Bo Dong

Yongdong Li

Zhixue Zheng
email: zheng-zx09@mails.tsinghua.edu.cn

Control Strategies of DC-Bus Voltage in Islanded Operation of Microgrid

Keywords: microgrid, grid-connected operation, islanded operation, storage system I

This paper focuses on the energy management system and stability of DC bus in both grid-connected and islanded operations in microgrid system. The microgrid system consists of wind turbines, photovoltaic panels, batteries and supercapacitors, also includes both AC and DC zones. Voltage of DC bus must be kept stable especially in islanded operation. In gridconnected operation voltage of DC bus is controlled by inverter. Real power from renewable energy generations and storage system can be transferred to AC zone through DC bus. In islanded operation, inverter must be controlled to keep magnitude and frequency of AC bus stable, so storage system is used to regulate voltage of DC bus. Simulation results in the paper show that voltage of DC bus can be kept steady and power can be kept balance with the strategy in microgrid system.

INTRODUCTION (HEADING 1)

More and more renewable energy generations such as wind turbines and photovoltaic panels are integrated into grid, which will lead grid instability because their fluctuate power.

Microgrid is one of the solutions. The advantages of the microgrid consist of -the association of the local generators and local loads in order to minimize the energy losses in the electricity transport; -the possibility of electricity and heat cogeneration in order to increase the energy generation efficiency; -the ease of using communication means in order to optimize the total electricity generation and to increase the quality and the reliability of the power system [START_REF] Tao | Power management strategies of a DC-coupled hybrid power system in a microgrid for decentralized generation[END_REF][START_REF] Zhenhua | Hybrid DC-and AC-Linked Microgrids: Towards Integration of Distributed Energy Resources[END_REF] .

One unit of the microgrid is established in the lab as shown in Fig 1 . It consists of a wind turbine (2KW), photovoltaic panels (1KW), batteries (one cell is 12V, 65AH and six cells series connected) and super-capacitor (72V, 70F). The stability of DC-and AC-bus voltage is very important. When microgrid is connected to the utility grid, the voltage of DC-bus voltage (200V) is regulated by the inverter. However when the microgrid works in islanded operation, DC-bus voltage must be regulated by renewable energy generations and storage system. And the magnitude and frequency of AC bus are controlled by droop character of parallel inverters [START_REF] De Brabandere | A voltage and frequency droop control method for parallel inverters[END_REF][4][START_REF] Piagi | Autonomous control of microgrids[END_REF] .

A. In Grid-Connected Operation

Voltage of DC bus can be controlled by inverter in gridconnected operation. Control strategy of inverter is shown in Fig. 2. Real power from photovoltaic panels, wind turbine and storage system is all delivered to DC load and AC bus through DC bus when voltage of DC bus is stable. If the power is not enough for the loads, microgrid will absorb power from utility grid. If power from microgrid is more than needed, it will transfer power to utility grid.

B. In Islanded Operation

Inverter must supply real and reactive power to AC zone in order to keep magnitude and frequency of AC bus stable in islanded operation when there some faults happened in utility grid. So voltage of DC bus must be regulated by Bi-DC/DC converters which connect batteries and super-capacitor to DC bus. Also because power from wind turbine and photovoltaic panels is fluctuant, storage system must absorb or release the power difference between renewable energy generations and loads which can be calculated by equation (1). Power flow in the microgrid system is shown in Fig. 3. Very little power is absorbed or released by DC bus. When voltage of DC bus is stable and power absorbed by DC bus is zero in a whole period and real power from renewable energy generations and storage system is all delivered to inverter through DC bus. Power difference includes high frequency and low frequency power. Super-capacitor as a fast-dynamic storage unit is used to absorb high frequency power, and battery as a long-term storage unit is controlled to absorb low frequency power for a long time. We can get power reference with the control strategy as shown in Fig. 4 where K is a gain determined by rating and state of charge of super-capacitor and batteries [START_REF] Zhou | Composite energy storage system with flexible energy management capability for micro-grid applications[END_REF] . Bi-DC/DC converter is a bidirectional buck-boost converter as shown in Fig. 5. We should control PWM of T1 and T2 to make voltage of DC bus stable and absorb the power calculated by control strategy in Fig. 4.

