Distribution Voltage Control for DC Microgrids Using Fuzzy Control and Gain-Scheduling Technique

7/28/2019 Distribution Voltage Control for DC Microgrids Using Fuzzy Control and Gain-Scheduling Technique

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2246 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 28, NO. 5, MAY 2013

Distribution Voltage Control for DC MicrogridsUsing Fuzzy Control and Gain-Scheduling Technique

Hiroaki Kakigano, Member, IEEE, Yushi Miura, Member, IEEE, and Toshifumi Ise, Member, IEEE

AbstractInstallation of many distributed generations (DGs)could be detrimental to the power quality of utility grids. Micro-grids facilitate effortless installation of DGs in conventional powersystems. In recent years, dc microgrids have gained popularity be-cause dc outputsourcessuch as photovoltaicsystems,fuel cells, andbatteries can be interconnected without ac/dc conversion, whichcontributes to totalsystem efficiency. Moreover, high-quality powercan be supplied continuously when voltage sags or blackouts occurin utility grids. We had already proposed a low-voltage bipolar-type dc microgrid and described its configuration, operation, andcontrol scheme, through experiments. In the experiments, we usedone energy storage unit with a dc/dc converter to maintain thedc-bus voltage under intentional islanding operation. However, dc

microgrids shouldhavetwo or more energystorageunitsfor systemredundancy. Therefore, we modified the system by adding anotherenergy storage unit to our experimental system. Several kinds ofdroop controls have been proposed for parallel operations, someof which were applied for ac or dc microgrids. If a gain-schedulingcontrol scheme is adopted to share the storage unit outputs, thestorage energy would become unbalanced. This paper thereforepresents a new voltage control that combines fuzzy control withgain-scheduling techniques to accomplish both power sharing andenergy management. The experimental results show that the dcdistribution voltages were within 340 V 5%, and the ratios ofthe stored energy were approximately equal, which implies thatdc voltage regulation and stored energy balancing control can berealized simultaneously.

Index TermsDC power systems, fuzzy control, gain-schedulingcontrol, microgrids.

I. INTRODUCTION

INSTALLATION of distributed generations (DGs) has re-

markably increased in recent years because of the reduction

in greenhouse gases and depletion of fossil fuels. After the

2011, Tohoku earthquake and tsunami, the trends have accel-

erated even more in Japan. As it is well known, installation of

many DGs could be detrimental to the power quality of util-

ity grids; therefore, microgrids have been studied as one of the

candidates to support smooth installation of DGs [1][6]. In

particular, dc distribution microgrids have been studied for a

few years [7][12]. Apart from a reduction in ac/dc conver-

sions losses, dc microgrids can supply continuous high-quality

power when voltage sags or blackouts occur in utility grids. For

Manuscript received April 28, 2012; revised July 4, 2012; accepted August27, 2012. Date of current version November 22, 2012. Recommended for pub-lication by Associate Editor L. Chang.

The authors are with the Division of Electrical, Electronic, and In-formation Engineering, Osaka University, Osaka 565-0871, Japan (e-mail:[email protected]; [email protected]; [email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TPEL.2012.2217353

instance, dc power supplies are commonly used in telecommu-

nication buildings and internet data centers where high-quality

power is needed [13].

We previously proposed a low-voltage bipolar-type dc mi-

crogrid, and described the configuration, operation, and con-

trol scheme, which were demonstrated through experimental

results [14], [15]. In the experiment, we used one energy stor-

age unit with a dc/dc converter to sustain dc-bus voltage when

the system was in intentional islanding operation. However, it is

desired that the system should have two or more energy storage

units for redundancy. Several types of droop controls have been

proposed for the parallel operations among, which some wereapplied for ac/dc microgrids [16][21]. Therefore, we added an-

other energystorage unit in theexperimental system andadopted

gain-scheduling control technique as the droop controls for shar-

ing the outputs. In this case, although output power sharing of

two energy storage units was achieved, it was found that the

storage energy control was necessary for the system operation

proposed in [14], [15].

In this paper, we propose a new voltage control method that

combines fuzzy control with gain-scheduling control technique

in order to accomplish both power sharing and energy manage-

ment simultaneously. Some researches proposed to apply fuzzy

controls to microgrid operations. A fuzzy-based controller wasapplied to all DGs in the microgrids, and the system perfor-

mance was analyzed by numerical simulations [22]. A fuzzy

control in addition to a PI controller was used to stabilize the

rate of a diesel generation [23]. A fuzzy control was applied to

maximum power point tracking control of a photovoltaic sys-

tem in an isolated microgrid to improve the response against

rapidly changing weather conditions [24]. In this paper, we fo-

cus on application of fuzzy control to gain-scheduling control

and demonstrate the validity of the proposed control through

experiments.

In our experimental system, electric-double-layer capacitors

(EDLC) were used as the energy storage unit, and each dc/dc

converter of EDLC controlled the dc distribution voltage when

the system was operated under an intentional islanding mode.

