A Practical Load Sharing Control Strategy for DC

Microgrids and DC Supplied Houses

Po-Hsu Huang, Weidong Xiao, and Mohamed Shawky El Moursi

Electrical Power Engineering

Masdar Institute of Science and Technology Abu Dhabi, United Arab Emirates

Abstract— Microgrid (MG) research mainly focuses on AC-

based power flow control techniques. Nevertheless, the rise of

DC output sources such as photovoltaic (PV) systems, fuel cells,

and distributed batteries leads to the immediate need for DC

MGs. In this paper, a hierarchical control strategy for a droop-

controlled DC MG is proposed, which fits the smart house

infrastructure to adopt online renewable generation and load

sharing. The improved control strategies combined with the

hierarchical approach includes three loops of controllers: the

primary control, the secondary control, and the tertiary control.

The issues of internal current limiter and anti-windup are also

discussed in this study. The simulation results of the proposed

approach are presented to verify the feasibility, and the

experimental results are carried out to evaluate the droop

concept and secondary voltage compensation.

Index Terms-- Distributed generation, droop method, microgrid.

I. INTRODUCTION

In recent years, a new system concept, the microgrid (MG)

[1], has been proposed to integrate DG and storage devices.

The concept is developed to tackle the problems of new

strategies, coordination, and controls that could be very

different from conventional electrical grids. AC MGs [2-3]

have been proposed to facilitate the connection of renewable

power sources to conventional AC systems. However, the

inherent advantage of supplying DC power directly from DC

sources leads to the irreplaceable need of DC MGs. Also, the

development and deployment of distributed DC sources such

as PV, fuel cells, and batteries have brought attention to their

advantage for dc loads in commercial, industrial and

residential applications. Different applications in DC MGs

have been proposed [4-5] to integrate various distributed

resources. It is shown in [6] that the voltage control for DC

MGs can achieve super high quality power supply. Various

advantages of DC MGs such as higher efficiency, reliability,

and easiness of integration are also mentioned in [7-8].

When operating a DC MG connected to the utility grid,

distributed sources in the MG can supply a certain amount of

power. However, when the MG is operated in islanded mode,

it must perform voltage regulation inside the MG. Many DC

bus voltage control schemes have already been proposed in

[9-10]. Various control and operation strategies of DC MGs

are also proposed in [11]-[15] by using the droop method to

ensure load sharing and voltage regulation. However, the

inherent drawback of the droop method is its poor voltage

regulation. Thus, a proper control strategy will need to be

applied to handle such an issue.

A hierarchical control strategy for both AC and DC MGs

is proposed in [16]. The controller consists of the primary

control, the secondary control, and the tertiary control. The

controller is designed to operate in both islanded and grid-

connected modes. When the MG is islanded, the voltage

inside the MG is regulated to ensure high quality power

supply. Therefore, the voltage control and power sharing

method must be implemented to operate the MG. The most

widely used methods are the master-slave control and the

droop method. In [16], the author adopts the droop method to

perform voltage regulation and current sharing. In the case of

applying parallel DC-DC converters, the droop method

consists of negative proportional parts of the output current,

which can avoid output conflict between converters and to

achieve desired current and power sharing. However, the

subtracting part leads to the magnitude drop in voltage, so

called voltage deviation, indicating that the more output

power supplied the more voltage deviation induced.

To tackle this problem, the secondary control is utilized to

restore the voltage back to the nominal level. The secondary

controller is also responsible for synchronizing the MG

voltage with the external grid. It is stated in [16] that the

communication is necessary for the secondary control with

the droop method to avoid the output conflict. After

synchronization, the tertiary control then manages to dispatch

power flow between the MG and the external grid.

However, the operation between islanded mode and grid-

connected mode might cause the controller to fail or induce

lager transients. In addition, the desired power injection

reference from the tertiary controller might exceed the

maximum power capacity of the MG. Such issues should be

addressed and discussed. In this paper, proper solutions are

978-1-4799-0224-8/13/$31.00 ©2013 IEEE 7124

proposed to coordinate with the hierarchical control to obtain

an improved performance of the MG control. The concept of

implementing current sharing priority for both DC sources

and converters is also investigated. The method allows the

MG designer to choose the suitable current sharing strategy

according to the preference of different types of DC sources

and converters.

II. HIERARCHICAL CONTROL OF DC MG

The control diagram is shown in Fig. 1 including primary, secondary, and tertiary controls.

