INTERNATIONAL JOURNAL OF PROFESSIONAL ENGINEERING STUDIES Volume V /Issue 4 /SEP 2015

IJPRES

APPLICATION OF FUZZY BASED THREE-PHASE INVERTER IN DISTRIBUTED

GENERATION BY IMPLEMENTING UNIFIED CONTROL STRATEGY

MUKKU MURALI1, DR. K VENKATESWARLU2

1PG Scholar, Malineni, Lakshmaiah Engineering College, Andhra Pradesh, India

2Assistant professor, Malineni, Lakshmaiah Engineering College, Andhra Pradesh, India

Abstract-By eliminating need of separate

controllers or critical islanding detection, this paper

proposes a fuzzy based three-phase inverter in

distributed generation which can be implemented for

both islanded and grid-tied operations. In the

proposed strategy the three-phase inverter is

regulated as just current source by inner inductor

current loop in grid-tied and for islanding mode a

voltage loop in the synchronous reference frame will

automatically regulates the load voltage. This paper

proposes a unified load current feedforward to

maintain the grid current waveforms in grid-tied

mode and load voltage waveforms in islanding mode

to be undistorted even under nonlinear local load.

The effectiveness of the proposed strategy is

validated by simulation.

Index Terms— Fuzzy Logic Controller,

unified control, islanding, load current, seamless

transfer, Distributed generation (DG), three-phase

inverter, unified control.

INTRODUCTION

The distributed generation (DG) concept

emerged as a way to integrate different power plants,

increasing the DG owner’s reliability, reducing

emissions, and providing additional power quality

benefits [4]. The cost of the distribution power

generation system using the renewable energies is on

a falling trend and is expected to fall further as

demand and production.

DG delivers power to the utility and local critical

loads in grid-connected mode. Upon outage of any

generator connected to the utility the islanding is

formed. Under these situations, DG must be tripped

and must stop to energize according to IEEE standard

929-2000. In order to continue to feed the local

critical load by disconnecting DG’s and some local

load in order to improve the power reliability. Load

voltage is fixed by the DG in the islanded mode and

by the utility in the grid mode operation. So,

maintaining the load voltage is important. In order to

reduce transients in the load DG must take over the

load as soon as possible which is challenging

operation for the DG.

In this paper voltage control mode is nothing but

Droop-based control is used widely for the sharing of

power among parallel inverters and can be applied to

DG to realize power sharing between DG and utility

in grid-tied mode [11-12]. Under this operation, load

voltage is guaranteed during transitions of operation

modes and inverter is regulated as voltage source by

voltage loop is good only steady-state performance

whereas dynamic performance is poor because

bandwidth of voltage loop is higher than of the

external power loop, realizing droop control. In

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addition to the (phase locked loop) PLL and the

virtual inductance, the inrush grid currents during

transition from islanded mode to grid-tied mode

always exists it means grid current is not controlled

directly [13].

Better dynamic performance can be achieved by

hybrid voltage and current mode type control for DG.

In which inverter is controlled as current source by

one sets of controller in grid-tied mode, and as a

voltage source by other sets of controller in the

islanded mode. Inrush grid currents are almost

eliminated in the output by directly controlling the

output current in grid-tied mode. There is no need to

change the switch of the controller when the

operation mode of DG is changed, with the use of

hybrid voltage and current control mode.

With the occurrence of utility outage the interval

during to change it to voltage mode, the load voltage

is neither regulated by DG nor fixed by the utility but

the length of the time interval is determined by the

islanding detection process. The main issue in this

approach is that quality of load voltage can heavily

reliant on the speed islanding detection method

accuracy [7]-[10].second issue is under non-linear

local load with aforementioned approaches is the

quality waveform of the grid current and load

voltage.

The output current of DG is generally desired to be

pure in grid-tied mode [13]. The harmonic

component will fully flow into the utility when

nonlinear load is fed. The harmonic components of

the grid current can be mitigated by harmonics

Fig. 1. Schematic diagram of the DG based on the proposed control strategy.

injected by single-phase DG in [4]. DG will emulate

a resistance at harmonic frequency is being controlled

by voltage mode control and then the harmonic

current flowing into the utility can be mitigated. In

the islanding mode, the nonlinear load may distort.

With the use of multi-loop control method, resonant

controllers, sliding mode control and many control

schemes have been proposed to improve the quality

of the load voltage. Existing control strategies, DG

with nonlinear local load will mainly concentrate on

grid current in the grid-tied mode and on load voltage

in island mode and improving both of them for

unified strategy is rarely used.

This paper discusses about unified control strategy

that avoids the aforementioned shortcomings. With a

given reference in the synchronous frame (SRF) the

three-phase inverter is controlled in DG act as a

current source using traditional current loop. A novel

voltage controller is presented to supply reference for

the inner inductor current loop in D-axis and Q-axis

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proportional-plus-integral (PI) compensator and a

proportional (P) compensator are employed. The load

voltage is dominated by the utility and the voltage

compensator in D-axis is saturated, while the output

of the voltage compensator in Q-axis is forced to be

zero by the PLL. The reference of the inner current

loop cannot be regulated by the voltage loop. With

the occurrence of grid outage, the load voltage is no

more determined by the utility. The voltage controller

is automatically activated to regulate the load

voltage.

Hence proposed control strategy does not need a

forced switching between two different sets of

controllers. So, there is no need of detecting islanding

quickly and accurately is no more critical in

approach. For better dynamic performance, the

proposed control strategy utilizes the feedback

control for both current and voltage compares to

voltage control mode. And paper is enhanced by

introducing a unified load current feedforward, is

implemented by adding the load current into the

reference of the inner current loop in order to deal

with the issue caused by the nonlinear local load. The

benefits of the proposed load current feedforward can

be extended into the islanded operation mode, due to

the improved quality of the load voltage.

