMC. Each CDSM is equivalent to two cascaded half-bridge modules (SM) during normal operation. The DC-DC converters of isolation stage including inverters, high frequency transformers and rectifiers are used to convert the DC voltage got from input stage to a 750 V DC voltage. The inverters of isolation stage are in series and the rectifiers are in parallel. The amount and type of converters of output stage are decided by the load at the side of low voltage. One or more DC/ AC modules and DC/DC modules can be connected to the 750 V DC bus. Reference to the grounding modes in the flexible DC transmission system, the clamping resistance grounding mode is applied in AC/DC distribution network as shown in Fig. 1. Compared with the topology of cascaded H-bridges shown in Fig. 5, the new topology shown in Fig. 1 cannot only reduce the quantity of high frequency transformers, but also improve the quality of the output voltage, while greatly satisfying the needs of AC/DC hybrid distribution networks. Take the example of 10 kV network mentioned in this paper, the following formula is true for the topology of cascaded H-bridges: m Ea = U dc (1) 2 If the maximum operating line voltage is 12 kV, m that represents voltage modulation ratio is 0.85, and the rated voltage of power unit (H-bridge) is 2 kV, then 6 cascaded units (only consider the H-bridge of input stage and DC-DC converter) are needed for each phase.
C2 Udc2 C3 C2 Udc2 C3 C2
Udc2 C3
3n
Yiqun Miao et al. Research on power electronic transformer applied in AC/DC hybrid distribution networks
10 kV AC main network 2 10 kVac
PV
10 kVdc
PET1
PET2
LOAD 2
DG
DC load
AC load
LOAD 1
As shown in Fig. 6, two terminals of 10 kV AC distribution networks are connected by PET and DC lines to constitute the hand in hand structure of the AC-DC hybrid distribution network. The low voltage port of PET can be connected with different types of AC or DC loads according to actual needs. The centralized photovoltaic connection into DC distribution network is realized by the DC/DC converter. The load carried by PET1 and PET2 are recorded as LOAD1 and LOAD2, respectively. In AC/DC hybrid distribution networks, PET can run in a variety of different modes according to the network structure, such as constant DC voltage control and constant active power control. At any time, at least one of the two PETs should be operated in the mode of DC voltage control to keep the DC voltage of network constant.
DC/DC
10 kVdc
DC load
2.2 The structure of AC/DC hybrid distribution networks
10 kV AC main network 1 10 kVac
AC load
Therefore, 18 high frequency transformers are needed in total for the H-bridge based PET structure. Whereas, for the topology shown in Fig. 1, 10 DC-DC converters are needed only. Consequently, the topology presented in this paper is helpful to improve the power density of PET and reduce the system cost. Besides, the topology of cascaded H-bridges is unable to lead out the medium voltage DC bus, but the new topology has the common DC bus which contributes to high-quality DC voltage output and adapts to the application environment in AC/DC hybrid distribution networks. Through the comparison above, it is not difficult to find that the C-MMC based PET structure is more suitable for AC/DC hybrid distribution networks. The fault of DC lines is one of the faults that should not be ignored in AC/DC distribution networks, so the converters with DC fault ride-through capability are the hot topics for scholars at present [8-10]. For the topology proposed in this paper, DC faults can be cleared by blocking the CDSMs of input stage, which is another advantage of the new PET. When a DC fault occurs, the system will quickly send a blocking signal according to the monitoring information, then CDSMs will be blocked. Cascaded module capacitors of each phase will provide a back electromotive force which is higher than AC line voltage, so the PET will be fully blocked and the fault arcs will not be resumed. Through this way, the complexity of the protection system will be reduced. Comparing to H bridge, the CDSM structure only uses one more IGBT to realize fault isolation, which can save the cost of the converter, but H bridge is more mature, so they both have advantages and disadvantages.
DG
Fig. 6 The structure of AC/DC distribution network
3 T he design of control and modulation strategies of new PETs The new topology proposed in this paper includes the input stage consisting of C-MMC, the DC-DC converters of isolation stage and the DC/AC modules as well as DC/ DC modules of output stage. In the following part of the paper, the control and modulation strategies design of each stage will be discussed separately.
