Operating Strategies for Stabilizing the Operation of a Micro Grid with Various Distributed Generation Sources

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International Journal of Emerging Technology and Advanced Engineering

Website: www.ijetae.com (ISSN 2250-2459,ISO 9001:2008 Certified Journal, Volume 3, Issue 9, September 2013)

44

Operating Strategies for Stabilizing the Operation of a Micro

Grid with Various Distributed Generation Sources

San-Yi Lee

1

, Wen-Chih Yang

2 1,2

Department of Electrical Engineering, Taipei Chengshih University of Science and Technology, Taiwan, R.O.C.

Abstract—A micro grid (MG) needs a suitable operating

strategy to maintain its operation stably. However, the development of an operating strategy for a MG depends deeply on the type and number of distributed generation sources (DGSs). For this reason, this work develops four operating strategies for a MG with one or multi DGSs. The DGSs in MGs can be sustained energy sources (SESs) or intermittent energy sources (IESs). The proposed operating strategies can be used to maintain the stability of MGs effectively. To examine the practicability of these operating strategies, a sample MG is employed and is simulated via computer software. The important simulation results are presented and discussed in this paper.

Keywords—Micro grid, Distributed generation source, Sustained energy source, Intermittent energy source, Operating strategy.

I. INTRODUCTION

A micro grid (MG) is a controllable small-scale power system [1]. It generally comprises distributed generation sources (DGSs), energy storage devices (ESDs), controllers, switchgears, protective devices, and loads, among others [2]. It can be applied to remote locations to supply electricity to customers or can be implemented in utility power systems to increase their reliability. The implementation of MGs can heighten the penetration of

DGSs in the grids of utilities, reduce the emission of CO2,

and protect the environment. Hence, the concept of MG is currently popular in utilities and with customers [3-5].

The DGSs in MGs can be sustained energy sources (SESs), such as a diesel engine generator (DEG), internal combustion engine generator (ICEG), micro turbine (MT), gas turbine (GT), or fuel cell (FC), or intermittent energy sources (IESs), such as wind turbine (WT), photovoltaic (PV), hydraulic generator (HG), wave-energy generation system (WEGS), or biomass generation system (BGS) [6,7]. One or more DGSs can be used simultaneously. A MG consists of different types of DGSs, and its operating condition is different as well.

Hence, developing a suitable operating strategy for MGs is an important task for electric utilities and is useful for MG operators.

In this paper, four operating strategies for MGs are presented. The first is suitable for MGs containing only one SES; the second is suitable for MGs containing one IES; the third is suitable for MGS containing two SESs; and the fourth is suitable for MGs containing one SES and one IES. To examine the practicability of these operating strategies, a sample MG is employed by this paper and is simulated using professional software, MATLAB/Simulink. The simulation results, such as bus voltages, feeder currents, grid’s power flow, and frequency, are presented and discussed. These operating strategies and simulation results are useful for operators of MGs because they can be used to maintain their MGs operating stably.

II. COMBINATIONS OF DGSS IN AMG

A MG may comprise one or more DGSs. The DGSs can be SESs, IESs, or both. Fig. 1 shows a typical configuration of a MG with multi DGSs. The DGSs comprise two DEGs, one PV and one WT. The DEGS are SESs, whereas the PV and the WT are IESs.

The operating conditions of a MG with only one type of DGS are relatively simple. Hence, the operating strategy for this kind of MG is simple as well. However, the operating conditions of a MG with both SESs and IESs are complex. As such, the operating strategy for this kind of MG is also complex [8,9].

III. OPERATING RESTRICTIONS FOR MGS

A MG can be operated either independently or with a utility grid in parallel. The former is called the islanded mode, and the latter is called the grid-connected mode [10]. Whatever the operating mode of a MG is, three operating restrictions should be followed [11,12].

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International Journal of Emerging Technology and Advanced Engineering

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Diesel engine generator Utility grid

Power transformer

Load Circuit breaker

Bus Legends:

Photovoltaic array

PV WT

DEG DEG

Micro grid

DEG

PV WT Wind turbine

[image:2.612.55.284.122.542.2]

Grid feeder

Fig. 1 A typical configuration of a MG with multi DGSs

   



 

m

1

n load

loss k

1

j DGS

grid P j P P n

P (1)

   



 

m

1

n load

loss k

1

j DGS

grid Q j Q Q n

Q (2)

Where

 Pgrid and Qgrid denote the active and reactive power

injected into a MG from a utility grid, respectively;

 PDGSj and QDGSj denote the active and reactive

power outputs of the jth DGS in a MG, respectively;  Ploss and Qloss denote the total active and reactive

power losses in a MG, respectively;

 Ploadn and Qloadn denote the active and reactive

power demands of the nth load in a MG, respectively;

 j is the number of DGSs in a MG;

 k is the amount of DGSs in a MG;

 n is the number of loads in a MG; and

 m is the amount of loads in a MG.

