control of hybrid ac/dc microgrid involving energy storage...

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This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TIA.2016.2613981, IEEETransactions on Industry Applications

1

Abstract—This paper presents real-time coordinated control of

hybrid AC/DC microgrids involving energy storage and pulsed

loads. Grid isolated hybrid microgrid applications require special

considerations due to the intermittent generation, online energy

storage control and pulsed loads. In this work, we introduce a

comprehensive frequency and voltage control scheme for a hybrid

AC/DC microgrid consisting of a synchronous generator, solar

generation emulator and bidirectional (AC/DC and DC/DC) con-

verters. A bidirectional controlled AC/DC converter, with an

active and reactive power decoupling technique is used to link the

AC bus with the DC bus while regulating the system voltage and

frequency. A DC/DC boost converter with a maximum power

point tracking (MPPT) function is implemented to maximize the

intermittent energy generation from solar generators. Current

controlled bidirectional DC/DC converters are applied to connect

each lithium-ion battery bank to the DC bus. Lithium-ion battery

banks act as energy storage devices that serve to increase the sys-

tem resiliency by absorbing, or injecting, power. Experimental

results are presented for verification of the introduced hybrid

AC/DC power flow control scheme.

Index Terms—Microgrid, energy management, pulsed load,

energy storage, battery bank, synchrophasor.

I. INTRODUCTION

YBRID power systems are gaining popularity due to in-

creasing microgrid deployments featuring renewable

power systems connected to low voltage AC distribution sys-

tems. Furthermore, DC grids are resurging due to the devel-

opment of new semiconductor technologies and sustainable DC

power sources such as solar energy. There has also been an

increase in DC loads, such as plug-in electric vehicles (PEVs)

[5]-[7] and light emitting diodes (LEDs), connected to the grid

to save energy and decrease greenhouse gas emissions. This

growth has been motivated by environmental concerns caused

by conventional fossil fueled power plants [24].

Thus far, a variety of control strategies have been introduced

for microgrids [1]-[4].One of the major technical challenges in

microgrids is the interconnection of pulsed loads, which can

cause voltage collapse, oscillation of the angular velocity in the

generators, and degradation of the overall system performance

[22],[23], [25]. The pulsed loads draw high currents during a

This work was partially supported by grants for the Office of Naval

Re-search and the US Department of Energy (DOE). The authors are with the

Energy Systems Research Laboratory, Department of Electrical and Computer

Engineering, Florida International University, Miami, FL 33174 (e-mail:

[email protected]).

short period of time, which can cause considerable voltage and

frequency fluctuation [22]. These disturbances can trip other

normal loads offline, causing a serious outage. The power re-

quirements of such loads can range from kilowatts to megawatts

with a charge interval on the order of seconds to minutes [23].

Power converters opened new horizons for effective inte-

gration of AC and DC distribution networks in a microgrid

operation concept [8]. Thus far, in [9]-[10], several ideas and

models of AC/DC microgrids have been proposed, but their

systems operate without the influence of the pulsed loads.

System stability and coordination control of power electronic

devices, during islanded operation modes with the influence of

pulsed loads, is still an open issue.

At the same time, various utility grids and some hybrid mi-

crogrids are increasing the penetration of renewable energy

resources [11]-[12]. The intermittent nature of wind and solar

power can quickly add up to system-wide instability that can

force generators to ramp up and down wildly, push grid pro-

tection gear into states it’s not meant to handle, or force the wind

and solar generator to shut off altogether [13].

Hybrid power systems face far more challenges when oper-

ating in islanded mode than they do in grid connected mode.

During islanded mode, the AC side can no longer be viewed as

an infinite bus, which results in load variations adversely af-

fecting the frequency and voltage of the system. If the system

has a high penetration of renewable power, the situation can be

even worse. At any time, reactive and active power flow should

be balanced between the AC and DC sides to maintain stability

on both sides of the grid [14]-[15].