This paper is arranged as follows. Section II

discusses about Distributed generation (DG) and its

applications. Section III describes the proposed

unified control strategy for three phase inverter in

DG which includes the power stage, the basic idea

and control diagram. Section IV discuss about fuzzy

logic controller. The parameter design and small

signal analysis of the proposed control system are

given in Section V. The simulation results for the

proposed system are shown in Section VI. Finally,

the conclusion and remarks are given in section VII.

DISTRIBUTED GENERATION (DG)

AND IT’S APPLICATIONS Distributed generation (or DG) generally refers to

small-scale (typically 1 kW – 50 MW) electric power

generators that produce electricity at a site close to

customers or that are tied to an electric distribution

system [12]. Distributed generators include, but are

not limited to synchronous generators, induction

generators, reciprocating engines, microturbines

(combustion turbines that run on high-energy fossil

fuels such as oil, propane, natural gas, gasoline or

diesel), combustion gas turbines, fuel cells, solar

photovoltaic, and wind turbines.

There are many reasons a customer may choose to

install a distributed generator. DG can be used to

generate a customer’s entire electricity supply; for

peak shaving (generating a portion of a customer’s

electricity onsite to reduce the amount of electricity

purchased during peak price periods); for standby or

emergency generation (as a backup to Wires Owner's

power supply); as a green power source (using

renewable technology); or for increased reliability. In

some remote locations, DG can be less costly as it

eliminates the need for expensive construction of

distribution and/or transmission lines.

Islanding: Islanding occurs when a DG system is

still generating power to the distribution system when

the main breaker from the Wires Owner is open. In

this case, the DG system would be the sole supplier

of electricity to the distribution system. This is a

concern for several reasons.

i. Safety concern for system maintenance if the Wires

Owner's line workers are not aware that the DG

system is still running, they may be electrocuted

working on the line or other equipment connected to

the line.

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ii. Equipment damage to other Wires Owners

customers. If the DG is still generating while the

main breaker from the wire owner is open, the

voltage and the waveform from the DG may fluctuate

and may not meet the acceptable standard. Existing

customers who are connected to the distribution line

are then fed by very poor quality of power from the

DG. As a result, their light fixtures, motors and other

electric equipment may be damaged or its life may be

shortened. If the situation persists unnoticed for an

unacceptably long time, a fire hazard may exist. iii.

Damage to the DG owner's generator if the DG is still

generating while the main breaker from the wires

owner is open, the DG equipment may be damaged

when the wires owner’s main breaker is closed due to

closing out of synchronism.

SYSTEM PROPOSED CONTROL

STRATEGY A. Power Stage:

To operate in both grid-tied and islanded modes this

paper proposes unified control strategy for three-

phase inverter in DG. The DG is equipped with a

three-phase interface inverter with a LC filter. The

energy from prime mover is converted in electrical

energy and then into DC by front end power

converter, the DC voltage is regulated represented

by 푉 as shown in figure. Local grids are directly

connected in the ac side of the inverter. The two

switches 푆 and 푆 functions are different. DG will

control the inverter transfer switch 푆 and the utility

will control the utility protection switch 푆 . Under

normal operation, the DG in the grid-tied mode

injects power to the utility and both 푆 and 푆

switches are ON. When the utility is in fault, the

utility instantly trips the switch 푆 and then the

islanding is formed. 푆 Switch will be disconnected

after the islanding has been detected by the DG and

DG will be transferred to islanded mode from grid-

tied mode. The DG will be resynchronized with the

utility only after when the utility is restored and the

switch 푆 will be turned ON to connect the DG with

the grid.

Fig. 2. Overall block diagram of the proposed unified

control strategy. B. Basic Idea

with the proposed control modes (hybrid voltage and

current mode) the inverter is controlled as a current

source to generate reference power 푃 + 푗푄 in

grid-tied mode, output power 푃 + 푗푄 should be

the power injected into the grid 푃 + 푗푄 and load

demand can be expressed as follows by assuming the

load is represented as a parallel RLC circuit:

푃 =32 ∙푉푅

------- (1)

푄 =32 ∙ 푉 ∙

1휔퐿 − 휔퐶

--------- (2)

Where 푉 the amplitude of load voltage and f is is the

frequency of load voltage. Considering the

fundamental component still equivalent to the

parallel RLC circuit when the nonlinear local load is

fed. The load voltage will neither be fixed by the

utility nor regulated by the inverter during the time

interval the moment of switching the control to the

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instant of islanding mode. The load voltage may drift

from the normal range.

The inverter will still controlled as current source

and kept it output power almost unchanged during

this time interval. The power injected to utility

decreases to zero rapidly, and then the power

consumed by the load will be imposed to the output

power of DG. If considered both active power and

reactive power injected into the grid is positive in the

grid-tied mode, then 푃 and 푄 will increase the

after the islanding mode. The amplitude and

frequency of the load voltage will rise and drop

according to equations (1) and (2).

Comparing to the traditional analysis, the output

power of inverter 푃 + 푗푄 can be regulated to

match the load demand by changing the current

reference before islanding is confirmed. The load

voltages excursions will be mitigated which is

implemented in this paper. By regulating the three-

phase inductor current 푖 only the output power of

the inverter is controlled in the proposed control

strategy, while the magnitude and frequency of the

load voltage 푣 are monitored. While islanding is

about to operate, the magnitude and frequency of the

load voltage may drift from normal range and then

they are controlled automatically and recovered to

normal range by regulating the output power of the

inverter.

C. proposed Control strategy:

Figure 2 shows the proposed unified overall control

block diagram. The sensed values from the block

diagram are the utility voltage 푣 , the inductor

current 푖 and the load current 푖 . The three-

phase variables of the three-phase inverter will be

represented in dc quantity is controlled in the SRF.