The experimental results show that the dc distribution voltages

were within 340 V 5%, and the energy ratios of the stor-age units were approximately equal, implying that dc voltage

regulation and stored energy balancing control can be realized

simultaneously by applying the proposed control.

II. DC MICROGRID FOR A RESIDENTIAL COMPLEX

A. System Configuration

We proposed a dc microgrid for a residential complex, as

shown in Fig. 1 [15]. There are around 50100 houses in the

0885-8993/$31.00 2012 IEEE


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KAKIGANO etal.:DISTRIBUTIONVOLTAGECONTROLFORDC MICROGRIDSUSINGFUZZYCONTROLANDGAIN-SCHEDULINGTECHNIQUE 2247

Fig. 1. System configuration of the dc microgrid for a residential complex.

system, each having a micro combined heat and power unit

(micro-CHP unit) such as a gas engine or a fuel cell. The micro-

CHP units are connected to a dc distribution line (three wire,

170 V), and the electric power is shared among the houses.Cogenerated hot water is either used by individual house or

shared between adjacent houses. The utility grid is connected

to the system by a rectifier. At the load side, various forms of

electric power (such as ac 100 V and dc 48 V) can be obtained

from the converters. EDLCs are used as the main energy storage

unit because of their advantages such as fast response, safety

(especially compared with Li-ion batteries), easy measurement

of the stored energy, and no toxicity of the constituent materials.

In a verification test of energy storage system using an EDLC

unit, the voltage, and maximum energy were 500 V and 5 MJ,respectively [25]. From this, EDLC is considered viable as an

energy storage system in a small grid. The capital cost per

kilowatthour (kWh) for EDLC is 3002000 dollars, while that

for lead-acid and Li-ion batteries is 200400 and 6002500

dollars, respectively [26]. In addition, EDLCs can handle many

chargedischarge cycles, and therefore has a low-cost per cycle

considering its long life. The capital cost per kWh-per cycle

for EDLC is 24 cents, while that for lead-acid and Li-ion

batteries is 20100 and 15100 cents, respectively [26]. The

disadvantageof EDLC is itslow-energy density. If a large energy

capacity is needed for a microgrid, a relatively large EDLC

is required. However, a large-energy capacity is not necessaryfor the proposed dc microgrid because the micro-CHP units

are operated to prevent over charge/discharge of the EDLCs as

described in the following section.

B. System Operation

The total output power of micro-CHP units can be controlled

by changing the number of running micro-CHP units. When

the system is connected to the utility grid, any deficiency in the

power supplied by the microgrid is compensated by the power

from the utility grid as shown in Fig. 2. In the interconnected

operation, the rectifier controls the dc distribution voltage, and

the supervisor computer changes the number of the running

Fig. 2. Interconnected operation.

Fig. 3. Intentional islanding operation.

micro-CHP units such that the power from the utility grid is

withinthe contract demand. In addition, thesupervisor computer

decides the order of the operation of the micro-CHP units so as

to meet the heat commitment.

When the system is disconnected from the utility grid, the sur-

plus, or deficient power is compensated by the EDLC as shown

in Fig. 3. In intentional islanding operation, the EDLC converter

controls the dc distribution voltage, and the number of the op-

erating micro-CHP units is determined by not only the loadconsumption, but also the stored energy of the EDLCs. When

the stored energy exceeds a maximum limit, the system stops

one of the operating micro-CHP units. Then, the total output

of the micro-CHP units becomes less than the load consump-

tion, and the EDLC discharges until the stored energy becomes

less than the minimum limit. On the contrary, when the stored

energy falls under the minimum limit, the system starts a micro-

CHP unit. Then, the total output of micro-CHP units becomes

more than the load consumption, and the EDLC charges until

the stored energy exceeds the maximum limit. These two modes

are repeated alternately in intentional islanding operation. The

heat commitment is not fundamentally considered in intentional


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Fig. 4. Configuration of the experimental system in the laboratory.

islanding operation because continuous electricity supply is a

priority during the operation.

The energy storage unit does not need a large capacity, if this

system chooses the suitable micro-CHP units that can start up

in a few minutes. For instance, one of the commercial micro

gas engine cogeneration (GEC) systems can start up in about

5 min. Most of the heat in the starting-up period is utilized in

heating the catalyst for the exhaust gas. Therefore, it is feasible

that the GEC system will be able to start in a few minutes every

time, after improving the exhauster for quick start up. In this

case, we assumed the energy of the storage system to be less

than 10 kWh and the charging/discharging time to be less than

1/2 h, although the parameters depend on the specification of

each system in practice. Therefore, EDLC can be a candidate of

the main energy storage unit in this system [27].