To the best knowledge of the authors, a realistic coordinated,

hybrid AC/DC microgrid control considering pulsed load mit-

igation with energy storage has not yet been studied. In this

paper, a real-time coordinated control of a grid-isolated, hybrid

AC/DC microgrid, involving energy storage and pulsed loads is

studied. This microgrid can be viewed as a PEV parking garage

power system or a ship power system that utilizes sustainable

energy and is influenced by pulsed load. We introduce a com-

prehensive frequency and voltage control scheme for a hybrid

AC/DC microgrid consisting of a synchronous generator, solar

generation emulator and bidirectional (AC/DC and DC/DC)

converters. A bidirectional controlled AC/DC converter with

active and reactive power decoupling technique is used to link

the AC bus with the DC bus, while regulating the system voltage

and frequency.

Control of Hybrid AC/DC Microgrid Involving

Energy Storage and Pulsed Loads

Tan Ma, Mehmet H. Cintuglu, Student Member, IEEE, and Osama A. Mohammed, Fellow, IEEE

H



2

A DC/DC boost converter with a maximum power point

tracking (MPPT) function is implemented to maximize the

intermittent energy generation from solar generators. Current

controlled bidirectional DC/DC converters are applied to con-

nect each lithium-ion battery bank to DC bus [16].

II. HYBRID AC/DC MICROGRID CONFIGURATION

Fig. 1 shows the complete schematics of the studied hybrid

microgrid. Fig.2 shows the DC side of the hybrid microgrid

configuration, where a photovoltaic (PV) emulator, battery

banks, and loads are connected [17], [18]. The AC and DC sides

are linked through a bidirectional three phase AC/DC converter

and a transformer. The system features constant and pulse loads

on both the AC and DC sides. The PV emulator is connected to

the DC bus as the DC energy source through a DC/DC boost

converter with MPPT functionality. Five 50Ah lithium-ion

battery banks with 51.8V terminal voltage, are connected to the

DC bus through five bi-directional DC/DC boost converters

[19]. The rated voltages of DC and AC sides are 300V and

208V phase to phase respectively. The system can be operated

in either grid-connected mode or islanding mode. To maximize

the utilization of the renewable sources, the PV emulator can be

operated in on/off maximum power point modes based on the

whole system power.

Energy balancing is handled by controlling the DC/DC boost

converter. In grid-connected mode, the proposed system can be

viewed as a PEVs car park system. The five batteries can be

viewed as five PEVs that can play the role of energy storage. By

controlling the charging process of the PEVs in the car park, the

hybrid microgrid can limit the PEVs’ charging impact to the

utility grid, and at the same time, provide some ancillary support

to the utility grid through V2G services, via frequency regula-

tion, reactive power compensation, and spinning reserve. In

islanded mode, the proposed system can be viewed as a ship

power system with solar panels. The bi-directional AC/DC

converter can take control of the AC side frequency and voltage

amplitude. The DC bus voltage is regulated by controlling the

charging and discharging of the battery banks, which also means

controlling the current flow through the bidirectional DC/DC

converter.

A. PV Panel Emulator

The PV panel can be viewed as a current source in parallel

with a diode. In this paper, the SunPower SPR-305-WHT solar

cell, with 305W maximum output power, is used. 33 cells are

used in the configuration of 11 parallel strings, with 3 serially

connected cells per string. Fig. 3 shows the non-linear P-V and

I-V electric characteristics of a single SunPower SPR-305-

WHT solar sell. Under different solar irradiations, the maxi-

mum power points of the power-voltage curves are associated

with different output voltages. Also, under certain solar irra-

diance, the output of the PV panel is varying with different

terminal voltages.

ST1 ST3 ST5

ST4 ST6 ST2

STC

STd

Rb

AC filter

RacLac

Cac

vpv+

-

Lpv Rpv

STpv

Vd

Cd

Remote Grid

Local Load

AC Pulsed Load

Synchronous Generator

Gps Clock

HMI

Phasor Data Concentrator

PMU (1) PMU (2)IED (1)

IED (2)

IED (3)

Fig. 1. Overview of AC side of hybrid microgrid configuration involving synchronous generators, inverter-based distributed energy resources, distribution line

and load models, synchrophasor, intelligent electronic devices (IED) protection setup, SCADA systems and human machine interface (HMI)

Lithium-ion battery bank

STM32F microcontroller

Signal converting circuit

Bidirectional DC-DC converter

Zigbee wireless transmitter

Converter power supply

Step-up Transformer

Grid-Tie Inverter

DC Load

L-C FilterEmbedded Controller

Fig. 2. Overview of DC side of hybrid microgrid configuration

involving bidirectional AC/DC, DC/DC converters



3

Equations (1)-(3) show the mathematical model of the PV

panel with its output current Ipv and output voltage Vpv [19].

