Voltage Control of Microgrid Systems Connected to Power Network System in Fault Condition(1)

International Electrical Engineering Journal (IEEJ) Vol. 4 (2013) No. 3, pp. 1087-1097 ISSN 2078-2365 http://www.ieejournal.com/

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Mehdi and Majid Voltage Control of Microgrid Systems Connected to Power Network System in Fault Condition

Voltage Control of Microgrid Systems

Connected to Power Network System in Fault

Condition

Abstract: In today's world, according as interest about renewable

energy source rises, many researches are progressing about

energy source development and power distribution system

configuration for improving renewable energy source's

efficiency. The microgrid is small power system that consists

of several consumers' loads and small size Distributed

resources and accomplishes one independent system. It is

operated by interconnecting with power distribution system

and could be islanding operation according to situation.

Therefore, the control of microgrid makes microgrid can be

connected to the power distribution system without

equipment correction of power distribution system, and can

compensate voltage quality and must satisfy dynamic

request of loads as active and reactive power are controlled

independently. Specially, when consumer's load includes

sensitive loads, consumer is going to expect very high

reliability from microgrid. In this paper, we studied about

effects that get to power distribution system relationship

fault condition this microgird using EMTP modeling and

simulation. And we proposed microsource's control

algorithm and simulated to improve voltage quality that is

supplied in microgird interior load in fault condition of

power distribution system. This shows that microgird can

provide local high quality power supply or voltage

compensation ability.

Keywords: Microgrid, Voltage control, power quality,

EMTP

I. INTRODUCTION The microgrid is defined as one independent grid

providing continuous power to loads on grid and

compromising two or more microsources with enough

capacity so as to operate independently, storage assets

and loads [5], [11].

Any energy sources should be connected on microgrid

and not affect operations of the existing grid including

protection coordination, reliability, power quality

problem, voltage control and so on correspond to

adding of new energy sources. And when microgrid are

separated from grid operating protection devices by

faults of the grid side, microsources should charge

electrical power needs of loads in microgrid and

operate maintaining power and voltage quality [5].

But in case of instantaneous fault or voltage or power quality deteriorations, not permanent faults of the grid

side, microgrid should operate economic and effective

compensation over a falling-off in quality maintaining

connection to grid.

It is similar to compensating voltage drop changes in

emergency conditions by interconnecting DVR

(Dynamic Voltage Restorer) which is custom power

device to the existing to distribution system. But in case

of microsources, unlike DVR, it has no limitation of

energy capacity on direct current side. So there is a

need of many studies over improvement of voltage

Mehdi Hadinezhad Islamic Azad University, Saveh Branch

Saveh, Iran

[email protected]

Majid Gandomkar

Department of Electrical Engineering Islamic Azad University, Saveh Branch

Saveh, Iran

[email protected]


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quality controlling micro sources, not custom power

devices.

In this paper, we designed and simulated distribution

system interconnected to micro sources and microgrid

using Electro Magnetic Transient Program. And we

figured out dynamic characteristics of microgrid in

emergency conditions of distribution system and made

a comparative study between the proposed control

algorithm of microsources and the existing control

algorithm.

II. Voltage and Power Quality Lately, as the equipment using micro-processor and

semiconductor, high sensitive and large load etc.

increase, about high quality electric power is increasing. The reason is that voltage sag or distortion

influence quality of product, result in information

damage, and consequently these bring big economic

loss. Existent analog signal was not influenced by

voltage sag caused by successful open/close result for

instantaneous system fault.

However, control element, speed control appliances

and computer etc are influence easily by voltage sag.

Especially, in case of computer, power loss caused by

voltage sag can influence seriously to the computer

processing or information etc.

Fig. 1. IEEE Std. 1100-1992 The CBEMA curve [13].

Like above the CBEMA curve, the computer can

perform the normal operation within +6% ~ -13% of

normal voltage. And the permissible limitation of

voltage is big in condition of momentary voltage

variation within 0.5cycle. Therefore the compensation

for the magnitude of voltage variation according to

loads and time is necessary.

III. MICROSOURCE CONTROLLER The inverter converts DC into AC and this is the key

element of micro source operation. The inverter

controller should be designed to response effectively in

Microgrid loads variations. According to Microgrid

operation modes especially, as micro source operate by

voltage source, in order to be connected into microgrid,

it need inductor (XL) for interconnecting to the power

distribution system. [3].


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Fig. 2. Micro source control structure.

where P : power angle, V : output voltage of inverter, E

: Grid voltage Like (1) ~ (3) and active power in the

load angle, reactive power depend in magnitude of

output voltage V of the inverter.

Therefore output voltage magnitude of voltage source

inverters and phase control will lead and will be able to

control micro source outputs.

A. Active power control

Fig. 3. Active power versus Frequency droop controller.

Microgrid system with distribution system

interconnection in case is driven being done each Micro

source must do fixed output according to the Active

Power output direction amount, must decide the

readjustment amount of output automatically in

Islanding mode.

Fig. 3 is active power control block diagram that use power-frequency droop [3]. This block is that apply

power frequency droop control principle that is used to

existent synchronous generator to inverter control.

Equation (4), (5) describes input/output of power-

frequency droop controller.

Where Pm is Micro source active power output

reference, PL is calculated active power, 0 is phase angle of grid.

While microgrid is supplied active power in

distribution system, by islanding mode operation because microgrid gets into overload in case is done

frequency drop. Is proportional and increase fuel


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control standard signal in droop set point in control

system. Droop control has special quality that

frequency decreases about output increase.

B. Q versus E Droop and Voltage Control

Fig. 4. Q versus E droop and voltage controller.