Second, each bus voltage should not exceed its tolerance. That is, (3) must be satisfied.

max i i min

i V V

V   (3)

Where

 |Vi|min、|Vi|max denote the lower and upper voltage

limits of the ith bus in a MG, respectively; and

 |Vi| denotes the voltage magnitude of the ith bus in a

MG.

Third, each feeder current should not exceed its thermal limit. That is, (4) must be satisfied.

thr i

i I

I  (4)

Where

 |Ii|thr denotes the thermal limit of the ith feeder in a MG;

and

 |Ii| is the current magnitude of the ith feeder in a MG.

IV. PROPOSED OPERATING STRATEGIES FOR MGS

A MG can be operated either independently or with a utility grid in parallel. The former is called the islanded mode, and the latter is called the grid-connected mode [10]. Whatever the operating mode of a MG is, three operating restrictions should be followed [11,12].

4.1A MG containing one SES

A SES can be a DEG, ICEG, MT, or FC, etc. The fuel costs of SESs are generally higher than those of utilities. Thus, SESs should be disconnected from their MGs to reduce operating costs and environmental pollution while the MGs are operated in the grid-connected mode. In contrast, SESs should be connected to their MGs while the MGs are operated in the islanded mode because utility grids cannot supply electricity to customers in MGs. Based on the operating conditions described previously, the operating strategy for a MG containing one SES is developed, as shown in Fig. 2. Some assumptions adopted by this paper are described as follows:

1. The SES has the ability to control the voltage and frequency of the MG within an allowable range while the MG operates in the islanded mode. A v-f controller has been installed in the SES.

2. The SES also has the ability to reconnect to its MG at

any time. A synchronized phasor measurement unit and switchgear have been installed in the SES.

3. The SES has enough capacity to meet the peak

demand of the MG while the MG is operated in the islanded mode.

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Does utility

service continue ?

No

Is the SES connected to the

MG ?

No

Yes Yes

Disconnects the SES from the MG

Is the SES's power generation less than the MG's

demand ?

Increases the SES's power output

No Yes

Does the SES's power generation

reach its lower limit?

Decreases the SES's power output

End Disconnects the SES from

the MG Yes

No Is the SES's power generation equal to the MG's

demand ? Yes

No

MG ?

No

Yes Start

Connects the SES to the MG

[image:3.612.56.280.136.450.2]

Fig. 2 The operating strategy for a MG containing one SES

4.2A MG containing one IES

An IES can be a WT, PV, HG, WEGS, or BGS, etc. IESs incur almost no fuel cost and they do not emit pollutants because they use natural energy to produce electric power. Hence, they should be connected to their MGs while the MGs are operated in grid-connected mode. In doing so, the operating cost of MGs can be reduced, and the environment can be protected. However, IESs are not stable electric sources. They cannot supply stable electricity for long because of the effects of weather and the environment. Thus, IESs have to be operated with ESDs, such as batteries, simultaneously. Based on the operating restrictions, the operating strategy for a MG containing one IES is developed, as shown in Fig. 3. Some assumptions adopted by this paper are described as follows:

1.The IES exports electric power using a power

converter. The power converter has the ability to control the voltage and frequency of the MG within an allowable range while the MG is operated in islanded mode.

2. The power converter of the IES does not control the voltage and frequency of the MG while the MG is operated in grid-connected mode. It only exports the maximum electric power according to the input energy

3. The rating capacity of the IES is enough for the peak

demand of the MG while the MG is operated in islanded mode.

4. The ESD also uses a power converter to export

electric power. The ESD does not have the ability to control the voltage and frequency of the MG whether the MG is operated in islanded or grid-connected mode.

5. Both the IES and ESD are limited by maximum

power output.

6. Both the IES and ESD are installed with a

synchronized phasor measurement unit and

switchgear. Hence, they can reconnect to their MG at any time.

7. This operating strategy does not adopt any load

shading strategy.

4.3A MG containing multi SESs

When a MG contains two or more SESs, its operating condition becomes complex. Two operating strategies are available for this kind of MG. First, one of the two SESs is reconnected initially to its MG while the MG is operated in islanded mode. Some loads in the MG are then restored according to the preplanning restoration scheme. After the voltage and frequency of the MG stabilize, the other SES is reconnected to the MG, and then restoring the residual loads of the MG. Second, the two SESs are reconnected gradually to its MG before load restoration. After the voltage and frequency of the MG stabilize, the loads of the MG are restored at the same time.