The main modes of the control diagram are the

inductor current loop, the PLL, and the current

reference generation module. In order to mitigate the

couplings due to the inductor, is implemented by the

PI compensator in both D- and Q-axes and

decoupling of the cross coupling 휔 퐿 /푘 .

Decoupling capacitor 1/푘 and output of inner

current loop 푑 sets the reference for the standard

space vector modulation (SVM) that control the

switches of the three-phase inverter. Where 푘

denotes the voltage gain of the inverter which equals

to half of the dc voltage in this paper.

The widely used SRF PLL in three-phase power

converter to estimate the utility frequency and phase

is also proposed in the control strategy [15], in order

to hold the frequency of the load within the normal

range in the islanded operation a limiter is inserted

between the PI compensator 퐺 and integrator.

From figure it can be concluded that the inductor

current is regulated to follow the current reference

푖 and the current phase is synchronized to the

grid voltage 푣 .

Fig 3 Block diagram of the current reference generation module. If current reference is constant, the inverter is just

controlled to be a current source, which is same with

the traditional grid-tied inverter. The new thing in

this paper is the current reference to guarantee the

power match between the DG and local load and

enables to operate in islanded mode. In this module

even unified load current feedforward to cope with

nonlinear local load is implemented. Figure 3

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provides the current reference for the inner current

loop in both grid-tied and islanding modes. An

unsymmetrical structure is used in D- and Q-axis

where PI compensator in D-axis with an extra limiter

and P is employed in Q-axis. Load current 푖 is

being added to the final inductor current reference

푖 by the load current feedforward. The benefits

from figure 3 are represented by two parts: 1) without

critical islanding detection seamless transfer

capability; and 2) in both grid-tied and islanded

operations improving the power quality. In D and Q-

axes the current reference 푖 composes of four

parts namely: 1) controller output voltages 푖 ; 2)

the reference grid current 퐼 ; 3) the load current

푖 and 4) the current through filter capacitor 퐶 .In

grid-tied mode, the load voltage 푣 is decided by

the utility. The load voltage and current reference are

irrelevant due to saturation of PI compensator in D-

axis and the output of P compensator being zero in

Q-axis. Thus, the inverter operates as a current

source. Voltage controller takes automatically to

control the load voltage by regulating current

reference when islanding occurs and makes the

inverter to operate as a voltage source to provide

stable voltage to the local loads.

The advantage of this control scheme is that it

relieves from different control architecture. The other

distinguished function of the current generation

module is the load current feedforward. In order to

compensate the harmonic component the sensed load

current is added as a part of the inductor reference

current 푖 in the grid current under the nonlinear

local load. But in the islanded mode still the load

current feedforward operates and the disturbance

caused by the nonlinear load can be suppressed by

the fast inner inductor current loop and finally the

quality of the load voltage is improved.

In [18] the inductor current control shown in Fig 2

was proposed for grid-tied operation of DG. Inspired

from [18] this paper proposes a unified control

strategy for DG in both grid-tied and islanded modes

can be represented by the current reference

generation module in figure 3.This module can be

summarized in two aspects for this contribution.

First, PI compensator in D and P compensator Q-axis

respectively, upon occurrence of islanding voltage

controller is activated automatically and maintained

inactive during grid-tied mode. There is no need for

switching different controllers and load voltage

quality during transition from grid-tied mode to the

islanded mode can be improved. Another

contribution of this module is to provide load current

feedforward to deal with the issue caused by the

nonlinear local load, by which load voltage quality in

islanded mode is enhanced and the grid current

waveform in grid-tied can also improved.

It should be noted that the unbalance three-phase

local load currents cannot be fed by the DG with the

proposed control strategy, because there is no flow

path for the zero sequence current of unbalanced

load, and the regulation of zero sequence current is

beyond the scope of the proposed control strategy.

FUZZY LOGIC CONTROLLER

The error value of the dc-bus voltage Δvdc= v∗dc−vdc

is passed through a Fuzzy-type compensator to

regulate the voltage of dc bus (vdc) at a fixed value.

The operation of FLC is as follows. FLC contains

three basic parts: Fuzzification, Base rule, and

Defuzzification. FLC has two inputs which are: error

and the change in error, and one output. The Fuzzy

Controller structure is represented in fig.6. The role

of each block is the following:

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Fig 2: The general structure of Fuzzy Logic Controller

Fuzzifier converts a numerical variable into

a linguistic label.. In a closed loop control system, the

error (e) between the reference voltage and the output

voltage and the rate of change of error (del e) can be

labeled as zero (ZE), positive small (PS), negative

small (NS), etc. In the real world, measured

quantities are real numbers (crisp). The FLC takes

two inputs, i.e., the error and the rate of change of

error. Based on these inputs, The FLC takes an

intelligent decision on the amount of field voltage to

be applied which is taken as the output and applied

directly to the field winding of generator. Triangular

membership functions were used for the controller.

Fig 3. Membership function of voltage

Fig 4. Membership function of voltage error

Fig 5. Membership function of output field voltage

Rule base stores the data that defines the

input and the output fuzzy sets, as well as the fuzzy

rules that describe the control strategy. Mamdani

method is used in this paper. Seven membership

functions were used leading to 49 rules in the rule

base.

Table 1

Rule base for fuzzy controller

Inference engine applies the fuzzy rules to

the input fuzzy variables to obtain the output values.

Defuzzifier achieves output signals based on the

output fuzzy sets obtained as the result of fuzzy

reasoning. Centroid defuzzifier is used here.

