A Review of Modular Multilevel Converter based STATCOM Topology * Ms. Bhagyashree B. Thool ** Prof. R.G. Shriwastva *** Prof. K.N. Sawalakhe * Dept. of Electrical Engineering, S.D.C.O.E, Selukate, Wardha, Maharashtra, India ** Dept. of Electrical Engineering, B.D.C.O.E, Selukate, Wardha, Maharashtra, India *** Dept. of Electrical Engineering, S.D.C.O.E, Selukate, Wardha, Maharashtra, India Abstract This paper presents the review of Transformer less Static Shunt Compensator (STATCOM) based on Modular Multilevel Converter (MMC) topology. MMC based STATCOM have many feature over conventional Voltage source converter like Transformer less operation, Sub Modular structure, low expensive due to un necessity of filters redundancy and fault tolerant operation, standard components are used, low PWM carrier Frequency and quality of output waveforms. Due to the advantages of Modular Multilevel Converter over other converter topology, it is used as Power Stages in STATCOM. Different converter models, control methods and modulation strategies are reviewed in this paper. Keywords: Modular Multilevel converter, STATCOM, Voltage source Converter (VSC) 1. Introduction In recent years, Modular Multilevel Converter is an emerging and promising converter topology use for medium to high power converter, which is introduced by Prof. R Marquardt Etal in 2001. MMC is used for various applications, such as renewable energy, smart grid, FACTS, and HVDC systems [1-6]. Multilevel converter can be classified as shown in Fig.1. In which Multi cell Topology is advance topology over two level or three level voltage source converter since its simple realization of redundancy due to large number of cells used, power semiconductor losses are reduced and no filter requirement, the lowest expense, higher voltage levels, modular construction, longer maintenance intervals and improved reliability and the lowest harmonic content due to the large number of output voltage levels they produce. Large number of cells require increase in converter control unit offers simple structure hence cost of manufacture is reduced. Modular multilevel converter among Multi cell topology of Multilevel converter offers additional feature. 1) Transformer less operation and common DC Bus 2) The PWM carrier frequency is low. Hence reduced losses 3) AC voltage and current contain low harmonics, passive filters becomes unnecessary 4) System has fast recovery 5) No bulky capacitors are required [1] Page 1 of 7
Fig.1.Multilevel Converter classification. 2. Structure of MMC Over the two level and three level converter topology Multilevel Converter have low switching frequency which reduces losses. The existing Multilevel Converter Topologies are 1) Diode Clamped or Neutral point clamped (NPC) 2) Flying capacitors or Capacitor Clamped (FC) 3) Cascaded H-Bridge with separate DC sources 4) Modular Multilevel cascaded bridge Converter (M2C) (without separate DC sources). However, in these topologies with the increase of level the number of diode clamp capacitors are required which make system costly and bulkier also the control operation is difficult. Hence over Multilevel Converter topology Modular Multilevel Converter topology is preferred. A MMC consist of Sub-modules which can be 2.1 Half Bridge Sub-module It consist of two IGBT, two anti-parallel diode and DC storage Capacitor two IGBT switches, two anti-parallel diodes and a DC storage capacitor Co. In HBSM, the output voltage can switched between zero or terminal voltage. Fig 2.Half Bridge Sub-module Page 2 of 7
2.2 Full bridge sub-module (FBSM) It consists of four IGBTs with anti-parallel diodes and a capacitor Co. In FBSM, the output voltage can switched between both positive negative terminal voltage. 2.3 Clamp double sub-module (CDSM) Fig 3 Full bridge sub-module Fig 4.Clamp double sub-module Clamp double sub-module consist of two equivalent Half bride sub-module which are connected positive to negative terminal. So that bypass switch position can be interchange. 2.4 Modular Multilevel Converter Fig 5.Modular Multilevel Converter Page 3 of 7
In MMC, number of Sub-modules can be connected in cascaded manner without increasing the complexity of the circuit. The converter arm represents a controllable voltage source controlled with modulation techniques to get the desired output. The main advantages of MMC 1) The internal arm currents are continuous 2) The inductors inserted in the arms limit the AC-current in case of a short circuit at the DC side 3) No separate energy sources are required for sub-module capacitors 4) No common DC link capacitor is required The SM terminal voltage is determined by the states of the four switches. For a SM, there are three operating modes, namely PWM mode, natural rectifying mode, and forbidden mode. In the PWM mode, four IGBT, T1 to T4 receive PWM gating signals. The pair of T1 and T2 has complementary signals, as well as the pair of T3 and T4. The SM terminal voltage is either equal to its capacitor voltage, or the negative of capacitor voltage, or zero. When there is at least one pair of IGBT being blocked, the SM is in the natural rectifying mode. The terminal voltage is determined by the current direction, which forces certain anti parallel diodes to conduct. When there is no current, the SM has high impedance and the terminal voltage is determined by the external circuit. Two IGBT in one pair cannot have ON signals at same time. This mode will short circuit the capacitor and damage the device and therefore is forbidden. 