06517935 a seamless control methodology for a grid connected and isolated pv-diesel microgrid

8/9/2019 06517935 a Seamless Control Methodology for a Grid Connected and Isolated Pv-diesel Microgrid

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4394 IEEE TRANSACTIONS ON POWER SYSTEMS, VOL. 28, NO. 4, NOVEMBER 2013

To achieve the maximum power point (MPP), in case of a PV

system, various schemes have been suggested in [6], [7], and

[8]. One technique proposes using sliding mode control for ob-

taining the MPP on the basis of maximizing the power equation

of the PV cell [9]. This is a first order sliding mode technique. A

second order technique has been presented in [10] for the max-

imum power point tracking (MPPT) process.

When the microgrid gets isolated from the main grid, due to

any reason, the setpoints for the controller have to be changed,

as we need to move from - to - mode. In [11], a flex-

ible control structure has been proposed that can operate both

in the grid connected mode and in the isolated mode without

the usage of a mechanism for islanding detection. Knowledge

of the electrical power, frequency and power angle is required

to achieve this control. The control that has been implemented

by the authors is based on the philosophy of modelling a VSC

as a synchronous generator. This results in the development of a

pseudo swing equation depicting the performance of the VSC.

The swing equation of any machine requires the specification

of a reference power level which acts as the setpoint for thatmachine. For all power sources that have been considered by

the authors of [11], a reference power level can be defined a

priori. However, for varying sources like PV arrays, it is im-

possible to define a reference power level as the power output

from these sources aredependent on the vagaries of nature. With

these sources of power, such a control scheme cannot be imple-

mented as maximum power has to be always extracted. This

philosophy has been adopted in [12]. In the isolated mode of a

PV-DG system, the PV array output will be bound between zero

and MPPT while the DG operation will be restricted within its

minimum and maximum limit of power output [12]. The PV

power output can be reduced from MPPT by increasing the dcvoltage of thePV array whereas the diesel inputto the DG can be

regulated to alter its real power output. Therefore, depending on

the load demand and insolation level, the dc voltage and diesel

input needs to be regulated. In [12], the authors have proposed

two approaches. In the first approach, a mathematical method is

used to obtain the dc voltage reference value by constructing the

PV array characteristics. This method requires the information

from the irradiation and temperature sensors that are located on

the PV panels. As per the data sheet of one of the manufacturer

[13] the accuracy is %. On the other hand the accuracy of

the temperature, voltage and current sensors are %. There-

fore, it can be concluded that the involvement of an irradiation

sensor makes the calculation of dc reference voltage erroneous

making the control loop inaccurate. Besides, as the irradiation

and temperature sensors are in general costly as compared to

voltage/current sensors and we need large number of such sen-

sors in an array, the cost of the entire control scheme will be

quite high.

In the second approach that has been proposed by the au-

thors of [12], the current level is maintained using a dc-dc con-

verter. However, the reference current level is obtained by using

look up tables that requires the information about irradiation and

hence all the limitations discussed above are also valid for this

scheme. Further, with change in environmental conditions, the

validity of the results from the look up tables is subject to dis-cussion. Changing the data in the look up tables to match the

environmental changes is a time consuming process. Thus the

overall complexity of this control scheme is high.

Keeping in mind the above mentioned issues, in this paper,

a seamless controller is proposed. The controller aims at elim-

inating the drawbacks of the control schemes presented in [11]

and [12]. Incorporation of a renewable energy source, namely a

PV array, calls for the need for change in control structure and

this issue has been addressed by the proposed controller. Fur-

ther, it eliminates the need of relying on the PV array character-

istics and local load information in order to obtain the required

setpoints. In addition, the control scheme that is proposed in this

paper does not require knowledge of the PV array characteris-

tics as is required in [12]. Thus this controller is suitable for

photovoltaic sources.

The salient features of this controller are:

1) suitable control scheme for energy sources from which

maximum power has to be extracted and thus a reference

power level cannot be defined;

2) no requirement of measurement of the local load level;

3) no requirement for a look up table or information about thePV array characteristics to decide the voltage reference in

the isolated mode;

4) reduced cost of control scheme as there is no requirement

for additional sensors to measure non-electrical quantities

like solar irradiation and temperature.

The technique has been tested with various DG generation

levels, multiple load power levels and multiple solar irradiation

levels both in the grid connected mode and in the isolated mode.