Related parameters are shown in Table I.

]1)))([exp(( spvs

pvsatpphppv RI

n

V

AkT

qInInI (1)

1000))((

STTkiII rssoph (2)

))

11()exp(()( 3

TTkA

qE

Tr

TII

r

gaprrsat (3)

B. Lithium-ion Battery Banks and Pulsed Load

An accurate battery cell model is needed to regulate the DC

bus voltage in islanding mode. The battery terminal voltage and

SOC need to be estimated during operation. A high fidelity

electrical model of lithium-ion battery model, with thermal

dependence, is used. The pulse load can be connected to both

the AC and DC side. On the DC side, the pulse load can usually

be viewed as a purely resistive load. On the AC side, the pulse

load can be either a resistive or inductive load, such as an in-

duction motor. However, those inductive loads are commonly

connected to the AC side through power electronic drives, such

as back-to-back converters. In this way, the inductive load can

be converted and act as a resistive load. In this paper, only 18

ohm resistive loads are used, and the programmable load is

designed accordingly.

III. COORDINATED CONTROL OF CONVERTERS

Three types of converters are utilized in this proposed hybrid

microgrid as shown in Fig. 1. These converters must be actively

controlled in order to supply uninterrupted power with high

efficiency and quality to pulse loads on the AC and DC sides

during grid-connected and islanding modes. The coordinated

control strategies for converters are discussed.

A. Boost Converter Control with MPPT

To maximize the utilization of renewable energy from the PV

farm, the boost converter should be operated in on-MPPT mode

when the hybrid microgrid is connected to the utility grid. The

battery banks in the microgrid can be used as an energy buffer,

and the charging/discharging rates can be controlled based on

the status of the output power from the PV farm and the power

flow in the AC side. In islanded mode, the boost converter of the

PV farm can be operated in on-MPPT or off-MPPT, which is

based on the system’s power balance and the SOCs of the bat-

tery banks. In most cases, this boost converter can operate in the

on-MPPT mode since the variation ratio of the solar irradiance

is much lower compared to the power adjustment ability of the

small size AC generator. Therefore, for a constant load either on

the AC or DC side, the PV should supply as much power as

possible to maximize its utilization. However, if the battery

banks’ SOCs are high, (near fully charged) and the PV’s

maximum output power is larger than the total load in the hybrid

microgrid, the PV should be turned to off-MPPT to keep the

system power in balance. In this paper, a perturbation and ob-

serve (P&O) method is used to track maximum power point.

B. Bi-Directional DC/DC Converter Control

The bi-directional DC/DC converters are used to connect the

battery banks to the DC bus. The hardware setup of a battery

0 10 20 30 40 50 60 700

2

4

61 kW/m2

Curr

ent

(A)

Voltage (V)

Module type: SunPower SPR-305-WHT

0.75 kW/m2

0.5 kW/m2

0.25 kW/m2

0 10 20 30 40 50 60 700

100

200

300 1 kW/m2

Pow

er (

W)

Voltage (V)