For inductor L, equation (4) describes the relationship between voltage and current:

in m L L dI d t U U = - ( 4
)
where L is the equivalent inductance, ( )

m ref in L ref L U U PI I I - - = + - (5)
The average modulation function of converter is obtained by equation ( 6):

m ref dc U m U - = (6)
So we can get control strategy of Bi-DC/DC converter for voltage of DC bus from upper equations as shown in Fig. 6. In islanded operation storage system is used to not only keep voltage of DC bus stable but also absorb or release the power difference between renewable energy generations and loads with the control strategy [START_REF] Nikkhajoei | Distributed Generation Interface to the CERTS Microgrid[END_REF][START_REF] Fang | A Control Strategy for a Distributed Generation Unit in Grid-Connected and Autonomous Modes of Operation[END_REF] .

1/Uout

where

storage m ref batt m ref sc m ref I I I -- -- -- = + (8)
In Fig. 4 we can get equation ( 9) and ( 10

(1 )

s c m ref storage m ref I K I -- -- = - (10)
K is determined by rating and SOC of super-capacitor and batteries. Also the gain will affect voltage of DC bus in transient operation when DC loads change or power from wind turbine and photovoltaic panels is fluctuant rapidly. The super capacitors as fast-dynamic storage system must absorb or release power quickly. We can make K=0.1~0.3 in transient operation.. After transition the strategy controls K=0.7~0.9, because the batteries store more energy.

III. SIMULATION AND EXPERIMENTAL RESULTS

When there are some faults happened in utility grid, mircogrid will work in islanded operation. At the time, voltage of DC bus is not kept by inverter. Instead, the storage system and renewable energy generations will keep DC-bus voltage constant with control strategies. Fig. 7 shows that when t=0.2s microgrid system began to work in islanded operation, voltage of DC bus can be kept stable. When DC loads change in islanded operation, the voltage of DC bus should be kept constant by storage system, and different K will lead different response speed. From Fig. 8 and Fig. 9, we can see when DC loads changed or power from wind turbine and photovoltaic panels was fluctuant rapidly, the super capacitors must be release or absorb more power. This paper introduces the strategies of different converters in the hybrid DC and AC bus linked microgrid. Also strategies for grid-connected, islanded and transition operations are described in the paper. With these strategies, the power between microsources, storage systems, loads and utility grid could be kept balance. In island operation, the microsources and storage systems are used to keep the voltage of DC bus steady and the simulation results are described.

Fig 1 .

 1 Fig 1.Hybrid DC and AC linked microgrid

Fig 2 .

 2 Fig 2.Block diagram of the controller for inverter

  power of storage system including batteries and super-capacitor. b a tt P and s c P are power released from battery and super-capacitor respectively. And w in d P is generated by wind turbine. s o la r P is generated by photovoltaic panels. lo a d P is power absorbed by DC and AC loads.

Fig 3 .

 3 Fig 3.Power flow in the system

Fig 4 .

 4 Fig 4.Control strategy of storage system

Fig 5 .

 5 Fig 5.Bi-DC/DC converter Control strategy of Bi-DC/DC converter as shown in Fig. 5 is very important to keep voltage of DC bus stable in islanded operation. The storages exchange power with others by the DC-capacitor:

inU

  is the voltage of battery or super capacitor, m U is the modulated voltage of chopper. According to equation (4) and linear relationship between m ref I -and L ref I -, m ref U -is obtained by equation (5):

Fig 6 .

 6 Fig 6. Block diagram of the controller for charging In equation (3) m ref I -

Fig 7 .

 7 Fig 7.Voltage of DC bus when microgrid works from grid-connected to island

Fig 8 .

 8 Fig 8.Voltage of DC bus (up) and current of load (down) when K=0.3

Fig

  Fig 10a.Currents of inductance L

ACKNOWLEDGMENT This paper and its related research are supported by grants from the Power Electronics Science and Education Development Program of Delta Environmental & Educational Foundation.