Fig. 4 shows a configuration of the experimental system used

in our laboratory. It is assumed that there are three households,

and each house has a dc power source such as a fuel cell. Thefeature of the dc system is to adopt a three-wire dc distribution,

which consists of a +170-V line, a neutral line, and a 170-Vline. In a three-wire composition, the voltage relative to ground

becomes low compared with that in two-wire system, where

one of the wires is grounded, and when half bridge inverters are

adopted for it, one of the single-phase 100-V output lines can

also become a grounded neutral line as long as abiding to the

Japanese standard [15]. In addition, load side dc/dc converters

can choose the source voltage from +340, +170, or 170 V. Arectifier connected to the utility grid is controlled to maintain the

dc voltage constant (340 V) in the interconnected operation. To

balance the positive voltage (+170 V) with a negative voltage(170 V), a voltage balancer is connected at the dc side ofthe rectifier. Two EDLC banks are connected through a dc/dc

converter as energy storage.

C. DC Voltage Control

As mentioned in the previous section, the dc distribution volt-

age is normally controlled by a grid connected rectifier in the

interconnected operation. The dc/dc converters of the storage

systems are controlled to maintain the dc distribution voltage

within a specified range [15]. In intentional islanding operation,

the dc/dc converters of the storage systems need to maintain the

dc distribution voltage. Therefore, if the dc microgrid has two or

Fig. 5. Control for dc/dc converter for energy storage.

Fig. 6. Droop control feature.

more energy storage units and those converters can be operatedin parallel, it contributes to the voltage regulation and system

redundancy. Droop control is a well-known method for voltage

control when two or more converters are used. In general, the

droop controller detects the output power or current as a feed-

back parameter, and the deviation of dc voltage is controlled

in proportion to the output power. However, if the converters

connect to energy storage units, the controller should consider

not only the output power balance, but also the stored energy.

In particular, stored energy balance is important to carry out

the operation described in the previous section because micro-

CHP units have to start or stop frequently under an unbalanced

condition of the stored energy. Therefore, we propose a novel

control method combining gain scheduling and fuzzy control,

which accomplishes good voltage regulation, load sharing, and

energy balance simultaneously.

III. CONTROL STRATEGY OF CONVERTER FOR ENERGY

STORAGE UNIT

Fig. 5 shows the circuit and proposed control diagram of

a dc/dc converter for EDLC banks. We designed the circuit

to be symmetric with respect to the neutral line, because the

converter is supposed to function as a voltage balancer under

the appropriate control.


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Fig. 7. Circuit and control diagrams to obtain the relation between the steady-state error and the gain Kc .

TABLE IPARAMETERS

A. Gain-Scheduling Control for Output Power Sharing

When dc voltage is controlled by several converters, it is dif-

ficult to achieve good voltage regulation and sharing at the same

time. As illustrated in Fig. 6, better voltage regulation requires

a higher dc gain, but it can decrease load sharing. Therefore,

we adopt a gain-scheduling control that changes the gain Kcaccording to the output power in order to obtain better voltage

regulation and load sharing simultaneously [16].

We examined the steady-state dc voltage error when the gain

Kc or the output power was changed by numerical simulations.

Fig. 7 and Table I show the circuit, control diagram, and param-

eters for the simulation. The rated output voltage and capacity

were 340 V and 3 kW, respectively. Fig. 8 shows the results,

which indicated that the steady-state error of the dc voltage be-

comes larger when the load is heavier or the gain Kc is smaller.

When the variation in the dc voltage is permitted within 2%, we

can obtain the relation between the gain Kc and output power

(p.u.) as shown in Fig. 8. The gain Kc can be expressed by a

linear function of the output power. Similarly, we can obtain

the relation between the gain and input power when the voltage

variation is permitted within 2%. Fig. 9 shows the results, ex-

pressing it with a linear function. From the results, the equationbetween the output power (p.u.) Pou t and the gain Kc can be

obtained as follows:

Kc = max(1.3 |Pou t | , 0.1) (1)

where theinput poweris expressed by a negative number, andthe

minimum valueofKc is determinedas 0.1. This gain-scheduling

control allows better load sharing and voltage regulation simul-

taneously. However, a stored energy control is also needed for

the converter energy storage unit device to prevent a surplus or

deficient of stored energy.

Fig. 8. Gain-dc/dc converter output power characteristics. (Voltage variation2%).

Fig. 9. Gain-dc/dc converter input power characteristics. (Voltage variation2%).

B. Proposed Control for Stored Energy Balancing

We propose a dc-voltage control that incorporates a fuzzy

control that changes the dc voltage reference to balance the

stored energy. Fig. 10 shows the membership functions of fuzzy

control. The input Xk is the ratio of the stored energy and the


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Fig. 10. Control rule of fuzzy control.

average of all stored energies as shown

Xk =Wk

1n1 (

ni=1 Wi Wk )

(2)

Wk is the charged energy of the kth EDLC bank, which is

described as follows:

Wk =12 Ckv

2k

12 CkV

2m in

12CkV

2m ax

12CkV

2mi n

=v2k V

2mi n

V2ma x V2

mi n

(3)

where the value n refers to the total number of EDLC banks. Ckand vk are the capacitance and voltage of the kth EDLC bank,respectively. Vm ax and Vm in are the maximum and minimum

operation voltage of EDLC banks, respectively.

