Soft-start Optimized Strategies of MMC-HVDC with Overhead Lines Based on Full-Bridge Sub-modules Kun Han, Caiyun Fan, Shuai Zhen, Zhilei Si, Lulu Liu

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1 26 International Conference on Power Engineering & Energy, Environment (PEEE 26) ISBN: Soft-start Optimized Strategies of MMC-HV with Overhead ines Based on Full-Bridge Sub-modules Kun Han, Caiyun Fan, Shuai Zhen, Zhilei Si, ulu iu (Xu Ji Electric Co. td., Xuchang 46, Henan Province, China) Keywords: Modular Multilevel Converter (MMC); full-bridge sub-modules; soft-start strategies. Abstract: Modular Multilevel Converter (MMC) based on full-bridge sub-modules(fb-mmc), which has advantages of outputting negative level, improving the utilization rate of voltage and having the capability of fast recovering short-circuit faults, can be applied to high voltage and large capacity MMC-HV system based on overhead lines. Based on the operation mechanism of FB-MMC, the charging characteristics of two kinds of charging modes including active charging and passive charging, are deeply analyzed, and corresponding soft-start strategies to be selected are given. Through theoretical and simulation comparison and analysis of performances of different soft-start strategies used in each kind of charging mode, the optimized soft-start strategies are proposed, which provide a theoretical basis for engineering design and application of high voltage and large capacity MMC-HV with overhead lines. I Introduction HV based on Modular Multilevel Converter (MMC-HV), has become worldwide research focus and its progress of industrialization is being accelerated [-4]. With the development of power electronics devices and control technology, having capability of recovering faults has become urgent demand for the development of high voltage and large capacity MMC-HV. For high voltage and large capacity MMC-HV system with overhead lines, the technical problem that how to fast recover the short-circuit faults by the converter itself should be solved firstly. Actually, MMC-HV based on full-bridge sub-modules (FB-MMC) has technical advantages such as outputting negative level, improving the utilization rate of voltage and having capacity of recovering faults by the converter itself, which is suitable for MMC-HV with overhead lines. As known, soft-start of FB-MMC is necessary for its being normally unlocked and putting into operating state. However, the existing literatures about FB-MMC mostly focus on its operation mechanism and control strategy of faults ride-through [5-7], yet seldom refer to its soft-start strategies. In this paper, firstly, the operation mechanism of FB-MMC is described, and then the charging characteristics of two kinds of charging modes including active charging and passive charging, are deeply analyzed, and corresponding soft-start strategies to be selected are given. Through theoretical and simulation comparison and analysis of performances of different soft-start strategies used in each kind of charging mode, the optimized soft-start strategies are proposed, which provide a theoretical basis for engineering design and application of high voltage and large capacity MMC-HV with overhead lines.

2 II FB-MMC operation mechanism Main circuit topology of FB-MMC is shown as figure, where each bridge-arm consists of n full-bridge sub-modules (FB) and reactor. The principle diagram and equivalent circuit of FB is also shown as figure, there are four kinds of operation modes according to working states of IGBTs (T-T4) which are equivalent to controllable switches (S-S4). The operation modes are shown as table. + Udc a b ia ib ic u u c u b a T T 3 u c S u c S 3 c u sm T 2 T 4 u sm S 2 S 4 - Figure. Main circuit topology of FB-MMC and FB schematic chart. Table. FB operation modes. Sequence number Operation modes T-T4 Working states Positive output (u sm = u c ) T T4 turn-on; T2 T3 turn-off 2 Negative output(u sm = -u c ) T T4 turn-off; T2 T3 turn-on 3 Zero output (u sm =) T T3 turn-on or T2 T4 turn-on 4 Unlocked(u sm = u c or u sm =- u c ) T T2 T3 T4 turn-off According to the control principle of HB-MMC, the reference voltage of upper and lower bridge-arms of the same phase unit is shown as expression () respectively: U u pm _ ref u pm _ ref em u N() t pm Round diffm 2 UC U unm _ ref unm _ ref em u N() t diffm nm Round 2 UC () (2) In expression(), u pm_ref (u nm_ref ) is the reference voltage of upper (lower) bridge-arm, U is voltage, e m is output voltage of the converter, u diffm is the voltage of bridge-arm reactor. The number of s required to be input in each bridge-arm at any moment N(t) p(n)m can be shown as expression (2), where U C is average voltage of s. According to expression () and (2), due to its capacity of outputting negative level, FB-MMC can realize over modulation, in the other words, the valve-side AC phase-voltage peak value can be greater than the bus voltage to ground. III Charging characteristics analysis and optimized soft-start strategies 3. Active soft-start Active soft-start is defined as the process that converter is charged from AC side until the voltage gets to its rated value. For simplicity, the research object only uses the single-station based on FB-MMC. () Charging characteristic analysis The schematic diagram of active charging circuit is shown as figure 2, this paper takes each bridge-arm with two FBs as an example.