PI

Controller

PI

Controller

Current

loop

MGV

*

MGV

DCV

V

DC

Micro

grid

DC Stiff Grid

GI

2OV

Voltage

loopPWM

(Duty Cycle)

2DR

DC

source

Voltage

loop

PWM

(Duty Cycle)

1DR

DC

sourceCurrent

loop

Co

mm

un

icatio

ns R

equ

ired

Primary Control

Secondary Control

Tertiary Control

MGV

refV

refV

+

+ −

−

SVδ

+

+

V

V

V

A

A

+

−

Primary Control

GI

1OV

1OI

2OI

*

GI

TVδ

Bypass Switch

A

Fig. 1. Primary, secondary, and tertiary controls of a DC MG.

A. Primary Control

The goal of the inner loop controller is to maintain the

voltage level inside the MGs. When two converters work in

parallel, equal current distribution between them is expected

since all converters are assumed to be identical. However,

due to component tolerances and non-identical characteristics

of the inductors and capacitors, the exact same current

distribution and output voltage are impossible. As shown in

Fig. 2(a), even small amount of voltage difference can cause

comparably high circulating current among the converters.

Therefore, the droop method is implemented, being necessary

to reduce circulating current, and further to manage the

current sharing. Assuming each converter has the same value

of the droop resistance, 1 2D DR R= , the droop method

adjusting the voltage reference for the inner loop controller

can be expressed by the following equation: *

o ref D oV V R I= − (1)

where oI is output current, DR is the droop resistance, and

refV is the reference voltage. The resistance value can be

chosen by assuming the maximum allowed voltage deviation,

which can be shown as follows:

/ 2

/

ref n v

D v o

V V

R I

ε

ε

= −

= (2)

where nV is the nominal output voltage, vε is the maximum

allowed voltage deviation, eqR is the equivalent droop

resistance of all parallel converters, and maxI is the maximum

total output current. The droop characteristic is shown in Fig.

3(b). Each converter can choose different droop resistances

depending on its priority. By proper ratio of droop values

among the controllers, a diverse current distribution can be

obtained. The details will be discussed in the latter section.

LR

OV1OI 2OI

1OV

1R 2R

2OV

*oV

I

V

refV

oI limitI

(a) (b)

Fig. 2. Principle of load sharing and droop control: (a) Equivalent circuit of

two parallel converters supplying a load; (b) droop characteristic in function

of output current.

B. Secondary Control

From previous discussion, the droop method induces the

voltage deviation when the current sharing strategy shown in

Fig. 2 is implemented. To solve the problem, the secondary

control is proposed to compensate. The compensator senses

the voltage DCV in the DC stiff grid and compares the error

with the voltage MGV inside the MG to provide voltage

restoration by Vδ . As shown in Fig. 2, in order to achieve

an exact voltage restoration, which contributes to an identical

output voltage level, low bandwidth communications are

required to prevent circulating currents [16]. The Vδ is

shown as follows: * *

1 1( ) ( )S p MG MG i MG MGV k V V k V V dtδ = − + −∫ (3)

where 1pk and 1ik are proportional and integral terms of the

secondary controller. Note that the output SVδ should be

limited within the maximum voltage deviation vε . From (3),

(1) can now be modified as *

o ref S D oV V V R Iδ= + − (4)

The reason why the controller senses the voltage in the DC

stiff grid is that the MG is designed to be islanded in the

beginning and then be connected with the DC stiff grid.

When the voltage levels in both sides become synchronized,

the connection can be accomplished by the bypass switch.

C. Tertiary Control

In this stage, the tertiary controller provides the power

flow control by changing the voltage reference inside the

MG. The compensator senses the error between the reference

and the current flowing toward the dc stiff grid to control the

power flow across the bypass switch. The controller can be

expressed as: * *

2 2( ) ( )T p G G i G GV k I I k I I dtδ = − + −∫ (5)

where 1pk and 1ik are proportional and integral terms of the

tertiary PI controller. The TVδ is also limited within

DC VV ε± so that the value will not exceed the maximum

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voltage deviation.

III. DC MICROGRID CONTROL

In this paper, we adopt Buck converters to implement the

DC MG system. Fig. 3(a) illustrates the equivalent circuit of

buck converters. Applying Kirchhoff’s law, we obtain

LIN O

O OL

diL mV V

dt

dV VC i

dt R

= −

= −

(6)

Therefore, the small-signal average model of buck

converters can be expressed as

2

( ) 1

( ) / 1

O

IN

V s

mV s LCs L Rs=

+ + (7)

The mathematical model is useful in analyzing the system

and designing the controller. However, the details such as

switching transient states, conduction losses, and ripples on

the inductor are not well revealed. Thus, in order to design an

effective controller, such effects should be considered and

examined. L

+

−

+ −

i

−

+

V

i

i

C R

VL

C

L O

C

VINm

VO

⋯⋯1DR

2DR

DnR

V∆

oI

1oI

2oI

onI

(a) (b)

Fig. 3. Equivalent circuits (a) buck converters; (b) droop resistances with

converter output current.