To obtain the dc voltage reference Vdc ,Xk is initially calcu-

lated from (2) and (3). Then, a membership value is calculated

from the membership function of PB, PS, Z, NS, and NB. Fi-

nally, the dc voltage reference is obtained from the resulting

membership function. For example, ifX1 is 1.7, the values of

PS and PB in the antecedent membership function are 0.4 and

0.6, respectively. Then, Vdc of EDLC1 is given from the conse-quent membership function as follows:

V

dc = 345 0.4 + 350 0

.6 = 348 [V]

.(4)

We assumed that the dc-distribution voltage has a tolerance

of5% with reference to ac systems. In this case, the upperand lower reference should be within 3% because the gain-

scheduling control includes a tolerance of2%. Therefore, theupper and lower limit was selected 350 and 330 V in the mem-

bership function of consequent, respectively. An isosceles tri-

angle is usually used as a membership function. However, to

realize a good voltage regulation, the peaks of PS and NS in

the antecedent are shifted to 1.1 and 0.9, respectively. We first

obtained this membership from computer numerical simulation

and then applied it to experiments. Although this fuzzy rule

was determined on the basis of experience, its effectiveness was

Fig. 11. Simulation circuit.

TABLE IIMAIN PARAMETERS

Fig. 12. Events of the simulation (initial conditionW2/W1 2).

Fig. 13. Droop control to obtain the voltage reference.

demonstrated through simulations and experiments described in

the following sections.

IV. SIMULATION

To confirm the performance of the proposed method, simula-

tions were conducted using PSCAD/EMTDC. The circuit and

the main parameters are shown in Fig. 11 and Table II, espec-

tively. The configuration and the parameters are based on the


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Fig. 14. Simulation results (gain-scheduling control only) (Initial conditionW2/W1 2).

experimental system shown in Fig. 4. Resistances and current

sources were used as the loads and the DGs, respectively. The

resistances were set to 115.6 , which consumed 1 kW at 340 V.The output power of the current source was changed gradually

from 0 to 1 kW or from 1 to 0 kW for 1 s.Two EDLCs were connected through dc/dc converters:

EDLC1 (rated voltage 160 V, and rated capacitance 18 F) and

EDLC2 (rated voltage 216 V, and rated capacitance 18.75 F).

The minimum voltage of both EDLCs was set to be 100 V.

Therefore, EDLC1 and EDLC2 can store about 140 kJ and

344 kJ, respectively.

Fig. 12 shows the ON/OFF events of the loads and DGs in

the simulation. All loads and DGs were turned ON in the first

half, and the loads and DGs were turned OFF in the last half.

The initial voltages of EDLC1 and EDLC2 were 125 and 200 V,

respectively. Therefore, the initial stored energy ratio (W2/W1 )

was about 2.

Fig. 15. Simulationresults (gain-scheduling control and fuzzycontrol)(InitialconditionW2/W1 2).

Although the fuzzy control was chosen for the stored energy

management in this paper, the other control could be a candidate

to obtain the voltage referenceVdc . For example, a simple droop

control can be illustrated in Fig. 13. The control diagram is

simpler than the proposed fuzzy control, and the only gain Kvis needed as the control parameter. In other words, it has only

one degree of freedom to adjust the control performance.

A. Simulation Results of Proposed Method

Fig. 14 shows the simulation results (the dc distribution volt-

age, the stored energy ratio, the current from EDLC, and the

terminal voltage of EDLC) when only the gain-scheduling con-

trol technique was used. The maximum voltage ofVdc (EDLC1)

and Vdc (EDLC2) was 346.7 V which was about 340 V + 2%,and the minimum voltage of them was 332.9 V which was about

340 V 2%. Therefore, the distribution voltages at the output

converter of the EDLCs were within 340 V 2%. However, the


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Fig. 16. Simulation results (gain-scheduling control and droop control,Kv = 10) (Initial condition W2/W1 2).

stored energy of EDLCs was not balanced, and the maximum

of the ratio (W2/W1 ) reached 4.2.Fig. 15 shows the simulation results when the proposed con-

trol was used. The stored energy ratio tended to be 1. The fea-

tures of EDLC currents were different from those in Fig. 14 be-cause EDLC2 had 1.3 times energy at the initial state. EDLC2

initially discharged its power, and then the energies of both

EDLCs balanced at around 80 s. The reason for the difference

in the EDLC terminal voltages, VEDLC1 and VEDLC2 , despite an

almost unity ratio of EDLC charge (W2/W1 ), is the differencein the maximum voltages of EDLC1 and EDLC2. Regarding the

distribution voltages at the output converters of the EDLCs, the

maximum and the minimum were 349.9 and 331.7 V, respec-

tively. Although the range of the voltage was a little wider than

the previous results, the range was within 340 V 3%, whichwas satisfied with the assumed specification (340 V 5%)

described in Section III-B.