0.75 kW/m2

0.5 kW/m2

0.25 kW/m2

Fig. 3. I-V and P-V curves for PV panel Sunpower SPR-305-WHT

TABLE I

PARAMETERS FOR PHOTOVOLATIC PANEL

Symbol Description Vaule

Voc Rated open circuit voltage 64.2 V

Iph Photocurrent 5.9602 A

Isat Module reverse satuation current 1.1753×10-8

q Electron charge 1.602×10-19 C

A Ideality factor 1.50

k Boltzman constant 1.38×10-23 J/K

Rs Series resistance of a PV cell 0.037998 Ω

Rp Parallel resistance of a PVcell 993.51 Ω

Isso Short-circuit current 5.96 A

ki SC current temperature coefficient 1.7×10-3

Tr Reference temperature 301.18 K

Irr Reverse saturation current at Tr 2.0793×10-6 A

Egap Energy of the band gap for silicon 1.1eV

np Number of cells in parallel 528

ns

S

T

Number of cells inseries

Solar radiation level

Surface temperature of the PV

480

0~1000 W/m2

350 K

Rac Lac

SPWM

eab PLL

ebc

Sa Sb

Sc

EqEd

θ

abc abc

abc

dqo dqo

dqo

vdc

refVdc+

idref

iqid

+

wLac+Rac

θ

-

PI

-

iqref

+

PI

PI

-

-wLac+Rac

+

+

+

+

vdcon

vqcon

0

+

RL

Fig. 5. The control block diagram for bi-directional AC/DC converter

in grid-connected mode



4

bank, bi-directional DC/DC converter together with the meas-

urement circuit, and control driver circuit is shown in Fig. 2. In

grid-connected mode, these converters only regulate the battery

banks charging rates. Based on the SOCs of the battery banks

and the power flow conditions in the AC side, the charg-

ing/discharging current references are generated to regulate the

current flow in the converters.

Each battery has its own bi-directional DC/DC converter,

which means they can have different charging rates. The battery

banks can inject power to, or absorb power from, the DC bus.

Also, they can transfer energy between different battery banks if

necessary. In this case, only one closed current control loop with

PI controller is enough to regulate the current. The

bi-directional DC/DC converters of the battery banks play an

important role in islanding mode to regulate the DC bus voltage.

A two-loop control is used to regulate the DC bus voltage. The

control scheme for the bi-directional DC/DC converter is shown

in Fig. 4.

The outer voltage controlled loop is used to generate a ref-

erence charging current for the inner current controlled loop.

The error between the measured DC bus voltage and the system

reference DC bus voltage is set as the input of the PI controller,

and the output is the reference current. The inner current control

loop compares the reference current signal with the measured

current flow through the converter and, finally, generates a

PWM signal to drive the IGBTs to regulate the current flow

through the converter. For example, when the DC bus voltage is

higher than the reference voltage, the outer voltage controller

generates a negative current reference signal. The inner current

control loop adjusts the duty cycle to force the current flow from

the DC bus to the battery, which results in charging of the bat-

tery. In this way, the energy transfers from the DC bus to the

battery, and the DC bus voltage, then decreases to the rating

value. If the DC bus voltage is lower than the normal value, the

outer voltage control loop generates a positive current reference

signal, which regulates the current flow from the battery to the

DC bus. Because of the extra energy injected from the batteries,

the DC bus voltage increases to the rating value.

If several energy storage systems are connected to the

common DC bus of the hybrid power system individually

through their own bidirectional DC-DC converter, a conven-

tional PID controller could not be used to regulate the DC bus

voltage. If all of them are used to regulate the DC bus voltage,

they may conflict with each other and cause instability prob-

lems; however, if only one of them is used to regulate the DC

bus voltage, distributing power flow becomes unclear to the

other controllers, which may cause SOC unbalance between

energy storage systems. Therefore, droop control is used to

regulate and dispatch power flow for multiple lithium-ion bat-

tery modules on the DC side of the hybrid power system. With

the SOC information, the five battery modules are ranked based

on their SOC. With the SOC rank and the measured DC bus

voltage, a central aggregator calculates and assigns the droop

coefficient to each battery module. Each bidirectional DC-DC

converter can generate its charging rate with the droop coeffi-

cient.

C. Bi-Directional AC/DC Converter

In grid-connected mode, the AC side can be viewed as an

infinite bus; therefore, the deviation of the voltage amplitude

and frequency can be ignored. In this case, the bi-directional

AC/DC converter only needs to regulate the DC bus voltage. In

order to operate in unit power factor, reference iq can be set as 0.

The controller only needs to control the id, which controls the

active power flow through the converter. The control block

diagram for bi-directional AC/DC converter in grid-connected

mode is shown in Fig. 5. As discussed earlier, a two-loop con-

troller is used to regulate the DC bus voltage. Based on the error

between the DC bus reference voltage and measured voltage,

the outer voltage control loop generates the id reference, which

is used to regulate id in the bi-directional converter. In d-q co-

ordinates, Id is controlled to regulate the active power flow

through the inverter, and Iq is controlled to regulate the reactive

power flow through the inverter. In the AC side, the active and

reactive power flow will influence the frequency and voltage

amplitude respectively.

In islanded operation mode, the frequency and voltage am-

plitude of the three phases AC side are volatile. The

bi-directional AC/DC inverter is used to regulate the active and

reactive power by controlling the id and iq, respectively. The

control scheme for the bi-directional AC/DC inverter is shown

in Fig. 6. Two-loop controllers are applied for both frequency

and voltage regulation. For frequency control, error between

measured frequency and reference frequency is sent to a PI

controller which generates the id reference.