3 Supposing that valve-side AC phase voltage U a is greater than U c and U c is greater than U b, the s of A phase and B phase is charged. The charging circuit is shown as figure 2, where the charging circuit marked is s charging circuit including A phase upper bridge-arm and B phase upper bridge-arm. In the charging process, the voltage of A phase upper bridge-arm is negative and voltage of B phase upper bridge-arm is positive, the reference direction is from positive bus to the valve-side AC outlet end or from the valve-side AC outlet end to negative bus. The charging circuit 2 can be described the same as the charging circuit. Two charging circuits are in parallel and charged at the same time, the number of bridge-arm s charged is double of bridge-arm normal required number. Besides, the theoretical sum of s natural charged steady voltages is the peak value of valve-side AC line voltage, and thus the natural charged voltage of each is half of its rated value. At any time, there are four bridge-arms s being charged in two phase unites, and the charging period is two thirds of line frequency period. In the charging process, the voltages of upper and lower bridge-arms in each phase unites have equal amplitude but opposite polarity, and thus -side voltage is zero when -side has no voltage source. + Charging circuit Charging Circuit2 A B C Figure 2. Active charging circuit of FB-MMC. _ (2) Active soft-start Strategies Due to over modulation strategy of FB-MMC, it has no limitation that valve-side AC phase voltage peak value should be less than half of bus voltage, and can even realize zero dc voltage control. Thus, the active soft-start strategy of FB-MMC can be chosen from two kinds as follows: ) Strategy : Using the similar active soft-start strategy as HB-MMC, which realizes s voltages balance control by cutting-off a fixed number of s having the highest voltages, and put into closed loop control after charging the s voltages to nearby the rated value. The differences between this strategy of FB-MMC from that of HB-MMC are those: the number of s cutting-off has doubled during the soft-start process. 2 voltage is zero before putting into closed loop control and voltage jumps when the converter is unlocked. 2) Strategy 2: Unlocking the converter and control the voltage following a certain slope from zero to the rated value directly after charging the s to their working voltages through natural soft-start. The comparative analysis of two active soft-start strategies is described as follows. (3) Simulation analysis The simulation analysis of active soft-start uses single-station based on 23 levels FB-MMC converter simulation platform built by MATAB/Simulink. the simulation process of strategy is that, the bypass switch of soft-starting resistance is closed at.3s, the converter is unlocked at.32s

4 to controlling voltage at ±2V, and 6kVar step of inductive reactive power is given at.5s. The active soft-start process is shown as figure 3. From the simulation results, it can be seen that, the problems of active soft-start strategy mainly lie in two aspects. On one hand, the number of s controlled to be cut off exceeds half number of bridge-arm s during boost control, which will bring unstable s voltages and driving signals in the switching progress, and increase the risk of soft-start failure. On the other hand, the voltage is zero before the converter is unlocked, and jumps at the converter unlocked moment, which also will impact problems when charging lines and soft-starting multi-terminal converters. Voltage(V) bus voltage positive bus voltage negative bus voltage s Voltage(V) driving coded signal Figure 3. Active soft-start simulation of FB-MMC using strategy. The simulation analysis of strategy 2 is described as follows. The converter is charged naturally through AC side, and become controllable when s voltages reach to the working value, and is unlocked at.s to controlling voltage from zero to rated value according to a certain slope. The analysis results are shown as figure 4. The active soft-start strategy 2 can effectively solve problems of strategy. On one hand, the converter can be unlocked as long as s voltages reach to the working value. On the other hand, voltage can be set up from zero to the rated value with a certain slope, which brings advantages for soft lines charging and soft-starting of multi-terminal converters. output voltage(v) voltage(v) A phase upper bridgearm voltage(v) driving coded signal s Voltage(V) Figure 4. Active soft-start simulation of FB-MMC using strategy Passive soft-start Passive soft-start can be defined as the process that the converter is charged from side until the AC voltage is setup. For simplicity, this paper still uses a single converter as study object and voltage source is provided by uncontrolled rectifier.