Both strategies have their advantages and disadvantages. The former needs a long duration to restore its MG. However, its restoring process is more reliable than that of the letter. In contrast, the latter spends only a short time for restoring. However, the voltage or frequency of the MG may oscillate largely when most of the loads are reconnected to the MG simultaneously. Once the oscillation exceeds the tolerance of the MG, the starting work fails.

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1.The capacity of each SES is smaller than the peak load demand of its MG. However, the total generation capacity of the two SESs is larger or equal to the peak load demand of its MG.

2.The two SESs export electric power in accordance

with their voltage-drop characteristics.

3.Both SESs have the ability to control the voltage and

frequency of the MG within an allowable range while the MG is operated in islanded mode.

4.Both SESs have the ability to reconnect to their MG at any time.

5.Each SES is limited by minimum power output.

Connects the IES to the MG

Charges the ESDs

Does utility service continue ?

No

The IES exports the maximum electric power Yes

Is the IES's power generation more than the MG's

demand ?

Decreases the IES's power output

No Yes

Does the IES's power generation

reach its upper limit ?

Increases the IES's power output

Yes No Is the IES's power generation equal to the MG's

demand ? Yes

No

Does the ESDs' power generation

reach its upper limit ?

Increases the ESDs' power output

No Yes

Disconnects the IES and ESDs from the MG

Start

[image:4.612.357.531.138.552.2]

End

Fig. 3 The operating strategy for a MG containing one IES

No

Is the first SES connected to the

MG ?

No

Yes Yes

Disconnects the first SES from the MG Start

No

Yes Disconnects the second

SES from the MG

Is the second SES connected to the

MG ? Is

the first SES connected to the

MG ?

No Yes

Connects the first SES to the MG

No

Is the second SES connected to the

MG ?

Yes

Is the SESs' total power generation

less than the MG's demand ?

Increases each SES's power output

No Yes

Does every SES's power generation reach its

lower limit?

Decreases each SES's power output

End Disconnects all SES from

the MG Yes

No Is the SESs' total power generation

equal to the MG's demand ?

Yes

No Yes Connects the second SES

[image:4.612.83.247.284.641.2]

to the MG

Fig. 4 The operating strategy for a MG containing two SESs

4.4A MG containing bothSES and IES

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However, the IES should be connected to its MG in islanded or grid-connected mode to supply green energy and to save fuel cost. In practice, IESs are used as the major electric sources of MGs when the MGs are operated in islanded mode. They are also used as auxiliary electric sources when the MGs are operated in islanded or grid-connected mode. The operating strategy for a MG containing one SES and one IES is developed in this paper, as shown in Fig. 5. Some assumptions adopted by this paper are as follows:

1. The SES can control the voltage and frequency of the

MG within an allowable range while the MG is operated in islanded mode. The generation capacity of the SES is larger than the peak demand of the MG.

2. The IES connects to its MG through a power

converter. However, the power converter does not have the ability to control the voltage and frequency of the MG whether the MG is operated in islanded or grid-connected mode.

3. The IES exports maximum electric power in

accordance with the input energy. The output power changes with the change in its input energy.

4. The SES is limited by minimum power output, but

the IES is not.

5. Both SES and IES have the ability to reconnect to their MG at any time.

V. THE SAMPLE MGFOR EXAMINATION

In practice, a MG is generally composed of both SES and IES. Hence, the sample MG is designed to have one SES and one IES. The structure, parameters, and equivalent model of the sample MG are described as follows.

5.1Structure and parameters of the sample MG

Fig. 6 shows the one-line diagram of the sample MG used in this paper. The power transformer P1 is used to

connect the utility grid and the sample MG. Its rating and parameters are shown in Table I. The sample MG has two DGSs: one is a DEG with a rating capacity of 150 kVA, and the other is a PV with the same rating capacity of 150 kVA. Each DGS is connected to the sample MG through a power transformer. The power transformers P2 and P3 are

used as isolation devices. Hence, their voltage ratings on the primary and secondary sides are identical. The power transformers P2 and P3 have the same size; their parameters

are shown in Table II. Moreover, the sample MG has some loads. These loads are fed with electricity by two distribution feeders, the sample feeders F1 and F2. The

parameters of the two sample feeders are identical and are shown in Table III.

To keep the voltage and frequency of the sample MG stable, the DEG has a V-f controller, but the PV does not.

No

MG ?

No

Yes Yes

Disconnects the SES from the MG Start

No

Yes Connects the IES to the

MG and exports the maximum electric power

Is the IES disconnected to

the MG ? Is the IES

connected to the MG ?