3.1 The control and modulation strategies of input stage The CDSM of input stage is equivalent to two cascaded half-bridge modules during normal operation, so the control strategy can be designed in reference with MMC based on the half-bridge module. The control strategy of input stage uses the direct current control mode, and the controller adopts double closed-loop control. In the network shown in Fig. 6, PET enables the control DC voltage and AC power, so high voltage DC voltage control loop and AC power control loop should be designed for the outer loop. The switch of control mode can be realized by the instructions from the upper controller. The controller of input stage is shown in Fig. 7. Udcref
+ Udc
Pref
+ -
Kp1 + Ksi1 Kp2 + Ksi2
P
idref + id
−
Kp4 + Ksi4
Ud
+ - +
Vd
+ Uq
Vq
·L ·L
iq Qref
Kp3 + Ksi3
iqref
+ -
Kp5 + Ksi5
Q
Fig. 7 The structure of controller of input stage
The controller of outer loop receives the reference values from the system, namely U dcref ( Pref ) and Qref , then 399
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generates decoupled current references that are idref and iqref according to their own targets. Vd and Vq are the expected references of output voltage at the side of AC, which are got by the PI link and mathematical calculation. Finally, the three-phase modulation voltage of input stage can be got by dq/abc coordinate transformation from Vd and Vq. The input stage of PET proposed in this paper consists of 10 modules in each bridge arm, namely 5 CDSM modules, which belong to the multilevel situation, so the nearest level modulation (NLC) is adopted. The modulation strategy which can be realized easily can not only make the switching frequency and switching loss of power electronic devices lower, but also make the dynamic response faster.
3.2 The control and modulation strategies of isolation stage The H-bridge structure is used at both primary and secondary sides of the high frequency transformer so as to make power flow in both directions. Generally, there are two methods for the control of isolation stage. One is duty cycle control, and the other is to convert the DC voltage from input stage to high-frequency square wave with a duty cycle of 50% via open-loop control. In order to simplify the control system and reduce the influence of harmonics, the latter method is adopted as the control strategy of isolation stage. First, the DC voltage from input stage modulates into a high-frequency square wave through the primary H-bridge, and after the coupling of high frequency transformer, the DC voltage can be output through the rectification of secondary H-bridge. It is worth mentioning that the control signals of the primary and secondary H-bridges are identical, thus solving the synchronization problem in the voltage conversion.
3.3 The control and modulation strategies of output stage 3.3.1 DC/AC module Three-phase four-wire structure is applied in the DC/ AC module to make it adapt to unbalanced load. Taking into account that the n-phase arm voltage has nothing to do with load current, a, b, c three-phase can be used as a single-phase inverter control alone. In order to supply high quality output voltage for load, the control strategy adopts double closed-loop control. The outer voltage loop adopts proportional resonance (PR) control. Compared with proportional integral (PI) control, PR control can realize zero steady-state error. As for the inner current loop, the PI control is adopted. The structure of the controller is shown in Fig. 8. 400
Urref
+ −
Kp1 +
Kr1 s s 2 +ω 02
Ur
+
−
K p2 + Ksi2
SPWM
Driving signal of aǃbǃc
iLr 0
SPWM
Driving signal of n
Fig. 8 The structure of controller of DC/AC module
In Fig. 8, Urref is the reference value of phase voltage, Ur is the three-phase AC voltage of low voltage side, and iLr is the current of inductor, where r = a, b, c. Finally, the phase voltage reference value output of the controller will modulate the driving signal of the three-phase IGBT. In addition, sinusoidal pulse width modulation (SPWM) is used in the DC/AC inverter, which is widely used and easy to implement. 3.3.2 DC/DC module DC/DC module uses a Buck-Boost circuit. Generally, there are two methods for the control of Buck-Boost circuit. One is voltage mode control and the other is current mode control. However, the voltage mode control, that is a single closed-loop control system with output voltage as feedback, has several disadvantages, such as the low response speed and the poor system stability. In comparison, the current mode control is a double closed-loop control system by adding the current loop with the output current as feedback so that the system will be more stable, the dynamic characteristics will be better, and the response will be faster. Therefore, the DC/DC module of output stage adopts the current mode control. Besides, DC/DC module uses the independent PWM mode which is widely used at present. It ensures that T1/D1 and T2/D2 will not act at the same time in order to reduce the converter switching loss effectively [11].