PI -1 PI 1/(sLb +Rb) db 1/sCd

Vd*

Vd

Id*

Id

+ +

- -

Vd

Fig.4. The control block diagram for bi-directional DC/DC converter

Rac Lac

SPWM

eab PLL

ebc

Sa Sb

Sc

EqEd

θ

abc abc

abc

dqo dqo

dqo

idref

vdc

fref

f

+

-

PI

PI

refVam

Vam

+- iq

ref

iqid

id

iq

+

+

PI

PI

-

-

iq -wLac+Rac

wLac+Rac

θ

Ed

Eq Vq Vd

Δq

Δd

Fig. 6. The control block diagram for bi-directional AC/DC converter

in islanded mode



5

To control the voltage amplitude, the error between the

measured voltage amplitude and the reference voltage ampli-

tude is sent to a PI controller to generate iq reference.

Equations (4) and (5) show the AC side voltage equations of

the bi-directional AC/DC inverter in ABC and d-q coordinates,

respectively. Where (Va, Vb, Vc) are AC side voltages of the

inverter, and (Ea, Eb, Ec) are the voltages of the AC bus. (Δa, Δb,

Δc) are the adjusting signals after the PI controller in the current

control loop.

c

b

a

c

b

a

c

b

a

c

b

a

ac

c

b

a

ac

E

E

E

V

V

V

i

i

i

R

i

i

i

dt

dL (4)

q

d

q

d

q

d

q

d

acac

acac

q

d

ac E

E

V

V

i

i

RL

LR

i

i

dt

dL

(5)

When the pulse load is connected or disconnected to the AC

side, the frequency or voltage amplitude changes. After de-

tecting the deviation using the phase lock loop (PLL) or voltage

transducer, Id and Iq reference signals are adjusted to regulate

power flow through the bi-directional AC/DC inverter. Because

of the power flow deviation, the DC bus voltage is also influ-

enced. The DC bus voltage transistor senses the voltage varia-

tion in DC bus, and the bi-directional DC/DC converter regu-

lates the current flow between the battery and the DC bus. In the

end, the energy is transferred between the battery and the AC

side to balance the power flow.

For further frequency and voltage regulation, a droop control

is implemented for solving the microgrid primary control

problem as shown in Fig. 7. Synchronous generator and grid-tie

inverter adjust their power output according to the no-load

speed setting with respect to the system frequency. The power

output for any given system frequency can be controlled. The

no-load frequency of a given generator can be set to obtain any

desired power output according to its droop slope R.

max min( ) / ( ) sysNL NL FLf P f f P P f (6)

sysNLf P R f (7)

Here, R is typically formulated with the maximum and

minimum power outputs of the DG, Pmax and Pmin. The no-load

and full-load frequencies, fNL and fFL, are normally chosen as the

bounds which the system frequency must not cross. The sec-

ondary level control covers the residual frequency error and

puts back the frequency value to 60 Hz.

IV. SIMULATION AND EXPERIMENTAL RESULTS

The operation of the hybrid microgrid is tested on both

simulation environment and experimental setup. 10.07 kW PV

farm under the influence of a 10 kW pulse load is applied to

verify the proposed hybrid microgrid control. The total rated

power of the synchronous generators are 13.8 kW, and a 4 kW

constant load is connected in the AC side. Five 51.8V, 21Ah

Lithium-ion battery banks are connected individually to DC bus

through bidirectional DC/DC converters with the rated power of

10 kW. The rated power of the grid-tied bidirectional AC/DC

inverter is 26 kW. In steady state, the hybrid AC/DC system is

operated with 300V rated DC bus voltage and 208V,

phase-to-phase AC bus voltage. Further system parameters for

hybrid microgrid are listed in Table II.

A. Simulation Results

In both grid-connected mode and islanded mode, to maxim-

ize the utilization of renewable energy, the boost converter

works in on-MPPT mode to keep seeking the maximum output

power from the PV farm. The MPPT of the boost converter is

enabled at 0.4s. The output power, the terminal voltage of the

PV panel, the duty cycle of the boost converter, and the solar

irradiance levels are shown in Fig. 8. For general study, two

kinds of solar irradiance deviation with different charging rates

are used. Before 0.4s, the duty cycle is set at 0.5, the terminal

voltage of the PV panel is 149V, and the output power from the

System f = 60 Hz

Frequency (Hz)

fNL=61.0

fNL=60.5

fNL=59.5

10.750.50.250-0.25

Power Output (p.u.)