The paper is organized as follows. The system considered is

presented in Section II. Section III looks at the development

of the control structure with the simulation results presented in

Section IV. Section V provides an estimation of the cost of thecontrol structure. The limitations and scope for future research

are presented in Section VI while Section VII concludes the

paper.

II. SYSTEM MODEL

The microgrid considered is shown in Fig. 1. In the grid con-

nected mode, maximum power is extracted from the PV array

as it operates at its maximum power point. The power delivered

by the diesel generator depends on the total amount of power

the IPP has to deliver according to its contract. In the isolated

mode, which is brought about by the isolator switch, the total

power produced depends on the local load level. The DG then

operates within the minimum and maximum level with the PV

array supplying the remaining power. The sources of power are

connected to the grid through a transformer.

The power produced by the PV array is shown in Fig. 2 for

three different insolation levels of 500 Wm , 750 Wm and

1000 Wm . As it can be seen from the figure, the amount of

power extracted from the the PV array depends upon the mag-

nitude of the voltage across the capacitor . The value of

the voltage for maximum power extraction and the maximum

power that can be produced at a particular insolation level are

shown in Fig. 2. This voltage level can be obtained by imple-

menting a suitable MPPT algorithm and thus maximum power can be extracted from the PV array. The Perturb and Observe


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MISHRA et al.: A SEAMLESS CONTROL METHODOLOGY FOR A GRID CONNECTED AND ISOLATED PV-DIESEL MICROGRID 4395

Fig. 1. Single line layout of utility and microgrid.

Fig. 2. Power-voltage curves of the photovoltaic array.

TABLE IASSUMPTIONS

MPPT algorithm used in this paper is on the lines of the algo-

rithm mentioned in [14].

III. CONTROL STRUCTURE

A multiple control loop structure is proposed in this paper.

The PV control loop is responsible for the generation of

the firing pulses for the switches in the VSC while the sec-

ondary/setpoint control loops are responsible for generating the

setpoints/reference values of the PV control loop. In order to

observe the capability of the VSC to meet the reactive power

demand, the VSC is made responsible for the maintenance of

voltage at point A and thus, no automatic voltage regulator

is present on the DG. To ensure the completeness of the -

mode, the DG is made responsible for the maintenance of

frequency in the microgrid.

The assumptions that have been made in this paper are tabu-

lated in Table I.

Fig. 3. Reference frames.

A. PV Control Loop

The PV array converter is controlled using a second order

sliding mode controller. The second order controller is realised

by first implementing the feedback linearisation technique in

order to obtain the output variables in terms of the control vari-

ables. The reference point for this controller is taken as the local

point A. As , a new frame of reference located at

an angle to the network frame is defined as shown in Fig. 3.

The angle is obtained from the phase locked loop (PLL) lo-

cated at point A. This new reference frame is formed such that

and . This aids in the decoupling

of the equations of the system. The differential equations shown

in (1)–(3) represent the system from the PV array to point A, in

this new frame of reference:

(1)

(2)

(3)

The terms and can be neglected as the resistance

of the filter is negligible. Neglecting the losses in the converter,

the equations for power can be written as

(4)

In order to construct the second order system, (1) is differenti-

ated once with respect to time. Equation (2) is then used to sub-

stitute for the term that is obtained [5]. As a result, the


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output variables are expressed in terms of the control variables.

In these set of equations, the active power is controlled by the

voltage level of the dc bus capacitor while the reactive power

is controlled by quadrature axis current. Thus and are

the output variables. The control variables are the modulation

indices, and , of the converter. The resultant system of

equations, in per unit, written in the form of a matrix is given

by (5):

(5)

where

and

These equations can be represented in a general form as

(6)

where . A reduction in the number of equations

and a direct link between the output and control variables is thus

obtained.

A sliding surface is defined for the output variables and

. This surface represents the surface. The slidingsurfaces, as defined in [5], are given by (7a) and (7b):

(7a)

(7b)

To satisfy the condition for Lyapunov’s stability, , the

differential of the sliding surface is defined in terms of a

function. Hence using these definitions the left hand side of (5)

can be represented as given by (8):

(8)

The final value of the control variables is given by

(9)

where, with reference to the solution of from (6) after substi-

tuting for from (8)

(10)

In this manner, the values of the control variables can be ob-

tained.

Fig. 4. Active power control loop.

B. Setpoint Control Loops

Three setpoints are required in this control structure, namely

and . The first two setpoints are for the VSC

while the third sets the mechanical power input of the diesel

generator.