I. INTRODUCTION

Batteries in series connected have been used widely, such as electric vehicles, hybrid electric vehicles, microgrid system with distributed renewable energy generations and so on. Because of variant of manufactures and working environment, aging, thermal difference, imbalance between charging and discharging, and differences in internal impedances and selfcharging rate, the energy (or charge) stored in cells are not equal [START_REF] Cao | Battery balancing methods: A comprehensive review[END_REF]. Because all the cells are in series, the battery performance is limited to the weakest cell. In the worst case, when SOC of one battery is very low and SOC of another is very high, the battery pack nearly can't be used. These battery mismatches reduce the lifetime, efficiency, and capacity of the battery. So equalization of SOC is very important for seriesconnected batteries.

There are many equalization methods. References [START_REF] Stuart | Fast Equalization for Large Lithium Ion Batteries[END_REF][START_REF] Lindemark | Individual cell voltage equalizers (ICE) for reliable battery performance[END_REF] focus on a kind of dissipative method, which is simple but with high loss. Switched capacitor method is described in [START_REF] Kimball | Analysis and design of switched capacitor converters[END_REF][START_REF] Kimball | Increased Performance of Battery Packs by Active Equalization[END_REF][START_REF] Baughman | Double-Tiered Switched-Capacitor Battery Charge Equalization Technique[END_REF][START_REF] Pascual | Switched Capacitor System for Automatic Series Battery Equalization[END_REF]. Control is easy and batteries can work in both recharging and discharging operation with this method. But energy is transferred from one cell to the adjacent one, so equalization time is long. Multi Bi-DC-DC converter method is described in [START_REF] Moo | Operation of Battery Power Modules with Series Output[END_REF][START_REF] Hong | Charge Equalization of Battery Power Modules in Series[END_REF][START_REF] Moo | Parallel Operation of Battery Power Modules[END_REF], which can control every cell to charge or discharge. But cost of multi Bi-DC-DC converters is high because there are many switching devices and inductors. Reference [START_REF] Kutkut | Charge Equalization for an Electric Vehicle Battery System[END_REF] focuses on multi-winding transformer method. The circuit of this method is complex and cost is high. Modularized charge equalization method in [START_REF] Kim | A Modularized Charge Equalizer Using Battery Monitoring IC for Series Connected Li-Ion Battery Strings in an Electric Vehicle[END_REF] can charge any cell in a battery pack which is used for a large number of cells, but the equalization time is long.

Although battery has low price and high energy density, but it can't absorb or release power very quickly. So supercapacitor is needed as fast energy storage to reduce the influence of power fluctuation, especially in renewable energy generation system. Super-capacitor has advantage of high power density, high cycle life and large charging or discharging current [START_REF] Pedram | Hybrid electrical energy storage systems[END_REF]. But its cost is much higher than leadacid battery. So a hybrid storage system is needed including batteries and super-capacitor.

A DC-and AC-bus microgrid system with distributed renewable energy generations is established in the lab which consists of wind turbine (2KW), photovoltaic panels (1KW), super-capacitor (72V, 70F) and batteries (one battery is 12V, 65AH and six batteries are connected in series) as shown in Fig. 1. Both batteries and super-capacitor are connected to DC bus with an isolated DC-DC converter. Also there is a kind of composite converter between batteries and super-capacitor. For Energy Management System (EMS) of microgrid system, it is very important to keep voltage of DC-and AC-bus stable [START_REF] Bilbao | Design and development of a supercapacitor-based microgrid dynamic support system[END_REF][START_REF] Zhenhua | Hybrid DC-and AC-Linked Microgrids: Towards Integration of Distributed Energy Resources[END_REF]. In islanded operation in order to make local loads working steadily, the hybrid storage system must be controlled to absorb or release power difference between renewable energy generations and local loads by EMS. According to characters of hybrid storages, super-capacitor is used to absorb or release high frequency power and batteries are used for long time storage. However super-capacitor can only store very little energy, so it is usually out of charge which will lead hybrid storage system to lose the ability of response to high frequency power. It is very dangerous especially in islanded operation. With composite converter, energy can be transferred between batteries and super-capacitor. So when super-capacitor has low or high SOC, it can absorb or release energy with batteries until SOC is normal. Meanwhile, after a long time usage, SOC of batteries will be different. Because all the batteries are connected in series, the battery performance is limited to the weakest one. The Composite Converter is also used to make batteries charge equalization through choosing switches.