4 Simulation verification In order to verify the rationality of the PET topology proposed in this paper, the rationality of the control strategies and the power flow regulation of PET in the AC/DC distribution network, in this section, we will build the simulation model in PSCAD/EMTDC of the structure of AC/DC distribution network shown in Fig. 6. System parameters and PET main parameters are shown in the following tables. Table 1 System parameters Rated Voltage
Rated Capacity
10 kV AC
1.5 MVA
±10 kV DC
1.5 MW
750V DC
1.5 MW
Yiqun Miao et al. Research on power electronic transformer applied in AC/DC hybrid distribution networks
5
The capacitance value of sub-module (Csm)
650 μF
The inductance value of bridge arm (Larm)
8 mH
The number of DC-DC converters (k)
10
The capacitance value of primary side (C1)
650 μF
The capacitance value of secondary side C2/C3
420 μF
The model of the AC/DC distribution network includes two PETs and one DC/DC PV boost converter, which are key components to the hybrid AC/DC distribution systems. In this system, PET1 works in DC voltage control mode and PET2 works in power control mode, which means PET1 works like a slack bus and PET2 works like a PQ bus. PET1 and PET2 can exchange real power so as to completely use the PV energy supplied by the DC/DC PV boost converter. In the simulation model, the output of PV is 1 MW, LOAD1 is 800 kW, and LOAD2 is 1 MW. The rationality of the control strategies can be verified by the dynamic characteristics of PETs. Initially, PET1 works in the mode of constant DC voltage control, while PET2 works in the mode of constant active power control and the power reference is 1 MW. After the system reaches steady state, the switches at the ±10 kV lines are open, so PET2 switches to the mode of constant DC voltage control. The process above can be shown in Fig. 9-12. As this can be seen from the simulation results above, in the dynamic process of the system switching to another state, the power flow of the system can be regulated by PET, which costs tens of milliseconds. At the same time,
DC voltage 12.5
PET110kV negative voltage
PET110kV positive voltage
PET210kV negative voltage
PET210kV positive voltage
PET1750V⭥
PET2750V⭥
kV
The number of CDSM modules(n)
–12.5
12.5 kV
5.774 kV
–12.5
0.900 kV
The voltage of input stage (Usa /Usb /Usc)
0.500 0.00
0.10
0.060 0.040 0.020 0.000 –0.020 –0.040 –0.060 0.0150 0.0100 0.0050 0.0000 –0.0050 –0.0100 –0.0150 0.060
0.50
0.60
PET210kV positive current
PET210kV negative current
PV positive current
PV negative current
0.10
0.20
0.30
0.40
0.50
0.60
Fig. 11 The simulation result of DC current
DCAC : Graphs
Pac2
1.20 1.00 0.80 0.60 0.40 0.20 0.00 –0.20
Qac2 y
MW MW
0.40
–0.060
System power flow Qac1
Pac1
0.30
DC current PET110kV positive current PET110kVnegative current
0.00
0.250 0.200 0.150 0.100 0.050 0.000 –0.050
0.20
Fig. 10 The simulation result of DC voltage
kA
Values
kA
Parameters
the DC voltage remains almost constant and the voltage of AC load is not affected. Therefore, the rationality of the topology and the strategies presented in this paper are fully validated.
kA
Table 2 The main parameters of pet
PET110kV DC power flow
PET210kV DC power flow
PV10kV DC power flow
MW
y
1.25
–1.25 0.00
0.10
0.20
0.30
0.40
0.50
Fig. 9 The distribution of system power flow
0.60
0.40 0.30 0.20 0.10 0.00 –0.10 –0.20 –0.30 –0.40 1.00 0.75 0.50 0.25 0.00 –0.25 –0.50 –0.75 –1.00 0.00
Uan
Ubn
Ucn
Iaload
Ibload
Icload
0.10
0.20
0.30
0.40
0.50
0.60
Fig. 12 The simulation result of AC load 401
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Global Energy Interconnection
5 Conclusion In this paper, a PET topology suitable for AC/DC distribution networks is proposed from its particularity and the general demands. Its advantage is the ability to accept DG and DC loads. In addition, it can clear DC faults by blocking the switching devices. The new PET consists of three stages, where the input stage adopts the structure of C-MMC, the isolation stage includes DC-DC converters with HFTs, and the output stage is used for AC loads and DC loads. Then, the control and modulation strategies are designed in detail. It is worth noting that the outer loop of input stage can switch between constant DC voltage control and constant active power control. Finally, the simulation model is built in PSCAD/EMTDC and the dynamic and steady state simulations are carried out, which fully verifies the rationality of new topology and the validity of control strategies. However, it is not hard to find that the loss of new PET is slightly large, so it also needs to be studied further.
Acknowledgements This work was supported by National Key Research and Development Program of China (2016YFB0900500, 2017YFB0903100); the State Grid Science and Technology Project (SGRI-DL-F1-51-011).