1) Setpoint: The control block diagram to set the dc

voltage reference for the PV array is shown in Fig. 4. In any

mode of operation, the DG output needs to be greater than or

equal to the minimum power level to achieve fuel consumption

economy [12]. Similarly, since our energy management strategyis PV first, it has to operate at MPPT whenever possible. There-

fore, in the grid connected mode the setpoint needs to be ob-

tained from the MPPT algorithm. This is achieved by setting a

lower limit of zero on the integrator block, as the DG output is

always to its minimum level. When the microgird gets iso-

lated from the main grid, the total generation must be equal the

total load plus losses in the line between the PCC and the local

load. In this case, when the total local load plus loss is less than

the sum of maximum power of the PV array and the minimum

power output of the DG, the PV array has to be derated to main-

tain the DG power output at its minimum level.

This can be achieved by taking the advantage of the inertialresponse of the DG following the isolation. As the PV power

will not change unless the dc reference is changed, immedi-

ately following the isolation under the above mentioned oper-

ating condition, the motoring of DG will occur as has been ex-

plained in Scenario 1 of Section IV [12], thus its electrical power

output will fall below its minimum level. At this instant, the

error (Fig.4) will be positive and thus the output of the integrator

will increase with a positive slope until the error exits. When

this adds to the MPPT algorithm output, the dc voltage refer-

ence will increase, leading to a reduction in the output power of

the PV array. On the other hand, when the load scenario forces

the power generated by the DG to be greater than its minimumlevel, the error signal will be negative and the integrator will

start to reduce its output until the error becomes zero. By set-

ting a lower limit of zero on the integrator, it is ensured that the

net dc voltage reference does not go below the MPPT value and

hence the PV array always operates in the stable region of its

power-voltage curve.

The error in frequency is used when the DG is to be operated

in an uncontrolled mode. Thus the value of is set as the

setpoint of the DG and an auxiliary signal proportional to the

frequency error is added to the loop.

2) Setpoint: The control block diagram to set the reac-

tive power is shown in Fig. 5. In both the grid connected and

the isolated mode, the functionality of this loop is based on the

error between the voltage at point A and its reference value.


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Fig. 5. Voltage control loop.

Fig. 6. Frequency control loop.

This voltage reference is set as 1 pu. The reactive power refer-

ence value is obtained from the error in voltage at point

A. The value of can be obtained from the reactive power

equation of (4).

3) Setpoint: The block diagram of this control loop isshown in Fig. 6. By taking the frequency error signal, the me-

chanical power setpoint of the DG is controlled. The governor

for theDG hasbeen modelled on thelines of thegovernor mech-

anism explained in [15]. In the grid connected mode, since there

is no deviation in frequency from the reference, the frequency

error will be zero and the setpoint will be as desired. In the

islanded mode, due to a mismatch in the generation and load

levels, the frequency will deviate from the reference and thus

the setpoint will change to maintain the frequency.

C. Design of Control Parameters

The parameters of the control structure have to be tuned to en-sure satisfactory operation. The control structure proposed has

a combination of nonlinear and linear control elements. The PV

control loop is non linear while the setpoint control loops are

linear in nature.

1) PV Control Loop: Three parameter values have to be

tuned in the second order sliding mode control scheme. These

parameters are and . Consider a process equation given

by (11):

(11)

The state variableis while thecontrol variable is . Theslidingsurface for this equation can be defined as

(12)

The condition for Lyapunov’s stability is . Substituting

from (11), we get

(13)

By taking and ensuring

, the stability criterion will be satisfied. In ad-

dition to this, stability will be ensured when the value of

. This ensures that the second term on the right hand side

of (13) will always be greater than the first term. Due to the neg-

ative sign the stability criterion will be maintained.

The second order sliding surface is defined as

(14)

On differentiating the above equation, we get

(15)

In this situation also, we can ensure that the stability criterion

is maintained by taking . However, to decide

the value of , the original first order equation of the control

variable has to be considered. In this case, the originalfirst order

equation is (2).

In this manner the value of and can be obtained. Once

these values have been obtained the value of can be suitably

adjusted to ensure a fast response.