Fig. 17. Simulation results (gain-scheduling control and droop control,Kv = 50) (Initial condition W2/W1 2).

B. Simulation Results of Droop Control

To compare the proposed fuzzy control described in the previ-

ous section and the droop control shown in Fig. 13, simulations

of the droop control were conducted in the same condition.

Fig. 16 shows the simulation results when the droop controlwas used, and Kv was set to 10. Although the stored energy

ratio tended to be about 1, it took about 160 s, which was twice

as the result of the fuzzy control. Fig. 17 shows the simulation

results when Kv was set to 50. The stored energy ratio also

tended to be 1, and it took about 80 s, which was almost the

same as the result of the fuzzy control shown in Fig. 15. In this

case, the maximum and minimum voltage of the output convert-

ers of the EDLCs was 349.9 and 331.6 V, respectively. From the

result, it seems that the proposed fuzzy control can be replaced

by the simple droop control. To confirm the advantage of the

proposed fuzzy control, another simulation was conducted in

the following section.


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Fig. 18. Simulation results (gain-scheduling control and droop control,Kv = 50) (Initial condition W2/W1 0.5).

C. Comparison Between Fuzzy Control and Droop Control

In this simulation, the configuration, parameters and events

were the same as the previous simulation. The only difference

was that the all parameters of EDLC1 and EDLC2 were ex-changed: EDLC1 (rated voltage, 216 V and rated capacitance

18.75 F) and EDLC2 (rated voltage 160 V and rated capaci-

tance, 18 F). The minimum voltage of both EDLCs was also set

to be 100 V, and the initial voltages of EDLC1 and EDLC2 were

200 and 125 V, respectively. Therefore, the initial stored energy

ratio (W2/W1 ) was about 0.5.Fig. 18 shows the simulation results when the droop control

(Kv = 50) was used. Although the stored energy ratio tendedto be 1, it exceeded up to 1.05 around 75 s. After the stored

energy ratio returned to 1, it dropped to 0.95 around 180 s.

Fig. 19 shows the simulation results when the fuzzy control was

used. Although there was no overshoot like the results of the

Fig. 19. Simulationresults (gain-scheduling control and fuzzycontrol)(InitialconditionW2/W1 0.5).

droop control, the results after 120 s were almost the same as

the previous droop control.

Fig. 20 shows that the relation between input Xk and the

output Vdc in each control. The fuzzy control changes the slope

in the range ofXk between 0.5 and 2.0. On the contrary, thedroop control has one slope in the shorter range ofXk between

0.8 and 1.2, and the Vdc was fixed on the minimum (330 V) or

the maximum (350 V) in the other area.

The characteristics of the two controls can be seen from the

results in Figs. 18 and 19. Because the initial value ofXk was

set to 0.5 in each case, the power from EDLC1 was supplied

to EDLC2 in order to balance the stored energy. In Fig. 18,

the current from the converter of EDLC1 and EDLC2 (IEDLC1and IEDLC2 ) were constant from 0 to 20 s because Xk waslower than 0.8, and the voltage references Vdc (EDLC1)

of

and Vdc (EDLC2) were fixed to the upper and lower limit, re-

spectively. On the other hand, the absolute value of IEDLC1


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Fig. 20. Relations between input Xk and voltage reference V

d c .

Fig. 21. Integral of the square of the current (IE D L C 1 and IE D L C 2 ).

and IEDLC2 in Fig. 19 were decreasing during the same period,

which contributed to reduce the losses due to the line resistances

(Rline1 and Rline2 ) and the inner resistances of both EDLCs.Fig. 21 shows the integral of the square of IEDLC1 and IEDLC2in each case. In case of the fuzzy control, the integral numbers of

the square ofIEDLC1 at 250 s was 20.1% lower than the results

Fig. 22. Circuit of experimental system.

of the droop control, while the integral numbers of the square

of IEDLC2 at 250 s was almost the same. It indicates that the

loss in case of the fuzzy control is lower than that of the droop

control. Therefore, this paper proposes the fuzzy control as the

energy balance control.