Grid-TieInverter

Synchronous Generator

Fig. 7. Droop control implementation

TABLE II

HYBRID MICROGRID SYSTEM PARAMETERS

Symbol Description Vaule

Cpv Solar panel capacitor 100 uF

Lpv Inductor for solar Panel boost converter 5mH

Cd DC bus capacitor 6000 uF

Lac AC filter inductor 1.2mH

Rac Inverter equivalent resistance 0.3ohm

Lb Battery converter inductor 3.3mH

Rb Resistance of Lb 0.5 Ω

f

Vd

Vm

n1/n2

Rated AC grid frequency

Rated DC bus voltage

Rated AC bus p-p voltage (rms)

Transformer ratio

60Hz

300V

208K

1:1

0 0.5 1 1.5 2 2.5 3 3.5 40

5

10

Po

wer

(kW

)

0 0.5 1 1.5 2 2.5 3 3.5 4

100

150

200

Vo

ltag

e (

V)

0 0.5 1 1.5 2 2.5 3 3.5 40.4

0.5

0.6

Du

ty c

ycle

0 0.5 1 1.5 2 2.5 3 3.5 4

500

1000

Time (s)

Irra

dia

nce (

W/m

2)

Fig. 8. PV output power control with MPPT



6

PV panel is only 9.56 kW. After the MPPT is enabled, the duty

cycle is decreased to 0.45 and the terminal voltage is increased

to 165V. This allows the PV panel to reach the maximum power

output of 10.07 kW. The simulation results show that the boost

converter, with MPPT functionality, can track the maximum

power point within a short response time.

In grid-connected mode, the DC bus voltage is regulated by

the bi-directional AC/DC converter. In this mode, the AC side

can be viewed as an infinite bus, therefore the 10 kW resistive

pulse load that is connected to the AC side would not have an

influence on the grid. Therefore, only the DC pulse load case is

studied. Fig 9 (a) shows the DC bus voltage. The bi-directional

AC/DC converter was enabled at 0.05s. Before being enabled, it

operates as an uncontrolled rectifier. After enabling the

bi-directional converter, the DC bus voltage reached steady

state in less than 0.3s. During this period, the solar irradiance is

1kW/m2. From 0.4s to 1.7s, the system under two kinds of solar

irradiance variations is simulated. The output power from PV

decreased from 10kW to 2.5kW in 0.3 second, and recovered

back from 2.5kW to 10kW also in 0.3 second. After that, the PV

output decreased from 10kW to 2.5 kW in 0.05 second at 1.3s,

and it went back from 2.5kW to 10kW at 1.65s. The PV output

power is shown in Fig. 9 (b). The DC bus voltage was stable and

stayed in the range of 293V to 307V during this process,

therefore, the bi-directional converter can keep the DC side

stable under rapid alteration of solar irradiance and PV output

power. From 2s to 2.8s, the charging/discharging of battery

banks impact to the DC bus is simulated. At 2s, the current

references of the bi-directional DC/DC converters of those five

batteries were changed from 0A to -4A, which means dis-

charging those battery banks with 4A. At 2.4 seconds, the cur-

rent references of the bi-directional DC/DC converters of the

five battery banks were changed from 0A to 4A. At this point,

the system began charging the battery banks. The current flow

of one bi-direction DC/DC converter is shown in Fig. 9 (c).

During this period, the DC bus voltage was still stable with less

than 3V voltage deviation. From 3s to 3.5s, a 10kW resistive

load is connected to the DC bus. During the connection and

disconnection of the 10 kW pulse load, the bi-directional

AC/DC inverter active power flow was greatly changed to

regulate the DC bus voltage. The DC bus was still stable, within

12V voltage deviation, during the transient response. The power

flow through the bi-directional AC/DC inverter is shown in Fig.

9 (d). After the system entered steady state, the system kept the

unit power factor as the reactive power was 0. The active power

flow varied with the solar irradiance influence, battery banks

charging/discharging influence and pulse load influence. The

bi-directional inverter can quickly adjust the power flow.