2) Setpoint Control Loops: The setpoint control loops com- prise of linear controllers. The values of these controller param-

eters can be obtained by evaluating the characteristic equation

of the control loop.

a) Active Power Control Loop: With reference to Fig. 4,

the signals and the output of the MPPT al-

gorithm can be considered as disturbance signals. The transfer

function of the incremental control loop can be written as

(16)

where is the integral gain, is a gain that can be used

to obtain the power extracted from the PV array from the dc

bus voltage and ms is the time constant of the sliding

mode controller. As this is a second order system, (16) can be

compared with the general equation of a second order system as

given by

(17)

The value of can be obtained by referring to Fig. 2. As the

initial transient will occur around the MPPT, the controller pa-

rameters are obtained with respect to this point. In incremental

form, the change in power extracted from the PV array can be re-lated to the change in the voltage level by the slope of the curve.

The power-voltage curve can be approximated as a straight line

in the region of the maximum power. For an insolation level of

500 Wm , the slope of the line is obtained as

(18)

To obtain in per unit, this value of is divided by the

base power. Thus, with and assuming a value

of damping ratio . When the DG is to

be operated in an uncontrolled mode, and the frequency

error gets added to the loop after being processed by a propor-

tional gain of . In the DG controlled mode, . The


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Fig. 7. Frequency response of the active power control loop.

frequency response of the open loop transfer function when DG

is controllable is as shown in Fig. 7.

b) Reactive Power Control Loop: With reference to

Fig. 5, the transfer function of the incremental control loop can

be written as

(19)

where is the integral gain, is a gain that will give voltage

at the point A from the current and ms is the time

constant of the sliding mode controller. It is assumed that a pos-

itive value of reactive power relates to an injection of reactive

power at point A by the VSC. An increase in the reactive power

injection will increase the voltage level at point A. However,

from (4), it can be seen that an increase in reactive power causes

to become more negative. Thus the value of is taken as

the negative of the impedance . On comparing (19) with

(17), with and . The fre-

quency response of the open loop transfer function is as shown

in Fig. 8.

c) Frequency Control Loop: With reference to Fig. 6, the

transfer function of the loop can be obtained. The governor

model consists of two blocks which simulate the actuator and

the engine dead time, respectively. While forming the transfer

function of the control loop, the dead time has to be represented

as a rational transfer function. The most common way of ob-

taining this representation is by using the Padé-approximation.

The dead time is represented by the engine torque constant

and the engine dead time . In addition, the signalsand can be treated as disturbance signals. A second order

Padé-approximation is used to represent the engine dead time

as given in (20):

Fig. 8. Frequency response of the reactive power control loop.

Fig. 9. Frequency response of the frequency control loop.

(20)

where and . The transfer functionis thus given as shown in (21) at the bottom of the page, where

and are the proportional and integral controller gains.

, where is the inertia constant of the generator.

and are the actuator constant, current driver constant

and the actuator time constant respectively. The gains of the

PI controller have been chosen based on the commonly used

values as mentioned in [15]. Thus, as a result, and

. This loop operates only when the DG is operating

in a controlled mode. The frequency response of the open loop

transfer function is as shown in Fig. 9.

IV. SIMULATION AND R ESULTS

The system described in Section II has been simulated inSIMULINK®. Various scenarios have been considered to vali-

date the proposed control scheme. In the first scenario, the need

for the seamless controller is showcased with both the PV array

(21)


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Fig. 10. Scenario 1. (a) DC bus voltage. (b) Power. (c) Frequency.

and the DG being uncontrolled. The second scenario validates

the performance in the grid connected mode with varying solar

insolation. Thus, the flow of power between the main grid and

the microgrid can be observed from this scenario. Further, with

the initial DG power level at 25 kW and the initial solar insola-

tion at 500 Wm , two separate scenarios varying in load pat-

tern and insolation level have been considered. These scenariosshow the performance of the control scheme when the microgrid

gets isolated from the main grid. In this isolated condition, the

power output of the sources have to be controlled according to

the varying demand and the varying insolation. Further, the phi-

losophy of extracting maximum power from the PV array while

simultaneously operating the DG in an economical manner is

also shown by these scenarios. The final scenario is considered

to validate the performance of the control scheme when only

the DG is non-controllable. This non-controllability of the DG

arises under the umbrella of market participation of the sources.

Though this mode of operation is essentially in a grid connected

scheme and has been observed from the second scenario, thisscenario shows the completeness of the seamless controller in

scheduling power between the sources even in the isolated mode

with the limiting factor being the schedule of the sources.

In the formulation of the control scheme, the resistance of the

filter and the converter losses have been neglected. However,

while running the simulation, these network elements have been

represented. With the addition of an integral controller in the PV

control loop, this does not cause a problem in the performance

as any slight variation in the network parameters is compensated

by the integral control [1].