V. EXPERIMENT

Two experiments were conducted to confirm the performance

of the proposed method practically. The circuit and the main

parameters are shown in Fig. 22 and Table III, respectively. The

system configuration and parameters are almost the same as

the simulation circuit. In house3, we used a commercial GEC,and the rated capacity was 1 kW. The generator outputs ac

340 V, 307.5 Hz, and the ac power was converted into a dc 390

400 V by a thyristor-diode rectifier. In general, the dc power

is converted to a single-phase ac 200 V and is supplied to the

utility grid, but we modified it to accept the dc power directly. In

the other two houses, we substituted the dc power supply for the

real GEC, which outputs 400-V dc constantly. Buck choppers

were connected between GECs and the dc distribution lines

because the dc voltage of the gas engine was different from the

distribution voltage. The output power was changed gradually

from 0 to 1 kW or from 1 to 0 kW for 1 s. The ratings of the two

EDLCs were the same as the simulation: EDLC1 (rated voltage

160 V, and rated capacitance 18 F) and EDLC2 (rated voltage

216 V and rated capacitance 18.75 F). The minimum voltage of

both EDLCs was also set to be 100 V. For the dc loads, we used

three electronic loads under constant resistance mode, each with

a rated capacity of 1 kW.

A. Case (a)

The ON/OFF events of the loads and DGs in case (a) were

the same as the previous simulation shown in Fig. 12. The

initial voltages of EDLC1 and EDLC2 were 125 and 200 V,

respectively. Then, the initial stored energy ratio (W2/W1 ) was

about 2. Fig. 23 shows the experimental results when only the


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TABLE IIIMAIN EXPERIMENT PARAMETERS

Fig. 23. Experimental results of case (a) (gain-scheduling control only).

gain-scheduling control technique was used. The dc distribu-

tion voltage was controlled within 340 V 5%, but the storedenergy of EDLCs was not balanced and the peak ratio was

around3.5. Fig. 24 shows the experimental results when the pro-

posed control was used. The energies of both EDLCs balanced

at around 100 s. The both experimental results were almost the

Fig. 24. Experimental results of case (a) (gain-scheduling control and fuzzycontrol).

same as the simulation results. However, the distribution volt-

age Vdc (EDLC2) and the current IEDLC2 in Fig. 23 were higherthan the simulation results in Fig. 14 during the first 120 s. The

reason is conjectured that the feedback value of Vdc (EDLC2)

were a little lower than the real values in the experiment, which

phenomenon was confirmed by numerical simulations. Apart

from it, the experimental results demonstrated that the validity

of the proposed methods. Regarding the performance during the

change of loads/sources, there were no disturbances or instabil-

ities due to the gain-scheduling control or the fuzzy control. In

case of the fuzzy control, the sign ofIEDLC1 and IEDLC2 were

opposite at first to balance their energy. Then, the envelopes of

IEDLC1 and IEDLC2 were gradually close each other during the

change of loads/sources.


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Fig. 25. Experimental results of case (b) (gain-scheduling control only).

B. Case (b)

In this experiment, Load1 and the output of DG3 were turned

ON from 0 to 1 kW at 5 s. Other loads and DGs were turned

OFF through the experiment. The initial voltage of EDLC1 andEDLC2 were 130 and 180 V, respectively. Therefore, the initial

stored energy ratio (W2/W1 ) was about 1.3.Fig. 25 shows the experimental result when only the gain-

scheduling control technique was used. If there had been no line

resistances between house1 and house3, the distribution voltage

Vdc (house1) would have been the same as Vdc (EDLC2) after

the event. In this case, the output power of DG3 would have

been supplied to the Load1 directly, and the both EDLCs would

have not outputted their power ideally. However, there were line

resistances R2 (1 2) between house1 and house3 in theexperimental system, and the distribution voltage Vdc (house1)

and Vdc (EDLC1) became lower than Vdc (EDLC2). To keep the

Fig. 26. Experimental results of case (b) (gain-scheduling control and fuzzycontrol).

distribution voltage, EDLC1 supplied Load1 with electricity,

and EDLC2 was charged with power from DG3. Therefore, thestored energy ratio was increasing.

Fig. 26 shows the experimental results when the proposed

control was used. The stored energy ratio (W2/W1 ) approaches1, and the energy ratios of each EDLC became equal. The volt-

ages of EDLC1 and EDLC2 decreased slightly because of the

presence of switching losses of the converters and the line resis-

tance. From this result, we confirmed that the stored energy ratio

converged by the proposed control. The response took around

150 s in this case, which is satisfactory for the dc microgrids as-

sumed in this paper. If there is a case where the faster response

is needed, the member ship function of antecedent has to be

shrunk horizontally centering on X= 1.


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VI. CONCLUSION

This paper presented a new dc distribution voltage control for

dc/dc converters with an energy storage unit. The proposed con-

trol combines a gain-scheduling technique with fuzzy control.

The experimental results show that the dc distribution voltage

was within 340 V 5%, and the ratios of the storage units were

approximately equal. This indicates that dc voltage regulationand stored energy balancing control are realized simultaneously.

The main advantage of the proposed control is presented for

the cases where the model is unknown or is mathematically

complex. Therefore, the proposed control is relatively easy to

introduce to real-life projects compared with the modern control

theories that utilize the time-domain state space representation.

