Real-Time Simulation of A Modular Multilevel

Converter Based Hybrid Energy Storage System

Feng Guo, PhDNEC Laboratories America, Inc.

Cupertino, CA

5/13/2015

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Outline

• Introduction

• Proposed MMC for Hybrid Energy Storage System

• Real-Time Simulation Results

• Conclusions

Research TargetDesign and Management

Technologies that enable the development of robust multi-

carrier energy hubs ( akamicro-grids) addressing the

triple bottom line of reliability, economy, and environment.

• 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.

ExpertiseExpertise

Reliability

EnvironmentEconomy

Energy Management Department

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Outside Air Cooling System

PG&E Utility Supply (208V 125A)

ControllableHVAC System

NECLA Smart Grid Facility

Static Switch

Programmable AC Source(30kW)

Programmable AC Load (6kW)

NECES Lithium‐Ion Battery (48 V, 105 Ah)

PV System(6kW)

Programmable AC Load (6kW)

VRLA(48 V, 246 Ah)

Inverter (7 kW)

Inverter (5 kW)

PMU

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5

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.

Hybrid ESS with battery and Ultracapacitor (UC) is a better candidate for this application [2]:• Improved battery lifetime.• Reduced battery size.• Improved energy efficiency.

[1]

Battery UltraCap

Utility Grid

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.

Battery

UltraCap

Utility Grid

Utility Grid

Battery

UltraCap

One dc/ac inverter [3] One dc/dc converter and one dc/ac inverter [4]

Two dc/dc converters and one dc/ac inverter [5]

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Proposed MMC Based HESS

SubModule Two switches One UC

Arm N SMs in series One inductor

MMC Six identical arms One battery

UCSwitch

SM 1

Inductor

SM 2

SM n

BatteryA B C

Equivalent SMs

Inductor

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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.

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AdvantagesSingle Stage Power Conversion

Low Switching Frequency High EfficiencyHigh Efficiency

Reduced Switching Device Voltage/Current Ratings Comparable CostComparable Cost

Easy Scalability

Easy Adding Redundancy High ReliabilityHigh Reliability

High Modularity in Hardware and Software

MMC

Conventional Topology

Usage of Well‐Proven Components

Usage of High Performance Switching Devices

SM 5.n

SM 5.1

Utility Grid

Battery

SM 6.1

SM 6.n

SM 4.1

SM 4.n

SM 2.1

SM 2.n

SM 3.n

SM 3.1

SM 1.n

SM 1.1

SM 5.2 SM 3.2 SM 1.2

SM 6.2 SM 4.2 SM 2.2

UC

Battery

UC

Utility Grid

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Efficiency Comparison

94.00%

94.50%

95.00%

95.50%

96.00%

96.50%

97.00%

97.50%

98.00%

98.50%

99.00%

0 200 400 600 800 1000 1200

Efficiency (%

)

Pout(kW)

Calculated Efficiency Under Different Power Distributions

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)

An average of 2.2%improvement

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Real-Time Simulation Platform

Real‐Time Simulator

Scope

Control Station

• 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.

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Circuit Topology Simulation• CPU based simulation.• One core can handle the entire model.• Simulation time step: 20 us.

Number of submodules per arm, N 4Battery voltage, VBatt 1 kVRated power, Pout 1 MWGrid voltage, Vgrid 480 VrmsFundamental frequency, f 60 HzSwitching frequency, fs 1.25 kHzCapacitance of the UC, C 2.5 FResistance of the buffer inductor, Rc 2 mΩInductance of the buffer inductor, Lc 500 uHLine resistance, Rf 1 mΩLine inductance, Lf 120 uH

SM 5.n

SM 5.1

Utility Grid

Battery

SM 6.1

SM 6.n

SM 4.1

SM 4.n

SM 2.1

SM 2.n

SM 3.n

SM 3.1

SM 1.n

SM 1.1

SM 5.2 SM 3.2 SM 1.2

SM 6.2 SM 4.2 SM 2.2

UCVC11

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Control Framework SimulationA 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.

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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.

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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.

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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.

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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.