A. Scenario 1

Even with a philosophy of extracting maximum power from

renewable energy sources, a need arises at times to reduce the

power extracted from such sources. To achieve this a control

methodology is required that will schedule the power generated

by these sources accordingly. This scenario showcases the op-

eration of the system when no control has been applied on the

level of power generated by the sources.

The system is operated with the setpoint of the DG being 10

kW and the solar insolation level is assumed to be 500 Wm .

From the PV curves, the power that is extracted from the PV

array is seen to be 27 kW. The local load consists of a 10 kW, 5

kVAR static load and a 25 kVA induction motor dynamic load.

The local load level is assumed to be 20 kW with 10 kW of static

load and 10 kW of dynamic load. Thus, there is a generation

surplus of 17 kW in the microgrid. In the grid connected mode,

this power (to the tune of 15 kW after losses) is pumped into

the grid. At s, the microgrid is isolated from the main

grid and the system performance is as depicted in Fig. 10. It can

be seen from Fig. 10(a) that the dc bus voltage is the same as

in the grid connected mode. Thus maximum power is extracted

from the PV array even in the isolated mode, as can be seenfrom Fig. 10(b). Since there is very little change in the voltage

levels, the power consumed by the loads remains constant. As

the generation levels are higher than the load, it results in a rise

in frequency of the system as depicted in Fig. 10(c). The surplus

power of 15 kW that was previously exported to the grid, is now

consumed by the DG resulting in the motoring of the DG. This

can be observed from Fig. 10(b) wherein theDG power is 10 kW

before isolation and it drops to kW after isolation. A simple

calculation reveals that the electrical power is now balanced in

the system with the DG operating as a motor. However, the fre-

quency continues to increase as the input mechanical power to

the DG is constant resulting in the acceleration of the machine.Thus it can be seen from this scenario that when the microgrid

gets isolated from the main grid a suitable control scheme is

required in order to reduce the generation levels and thus switch

the operation of the control scheme from a - mode to a -

mode.

B. Scenario 2

The power generated by the DG is defined by the setpoint

of its governor while the power extracted from the PV array is

dependent on the incident solar irradiation. In this scenario, the

microgrid is always assumed to be connected to the main grid.

With a fi

xed setpoint for the governor and fi

xed level of localloading, the power that is either consumed or delivered by the

main grid is dependent on the level of solar insolation. With

decrease in solar insolation, the total power generation in the

microgrid may be insuf ficient to meet the demands of the local

load and thus the deficit in power will have to be supplied by

the main grid. Under these circumstances, since the frequency

of the system is maintained by the grid, the frequency control

loop does not alter the governor setpoints, and thus the DG does

not deviate from its schedule. The satisfactory performance of

the control scheme can be observed from Fig. 11 which shows

the plots of the dc voltage of the capacitor , the power flows

and the frequency of the system. The insolation level is assumed

to be 750 Wm until s. At this insolation level the

maximum power that can be extracted from the PV array is 41


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Fig. 12. Scenario 3. (a) DC bus voltage. (b) Power. (c) Frequency. (d) Motor speed. (e) Voltage at point A. (f) DC current.

kW. The DG setting is maintained at 10 kW. Thus the total

power generation in the microgrid is 51 kW. The power level of

the local load is 37 kW with 10 kW being a static load, 25 kW

being an induction motor load and 2 kW being the losses. This

load level is also maintained constant for the entire scenario.

It can be observed from Fig. 11(b) that with these power levels,

the microgrid has surplus power and thus this extra power, to the

tune of 14 kW, is sent from the microgrid. Due to further losses

in the transformer and line, 11 kW of power is sent to the main

grid. The dc voltage level, shown in Fig. 11(a), is maintained

at the MPP value of 796 V. At s, the insolation level is

decreased to 500 Wm thereby reducing the maximum power

extraction from the PV array to 27 kW. From the dc voltage plot

it can be seen that the MPPT algorithm successfully tracks the

PV curve in order to extract the maximum power at a voltage

level of 782 V. However, at this reduced insolation level, the

total power generated in the microgrid is slightly insuf ficient to

meet the local load demand and the losses. Thus 2 kW of power

flows in from the grid into the microgrid. From the frequency

plot of Fig. 11(c) it can be seen that with the disturbance there is

a perturbation which thus causes a perturbationin theDG output

power. This however does not alter the governor settings during

steady state as the grid frequency is maintained at 50 Hz. The perturbation in frequency occurs as a result of the sudden power

mismatch that occurs. However, with the inflow of power from

the grid, the mismatch is reduced. At s, the insolation

level is further decreased to 300 Wm resulting in a larger

inflow of power from the grid to the tune of 12 kW.