3. Control Strategies for MMC Statcom In MMC, the AC and DC current or voltage is controlled as per the application. The control of MMC is divided into different classes according to different control objective, which are discuss in reviewed papers. The main control objective in MMCs is the control of voltages, or currents, at its terminals. Depending on the application, it could be required to control the DC and AC voltages or currents. The converter works as an energy interface between input and output using the cell capacitors as energy storage elements. For stable operation in steady-state, the average capacitor voltage must be adjusted or controlled. Additionally, there are secondary control objectives such as the influence of circulating currents and equalizing stress on the power cells, low switching losses, etc. Therefore, the control of the MMC can be divided into several classes according to the different control objectives: control of external voltages and/or currents, control of internal currents, control of the average capacitor voltage and control of the capacitor voltage balance. Early MMC publications describe the structure of the control and develop an open-loop strategy calculating the required voltages in each arm depending on the input and output voltages. Nowadays a closed-loop control strategy is proposed to control the DC current, average voltage and voltage balance is used where the average voltage capacitor control is located in the outer control loop. 3.1 Arm Current Control Arm current control method is use for Input and output voltage and current control depending upon converter configuration arm current can be controlled by controlling arm voltage. The control strategy used in MMC for calculation of modulation signals at a given operating point from input and output references [2]. Which effectively controls input and output current but need compensation for eliminating steady state error[3]. In addition, it does not directly control internal dynamics of converters [4]. So a closed-loop strategy can be used to calculate the modulation indices based on the error between the arm currents and their references [5]. The references of the arm currents are calculated based on the output Page 4 of 7
and input current references, but the closed-loop configuration automatically rejects any steady-state error. The closed-loop controller can be implemented with several types of controllers ranging from the simple PI controller in rotating coordinates up to more sophisticated controllers such as LQR controllers. 3.2 Circulating Current Control MMCs produce circulating currents, which are internal currents that flow among the converter arms but do not flow outside the converter [6]. One source of circulating currents is the capacitor voltage ripple, where the frequency of the circulating current is twice the output frequency. The instantaneous changes between voltage levels due to multilevel modulation will also produce circulating currents but at switching frequency. The controller can, optionally, minimize the circulating current in order to reduce losses and to improve controllability. On the other hand, it can also be maintained at a given value to minimize the capacitor voltage ripple, reducing the required capacitance of the cell. However, this alternative increases the losses and the power rating of the semiconductors. The circulating currents can be controlled defining them as a part of the input current of each arm or with a completely independent control scheme. Both alternatives have a comparable performance but the use of an independent controller simplifies controller design. The most common method to control circulating currents is the use of one, or a group, of resonant controllers [7]. The circulating currents due to multi-level modulation can be minimized in the modulation stage. For example, a modified carrier disposition modulation can be used to avoid multiple simultaneous commutations. 3.3 Capacitor Voltage Control In order to make the MMC operate properly, the capacitor voltages must be controlled at a given voltage reference level. The control of these capacitors is usually separated in terms of average voltage and voltage balance. In turn, the voltage balance can be separated into two stages: the control of the imbalance among arms and the voltage balance of cells inside each arm. The first stage can be integrated into the control scheme, distributing the energy among arms by changing the current references. The second stage must balance the cell voltage differences inside each arm by acting on the modulation signal or directly on the switching pattern. This arm balance stage will be analysed in the next section. One of the control objectives is to keep the average voltage of all cells at the reference. The balance of voltages, or energy, among the arms can be performed by modifying the currents in the different arms. In order to minimize the impact on input and output currents, the components of circulating current and common mode voltages are usually employed for this task. There are several strategies for accomplishing this balancing: using only the positive and negative arm, improving the balance per phase or with a combination of both methods. 