References [1] Zhang L, Tang W, Liang J et al (2016) A medium voltage hybrid AC/DC distribution network and its economic evaluation. In: 12th IET International Conference on AC and DC Power Transmission (ACDC 2016), Beijing, pp: 1-6 [2] Wei Z, Yu T, Wang Z et al (2016) Hierarchical voltage control strategy based on typical scenes in AC/DC hybrid distribution network with operation of multiport flexible DC looped network. In: 2016 IEEE PES Asia-Pacific Power and Energy Engineering Conference (APPEEC), Xi’an, pp: 2220-2224 [3] López M, Briz F, Saeed M et al (2016) Comparative analysis of modular multiport power electronic transformer topologies," In: 2016 IEEE Energy Conversion Congress and Exposition (ECCE), Milwaukee, WI, pp: 1-8 [4] Tatcho P, Li H, Jiang Y et al (2013) A novel hierarchical section protection based on the solid-state transformer for the future renewable electric energy delivery and management (FREEDM) System. IEEE Transactions on Smart Grid, 4(2):1096-1104 [5] Wang X, Liu J, Ouyang S et al (2015) Research on unbalancedload correction capability of two power electronic transformer topologies. IEEE Transactions on Power Electronics, 30(6):3044-3056
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[6] Wang X, Ouyang S, Liu J et al (2013) Comparison on unbalanced-load handling capability of two power electronic transformer topologies. In: 2013 IEEE Energy Conversion Congress and Exposition (ECCE), Denver, CO, pp: 5266-5272 [7] Wang Z, Ouyang J, Zhang J et al (2014) Resonant power electronic transformer for power grid. In: 2014 IEEE Energy Conversion Congress and Exposition (ECCE), pp: 4531-4536 [8] Xue Y, Xu Z, Tu Q (2013) Modulation and control for a new hybrid cascaded multilevel converter with DC blocking capability. In: 2013 IEEE Power & Energy Society General Meeting, Vancouver, BC, pp: 1-1 [9] Xue Y, Xu Z (2013) DC fault ride-through mechanism and improved topology scheme of C-MMC. Proceedings of the CSEE, 33(21):63-71 [10] Merlin MMC, Green TC, Mitcheson PD et al (2010) A new hybrid multi-level Voltage-Source converter with DC fault blocking capability. In: 9th IET International Conference on AC and DC Power Transmission (ACDC 2010), London, pp: 1-5 [11] Yao C, Ruan X, Cao W et al (2013) An input voltage feedforward control strategy for two-switch Buck-Boost DC-DC converters. Proceedings of CSEE, 33(21):36-45 [12] Sun G, Qi C, Han B et al (2016) Study on key technologies of AC/DC hybrid distribution network planning and operation. Distribution & Utilization, 33(8):7-17 [13] Huang R, Cheng L, Li H (2015) Study on key technologies of AC/DC hybrid active distribution network. Electric Power Construction, 36(1):46-51 [14] Zhang Y (2017) Flexible power electronics substation: “The Top Technology to Realize AC/DC Transformation”. State Grid News, 2017-02-21 [15] Zhai C, Yang X, Tang X (2015) Application of real-time digital simulator RTDS in power system. In: 2015 Annual Meeting of the National Smart Grid User Side Energy Management, pp: 57-64
Biographies Yiqun Miao received his Ph.D. degree from Zhejiang University, China, in 2012. He is currently working in State Grid Shanghai Municipal Electric Power Company. His research interests include HVDC & FACTS, and integration of electric vehicles into power systems. Jieying Song, engineer, received her master degree from North China Electric Power University in 2012. Her research interests include power electronic transformer technology and its application in power system, and HVDC & FACTS.
Yiqun Miao et al. Research on power electronic transformer applied in AC/DC hybrid distribution networks
Zhengang Lu, senior engineer, He received his bachelor degree in electrical engineering from Xi’an Jiaotong University, China, in 2007. He received his master degree from CEPRI in 2010. His research interests include power electronic transformer technology and its application in power system, and HVDC & FACTS. Shufan Chen received his bachelor degree from North China Electric Power University, Beijing China, in 2011. He is currently working in State Grid Shanghai Municipal Electric Power Company. His research interests include HVDC & FACTS and power system analysis. Haijun Liu received his bachelor degree in electrical engineering from Beijing Jiaotong University, China in 2007. He received his master degree in power electronics from North China Electric Power University, Beijing, China in 2010. His research interests include power electronic transformer technology and its applications. Chun Ding, assistant engineer, electrical test senior technician. He graduated from Tongji University majoring in Electrical Engineering and Automation. He is currently working in State Grid Shanghai Municipal Electric Power Company. His research interests include HVDC & FACTS, and power system analysis.
Tianzhi Cao received his bachelor degree in electrical engineering from North China Electric Power University, Baoding, China in 2003. He received his master degree in electrical engineering from North China Electric Power University, Beijing, China in 2008. Since April 2008, he has been an electrical engineer at the North China Electric Power Research Institute, Beijing. His research interests include power electronic transformer technology. Linhai Cai received his master degree in electrical engineering from North China Electric Power University, Beijing, China. He is currently working in Global Energy Interconnection Research Institute, Beijing. He is a senior engineer. His research interest includes flexible AC transmission technology. Yuzhong Gong (S'13-M’16) received the bachelor and Ph.D. degrees in electrical engineering from Zhejiang University of Technology and Zhejiang University, Hangzhou, China, in 2010 and 2015, respectively. He is currently a Postdoctoral Fellow in the Department of Electrical and Computer Engineering, University of Saskatchewan, Saskatoon, SK, Canada. His current research interests include the optimization of power system operation and renewable energy integration. (Editor Chenyang Liu)
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