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PARAMETER DESIGN AND SMALL

SIGNAL ANALYSIS OF THE

PROPOSED CONTROL SYSTEM

The fuzzy based proposed unified control

strategy with operating principle of DG is illustrated

in detail in this section. The four states of DG are as

follows:1) grid-tied mode, 2) transition from the grid-

tied mode to islanded mode, 3) the islanded mode, 4)

from islanded mode to the grid-tied mode.

i. Grid-Tied mode: under normal case of utility, by

inductor current loop the DG is controlled as current

source and will supply active and reactive power

through current D- and Q- axis independently. For

that utility voltage phase angle is obtained through

PLL by park transformation, PI controller, a limiter

and an integrator.

푥푥 =

23

cos휃 cos 휃 +23휋 − cos 휃 +

23휋

− sin 휃 − sin 휃 −23휋 − sin 휃 +

23휋

×푥푥푥

-------- (3) An inductor current reference 푖 seems little

complex and compared with the instantaneous filter

inductor current which is transformed into SRF by

the park transformation. The inductor current is

regulated to track the reference 푖 by the PI

compensator 퐺 . The utility is assumed stiff, the

three-phase utility voltages are expressed as

푣 = 푉 cos휃∗

푣 = 푉 cos(휃∗ −2휋3 )

푉 cos 휃∗ −2휋3

------- (4)

The SRF transformation of the utility voltage is

expressed as

푣 = 푉 cos(휃∗ − 휃)푣 = 푉 sin(휃∗ − 휃)

----------- (5)

Where 푉 = magnitude of the grid voltage,

휃∗ = the actual phase angle.

푣 is regulated to zero by the PLL, so 푣 equals the

magnitude of the utility voltage 푉 . As the filter

capacitor voltage equals the utility voltage in the

gird-tied mode, 푣 equals the magnitude of the

utility voltage 푉 , and 푣 equals zero. In the D-axis,

the inductor current reference 푖 can be expressed

by (6) according to Fig. 3

푖 = 퐼 + 푖 − 휔 퐶 ∙ 푣

------- (6)

In steady state, the given voltage reference 푉 is

larger than the magnitude of the utility voltage 푣

and the first part is the output of the limiter. So the PI

compensator, denoted by GVD in the following part

will saturate and the limiter outputs its upper

value 퐼푔푟푒푓푑. The second part is that the

characteristics of local load will determine D-

axis 푖 load current.

The third is the proportional part −휔 퐶 ·푣 ,

where 휔 is the rated angle frequency, and 퐶 is the

capacitance of the filter capacitor. It is fixed as 푣

depends on the utility voltage. The given reference

퐼 and the load current 푖 is being imposed by

the current reference 푖 and independent of the

load voltage. In the Q-axis, the inductor current

reference 푖 consists of four parts as

푖 = 푣 ∙ 푘 + 퐼 + 푖 + 휔 퐶 ∙ 푣

------ (7)

Where 푘 = parameter of the P compensator,

denoted by 퐺 in the following part. The first part is

the output of 퐺 , which is zero as the 푣 has been

regulated to zero by the PLL. The second part is the

given current reference 퐼 , and the third part

represents the load current in Q-axis. The final part

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is the proportional part−휔 퐶 ·푣 , which is fixed

since 푣 depends on the utility voltage.

Fig. 4. Simplified block diagram of the unified

control strategy when DG operates in the grid-tied

mode Therefore, external voltage drop will not influence

the current reference 푖 .but, the current reference

푖 will determine the given reference 퐼 and

the load current 푖 .the control diagram of the

inverter is simplified n grid-tied mode, with the

analysis of previous cases and the inverter is

controlled as a current source with inductor current

reference 퐼 and the load current 푖

determined by the inductor current loop will track the

current reference and the load current.

퐼 represents the grid currents if steady state

error is zero will be explained in next section.

ii. Transition mode from grid –tied mode to the

islanded mode: By opening utility switch 푆 , the

islanding mode begins; frequency and load voltage

will drift because of active and reactive power

mismatch between DG and the load demand. The

transition is divided into two time intervals where

first is from the instant of turning off 푆 to the instant

of turning off 푆 when islanding mode is confirmed.

The second one starts from instant of turning off

inverter switch 푆 . As switch 푆 is in ON state, in first

interval the utility voltage 푣 will be same as load

voltage 푣 because dynamic of the inductor

current loop and the voltage loop is much faster than

the PLL [15] but load voltage and current are varying

dramatically considering load voltage angle

frequency to be not varied. In the grid-tied mode, it is

assumed that the DG injects active and reactive

power into the utility, which can be expressed by (8)

and (9), and that the local critical load, shown in (10),

represented by a series connected RLC circuit with

the lagging power factor

Fig. 5. Operation sequence during the transition from

the grid-tied mode to the islanded mode.

Fig. 6. Transient process of the voltage and current

when the islanding happens.

푃 =32 ∙ 푣 푖 + 푣 푖 =

32푣 푖

------- (8)

푄 =32 ∙ 푣 푖 + 푣 푖 =

32푣 푖

--------- (9)

푍 = 푅 + 푗 휔퐿 −1휔퐶

= 푅 + 푗 휔퐿 −1휔퐶

= 푅 + 푗푋

------- (10)

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In islanding mode, 푖 will decrease from positive to

zero, and 푖 will increase from negative to zero.

