Real-Time Simulation of A Modular Multilevel Converter Based Hybrid Energy Storage System Feng Guo, PhD NEC Laboratories America, Inc. Cupertino, CA 5/13/2015
Outline Introduction Proposed MMC for Hybrid Energy Storage System Real-Time Simulation Results Conclusions 2
Energy Management Department Economy Reliability Environment Research Target Design and Management Technologies that enable the development of robust multicarrier energy hubs ( aka micro-grids) addressing the triple bottom line of reliability, economy, and environment. Expertise Power/Energy Systems Dynamics and Operation, Power electronics. Optimization Linear and non-linear techniques, Stochastic and Dynamic programming, Robust optimization. Economics economic dispatch, energy markets. 3
NECLA Smart Grid Facility PG&E Utility Supply (208V 125A) PMU Programmable AC Source (30kW) Programmable AC Load (6kW) Inverter (5 kw) Static Switch Inverter (7 kw) NECES Lithium Ion Battery (48 V, 105 Ah) PV System (6kW) VRLA (48 V, 246 Ah) Programmable AC Load (6kW) Outside Air Cooling System Controllable HVAC System 4
Motivation The fluctuation of PV output power makes the use of Energy Storage System (ESS) necessary: Stabilize plant output power. Shave grid peak power. Compensate grid reactive power. The existing Battery Only ESS has the following issues: Limited battery life cycle < 4000 cycles. Reduced battery lifetime as much as 50% under high charging power. [1] Hybrid ESS with battery and Ultracapacitor (UC) is a better candidate for this application [2]: Improved battery lifetime. Reduced battery size. Improved energy efficiency. 5
Existing Circuit Topologies However, current Power Conversion System (PCS) of the HESS has the following issues: Extra dc/dc converters are needed. Not suitable for high power systems (>100 kw). Lower reliability. UltraCap Battery UltraCap Battery Utility Grid Utility Grid One dc/ac inverter [3] One dc/dc converter and one dc/ac inverter [4] Battery Utility Grid UltraCap Two dc/dc converters and one dc/ac inverter [5] 6
Proposed MMC Based HESS SM 1 SM 2 Equivalent SMs Inductor A B C Battery Switch UC SM n Inductor SubModule Two switches One UC Arm N SMs in series One inductor MMC Six identical arms One battery 7
Operation Principle Compared to a typical MMC case, the proposed MMC has different operation principles: 1) The average active power of each SM is not necessarily equal to zero, and the power from the dc side is not necessarily equal to the ac side. 2) The sum of UC voltages in one arm will not necessarily be equal to the battery voltage at dc bus. 8
Advantages Single Stage Power Conversion Low Switching Frequency High Efficiency Battery Usage of High Performance Switching Devices UC Utility Grid Conventional Topology High Modularity in Hardware and Software Easy Adding Redundancy High Reliability Usage of Well Proven Components Battery SM 1.1 SM 1.2 SM 3.1 SM 3.2 SM 5.1 SM 5.2 UC SM 1.n SM 3.n SM 5.n Reduced Switching Device Voltage/Current Ratings Comparable Cost SM 2.1 SM 2.2 SM 2.n SM 4.1 SM 4.2 SM 4.n SM 6.1 SM 6.2 SM 6.n Utility Grid Easy Scalability MMC 9
Efficiency Comparison 99.00% 98.50% 98.00% 97.50% Calculated Efficiency Under Different Power Distributions Efficiency (%) 97.00% 96.50% 96.00% 95.50% 95.00% 94.50% 94.00% 0 200 400 600 800 1000 1200 Pout(kW) An average of 2.2% improvement MMC (Pout=Pbatt, Puc=0) MMC (Pout=2Pbatt=2Puc) MMC(Pout=Puc, Pbatt=0) Traditional (Pout=Pbatt, Puc=0) Traditional (Pout=2Pbatt=2Puc) Traditional (Pout=Puc, Pbatt=0) 10
Real-Time Simulation Platform OP5600 from Opal RT. 2 CPUs, Intel Xeon, Six Core, 3.46 GHz, 12 M Cache. 4 G RAM. 16 Channels Analog Input, 16 Channels Analog Output. 32 Channels Digital Input, 32 Channels Digital Output. 2 Ethernet boards, with one dedicated for IEC61850 communication. Operation System: Redhat. Scope Real Time Simulator Control Station 11
Circuit Topology Simulation CPU based simulation. One core can handle the entire model. Simulation time step: 20 us. Battery SM 1.1 V C11 SM 3.1 SM 5.1 UC Number of submodules per arm, N 4 Battery voltage, V Batt 1kV Rated power, P out 1MW Grid voltage, V grid 480 Vrms Fundamental frequency, f 60 Hz Switching frequency, f s 1.25 khz Capacitance of the UC, C 2.5 F Resistance of the buffer inductor, R c 2mΩ Inductance of the buffer inductor, L c 500 uh Line resistance, R f 1mΩ Line inductance, L f 120 uh SM 1.2 SM 3.2 SM 5.2 SM 1.n SM 3.n SM 5.n SM 2.1 SM 4.1 SM 6.1 Utility Grid SM 2.2 SM 4.2 SM 6.2 SM 2.n SM 4.n SM 6.n 12
Control Framework Simulation A two layer control framework is proposed to operate the MMC based HESS. Coordination Layer Distribute the power depending on different characteristics of battery and UC. Converter Layer Generate desired number of inserted SMs based on battery and UC reference power. Balance the power output from different SMs. 13
Real-Time Simulation Results The power from the battery and UC can be controlled independently from each other. The multilevel AC output voltage can be seen clearly. 14
Real-Time Simulation Results (Cont d) The HESS helps to smooth the PV output power. The real time simulation helps us obtain the circuit operation detail, at the same time reach a long period of time. 15
Conclusions In this presentation, a Modular Multilevel Converter based Battery UltraCapacitor Hybrid Energy Storage System is proposed for Photovoltaic applications. Compared to the traditional HESS topologies, the proposed system features high efficiency, high reliability, and comparable cost. A two layer control framework is proposed to operate the MMC based HESS. Real time simulation results validate the effectiveness of the proposed control framework. 16
References [1] A. Omran, M. Kazerani, and M.M.A. Salama, Investigation of methods for reduction of power fluctuations generated from large grid-connected Photovoltaic systems, IEEE Trans. Energy Conversion, vol. 26, no. 1, pp. 318-327, Mar. 2011. [2] Y. Ye, P. Garg, and R. Sharma, An Integrated Power Management Strategy of Hybrid Energy Storage for Renewable Application, Proceedings of IECON 2014 -- The 40th Annual Conference of the IEEE Industrial Electronics Society, 2014, pp. 3088-3093. [3] R. Dougal, S. Liu, and R. White, Power and life extension of battery-ultracapacitor hybrids, IEEE Trans. Components and Packaging Technologies, vol. 25, no. 1, pp. 120-131, Mar. 2002. [4] L. Gao, R. Dougal, and S. Liu, Power enhancement of an actively controlled battery/ultracapacitor hybrid, IEEE Trans. Power Electronics, vol. 20, no. 1, pp. 236-243, Jan. 2005. [5] B. Hredzak, V. Agelidis, and G. Demetriades, A Low Complexity Control System for a Hybrid DC Power Source Based on Ultracapacitor Lead Acid Battery Configuration, IEEE Trans. Power Electronics, vol. 29, no. 6, June 2014.