B. Paralleling DC-DC converters

Unlike the single centralized power supply, paralleling

DC-DC converters require a reliable current sharing strategy

to ensure proper distribution and to prevent circulating

current among the converters. In order to achieve that, the

droop method is proposed to reduce the circulating current

and assist in distributing power supply among the converters.

As we mentioned in the previous section, the voltage-current

droop method includes a small virtual resistance which

allows the voltage set point to decrease when the load current

increases. If each converter has the same droop resistance,

identical current sharing is expected. When the values are not

the same, the equation (1) can be derived as *

1 1 2 2o ref D o ref D o ref Dn onV V R I V R I V R I= − = − = = −⋯ (8)

Therefore, *

1 1ref o eq o D o Dn onV V V R I R I R I− = ∆ = = = =⋯ (9)

where V∆ is voltage deviation, eqR is equivalent resistance

of all parallel droop resistors, OI is the total output current,

and DnR and OnI are the resistance and output current of the n-

th converter. Note that the equivalent circuit of equation (9) is

shown in Fig. 3(b) from which the output current of the n-th

converter is derived as

o eqOn

L Dn

V RI

R R

=

(10)

By properly selecting the droop values, the MG designer can

assign higher priority to the converters with higher efficiency

or the DC sources that is preferred to be utilized.

C. Current Limiter

Since the current sharing is based on the droop resistance

value, the controller itself cannot regulate the current output.

Therefore, the current limiter is required to prevent the

overcurrent induced in the converter output that could

damage the DC source or the converter itself. Especially, the

droop method permits some converters to inject more current

than others. When the heavy load changing happens, the

converters which are assigned to supply more power will

confront large output current so that large transients could be

induced. Also, when the tertiary control manages the current

flow in grid-connected mode, the desired injected current

reference could exceed the capacity of the DC MG. In such a

case, the current limiters will need to be applied to confine

the total output current injected to the external grid in order to

protect the MG.

D. Anti-Windup

When the MG resumes normal operation from current

limitation modes, special control action should be taken into

account. Otherwise, large transients might occur. The

transients happen because the tertiary controller proceeds to

normal operation without considering the occurrence of

current limitation. The output signal the tertiary controller

expectation is mismatched with the actual signal. Therefore,

the windup will be induced in the integrator. In order to

prevent such transients, the proper control strategy should be

applied to inform the controller the occurrence of saturation.

The anti-windup loop can be implemented by adding a

differential term between the real output and the expected

output of the controller into the integrator. By tuning the anti-

windup gain, the integral value in the integrator would reset

to the proper value before the system resumes normal

operation so as to avoid large transients. Besides, the anti-

windup loop can also be implemented in the secondary

controller and the inner loop controller when the output signal

of the controllers reaches the limit.

IV. EVALUATION

The feasibility of the proposed control strategy is verified

by simulation and experiment.

A. Simulation

The simulation consists of two dc/dc buck converters

connected in parallel supplying a load. The voltage level

inside the MG is selected at 400V, the droop values for

converter 1 and 2 are chosen as 1 Ω and 2 Ω respectively,

and the local load is designed to switch between 4kW and

12kW every 10ms during the entire simulation. The

simulation scenario is shown in Table I. Three cases are

simulated to verify the feasibility of MG operation.

Case 1—Injecting 10A to the external DC stiff grid: Fig 4

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(a) shows that, at 5ms, the secondary control is enabled to

restore the MG voltage back to 400V. Fig 4 (b) indicates that

the current sharing ratio is always maintained constant in both

standalone and grid-connected modes. In Fig 4(c), the tertiary

control is activated at 15ms to produce voltage deviation in

order to inject the desired current into the external grid.

TABLE I

SIMULATION SCENARIO

Time (ms) Description Mode

5 secondary control activated Islanded

10 load changing from 4kW to 12kW Islanded

15 connected to the grid Grid-connected

20 load changing from 12kW to 4kW Grid-connected

30 load changing from 4kW to 12kW Grid-connected

35 disconnected from the grid Islanded

40 load changing from 12kW to 4kW Islanded

50 simulation end Islanded

5 10 15 20 25 30 35 40 45 50

380

400

420

(a) MG Voltage

time (ms)

Am

plitu

de (

V)

5 10 15 20 25 30 35 40 45 50-10

0

10

20

30(b) Converter output current

time (ms)

Io1 Io2 (

A)

5 10 15 20 25 30 35 40 45 50-10

0

10

20(c) Current flow between the MG and the external grid

time (ms)

Ig (A

)

Converter 1

Converter 2

Fig. 4. Behavior of the voltage and currents of the dc MG in Case 1: (a) MG

voltage (b) Converter output current (c) Current flow between the MG and

the external Grid.