During this time load current will vary in the opposite

direction. From equations (11) and (12) it can be

concluded that D- and Q-axes load voltage each

consists of two terms. The load voltage in D-axis 푣

will increase as both terms increase. But in Q-axis

푣 is uncertain because the first term decreases and

the second term increases, and it is not concerned for

a while

푣 = 푖 ∙ 푅 − 푖 ∙ 푋

------ (11)

푣 = 푖 ∙ 푅 − 푖 ∙ 푋

------ (12)

The input of PI compensator 퐺 will become

negative with the increase of the load voltage in D-

axis 푣 , when it reaches and exceeds 푉 so its

output will decrease. Then the output of limiter will

not imposed to 퐼 any longer and the current

reference 푖 will drop. In the regulation of the

inductor current loop, D-axis푖 load current will

decrease. The load voltage in D-axis 푣 will drop

and recover to 푉 . If 푖 has almost fallen to the

normal value, the load voltage in Q-axis 푣 will

drop according to (12). The PI compensator 퐺 will

going to be negative if 푣 is decreased from zero to

negative and its output will drop. The angle

frequency ω will be reduced. If it falls to the lower

value of the limiter 휔 , the angle will be fixed

at 휔 . At the end of the first time interval the load

voltage in D-axis 푣 will increase and fix at 푉

and angle frequency of the load voltage ω will also

drop.PLL can still operate normally if the value is

higher than the lower value of the limiter 휔 , and

the load voltage in Q-axis 푣 will be zero.

If 휔 is fixed, load voltage in Q-axis 푣 will be

negative. With the help of power relationship the

variation of frequency and amplitude can be

understandable. When the islanding happens, the

local load must absorb the extra power injected to the

grid, as the output power of inverter is not changed

instantaneously. From (1) the magnitude of load

voltage 푉 will rise with the increase of 푃 . In

meanwhile the angle frequency ω should drop, in

order to consume more power with (2). Results from

power relationship coincide with the previous

analysis. The second time interval transition begins

from the instant when the switch 푆 opens after the

islanding detection method. If switch 푆 opens the

load voltage 푣 is independent with the grid

voltage 푣 . In the mean time 푣 will reduce to

zero theoretically as the switch 푆 has opened. The

angle frequency is invariable and then, input of the

compensator 퐺 becomes zero and fixed to the end

of the first time interval.

The inverter is controlled to be a voltage source

when 푣 is regulated by the voltage loop. Under

islanding operation, the load voltage is restricted to

particular range to drift the amplitude and frequency

and the inverter is transferred from the current source

operation mode to the voltage source operation mode

autonomously. With the increase in the time of delay,

the drift becomes worse in the hybrid voltage and

current mode control. So, the time delay of islanding

detection is critical to drift of the frequency and

magnitude in the load voltage. In proposed control

strategy this phenomenon is avoided.

iii. Islanded Mode: in this state switching 푆 and 푆

both in OFF state. The PLL cannot track the utility

voltage normally, and angle frequency is fixed. Since

voltage compensator 퐺 and 퐺 can regulate the

load voltage 푣 , the DG is controlled as a voltage

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source. In D-axis the voltage reference is 푉 and in

Q-axis voltage reference is zero respectively and the

magnitude of the load voltage equals to 푉

approximately, elaborated in next section. The

control diagram of three-phase inverter for islanded

mode can be simplified and is shown in figure 7. If

there is any disturbance in the load current, it will be

suppressed quickly by the inductor current loop and a

stiff load voltage can be achieved. Finally, the load

current 푖 is partial reference of the inductor

current loop.

iv. Transition from the Islanded Mode to the Grid-

Tied Mode:

a. If the utility switch 푆 made ON after the restoring

the utility, the DG should be connected with utility

by turning ON switch 푆 . There are several steps

before preparation before turning on switch 푆 .as

soon as utility voltage is restored, the PLL will track

the phase of the utility voltage which results that the

phase angle of utility voltage 푣 will follow the

grid voltage 푣 . If the load voltage 푣 is in

phase with the utility voltage, according to equation 5

푣 will equal the magnitude of the utility voltage.

Fig 7 Simplified block diagram of the unified control

strategy when DG operates in the islanded mode.

b. The magnitude of the load voltage 푉 is larger

than the utility voltage magnitude 푉 , the reference

voltage 푉 will be changed to 푉 by toggling the

selector S from terminals 1 to 2. The load voltage

will equal t the utility voltage in both phase and

magnitude.

c. The switch 푆 is turned on, and the selector S is

reset to terminal 1 where load voltage is held by

utility. As 푉 = 푉 which is larger than the

magnitude of the utility voltage 푉 , so PI

compensator 퐺 will saturate, and the output of

limiter is its upper value 퐼 meanwhile 푣 is

regulated to zero by the PLL from equation 5. The

output of 퐺 will be zero. By inactivating

퐺 and 퐺 , DG is controlled as a current source by

inductor current loop.

Analysis and Design: This section briefs about the

proposed fuzzy based control strategy is analyzed and

designed in both steady state and transient state along

with three-phase inverter.

In the steady state, the operating points of both grid-

tied and islanded modes of DG are analyzed where

limiters and references are selected. Whereas in

transient state, compensators in both inductor current

loop and the external loop are designed based on the

small-signal model and the effect of load current

feedforward is also analyzed as well.

A. Steady State

1) Analysis of Operation Points:

2) Selection of References and Limiters

1) Analysis of Operation Points: in the grid-tied

mode, the inverter is controlled as a current source,

and the current reference for the inductor current

loop 푖 is expressed according equation (6) and

(7). The steady-state error will be zero with the Fuzzy

Logic Compensator in the inductor current loop, so

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the inductor current in steady state can be expressed

as follows:

푖 = 퐼 − 휔 퐶 ∙ 푣 + 푖푖 = 푣 ∙ 푘 + 휔 퐶 ∙ 푣 + 퐼 + 푖

------- (13)

In the SRF, the relationship between the voltage and

the current of the filter capacitor in steady state can

be expressed by 푖 = −푣 ∙ 휔퐶푖 = 푣 ∙ 휔퐶

-------- (14)

Where 휔 represents the angle frequency of the DG

and 퐶 denotes capacitance of the filter capacitor. As a

result, the output current of the inverter 푖 can be

gained

푖 = 푖 − 푖 = 퐼 − (휔 − 휔)푖 = 푖 − 푖 = 푣 ∙ 푘 + 퐼

+(휔 −휔) ∙ 퐶 ∙ 푣 + 푖 .