Yes No

Disconnects the IES from the MG

No

MG ?

Yes

Yes Connects the SES to the

MG

Connects the IES to the MG and exports the maximum electric power

Is the SES and IES's total power generation

equal to the MG's demand ?

Yes

Is the SES and IES's total power generation less than the MG's

demand ? No

Yes

Does the SES's power generation reach its

lower limit?

Decreases the SES's power output

End Disconnects the SES from

the MG No No

Disconnects the IES from the MG

Yes

Is the SES's power generation more

than the MG's demand ?

Yes No

Increases the SES's power output

Is the difference between SES's power generation and MG's demand more than the IES's power

generation ?

[image:5.612.356.531.162.685.2]

No

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DEG

PV

P1

P2

P3

Diesel engine generator Utility grid

Power transformer

Load Circuit breaker

Bus Legends:

Photovoltaic array DEG

PV

Grid feeder

Feeder F1

Feeder F2

CB1

CB2

CB3

CB4

[image:6.612.60.279.121.396.2]

CB5

Fig. 5 One-line diagram of the sample MG

5.2Equivalent model of the sample MG

This paper develops an equivalent model of the sample MG for computer simulation. The simulation tool adopted by this paper is MATLAB/Simulink [13,14]. Fig. 7 shows the blocks of the equivalent model of the sample MG built in MATLAB/Simulink. The equivalent model of each component is provided by SimPowerSystems in Simulink, in which the grid of the utility is denoted by a constant voltage source with constant impedance.

Table I

Parameters Of The Power Transformer P1 In The Sample MG

Capacity (kVA)

Primary voltage

(kV)

Secondary voltage

(V)

Winding impedance

(%)

Winding connection

[image:6.612.45.291.545.667.2]

150 11.4 380 1.65+j2.5 Wye-delta

Table II

Parameters Of The Power Transformers P2 And P3 In The Sample

MG

Capacity (kVA)

Primary voltage

(kV)

Secondary voltage

(V)

Winding impedance

(%)

Winding connection

150 380 380 1.35+j1.9 Wye-delta

Table III

Parameters of the distribution feeder in the sample MG

Material

Cross section (mm2₎

Impedance ()

Thermal capacity

(A)

Copper 200 0.101+j0.092 300

VI. SIMULATION RESULTS AND DISCUSSIONS

To examine the practicability of the proposed operating strategies under different combinations of DGSs, two simulation scenarios are carried out: one is for the operating strategy of a MG with one DEG, and the other is for the operating strategy of a MG with one DEG and one PV. The simulation results are organized and presented as follows.

6.1Scenario 1

In this scenario, the PV in the sample MG is removed. The sample MG only has one DGS, i.e., the DEG. Hence, the operating strategy of a MG described in subsection 4.1 is adopted in the simulation scenario. The operating conditions of the sample MG are given below. The simulation results are presented in Figs. 8 to 11.

1. At 0 s: The utility grid and DEG operate in parallel to

supply the sample MG. The total load of the sample MG is 100 kVA with 0.8 lagging power factor. The sample MG is operated in grid-connected mode.

2. At 1 s: The DEG is disconnected from the sample MG

because the sample MG is operated in grid-connected mode.

3. At 2 s: The utility grid trips because of a fault. The sample MG loses power service. All loads of the sample feeders are shed in this duration.

4. At 3 s: The DEG starts to supply electric power to the

sample MG. The sample MG is operated in islanded mode in this duration.

5. At 5 s: The circuit breaker CB4 closed. Hence, the

loading of 50 kVA of the sample feeder F1 is restored.

6. At 6 s: The circuit breaker CB5 closed. Hence, the

loading of 50 kVA of the sample feeder F2 is restored

as well.

7. At 7 s: The loading of the sample MG is reduced by

50 kVA because the circuit breaker CB4 opened.

8. At 9 s: The loading of the sample MG is again

reduced by 50 kVA because the circuit breaker CB5

opened as well.