B. Experimental Result

Complementary to simulation results and extension of [16],

the real-time experimental performance of the hybrid microgrid

is tested under islanding and pulsed load conditions. The is-

landing experiment was realized by opening the circuit breaker

of PMU2, shown in Fig.1. Fig.10 illustrates the islanding tran-

sition of the microgrid and the corresponding primary and

secondary controls. Primary control is the immediate response

of the synchronous generator, and the grid-tie inverter arrests

deviations in power system frequency. Secondary control ad-

justs the dispatchable assets shortly after frequency and voltage

deviations to restore the system to nominal operating condi-

tions.

1) Islanding of Hybrid AC/DC Microgrid

The objective of this experiment is to present proper centrally

coordinated actions to reinstate frequency to its nominal value.

In a conventional power system operation approach, if an un-

controlled islanding is formed due to emergency conditions, the

primary control responds rapidly according to droop adjust-

ments of the generators inside the island. Droop based primary

control deviates the frequency from the nominal value, ac-

cording to the system loading conditions. Upon separation, it

may be necessary to shed some of the predetermined loads in the

islanded area in order to balance generation and load. The

measurements are taken from synchrophasors deployed on the

AC side of the microgrid [20]. During the synchronized opera-

tion, the islanded power system was importing power from the

remote power system. When the islanding situation takes place

at t= 130s, the imported power becomes zero.

( ) ( ) ( )( ) Gen t Load t import tP t P P P (8)

As per equation (9), the power imbalance inside the mi-

crogrid results in a frequency drift in the islanded area:

dt

df

f

HtPtPtP s

n

tot

loadgen

2))()(()(

(9)

The active power imbalance introduces frequency deviation

in islanded microgrid (9), where Htot is the total inertia, fn is the

nominal frequency, and fs is system frequency [21]. The phase

angle difference between the two areas increases. The figure

shows the oscillation of the phase angle of the system generators

during primary control. Between t= 130s and at t= 147s, the

primary control is established. The generation in the islanded

area increases and the frequency settles in a stable region. At

t= 181s, the system frequency reaches the minimal value. Au-

tomatic generation control (AGC) based secondary control

0 0.5 1 1.5 2 2.5 3 3.5 4280

300

Vo

ltag

e (

V)

(a)

0 0.5 1 1.5 2 2.5 3 3.5 40

5

10

Po

wer

(kW

)

(b)

0 0.5 1 1.5 2 2.5 3 3.5 4-5

0

5

Cu

rren

t (A

)

(c)

0 0.5 1 1.5 2 2.5 3 3.5 4-2

0

2x 10

4

Po

wer

(VA

)

(d)

Time (s)

P

Q

Vdc-ref

Vdc-mea

Fig. 9. Hybrid microgrid performance in grid-connected mode

Pulsed Load Phase Angle

Deviation



7

Islanding

Instant Secondary

Control

Primary Control

Grid-connected

Operation Islanded

Operation

Phase Angle

Difference

Loss of Power Import from Grid

Generation in Microgrid Increased

Fig. 10. Islanding of hybrid AC/DC microgrid

is used to restore system frequency to nominal value. A

common way to enable AGC in power systems is to implement a

proportional-integral (PI) controller. An area control error

(ACE) in a power system is given as (10), where B is the fre-

quency bias factor, ∆PT is the deviation of active power balance

in area, and ∆PAGC is the control command to be sent to the

grid-tie inverter. β1 and β2 are the PI control coefficients.

1 2

T

AGC

ACE P B f

P ACE ACEdt

(10)

In real-time applications, angular stability is measured by the

difference in generator voltage angles of the two points and

compared with a predefined threshold angle during pulsed load:

1 2PMU PMU (11)

where is defined as the tolerance in degrees. The tolerance

in this study is selected to be 10 degrees. When the angular

separation between two PMU measurements is exceeded, the

instability should check whether the system is in a stable or

unstable swing. Specifically, the microgrid buses are assumed

as two-machine system implementing swing equation to de-

termine the out-of-step condition in the two-area system.