It can be seen that the DG power level is maintained at the

initial setpoint level and thus the DG can maintain its schedule.

The flow of power between the microgrid and the grid is also

observed from this scenario. Thus the performance of the con-

troller in the grid connected mode is validated.

C. Scenario 3

The insolation level, in the grid connected mode, is taken as

500 Wm with the maximum power that can be extracted from

the PV array being 27 kW. With the DG setpoint as 25 kW, a

total of 52 kW of power is generated by the IPP. The microgrid

isislandedfrom the maingrid at s.The insolationlevel is

assumed to remain constant at 500 Wm throughout the sce-

nario. The dc voltage of the capacitor , power, frequency,

speed of the induction motor, voltage at point A and dc current

are shown in Fig. 12.

Referring to Fig. 12(b), the total power that is supposed to

be consumed by the local load is 20 kW, with 10 kW being the

static load consumption and 10 kW being the induction motor

load. However, the static load is represented as a constant


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impedance load with 10 kW being the power consumed by it

at rated voltage. Due to the drop in the line from the PCC, the

voltage at the local load is lower than the rated voltage. Thus,

the power consumed by the static load is lesser than 10 kW.

Hence, in Fig. 12(b), the total input electrical power to the load

(including the losses in the induction motor), is lower than 20

kW. When the microgrid gets isolated from the main grid, since

the load level remains the same, the PV array has to be derated

in order to maintain the DG output at its minimum level. The

maximum power of the DG is considered to be 80 kW with a

minimum power of 4 kW [12].

Increasing the voltage of the capacitor will reduce the

power extracted from the PV array as shown in Fig. 12(a) and

(b). After islanding, the dc voltage is raised from theMPPT level

of 782 V to 890 V. At this voltage level, the power extracted

from the PV array is 17 kW while the DG produces 4 kW. Thus

the total generation is 21 kW while the consumption is almost

20 kW. The losses in the line and the load are to the tune of

1 kW. At the moment of islanding, the difference between the

total load and the total generated power is to the tune of 32 kW.

Due to this, there is a rise in the frequency of the system and

thus a corresponding rise in the speed of the induction motor.

This can be observed from Fig. 12(c) and (d), respectively. Due

to surplus power, as depicted in Scenario 1, the DG consumes

power and the generated power goes below its minimum level

and thus according to the control loop in Fig. 4, the dc voltage

reference is increased. As the difference in power is more than

the initial generation level of the DG, the speed of the generator

and thus the frequency of the system increases by a large value

as shown in Fig. 12(c). The speed of the motor, as seen from

Fig. 12(d) rises from 1476 RPM to 1576 RPM in 0.5 s. This

corresponds to a of 20.94 radsec , which agrees with

the swing equation of the motor. From Fig. 12(a) and (c) the

difference in the speed of operation of the electrical loop con-

trolling the dc voltage reference and the mechanical loop, i.e.,

the governor, controlling the input to the DG can be observed.

Due to the dead time and also the fuel flow time, the governor

action is very slow compared to the action of the electrical loop.

The per unit voltage at point A is shown in Fig. 12(e). It can be

seen that the voltage control loop maintains the voltage level at

the reference value of 1 pu.

At s, the induction motor load is increased from 10

kW to 18 kW. The total load on the system at this point is 28

kW and the losses in the motor are to the tune of 2 kW. This power level is greater than the maximum power of the PV array

for this insolation level, i.e., 27 kW. However, since the DG

has to be operated at a minimum level, the PV array has to still

operate in the derated mode. This increase in load is met by

releasing power from the kinetic energy of the DG. This can be

seen from Fig. 12(b). Due to this, the speed of the DG drops

and this is reflected as a drop in the frequency of the system

as shown in Fig. 12(c). As the MPPT algorithm changes the

voltage reference, the DG power returns to its minimum value

and the PV array takes up the additional load demand as seen

from the PV power curve in Fig. 12(b). Thus, 26 kW is extracted

from the PV array while the DG operates at its minimum levelof 4 kW.