In this paper, section II introduces the structure, principles of operation and control strategy of the composite converter. Section III shows simulation results. Section IV concludes the full paper.

II. COMPOSITE CONVERTER

A. Structure of Composite Converter

The composite converter includes a Bi-DC-DC converter, positive and negative switches as shown in Fig. 2. Bi-DC-DC

Composite Converter of Hybrid Storage in Distributed Renewable Energy Generation System

Bo Dong, Yongdong Li, Zhixue Zheng Department of Electrical Engineering, Tsinghua University Beijing, China, 10084 E-mail: dongb07@mails.tsinghua.edu.cn Fig. 1. Microgrid system with hybrid DC-and AC-bus converter is used to transfer energy between batteries and super-capacitor according to SOC of them. There are six batteries in the hybrid storage system, and each battery has a pair of switches. All the positive switches are connected to positive output of converter, and all the negative switches are connected to negative pole of super-capacitor.

B. Operation Principle of Composite Converter

The composite converter can be used to balance SOC of batteries by controlling the switches. When SOC of battery is lower than 30%, it is in low zone. If SOC is between 30% and 85%, it is normal. And it is in high zone when SOC of battery is higher than 85%. Different control strategies will be used in different zones [START_REF] Tao | Power management strategies of a DC-coupled hybrid power system in a microgrid for decentralized generation[END_REF]. In order to avoid oscillation at the threshold, hysteresis control is used as shown in Fig. 3. For example, when battery is charging, high SOC zone is defined that SOC of battery must be higher than 90%. And if battery discharging from high to normal, SOC must be lower than 80%. SOC of battery can't be calculated directly from the voltage of positive and negative poles when the battery is working. Several methods for estimating SOC of battery are described in [START_REF] Juang | System identification-based lead-acid battery online monitoring system[END_REF][START_REF] Wu | State of charge Estimation for NiMH Battery based on electromotive force method[END_REF]. Just like battery, when SOC of super-capacitor is lower than 20%, it is in low zone. If SOC is between 20% and 90%, it is normal. And it is in high zone when SOC of super-capacitor is higher than 90%. Also different control strategies will be used in different zones. Hysteresis control of super-capacitor is used as shown in Fig. 4. If SOC of Bi (i=1,2,…) is higher, EMS controls positive switch SPi and negative switch SNi on, and others off. In this case, Bi connects to the converter and transfers energy to super-capacitor. And then super-capacitor will release energy to the lower battery through converter by choosing the switches until SOC of all the batteries are equal. So EMS could control super-capacitor and composite converter to balance the SOC of all the batteries in battery pack as a buffer. Suppose the SOC difference of batteries is not very large, super-capacitor has the storage capability to finish the equalization.

Meanwhile, composite converter can regulate SOC of super-capacitor. If SOC of super-capacitor is high, EMS will control switches to choose one or several batteries which have low SOC. Energy will be transferred from super-capacitor to chosen batteries until SOC of super-capacitor is normal. If SOC of super-capacitor is low, EMS will choose one battery with high SOC to connect to converter. The battery will transfer energy to super-capacitor until SOC of supercapacitor is normal. When SOC of super-capacitor becomes a little higher, more batteries can be chosen to charge for supercapacitor.

C. Control Strategy of Bi-DC/DC

If SOC of super-capacitor is high or one of batteries is in low SOC zone, composite converter transfers energy from super-capacitor to the battery. It is a buck converter. Batteries can be charged with constant voltage or constant current. If the battery discharges deeply, charging method with constant current will be used. The block diagram is shown in Fig. 5. Otherwise the battery will be charged with constant voltage as shown in Fig. 6. When SOC of super-capacitor is low or one of batteries is in high SOC zone, the battery will discharge through composite converter. It is a boost converter. The device IGBT connects to super-capacitor directly, so constant charging current for super-capacitor can't be controlled. But charging method with constant power can be used if energy loss of converter is ignored. Also super-capacitor can be charged with constant voltage. The block diagram of constant charging power and voltage are shown in Fig. 7 and Fig. 8, respectively.