Case 2—Extracting 20A from the external grid: Fig. 5 (a)

and (c) show that the MG voltage drops at 15ms because the

tertiary control produces negative voltage deviation so that

the current can be extracted into the MG. At 20ms, the load

switches from 12kW to 4kW, causing an increase of the MG

voltage. Due to the decrease of the load, the MG no longer

needs 20A current to supply the loads. Therefore, the

converters stop supplying power and the extracted current is

limited at 10A.

Case 3—Current limiters: In this case, the current limiter

is equipped into the primary controller. The injected current

reference is chosen at 10A. The current limitations are

selected to be 10A and 25A for converter 1 and 2,

respectively. Fig 6(b) shows that converter 1 reaches the limit

at 10ms when the load is changing from 4kW to 12kW. The

remaining power, therefore, is supplied by the converter 2. At

15ms, the MG is connected to the external grid and the

injected current is limited to 5A due to the 35A total capacity

of the MG (30A supplied to the load). It can be seen at 20ms,

the load is reduced to 4kW so that the injected current returns

to the reference value.

5 10 15 20 25 30 35 40 45 50

380

400

420

(a) MG Voltage

time (ms)

Am

plitu

de

(V

)

5 10 15 20 25 30 35 40 45 50-10

0

10

20

30(b) Converter output current

time (ms)

Io1

Io

2 (

A)

5 10 15 20 25 30 35 40 45 50-30

-20

-10

0

10(c) Current flow between the MG and the external grid

time (ms)

Ig (

A)

Converter 1

Converter 2

Fig. 5. Behavior of the voltage and currents of the dc MG in Case 2: (a) MG

voltage (b) Converter output current (c) Current flow between the MG and

the external Grid

5 10 15 20 25 30 35 40 45 50

380

400

420

(a) MG Voltage

time (ms)

Am

plitu

de

(V

)

5 10 15 20 25 30 35 40 45 50-10

0

10

20

30(b) Converter output current

time (ms)

Io1

Io

2 (

A)

5 10 15 20 25 30 35 40 45 50-10

0

10

20(c) Current flow between the MG and the external grid

time (ms)

Ig (

A)

Converter 1

Converter 2

Fig. 6. Behavior of the voltage and currents of the dc MG in Case 3: (a) MG

voltage (b) Converter output current (c) Current flow between the MG and

B. Experimental Results

A small-scale DC MG is constructed with two DC/DC

buck converters. The system parameters are demonstrated in

Table II. The control algorithm is implemented based on TI

TMS320F2808 microcontroller for evaluating the droop

concept and the secondary voltage compensation. Fig. 7(a).

shows the system response with the droop ratio of 1:1 under

load disturbance. The load changes from 0.6 Ω to 1.1 Ω .

The voltage rises very less and gradually recovers to the

original value due to the secondary voltage regulation. The

current sharing ratio maintains the same and continues on

supplying to the load. The system response with the droop

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ratio of 1:2 under load disturbance is shown in Fig. 7(b). The

sudden change of load causes the unequal current drops

among the converter in the beginning of the transient period.

The higher voltage drop in one converter caused by the

feedback droop loop then induces lower voltage reference set

point. The controller output will decrease and reduce the

output current so that the voltage deviation is mitigated.

Therefore, the current sharing proceeds into the new steady

state and maintains the same ratio. Such self-adjusting

algorithm can ensure the current sharing as defined droop

ratio under load changing condition.

TABLE II

EXPERIMENTAL DATA

Parameters Value

Input voltage 9V

Rated output voltage 2V

PWM Switching frequency 200kHZ

Converter inductance 0.9 uH

Converter output capacitance 470 uF

Nominal load 0.6 Ω

Load resistance changes from 0.6 to 1.1 ohms

(a)

Load resistance changes from 0.6 to 1.1 ohms

(b)

Fig. 7. Voltage and current responses under load changing: (a) with droop

ratio of 1:1; (b) with droop ratio of 1:2 under load changing.

V. CONCLUSION

This paper has presented an improved implementation of the hierarchical controller for DC MGs. The DC MG can be operated in both islanded and grid-connected modes. The proposed control system coordinates the current sharing control of the two converters and performs power exchange management with the grid. In addition, the effects of load changing and mode switching on the system are also discussed and treated by the proposed methods. The simulation results

illustrate that the MG can properly switch between the standalone and grid-connected mode without large transients induced. The hierarchical controller is capable of providing good performance during the load changing and preventing the MG from injecting overcurrent. The performance after disconnected from the grid was also demonstrated by simulation to verify the feasibility. The advantage of using anti-wind up solution demonstrated that the oscillations can be effectively reduced when the system starts resuming normal operation. It also helps to increase the speed of the response after load changing or mode switching. A small scale MG prototype is constructed to verify the proposed control experimentally.

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