However, for this purpose, trial and error methods might be

adopted to adjust the membership functions in practice, which

is a time consuming process. Our future study will attempt to

establish the design procedure of the membership functions.

In actual application, if a dc microgrid is extendedand another

energy storage system is included, it would require a communi-

cation line to obtain state information and detect faults. In this

case, equation (2) must be updated such that the new input Xkcan be calculated. Although the membership functions appear

to be the same as shown in Fig. 10 from the simulation analysis,

we will address this in another study after the accomplishment

of design procedure for the membership functions.

REFERENCES

[1] N. Hatziargyriou, J. Driesen, F. Katiraei, B. Kroposki, R. Lasseter, T. Ise,S. Morozumi, S. Papathanassiou, R. Iravani, A. Dimeas, C. Marnay, H.Asano, S. Papathanassiou, G. Strbac, and G. Venkataramanan, Micro-

grids, IEEE Power Energy Mag., vol. 6, no. 3, pp. 2694, 2008.[2] Lawrence Berkeley National Laboratory. Microgrid Symposium. (Sep.

2012). [Online]. Available: http://der.lbl.gov/microgrid-symposiums[3] Q. Zhang, R. Callanan, M. K. Das, S. Ryu, A. K. Agarwal, and

J. W. Palmour, SiC power devices for microgrids, IEEE Trans. PowerElectron, vol. 25, no. 12, pp. 28892896, Dec. 2010.

[4] T. Hornik and Q. Zhong, A current-control strategy for voltage-sourceinverters in microgrids based on H and repetitive control, IEEE Trans.Power Electron, vol. 26, no. 3, pp. 943952, Mar. 2011.

[5] P. Tenti, H. K. M. Paredes, and P. Mattavelli, Conservative power theory,a framework to approach control and accountability issues in smart mi-crogrids, IEEE Trans. Power Electron., vol. 26, no. 3, pp. 664673, Mar.2011.

[6] J. Rocabert,G. M. S. Azevedo,A. Luna, J. M. Guerrero,J. I. Candela, andP. Rodriguez,Intelligent connection agentfor three-phase grid-connectedmicrogrids,IEEE Trans. Power Electron., vol. 26, no.10, pp. 29933005,Oct. 2011.

[7] J. M. Guerrero,J. C. Vasquez, J. Matas, L. G. Vicuna,andM. Castilla,Hi-erarchical control of droop-controlled AC and DC microgridsA generalapproach toward standardization, IEEE Trans. Ind. Electron., vol. 58,no. 1, pp. 158172, Jan. 2011.

[8] A. Kwansinski and C. N. Onwuchekwa, Dynamic behavior and stabiliza-tion of DC microgrids with instantaneous constant-power loads, IEEETrans. Power Electron., vol. 26, no. 3, pp. 822834, Mar. 2011.

[9] A. Kwansinski, Quantitative evaluation of DC microgrids availability:Effects of system architecture and converter topology design choices,

IEEE Trans. Power Electron., vol. 26, no. 3, pp. 835851, Mar. 2011.[10] R. S. Balog and P. T. Krein, Busselection in multibus DC microgrids,

IEEE Trans. Power Electron., vol. 26, no. 3, pp. 860867, Mar. 2011.[11] T. Wu, K. Sun, C. Kuo, andC. Chang,Predictive current controlled 5-kW

single-phase bidirectional inverter with wide inductance variation for DC-microgrid applications, IEEE Trans. Power Electron., vol. 25, no. 12,pp. 30763084, Dec. 2010.

[12] A. A. A. Radwan and Y. A.-R. I. Mohamed, Linear active stabilizationof converter-dominated DC microgrids, IEEE Trans. Smart Grid, vol. 3,no. 1, pp. 203216, Mar. 2012.

[13] M. Noritake, K. Hirose, M. Yamasaki, T. Oosawa, and H. Mikami, Eval-uation results of power supply to ICT equipment using HVDC distributionsystem, in Proc. Int. Telecom. Energy Conf., Jun. 2010, pp. 18.

[14] H. Kakigano, Y. Miura, T. Ise, T. Momose, and H. Hayakawa, Funda-mental characteristics of DC microgrid for residential houses with cogen-eration system in each house, in Proc. IEEE Power Energy Soc. General

Meeting, Jul. 2008, Paper 08GM0500, pp. 18.[15] H. Kakigano, Y. Miura, and T. Ise, Low-voltage bipolar-type DC micro-

grid for super high quality distribution, IEEE Trans. Power Electron.,vol. 25, no. 12, pp. 30663075, Dec. 2010.

[16] Z. Ye, D. Boroyevich, K. Xing, and F. C. Lee, Design of parallel sourcesin DC distributed power systems by using gain-scheduling technique, inProc. IEEE Power Electron. Spec. Conf., Aug. 1999, vol. 1, pp. 161165.