At s, the induction motor load is increased from 18

kW to 25 kW. Since the total electrical load is now greater than

the sum of the maximum power of the PV array and the min-

imum value of the DG, the PV operates at MPPT and the deficit

is taken care off by the DG due to the action of the frequency

control loop. The DG thus now produces 12 kW of power. It

can be observed from Fig. 12(f) that when there is an increase

in the induction motor load, negligible transients appear in the

dc current. Thus the switches in the converter will be able to

withstand the changes in the current.

D. Scenario 4

The microgrid is islanded from the main grid at s. A

step change in the insolation level is assumed to occur at

s from 500 Wm to 750 Wm . Further at s a ramp

increase in the insolation level is assumed from 750 Wm to

1000 Wm at a rate of 250 Wm s . The total active power

consumed by the local load at the time of islanding is again

20 kW with 10 kW being the static load and 10 kW being theinduction motor load. At s the induction motor load is

increased to 25 kW and thus the total active power load on the

microgrid is 35 kW. The dc voltage of the capacitor , power,

frequency, speed of the induction motor, voltage at point A and

dc current are shown in Fig. 13.

The response of the system until s is the same as

the previous situation. It can be seen that in the isolated mode,

due to the increase in insolation level, the PV array is able to

fully support the load and thus the DG always operates at its

minimum level. Referring to Fig. 13(c), at s, when the

load power level increases, the frequency of the system drops

slightly as the DG tries to support this increase in load by re-

leasing power from its kinetic energy. The MPPT algorithm

reduces the voltage level of thereby increasing the power

extracted from the PV array. Since the insolation has also in-

creased, the algorithm brings the capacitor voltage to a level as

defined by the power voltage curve of this new insolation level.

The DG then settles at its minimum value. It can be observed

from Fig. 13(f) that when there is an increase in the induction

motor load at s, a very short duration transient appears

in the dc current. The IGBTs in the VSC however will be able to

handle this current transient as its magnitude and rate of change

is small.

When there is a ramp increase in the insolation level from

s, the MPPT algorithm tracks the change in the in-solation and thus the dc voltage level increases to maintain the

output power level of the PV array the same as before the inso-

lation change. Thus due to the increase in insolation, the load is

supported at all times by the PV array.

For the two scenarios considered above, the frequency of the

microgrid in the isolated mode is maintained within 50.05 Hz

and 49.95 Hz. The voltage at point A is also maintained at 1

pu. Hence, the voltage and the frequency of the microgrid are

controlled.

E. Scenario 5

In this scenario, the DG is operated in an uncontrolled modewith a constant setpoint. Thus in the active power control loop


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Fig. 13. Scenario 4. (a) DC bus voltage. (b) Power. (c) Frequency. (d) Motor speed. (e) Voltage at point A. (f) DC current.


of Fig. 4, the minimum value, , is set equal to the setpoint

of the DG. However, due to this loop, a three point oscillation

occurs in the output power of the DG due to the influence of the

P&O algorithm. Thus, for the operation of the loop, a band of

0.05 kW is specified around . The plots of the dc voltage,

power and frequency are shown in Fig. 14.

The setpoint of the DG is 10 kW while the PV array power is

27 kW at a solar insolation of 500 Wm . The local load level

is 20 kW as in Scenario 3. As expected, the system frequency isstable in the grid connected mode. At the moment of islanding,

at s, the DG output power remains at its setpoint, while

the PV array operates in a derated mode with 12 kW of power

being extracted from it. As the frequency is now not controlled

by the DG, an auxiliary signal proportional to the frequency de-

viation is added to the output of the MPPT algorithm. This en-

sures that the frequency is controlled and thus helps in the sta-

bilization of the system. As the load level increases at s,

the PV array is still able to support the load with the DG oper-

ating at its setpoint and the frequency stabilises. With a further

increase in load, at s, maximum power is extracted

from the PV array. However, the load level plus losses is now

higher than the combined output of the DG and the PV array.

There is a slight inertial disturbance in the output power of the

DG but it settles back at the initial setpoint. The frequency how-

ever begins to drop and the system loses stability. Thus we can

see that the control methodology with an auxiliary signal is able

to perform satisfactorily even when the DG is to be operated in

an uncontrolled mode as long as the load level is lower than the

generation levels.

This validates the performance of the control scheme in all

operating scenarios.