D. Energy Management of Hybrid Storage System

In the microgrid system, usually there are many renewable energy generations and local loads. In grid-connected operation, when power from renewable energy generations is fluctuant or local loads change quickly, hybrid storage system including batteries and super-capacitor must absorb or release power difference very fast. According to the characters of battery and super-capacitor, EMS controls super-capacitor to absorb or release high frequency power as a fast storage, and battery to absorb or release low frequency power as a longtime storage [START_REF] Haihua | Composite energy storage system with flexible energy management capability for micro-grid applications[END_REF]. Control strategy of hybrid storage system is shown in Fig. 9. When there are some faults happened in utility grid, mircogrid will work in islanded operation. In this case power difference between renewable energy generations and local loads must be absorbed or released by hybrid storage system. If SOC of batteries or super-capacitor is low or high, composite converter can transfer energy between them. However, in islanded operation if both of them have high SOC and power generated from photovoltaic panels and wind turbine is more than need of local loads, EMS will control photovoltaic panels and wind turbine to replace MPPT operation by load power matching operation. If both of batteries and super-capacitor have low SOC and power from renewable energy generations can't match the need of local loads, the normal loads must be cut from microgrid system in order to keep sensitive loads working.

III. SIMULATION RESULTS

Simulation results of battery equalization with composite converter are shown in Fig. 10 with Matlab/Simulink where we suppose super-capacitor can store enough energy. The initial SOCs of the batteries are SOC B1 =75%, SOC B2 =66%, SOC B3 =68%, SOC B4 =70%, SOC B5 =72%, SOC B6 =60%. With control strategy of hybrid storage system, supercapacitor and batteries absorb or release high and low frequency power through the isolated DC-DC, respectively. Fig. 11 shows currents of batteries and super-capacitor when local loads in microgrid system change from 500W to 1300W at 0.3s and from 1300W to 800W at 0.6s. A kind of composite converter is presented in this paper. Structure, operation principle and control strategy are described. With the composite converter, energy can be transferred between batteries and super-capacitor in the hybrid storage system. The composite converter could balance the state of charge of batteries by controlling the switches when SOCs of batteries are different. Also SOC of super-capacitor could be regulated when it is low or high in order to increase the stability of microgrid system, especially in islanded operation. Simulation results verify that SOC equalization of batteries can be finished with the composite converter.

Fig. 1 .

 1 Fig. 1. Structure of Composite Converter

Fig. 3 .

 3 Fig. 3. Hysteresis control of battery's SOCState of charge of super-capacitor can be calculated in[START_REF] Cao | Battery balancing methods: A comprehensive review[END_REF]. Just like battery, when SOC of super-capacitor is lower than 20%, it is in low zone. If SOC is between 20% and 90%, it is normal. And it is in high zone when SOC of super-capacitor is higher than 90%. Also different control strategies will be used in different zones. Hysteresis control of super-capacitor is used as shown in Fig.4.

Fig. 4 .

 4 Fig. 4. Hysteresis control of super-capacitor's SOC

Fig. 5 .Fig. 6 .

 56 Fig. 5. Block diagram of charging for battery with constant current

Fig. 7 .Fig. 8 .

 78 Fig. 7. Block diagram of charging for super-capacitor with constant power

Fig. 9 .

 9 Fig. 9. Control strategy of hybrid storage system

Fig. 10 .

 10 Fig. 10. Simulation results of battery SOC equalization

Fig. 11 .

 11 Fig. 11. Current of batteries and super-capacitor

ACKNOWLEDGMENT

  This paper and its related research are supported by grants from the Power Electronics Science and Education Development Program of Delta Environmental & Educational Foundation.