[17] W. Tang and R. H. Lasseter, An LVDC industrial power distributionsystem without central control unit, in Proc. IEEE Power Electron. Spec.Conf., 2000, vol. 2, pp. 979984.

[18] Y. Ito, Y. Zhongqing, and H. Akagi, DC microgrid based distributionpower generation system, in Proc. IEEE Int. Power Electron. MotionControl Conf., Aug. 2004, vol. 3, pp. 17401745.

[19] Y. A. I. Mohamed and E. F. El-Saadany, Adaptive decentralized droopcontroller to preserve power sharing stability of paralleled inverters indistributed generation microgrids, IEEE Trans. Power Electron., vol. 23,

no. 6, pp. 28062816, Nov. 2008.[20] A. M. Roslan, K. H. Ahmed, S. J. Finney, and B. W. Williams, Im-proved instantaneous average current-sharing control scheme for parallel-connected inverter considering line impedance impact in microgrid net-works, IEEE Trans. Power Electron, vol. 26, no. 3, pp. 702716, Mar.2011.

[21] T. M. Haileselassie and K. Uhlen, Impact of DC line voltage drops onpower flow of MTDC using droop control, IEEE Trans. Power Systems,vol. 27, no. 1, pp. 14411449, Aug. 2012.

[22] C. N. Papadimitriou and N. A. Vovos, A fuzzy control scheme for in-tegration of DGs into a microgrid, in Proc. IEEE Mediterranean Elec-trotechnical Conf., Apr. 2010, pp. 872877.

[23] A. Elmitwally and M. Rashed, Flexible operation strategy for an isolatedPV-diesel microgrid without energy storage unit, IEEE Trans. EnergyConvers., vol. 26, no. 1, pp. 235244, Mar. 2011.

[24] B. N. Alajmi, K. H. Ahmed, S. J. Finney, and B. W. Williams, Fuzzy-logic-control approach of a modified Hill-Climbing method for maximum

power point in microgrid standalone photovoltaic system, IEEE Trans.Power Electron., vol. 26, no. 4, pp. 10221030, Apr. 2011.

[25] T. Konishi,S. Hase,and Y. Nakamichi,Verificationtests of energy storagesystem for DC electrified railways using EDLC, Quart. Rep. RailwayTech. Res. Inst., vol. 48, no. 2, pp. 7177, 2007.

[26] H. Chen, T. N. Cong, W. Yang, C. Tan, Y. Li, and Y. Ding, Progress inelectrical energy storage system: A critical review, Progress Naural Sci.,vol. 19, no. 3, pp. 291312, 2009.

[27] S. M. Muyeen, R. Takahashi, T. Murata, and J. Tamura, Integration ofan energy capacitor system with a variable-speed wind generator, IEEETrans. Energy Convers., vol. 24, no. 3, pp. 740749, Sep. 2009.

Hiroaki Kakigano (M06) was born in 1976. Hereceived the B.S. and M.S. degrees in nuclear engi-neering from Nagoya University, Nagoya, Japan, in1999 and 2001, respectively. He received the Ph.D.degree in electrical engineering from Osaka Univer-sity, Osaka, Japan, in 2009.

After he worked at an Electric Company, he joinedOsaka University in 2004 for a doctorate course,where he is currently an Assistant Professor. FromAugust2011 toMarch2012, hewas a VisitingProfes-sor with KU Leuven, Leuven, Belgium. His research

interests include power electronics, microgrids, and dc distribution systems.


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Yushi Miura (M06) received the Doctorate de-gree in electrical and electronic engineering from theTokyo Instituteof Technology, Tokyo, Japan, in 1995.

From 1995 to 2004, he was at Japan AtomicEnergy Research Institute as a Researcher wherehe developed power supplies and superconductingcoils for nuclear fusion reactors. Since 2004, he hasbeen an Associate Professor in the Division of Elec-trical, Electronic, and Information Engineering atOsaka University, Osaka, Japan. His research inter-ests include applications of power electronics and

superconducting technology. Currently he is interested in control of distributedgenerations and energy storages in the power systems.

Toshifumi Ise (M87) was born in 1957. He receivedthe Bachelor, Master, and Doctor of Engineering de-grees in electrical engineering from Osaka Univer-sity, Osaka, Japan, in 1980, 1982, and 1986, respec-tively.

He joined , Osaka University in 1990, where heis currently a Professor in the Division of Electrical,Electronic, and Information Engineering, Faculty ofEngineering. From 1986 to 1990, he was at the NaraNational College of Technology, Nara, Japan. His re-search interests include the areas of power electronics

and applied superconductivity including superconducting magnetic energy stor-ages and new distribution systems.

Dr. Ise is a Member of the Institute of Electrical Engineers of Japan and theJapan Society for Power Electronics.

Documents

Distribution Voltage Control for DC Microgrids Using Fuzzy Control and Gain-Scheduling Technique