Fig. 4. is voltage control block that use Q-E droop

control.

where Ereq is voltage reference of node. applying Q-E

droop control, make new voltage reference and voltage

control [3]. This can be possible to reduce large output

of reactive power by allowing some voltage changes (

V/Qmax) from reference voltage. voltage controller and Active power model in droop control using EMTP

is illustrated in Fig. 5.

Fig. 5. EMTP/RV module development for P and Voltage magnitude.

(Droop)


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Fig. 6. Proposed active power controller.

The Block diagram of the proposed active power

controller is illustrated in Fig. 6, This can be expressed the following equation:

The proposed controller, when many micro sources

within microgrid in islanding operation operate in

parallel, limits active power output of micro sources

which operate in exceeding rated active power output

of inverter and provides lack of active power from

other micro sources.


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Fig. 7. EMTP/RV module development for P and Voltage magnitude. (proposed)

IV. MICROGRID AND FAULT MODELING A kind of faults in distribution system can classify into

instantaneous fault, permanent fault as fault

maintenance time and line to ground fault, line to line

fault as fault conditions. Fault in distribution system the moment fault about 98% occupy and also, most of

distribution line fault appear by line to ground fault.

In this case, when recloser produces fault being

engaged to protect distribution system, because fault is

intercepted instantaneously, Causes a lot of short-cycle

voltage sag.

Therefore, the momentary voltage sag compensation

micro source through the control effectively, the fault will be reduced by the loss.


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Fig. 8. Model of a Microgrid.

Microgrid modeling is a 15kW, 11.25kVar capacity is

configured as three micro sources, and 10kW, 1kVar of

the load capacity of the three places are located. Fig. 9,

is active power output of proposed micro source

controller.

Fig. 9. Active power output of micro source controller.

V. CASE STUDY

Following Fig. 10. It shows the result of simulation that

the micro source existing in the microgrid does not

work. In this case, we simulated general distribution


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system existing only loads except micro sources. And

the condition of simulation is set to at 5 seconds from

grid and restore at 6 sec.

At this time, the internal voltage in microgrid is

influenced by voltage variation caused by distribution

system fault.. In this simulation, average voltage for

fault period is 180V (RMS) as following Fig. 10

Fig. 10. Voltage of the microgrid when micro source does not exist. (RMS)

A. Droop control In the internal microgrid, three micro-source based on

droop control is connected. At this time, voltage

variation caused by distribution system fault is

compensated partly. In simulation, average voltage for

the fault period is 255V (RMS) as following Fig. 11,

And when the system is recovered normally, voltage

increase is very large according to the micro-source

control.


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Fig. 11. Microgrid internal voltage that is consisted of droop controller base micro source.

B. Proposed control In the internal microgrid, three micro-source based on

the proposed control is connected. Fig. 12, shows that

the compensation for voltage variation of micro-source

caused by distribution system fault is more developed

than the compensation by micro-source based on droop

control.

In this simulation, average voltage for the fault period

is 261V(RMS) as like Fig. 12, And when the system is

recovered normally, voltage increase caused by the

micro source control is smaller than the increase caused by micro source based on droop control.


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Fig. 12. Microgrid internal voltage that is consisted of proposed controller base micro source.

VI. CONCLUSIONS If the fault of grid is not permanent but instantaneous or

reduction of power quality, micro source can

compensate voltage sag by similar principle that use to

distribution system or custom power equipment.

However, compensation rate is different from the control method of micro-source. The controller of

micro-source proposed by simulation is more efficient

than existent control method of micro-source for

voltage sag compensation ability.

And when micro-source based on the proposed

controller transfer to the islanding operation, or the

loads increase /decrease in islanding operation, it is

operated normally within rating output range.

This paper is the result of simulation in the ideal

inverter model supposing three phase balance system,

don't considered the economical operation of micro-

source. In the next research, switching model of

inverter including 3 phase unbalance system and

modeling of real source would be simulated.

VIII. REFERENCES

[1] IEEE Standards Coordinating Committee 21, IEEE Std.1547.4-2011: IEEE Guide for Design, Operation,

and Integration of Distributed.

[2] I. Bae, J. Kim, Reliability Evaluation of Customers in a Microgrid, IEEE Transaction on Power Systems, Vol. 23, No. 3, pp. 1416-1422, Aug.

2008.

[3] M.Shao, R. L. Dianjun Lv, Control Strategy of Voltage and Frequency for Islanded Microgrid, 2012 IEEE 7th International Power Electronics and Motion

Control Conference -ECCE Asia June 2-5, 2012,

China.

[4] R. H. Lasseter, MicroGrids, IEEE Power Engineering Society Winter Meeting, Vol. 1, pp. 305 -

308, 2002.

[5] R. J. Vijayan, S. Ch, R. Roy, Dynamic Modeling of Microgrid for Grid Connected and Intentional

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S. Kim Active synchronizing control of a microgrid, IEEE Trans. power. Electron, vol. 26, no. 12, 2009, pp. 37073719.


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[7] M.B. Delghavi, A. Yazdani, A control strategy for islanded operation of a Distributed Resource (DR)

unit, Power & Energy Society General- Meeting, 2009, PES '09, IEEE 2009, pp. 18. [8] R. Caldon , F. Rossetto, A. Scala Reactive power control in distribution networks with dispersed

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[9] Robert H. Lasseter and Paolo Piagi, Control and Design of Microgrid Components. University of Wisconsin-Madison, 2006.

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Voltage Control of Microgrid Systems Connected to Power Network System in Fault Condition(1)

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