5 () Charging characteristic analysis + Charging circuit A B C Figure 5. Passive charging circuit of FB-MMC. _ As shown in figure 5, for passive charging, three phase units are charged at the same time from side. Taking A phase as an example, the charging process is from positive bus to the upper bridge-arm, then to the lower bridge-arm, at last to negative bus. The principle is similar to that of HB-MMC, here it is no longer described in detail. (2) Passive soft-start strategies ) Strategy : Using the similar passive charging strategy as HB-MMC, a fixed number of s in the upper and lower bridge-arms are controlled to be in cutting off state to realize s voltages balance when voltage is lower than 8% of the rated value. When voltage is higher than 8% of the rated value, the number of cutting off s in the upper and lower bridge-arms increases gradually according to a certain step length to charging the s to rated voltage value, and then the converter is unlocked. The passive soft-start process comes to an end when valve-side AC voltage is set up to rated value. 2) Strategy 2: The s of each bridge-arm are naturally charged firstly, and the converter is unlocked to control the AC voltage when the s voltages rise to the working value. The passive soft-start process comes to an end when valve-side AC voltage is setup to the rated value. The comparative analysis of two passive soft-start strategies is described as follows. (3) Simulation analysis The passive soft-start simulation platform and system parameters of FB-MMC are the same with that of active soft-start analysis, except that side is connected with an uncontrolled rectifier source. Firstly, the simulation of passive soft-start strategy is analyzed, s voltages balance and boost control are implemented between during ~.32s. After that, the converter is unlocked to control AC voltage and frequency, the voltage amplitude command is given by a certain slop from to the rated value, and the valve-side AC voltage reaches to rated value at.8s. The simulation results are shown as figure 6.

6 voltage(v) output voltage (V) driving coded signal s Voltage(V) A Phase upper bridgearm voltage(v) 时间 (s).9 Figure 6. Passive soft-start simulation of FB-MMC using strategy. As shown in figure 6, when using passive soft-start strategy, the whole soft-start process has no impact on voltage. Switching frequency of is higher before the converter is unlocked, but it significantly decreases after the converter is unlocked because the switching frequency optimization control is applied. The bridge-arm voltage is half of voltage before the converter is unlocked and gradually rises to rated value by a certain slope centered on half of voltage. For passive soft-start strategy 2, during ~.3s, the converter is unlocked and s are naturally charged to the steady voltage value. After then, the converter is unlocked to control AC voltage and frequency, according to a certain slop from to the rated value. The simulation results are shown in figure 7. voltage has obvious drop in the instant of unlocking the converter. That reason is that voltage is supported by all s of phase unit before unlocking the converter, however, only the controlled s support voltage in the instant of unlocking the converter, which causes a large bridge-arm current shock. Compared with passive soft-start strategy, strategy 2 is simpler and easier to be implemented, which needs no boost control of s, but there is an obvious bridge-arm current shock in the instant of unlocking the converter. Therefore, it needs to make a specific analysis according to the project s practical conditions. If the bridge-arm current shock is within the allowed range, this paper recommends strategy 2, otherwise strategy is suitable. voltage(v) output voltage (V) A phase upper bridgearm voltage(v) s voltage(v) driving coded signal Figure 7. Passive soft-start simulation of FB-MMC using strategy 2. IV Summary The charging characteristics and soft-start strategies of FB-MMC are studied through the

7 combination of theoretical analysis and simulation verification. The important conclusions can be drawn as follows. For active soft-start strategies, active soft-start strategy 2 of FB-MMC can effectively solve problems of strategy, in which the voltage can be set up from zero to the rated value with a slope, which brings advantages for soft charging and converter starting with multi-terminal converters. For passive soft-start strategies, compared with passive soft-start strategy, strategy 2 is simpler and easier to be implemented, which needs no boost control of s, but there is an obvious bridge-arm current shock in the instant of unlocking converter. So it needs to make a specific analysis according to the project s practical conditions. Acknowledgements This research work is sponsored by the major science and technology project in Henan Province (project number: 42). V References [] Allebrod S, Hamerski R, Marquardt R. New transformerless scalable modular multilevel converters for HV transmission[c]//ieee Power Electronics Specialists Conference. Rhodes, Greece:IEEE, 28: [2] Nikolas Flourentzou, Vassilios G., et al. VSC-Based HV power transmission systems: an Overview[J]. IEEE Transctions On Power Electronics, 29,24(3): [3] Xu Zheng, Tu Qingrui, Qiu Peng. New trends in HV technology viewed through CIGRE2. High Voltage Engineering, 2, 36(2): (in Chinese). [4] Teeuwsen S P Modeling the transbay cable project as voltage-sourced converter with modular multilevel converter design[c]//ieee Power and Energy Society General Meeting.Detroit, MI, United States:IEEE, 2:-8. [5] Gao Zhigang, i Yongdong. Modulation method model and optimization for cascaded H-bridge converters[j]. Electric Power Automation Equipment, 2,3():2-6 (in Chinese). [6] Zhao Chengyong, Xu Jianzhong, i Tan. faults ride-through capability analysis of full-bridge MMC-MT system[j]. Sci China Tech Sci, 23,43():6-4(in Chinese). [7] Y. H. iu, J. Arrillaga, and N. R. Watson. Cascaded H-bridge voltage reinjection Part II:Application to HV Transmission[J]. IEEE Transactions On Power Delivery, 28,23(2):2-26.