------- (15)

As angle frequency ω is very close to the rated angle

frequency휔 , it can be found that the output current

follows퐼 and the load current 푖 , as 푣 equals

zero in the grid-tied mode. The active and reactive

power injected into utility can be obtained as follows.

Consequently, the active power and reactive power

flowing from the inverter to utility can be given

by 퐼 and 퐼 , respectively

⎩⎪⎪⎨

⎪⎪⎧ 푃 =

32 ∙ 푣

(푖 − 푖 ) + 푣 푖 − 푖

=32 ∙ 푣 퐼

푄 =32 ∙ 푣

(푖 − 푖 )− 푣 푖 − 푖

=32 ∙ 푣 퐼

----- (17)

In the islanded mode, the inverter is controlled as a

voltage source by the external voltage loop. In the D-

axis,푣 is regulated by the Fuzzy Logic

compensator 퐺 , so the steady state error will be

zero and 푣 can be expressed as follows:

푣 = 푉

------- (18)

Where 푉 is in D-axis. In the Q-axis, the regulator

퐺 is 푃 compensator, so the steady state error may

not be zero. As the load current is added to the

inductor reference, the condition shown as below can

be achieved

푣 ∙ 푘 + 퐼 = 0

------ (19)

And then, the load voltage in Q-axis can be expressed

by (20). It should be noted that the absolute value

of 푣 rises with the increase of the current reference

퐼 which is related to the reactive power injected

into the utility

푣 = −퐼푘

------ (20)

The magnitude of the load voltage 푉 can be

represented as follows. It equals to 푉

approximately, because 푣 should be much lower

than 푉 with proper current reference 퐼

푉 = 푉 +퐼푘 ≈ 푉

------- (21)

During islanding operation, the angle frequency is

restricted in the given range by the limiter. During

transition from grid-tied mode to the islanded mode,

In first-time interval only the angle frequency is

determined. If current reference 퐼 is set to zero,

then 푣 is zero. It means that the angle frequency 휔

does not vary in the first time interval of the

transition, and it should equal the angle frequency of

the utility before islanding happens 휔 . the angle

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frequency of the load voltage 휔 in the islanded mode

is determined by the current reference 퐼 , where

휔 represent the upper values of the limiter and

휔 represent the lower values of the limiter shown

in fig 2

휔 =휔 , 퐼 > 0휔 , 퐼 = 0 휔 , 퐼 < 0

----- (22)

2) Selection of References and Limiters: In the grid-

tied mode, through the current reference 퐼 the

active power is injected into the grid 푃 . Therefore,

the selection of 퐼 depends on the power rating of

the inverter. According to equation 17, the current

reference 퐼 , first determines the amount of

reactive power to be injected into utility 푄 in the

grid-tied mode and even affects the magnitude of the

load voltage in the islanded mode according to

equation 21. Finally, the reactive power 푄 cannot be

very large, In order to maintain load voltage within

the normal range in the islanded mode. In grid-tied

mode, 푉 should be maintained larger than the

utility voltage 푉 . At the same time, load voltage will

be determined by 푉 in the islanded mode

according to equation 21. So 푉 should not be

much larger than 푉 . For this case only it is selected

as the maximum magnitude of the utility voltage in

this paper. As per IEEE standard 1547-2003 the

range of the normal grid voltage 0.88-1.1 p.u.

so 푉 can be selected as

푉 = 1.1 ∙ √2 ∙ 푉

--------- (23)

Where 푉 =The RMS value of the rated phase

voltage. In order to guarantee that the PLL operates

normally in the grid-tied mode, the utility angle

frequency 휔 should not touch the upper value 휔

or lower value 휔 of the limier in the PLL.

Besides, the angle frequency 휔 is restricted

between 휔 and 휔 in the islanded mode, and it

should not drift from the normal value too far. So,

휔 and 휔 are selected as the maximum and

minimum angle frequencies allowed by the utility

standard.

B. Transient State

1) Small-Signal Model of the Power Stage

2) Design and Analysis of the Current Loop

3) Design and Analysis of the Voltage Loop

4) Impact of Load Current Feedforward

1) Small-Signal Model of the Power Stage: The

transient performance is analyzed; the three-phase

inverter in the DG needs to be modeled. According to

the power stage shown in Fig. 1, by front-end

converter in DG the DC-link voltage 푉 is regulated.

By eliminating its dynamic performance in this paper

the dc voltage 푉 is assumed very stiff. The average

model of the power stage can be described by

푉2 ∙

푑푑푑

= 퐿 ∙푑푑푡

푖푖푖

+ 푅 ∙푖푖푖

+푣푣푣

(24)

푖푖푖

= 퐶 ∙푑푑푡

푣푣푣

+푖푖푖

+푖푖푖

(25)

In (24),푑 ,푑 , and 푑 = the average duty cycle of

each leg varying from −1 to 1 ,and 푅 = the

equivalent series resistance of the filter inductor.

Then, the average model in the SRF can obtained

with the Park transformation shown in (3), which is

represented by

푉2 ∙

푑푑 = 퐿 ∙

푑푑푡

푖푖 +

0 −휔퐿휔퐿 0 ∙

푖푖 + 푅

∙푖푖 +

푣푣

------- (26)

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푖푖 = 퐶 ∙

푑푑푡

푣푣 +

0 −휔퐶휔퐶 0 ∙

푣푣 +

푖푖

+푖푖 .