V. COST ESTIMATION

The control scheme that has been proposed in this paper

is cost ef ficient when compared with other similar schemes

that have been documented in the literature [12]. Sensors to

measure frequency, voltage and current are common to all

schemes. The data from these sensors are then used by a digital

signal processor chip to implement the control scheme. The

estimated cost of building this proposed control scheme falls

around $350–$400 based on the cost of the sensors reported in

[16]–[18]. However, the scheme mentioned in [12] requires an

additional temperature sensor and a solar irradiation sensor for

the each PV panel. As per the manufacturer data sheets [19],

[20], each sensor costs around $160. Thus the total cost of real

time implementation of the control scheme increases by $320


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[12] A. Elmitwally and M. Rashed, “Flexible operation strategy for an iso-lated pv-diesel microgrid without energy storage,” IEEE Trans. EnergyConvers., vol. 26, no. 1, pp. 235–244, Mar. 2011.

[13] [Online]. Available: http://www.fronius.com/cps/rde/xchg/SID-7BBC196D-F8FE4659/fronius_international/hs.xsl/83_16139_ENG_HTML.htm.

[14] A. Yazdani, A. Di Fazio, H. Ghoddami, M. Russo, M. Kazerani, J.Jatskevich, K. Strunz, S. Leva, and J. Martinez, “Modeling guidelinesand a benchmark for power system simulation studies of three-phasesingle-stage photovoltaic systems,” IEEE Trans. Power Del., vol. 26,no. 2, pp. 1247–1264, Apr. 2011.

[15] S. Roy, O. Malik, and G. Hope, “An adaptive control scheme for speedcontrol of diesel driven power-plants,” IEEE Trans. Energy Convers.,vol. 6, no. 4, pp. 605–611, Dec. 1991.

[16] [Online]. Available: http://in.rsdelivers.com/product/lem/lv-25-p/hall-effect-pcb-mount-volt age-transducer/0286361.aspx.

[17] [Online]. Available: http://in.rsdelivers.com/product/lem/cksr-50-np/current-transducer-50a-low-drift-5v-vs/6668214.aspx.

[18] [Online]. Available: http://www.ti.com/product/tms320f2812.[19] [Online]. Available: http://www.civicsolar.com/product/fronius-irra-

diance-sensor.[20] [Online]. Available: http://www.civicsolar.com/product/fro-

nius-module-temperature-sensor.[21] I.-S. Kim, “Sliding mode controller for the single-phase grid-connected

photovoltaic system,” Appl. Energy, vol. 83, no. 10, pp. 1101–1115,

2006.[22] , W. Perruquetti and J. P. Barbot, Eds. , S liding M ode Control in Engi-

neering . New York, NY, USA: Marcel Dekker, 2002.

S. Mishra (M’97–SM’04) received the B.E. degreefrom University College of Engineering, Burla,Orissa, India, and the M.E. and Ph.D. degrees fromRegional Engineering College, Rourkela, Orissa,India, in 1990, 1992, and 2000, respectively.

In 1992, he joined the Department of ElectricalEngineering, University College of EngineeringBurla as a Lecturer, and subsequently became aReader in 2001. Presently, he is a Professor withthe Department of Electrical Engineering, IndianInstitute of Technology Delhi, New Delhi, India. His

interests are in soft computing applications to power system control, power quality, renewable energy, and microgrids.

Dr. Mishra has been honored with many prestigious awards such as the INSAYoung Scientist Medal in 2002, the I NAE Young Engineers Award in 2002, andrecognition as the DST Young Scientist in 2001 to 2002. He is a Fellow of the Indian National Academy of Engineering, the Institute of Engineering andTechnology (IET), London, U.K., and the Institute of Electronics and Commu-nication Engineering (IETE), India.

D. Ramasubramanian (S’10) received the B.E.degree from Visvesvaraya Technological University,Belgaum, India, in 2011. Currently, he is pursuingthe M.Tech. degree at the Indian Institute of Tech-nology Delhi, New Delhi, India.

His research interests are in integration of renew-able energy into the existing grid, microgrids, and power system dynamics and control.

P. C. Sekhar (S’12) received the B.Tech. from Jawa-harlal Nehru Technological University, Hyderabad,India, and the M.Tech. degree from National Insti-tute of Technology Rourkela, India. Curr ently, he is pursuing the Ph.D. degree at Indian Institute of Tech-nology Delhi, New Delhi, India.

His research interests are soft computing applica-tions to design and control of microgrid based power systems.

06517935 a seamless control methodology for a grid connected and isolated pv-diesel microgrid

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