3.4 Capacitor Voltage Balance Control As shown in the previous section, the voltage balancing can be divided into the two categories: in arm voltage or arm energy balance and voltage balance inside each arm. The latter is, due to the dependency of the arm current, usually implemented in the modulation stage. However, it is possible to include it in the control system by adding, to the modulation signal of each cell, a term which depends on the capacitor voltage error and the arm current, as shown in If the error is positive this term charges the capacitor of the corresponding cell and, if the error is negative this term discharges it. Therefore, this additional term also depends on the current direction. It is important to note that this method modifies the modulation index previously calculated by the current controllers, leading to a small deviation from the current references. Page 5 of 7
A similar result, but with lower impact on the current control loop, can be obtained using a single compensation term added to the upper and subtracted from the lower arm modulation index, This method injects a common mode voltage in the arms, and therefore cannot be used in systems with neutral to ground connection. A second way to obtain capacitor charging/discharging action is injecting an adjustment to switching times directly prior to modulation. 4. Proposed Methodology Fig 6 Proposed Methodology for Full Bridge MMC STATCOM In proposed methodology MMC STATCOM is connected between source and load. Usually a STATCOM is installed to support electricity networks that have a poor power factor and oftenpoor voltage regulation. There are however, other uses, the most common use is for voltage stability. A STATCOM is a voltage source converter (VSC)-based device, with the voltage source behind a reactor. The voltage source is created from a DC capacitor and therefore a STATCOM has very little active power capability. However, its active power capability can be increased if a suitable energy storage device is connected across the DC capacitor. 5. Conclusion Modular multilevel converters based STATCOM have attractive operational features in terms of high voltage operation, power quality, efficiency, redundancy and modularity. These features have attracted the interest of the research community, resulting in a large number of new configurations, models, control schemes and modulation strategies. The full bridge Modular Multilevel converter based STATCOM is advanced topology reviewed in this paper. This work presented a review of the latest achievements regarding modular multilevel converters in terms of modeling, control. 6. References [1] Circuit Topologies, Modeling, Control Schemes and Applications of Modular Multilevel Converters Marcelo A. Perez, Senior Member, IEEE, Steffen Bernet, Member, IEEE, Jose Rodriguez, Fellow, IEEE, Samir Kouro, Member, IEEE, Ricardo Lizana, Student Member, IEEE10.1109/TPEL.2014.2310127, IEEE Transactions on Power Electronics. [2] M. Glinka, Prototype of multiphase modular-multilevel-converter with 2 mw power rating and 17-level-output-voltage, in Power Electronics Specialists Conference, 2004. PESC 04. 2004 IEEE 35th Annual, vol. 4, 2004, pp. 2572 2576 Vol.4. [3] Haider, N. Ahmed, L. Angquist, and H.-P. Nee, Open-loop approach for control of multiterminal dc systems based on modular multilevel converters, in Power Electronics and Applications (EPE 2011), Proceedings of the 2011-14th European Conference on, 2011, pp.1 9. Page 6 of 7
[4] L. Harnefors, A. Antonopoulos, S. Norrga, and H. Angquist, L. and Nee, Dynamic analysis of modular multilevel converters, Industrial Electronics, IEEE Transactions on, vol. PP, no. 99, p. 1, 2012. [5] Lachichi and L. Harnefors, Comparative analysis of control strategies for modular multilevel converters, in Power Electronics and Drive Systems (PEDS), 2011 IEEE Ninth International Conference on, 2011,pp. 538 542. [6] X. She and A. Huang, Circulating current control of double-star chopper-cell modular multilevel converter for hvdc system, in IECON 2012-38th Annual Conference on IEEE Industrial Electronics Society, 2012, pp. 1234 1239 [7] Z. Li, P. Wang, Z. Chu, H. Zhu, Y. Luo, and Y. Li, A novel inner current suppressing method for modular multilevel converters, in Energy Conversion Congress and Exposition (ECCE), 2012 IEEE, 2012, pp. 4506 4512 [8] Modular Multilevel Converter Based STATCOM Topology Suitable for Medium-Voltage Unbalanced Systems Hassan Mohammadi Pirouzy and Mohammad Tavakoli Bina. Journal of Power Electronics, Vol. 10, No. 5, September 2010 [9] Modeling and Control of a Full-Bridge Modular Multilevel STATCOM. Wei Li, Member, IEEE, L.-A. Grégoire, Member, IEEE, and J. Bélanger Member, IEEE, power and energy general society meeting 2014 Page 7 of 7