As classical generator dynamics are defined as:

.

i i

i i

ii m e

dH P P

dt

(12)

2

1

1

cos cos( )i

n

e i ii ii i j ij i j ij

j

j

P E Y E E Y

(13)

where Hi is the inertia constant of ith generator, i is the in-

ternal voltage angle of ith generator, i is the rotor speed of the

ith generator, /i ie mP P electrical/mechanical output power of

the ith generator, ,i jE E voltage behind transient reactance, Y

is the admittance matrix reduced at the internal generator node.

2) Performance during Pulsed Loads

To further verify the proposed control algorithm for hybrid

AC-DC power system operation with pulse load mitigation, a

hardware experiment is demonstrated. PMUs are connected to

synchronous generator and grid-tie inverter buses. System fre-

quency and voltage variations, phase angle displacement and

pulsed loading are shown in Fig. 11. A 4 kW pulsed load is

applied three consecutive times. It is noticed that the system

frequency experiences an oscillation during the transition,

which eventually settles to the reference value. The terminal

voltage experiences temporary spikes during transition as well,

and it settled within limits due to reactive power control inside

the island. The phase angle displacement between microgrid

buses are kept under 10 degree threshold. From the results, it

can be seen that during the pulse load duration, the DC micro

grid can inject power to help the system regulate the frequency

and the frequency only dropped to around 57.5 Hz. The voltage

dropped to around 115 V. The proposed coordinated converter

control was able to perfectly mitigate frequency and voltage

dips according to pulsed loading. The islanded low inertia hy-

brid microgrid was able to withstand the pulsed loading condi-

tions.



8

V. CONCLUSION

In this paper, a coordinated power flow control method of

multi power electronic devices is proposed for a hybrid AC/DC

microgrid operated in both grid-connected and islanded modes.

The microgrid consists of a PV module, battery bank and a

synchronous generator that supply energy to its DC and AC

side. Battery banks are connected to the DC bus through

bi-directional DC/DC converter. The AC side and DC side are

linked by the bi-directional AC/DC inverter. The control algo-

rithms are tested with the harsh influence of pulse loads and

islanding conditions. The simulation and experimental results

show that the proposed microgrid with the control algorithm can

greatly increase the system stability and robustness.

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Frequency

Disturbance

Phase Angle

Deviation Pulsed Load

Frequency Disturbance

Pulsed Load

Phase Angle Deviation

Fig. 11. Consecutive pulsed loads and system disturbance



9

Tan Ma (M’16) received the M.

E. degree in control theory and con-

trol engineering from Huazhong

University of Science and Technol-

ogy (HUST) in China in 2009 and the

Bachelor of Eng. degree in automa-

tion from HUST in China in 2007.

He received his Ph.D. degree in

electrical and computer engineering

at Florida International University.

His research interests include Power

System Operations, Control and Protection, Artificial Intelli-

gence Applications to Power Systems, Energy Conservation and

Alternate Energy Sources and smart grid power systems design

and optimization.

Mehmet H. Cintuglu received

the B.S. and M.S. degrees in elec-

trical engineering from Gazi Uni-

versity, Ankara, Turkey, in 2008

and 2011, respectively. He is cur-

rently pursuing the Ph.D. degree

from the Energy Systems Research

Laboratory, Electrical and Com-

puter Engineering Department,

College of Engineering and Computing, Florida International

University, Miami, FL, USA. From 2009 to 2011, he was a

Power Systems Project Engineer at an electric utility company

in Turkey. His current research interests include multi agent

systems, distributed control, cyber physical systems for active

distribution networks and microgrids.

Osama A. Mohammed is a Pro-

fessor of Electrical Engineering and

is the Director of the Energy Systems

Research Laboratory at Florida In-

ternational University, Miami, Flor-

ida. He received his Master and

Doctoral degrees in Electrical En-

gineering from Virginia Tech in

1981 and 1983, respectively. He has

performed research on various topics

in power and energy systems in ad-

dition to design optimization and physics passed modeling in

electric drive systems and other low frequency environments.

Professor Mohammed is a world renowned leader in electrical

energy systems. He has performed research in the area of elec-

tromagnetic signature, wideband gap devices and switching,

and ship power systems modeling and analysis. He has current

active research projects for several Federal agencies dealing

with; power system analysis and operation, smart grid distrib-

uted control and interoperability, cyber physical systems, and

co-design of cyber and physical components for future energy

systems applications.

control of hybrid ac/dc microgrid involving energy storage...

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