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1.0

wref (pu)1 Discrete, Ts = 1e-005 s.

pow ergui 1 Vtref (pu)1 1 Vref (pu) v + -Vpv Vabc (pu)

Vd_ref (pu) Vabc_inv m Voltage Regulator g A + -Universal Bridge 1 arm z 1 N A B C TaiPower A B C A B C TL3 A B C A B C TL2 30 T Pm Vf _ m A B C

SM 100 kVA

Signal(s) Pulses PWM Generator g A B C + -PWM IGBT Inverter Vm Ga Ta Im

PV array AVabc

B C a b c Measure In Mean Mean Value1 In Mean Mean Value Ipv Vpv M MPPT A B C A B C LC Filter1 L1 L 450 Ga Uref Pulses Discrete PWM Generator wref Vtref m Pm Vf Vt w Diesel Engine Speed & Voltage

Control

s

-+

Controlled Current Source

C4

C3 C2 C1

A B C 80 kW 380 V rms 60 Hz1 A B C 80 kW 380 V rms

60 Hz A B C a b c 3-Phase Breaker6

A B C

a b c

3-Phase Breaker5 A B C a b c 3-Phase Breaker4 A B C a b c 3-Phase Breaker2

A B C 10 KW

[image:7.612.79.531.143.439.2]

Pmec (pu) Vf (pu)

Fig. 7 An equivalent model of the sample MG

In this scenario, the sample MG is operated in islanded mode at 3 s. Afterwards, the DEG supplies the entire sample MG. Fig. 8 illustrates the variation of power generation of the DEG with the demand of the sample MG. Fig. 9 shows that the system frequency is affected by the variation of the loading of the sample feeder. When the loading varies significantly, the system frequency varies significantly as well. Figs. 10 and 11 show that the voltage and currents of the sample MG are roughly stable in islanded mode. This finding indicates that SESs are suitable for application to MGs if the capacity of the electric sources is large enough.

6.2Scenario 2

In this scenario, one DEG and one PV are simultaneously connected to the sample MG. The DEG has a V-f controller, but the PV does not. The sample MG has two DGSs. Hence, the operating strategy of DGS described in subsection 4.4 is adopted in this scenario. The operating conditions of the sample MG in Scenario 2 are different from those in Scenario 1, as indicated below.

The simulation results of this scenario are presented in Figs. 12 to 16.

1. At 0 s: The sample MG is supplied by the utility grid.

Its total load is 150 kVA with 0.8 lagging power factor. The sample MG is operated in the grid-connected mode.

2. At 0.1 s: The PV is connected to the sample MG and

supplies 24k W of electric power.

3. At 0.5 s: The utility grid trips because of a fault. The PV supplies electric power to the sample MG continuously.

4. At 0.9 s: The PV is disconnected from the sample MG.

There is no electric source in the sample MG.

5. At 1.2 s: The DEG starts and is connected to the sample MG to supply 150 kVA of electric power. The sample MG operates in the islanded mode in this duration.

6. At 1.5 s: The PV is reconnected to the sample MG to

supply 30 kVA of electric power with 0.8 lagging power factor.

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51

Fig. 8 The generation of the DEG in the sample MG

Fig. 9 The frequencies at bus 2 of the sample MG

Fig. 10 The voltages at bus 2 of the sample MG

Fig. 11 The currents flowing through bus 3 of the sample MG

[image:8.612.325.557.119.402.2]

Fig. 12 The generation of the DEG in the sample MG

Fig. 13 The generation of the PV in the sample MG

[image:8.612.327.556.584.697.2]

In this simulation scenario, the PV operates in parallel with the utility grid at the begging and then trips because the sample MG is operated in islanded mode. The DEG starts to supply the entire sample MG at 1.2 s. The PV then reconnects to the sample MG at the 1.5 s. Figs. 12 and 13 present the variations of the electric power output of the DEG and PV. The electric power output of the PV is not stable from 0.1 to 0.5 s because of the interconnection of the utility grid and the DEG. Fig. 14 shows that the system frequency is affected by the connection of the two DGSs. The frequency varies small except for a short time from 1.2 to 1.6 s. Figs. 15 and 16 illustrate that the voltage of the sample MG is very stable except for a short duration that the PV connects to the sample MG.

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[image:9.612.54.286.128.381.2]

52

Fig. 15 The voltage at bus 2 in the sample MG

Fig. 16 The current flowing through bus 3 of the sample MG

VII. CONCLUSIONS

Four operating strategies of a MG have been presented in the current paper. The first strategy can be used for MGs with only one SES; the second can be used for MGs with only one IES; the third can be used for MGs with multi SES; and the fourth can be used for MGs with both SES and one IES. SESs and IESs are all important electric power sources for MGs while operating in islanded mode. The SESs can maintain the balance between the power supply and demand, and the IESs can provide clean energy. The concept of MGs is more popular in electric industries [15]. DGSs are expected to play an important role in MGs. Adopting the appropriate operating strategy for a MG is important. Hence, the proposed operating strategies are useful for utilities when operating their MGs in the future.

Acknowledgment

The authors would like to thank the National Science Council of the Republic of China, Taiwan, for financially supporting this research under Contract No. NSC102-3113-P-194-002.

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