(27)

Considering the dc voltage푉 as stiff, the small-

signal model will be same as the average model. In

SRF model between D and Q-axes the inductor 퐿

and capacitors 퐶 couplings are introduced and these

couplings can be mitigated by the decoupling

components and 휔 퐶 in Fig. 3. Therefore, the

small-signal model can be simplified into two

identical SISO systems, which is represented by (28)

ignoring the subscript d and q 푉푑푐2∙ 푑 = 퐿푓 ∙

푑푑푡푖̂퐿 + 푅푙 ∙ 푖̂퐿 + 푣퐶

푖̂퐿 = 퐶푓 ∙푑푑푡푣퐶 + 푖̂퐿퐿 + 푖̂푔 .

(28)

TABLE 1 PARAMETERS OF THE POWER STAGE

parameters value

DC voltage 푉

Filter inductor 퐿

Filter capacitor 퐶

Switching frequency푓

Sampling frequency 푓

Rated power of DG 푃

Rated RMS phase voltage 푉

Rated utility angle frequency 휔

Rated linear local load 푅 _

Rated nonlinear local load 푅 _

400V

3.5mH

15휇F

10kHz

20 kHz

3000W

115V

50× 2휋

rad/s

60Ω

120 Ω

2) Design and Analysis of the Current Loop:

In both islanded and grid-tied modes to regulate the

inductor current loop it should operate normally.

From 28 equation, the small-signal model of the

control-to-current can be obtained. Which is shown

as

퐺 (푠) =푖̂퐿(푠)푑(푠)

=푉2 ∙

푠퐶푠 퐿 퐶 + 푠푅 퐶 + 1

(29)

However, In the grid-tied mode, because of the stiff

utility the dynamics of the capacitor 퐶 is ignored,

and the small-signal model of the control-to-current

is described by

퐺 (푠) =푖̂퐿(푠)푑(푠)

=푉2 ∙

1푠퐿 + 푅

------- (30)

The required parameters of the power stage

implemented in this paper shown in Table I. In fig.8

for both operation modes the bode plot of the control-

to-current transfer function can be obtained. It can be

found that huge difference appears in the low and

medium frequency range and it is difficult to design

the compensator 퐺 to achieve good performance in

both of operation modes. It is because of the inductor

current is coupled with the capacitor voltage in the

islanded mode. This difference can be mitigated by

the capacitor voltage is fed forward with the

coefficient in Fig. 2, by decoupling the inductor

with the capacitor voltage.

Fig. 8. Bode plot of the loop gain of the inner current

loop.

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Fig. 9. Block diagram of the simplified voltage loop.

The transfer function of control to current in the

islanded mode is changed to be close to the one in the

grid-tied mode, and the current compensator 퐺 can

be designed based on unified transfer function shown

by (30). The loop gain of the current loop is shown in

Fig. 9, with the crossover frequency of 1100 Hz, and

the phase margin of 65°

퐺 (푠) = 푘 ∙1 + 푠

휔퐺푠

----- (31)

3) Design and Analysis of the Voltage Loop:

To regulate the load voltage, the voltage loop just

operates in the islanded mode can be seen in the

simplified block diagram Fig. 10. Where 퐺 (푠)

and퐺 (푠) denote the closed-loop transfer function of

an inductor loop and 퐶 =the impedance of the filter

capacitor, respectively. These two compensators are

designed, and the loop gain of the current loop is

shown in Fig.11.results in little difference in the low

frequency range. The phase margin is set to 55° and

crossover frequency is around 600Hz in both D- and

Q-axes.

Fig. 10 Bode plot of the loop gain of the voltage loop

in D-and Q-axes.

퐺 (푠) = 푘 ∙1 + 푠

휔퐺푠

(32)

퐺 (푠) = 푘

(33)

4) Impact of Load Current Feedforward: the

disturbance from the load current can be suppressed

by the inductor current reference and the load current

횤̂ is a part of the inductor current reference. The

transfer function of the output impedance is derived

to estimate the response of the load current

feedforward in the islanded mode. The output

impedances with and without load current

feedforward are expressed by

푍 (푠) =푣 (푠)횤̂ (푠) = −

퐺 (푠) ∙ [1− 퐺 (푠)]1 + 퐺 (푠) ∙ 퐺 (푠) ∙ 퐺 (푠)

-------- (34)

푍 (푠) =푣 (푠)횤̂ (푠) = −

퐺 (푠)1 + 퐺 (푠) ∙ 퐺 (푠) ∙ 퐺 (푠)

(35)

With load current feedforward an extra factor

[1− 퐺 (푠)] appears in the output impedance. The

magnitude of output impedance will be reduced in

the low frequency range because the gains of the

closed-loop transfer function 퐺 (푠) to unity in the

bandwidth of the current loop.

With two conditions, the output impedance of the

bode plot is shown in Fig. 12. The output impedance

is reduced from dc to 600Hz with the load current

feedforward. At the same time, the quality of the load

voltage 푣 will be improved with the load current

feedforward. The inductor current loop is regulated

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directly by inductor current in grid-tied mode. The

incurrent reference is mainly composed by the

current reference 퐼 , and the load current 푖 .

The output current 푖 of the inverter will be fixed

by퐼 . The disturbance of the load current will be

fully injected into the utility, which can be

represented by feedforward, when DG operates in

islanded mode.

횤̂ (푠)횤̂ (푠) = −1

------- (36)

Fig. 11 Bode plot of the output impedance with and

without the load current

Fig. 12 Bode plot of the transfer function from load

current to grid current

With and without the load current feedforward DG

operates in the grid-tied mode. The disturbance of the

load current can be compensated by the inverter and

the transfer function from load current to grid current

can be explained from equation 37. The bode plots of

transfer function and the gain is mitigated upto

1050Hz with the load current feedforward and

therefore, the quality of the grid current can be

improved

횤̂ (푠)횤̂ (푠) = 퐺 (푠) − 1

(37)

SIMULATION RESULTS

The proposed control strategy is investigated

in MATLAB simulink and simulation results are

verified. For simulation purposes the three-phase

inverter power rating is considered as 3kW. The

parameters in the simulation are compared with

Tables I and II. The RMS rated phase voltage is

115V ad the voltage reference 푉 is set as

10%higher than the rated value. The utility rated

frequency is 50Hz, and the upper and the lower

values of the limiter in the PLL are given as 0.2Hz

higher and lower than the rated frequency,

respectively. By stepping down the grid current

reference from 9A to 5Ain the grid-tied mode the

conventional voltage mode control and proposed

fuzzy based unified control strategy are compared.

The simulated results for the voltage mode control

are shown in fig. 14(a). at the moment of 14s the

current reference is changed. It is found that dynamic

process lasts until around 15.2s. The simulation result

for the proposed control strategy is represented in fig

14(b). Here the time interval of the dynamic process

is less than 5ms. From the Comparison of the

simulation results above. It can be seen that the

dynamic performance of the proposed fuzzy based

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unified control strategy is better than the

conventional voltage mode control.

TABLE II PARAMETERS IN THE CONTROL SYSTEM

parameters Value

Voltage reference 푉

Rated current reference 퐼

Rated current reference 퐼

Upper value of the limiter 휔

Lower value of the limiter 휔

179V

9A

0A

50.2× 2휋 rad/s

49.8× 2휋 rad/s

(a)

(b)

Fig 13 Simulation waveforms of load voltage 푣 ,

grid current 푖 , and inductor current 푖 when DG is

in the grid-tied mode under condition of the step

down of the grid current reference from 9 A to 5 A

with: (a) conventional voltage mode control, and (b)

proposed unified control strategy

(a)

(b)

Fig. 14. Simulation waveforms of load voltage 푣 ,

grid current 푖 ,and inductor current 푖 when DG is

transferred from the grid-tied mode to the islanded

mode with: (a) conventional hybrid voltage and

current mode control, and (b) proposed unified

control strategy.

Under transition states the grid-tied mode to

the islanded mode, the unified control strategy is

compared with the hybrid voltage and current mode

control, and the simulation scenario is shown as

follows: Initially, the utility is normal, and the DG is

connected with the utility; At 0.5s, islanding happens;

and At 0.52s, the islanding is confirmed. Simulated

results of hybrid voltage and current mode control

can be seen in figure 15(a). It can be found that the

grid current drop to zero at 0.5s, and load voltage is

seriously distorted from 0.5 to 0.52s. The load

voltage is recovered to normal value after 0.52s. Fig

15(b) presents the simulated of proposed fuzzy based

control strategy. The magnitude of the grid current is

9A and follows the current reference 퐼 . The

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load voltage magnitude and frequency is held by the

utility. When islanding happens, to follow the voltage

reference 푉 amplitude levels of load voltage will

increase little more whereas the output current of DG

decreases autonomously to match the load power

demand. The voltage quality in the proposed control

strategy in three states (two modes and two transition

states) is no more critical.

CONCLUSION

Fuzzy based unified control strategy was

proposed for DG based three-phase inverters to

operate in both islanded and grid-tied modes, with no

need for switching between two different control

architectures or for critical islanding detection. A

novel fuzzy based voltage controller was presented.

When grid-tied mode is inactivated the DG operates

as a current source with fast dynamic performance. In

outage conditions, the controller can automatically be

activated to regulate the load voltage and even a load

current feedforward propos where it can improve the

wave form quality of the both the grid current in the

grid-tied mode and load voltage in islanded mode.

The proposed fuzzy based unified control strategy

was verified through the MATLAB simulation.

REFERENCES

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[3] C. Mozina, “Impact of green power distributed

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[6] J. Stevens, R. Bonn, J. Ginn, and S. Gonzalez,

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[7] A. M. Massoud, K. H. Ahmed, S. J. Finney, and

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[8] T. Thacker, R. Burgos, F. Wang, and D.

Boroyevich, “Single-phase islanding detection based

on phase-locked loop stability,” inProc. 1st IEEE

Energy Convers. Congr. Expo., San Jose, CA, USA,

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[9] S.-K. Kim, J.-H. Jeon, J.-B. Ahn, B. Lee, and S.-

H. Kwon, “Frequencyshift acceleration control for

anti-islanding of a distributed-generation inverter,”

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504, Feb. 2010.

[10] A. Yafaoui, B. Wu, and S. Kouro, “Improved

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[11] J. M. Guerrero, L. Hang, and J. Uceda, “Control

of distributed uninterruptible power supply

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systems,”IEEE Trans. Ind. Electron., vol. 55, no. 8,

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[12] M. C. Chandorkar, D. M. Divan, and R. Adapa,

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MUKKU MURALI Completed

B.Tech In Electrical & Electronics Engineering In

2009-2013 From QIS INSTITUTE OF

TECHNOLOGY, Ongole Affiliated To JNTUK,

Kakinada And M.Tech In Power Electronics And

Electrical Drives In 2015 From MALINENI

LAKSHMAIAH ENGINEERING COLLEGE,

Singarayakonda Affiliated To JNTUK, Kakinada.

Area Of Interest Includes Power Electronics. E-Mail

Id: Murali.M232@Gmail.Com

Dr K VENKATESWARLU

Completed B.Tech In Electrical & Electronics

Engineering In 1990-1994 From S V UNIVERSITY

And M.Tech In Power Systems In 1999 From

JNTUH, Hyderabad And Ph.D In Power Systems In

2015 From JNTUK, Kakinada. Working As

Professor Of EEE Department At MALINENI

LAKSHMAIAH ENGINEERING COLLEGE

Singarayakonda, Prakasam(District),Andhra Pradesh,

India. Area Of interest Includes Power Electronics

And Power System.

E-Mail Id: Kvlueee@Gmail.Com