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Academic and Applied Studies Vol.6 (Q.2), 2016, 33-44, ISSN: 1925-931X

Special issue on Electronic & Electrical Sciences

Journal of Academic and Applied Studies Content available online @ www.Academians.org

Reliability and Power Quality Improvement of Microgrids by Fault Current Limiter M. Yousefikia1, E. Gharibreza*2, M. Baledi3

Khorramshar University of Marine Science & Technology, Khorramshahr, Iran 2 Khuzestan Regional Electric Co, Ahvaz, Iran. 3 Ahvaz Electrical Power Distribution Co, Ahvaz, Iran

1

Abstract With the increase of DGs in distribution networks, management of this systems are faced with new challenges. A proposed method for the management of these systems is using Microgrids. A Microgrid is a set of distributed generation and load which acts as a large source or load in view of main grid. On the other hand, the fault current level exceeds the short-circuit capacity of network equipment by microgrids connection. To overcome this problem, fault current limiter (FCL) could be located between the main network and microgrid. So in this paper a typical distribution network with a connected microgrid has been simulated in the Matlab / Simulink, and then the most severe type of the faults execute in two different locations on the network. Various cases are surveyed and the impact of the FCL on the fault current and voltage sag of the microgrid at the junction with the main network (PCC) has been studied. The obtained results show FCL functionality to increase reliability and improve power quality of microgrid.

Keywords: Voltage sag, microgrid, power quality, fault current limiter. I. Introduction Among the problems with conventional power systems that work centralized, can be cited to high costs, difficulties in the operation of the system, low reliability and security of such systems (Issarachai et al. 2014). Wide blackouts have occurred in recent years, expresses this reality. As a result, the concept of distributed generation power (DG) has been proposed. Distributed generation units are connected to the distribution network in order to achieve beneficial effects (Ghanbari et al. 2013). Because in some cases the use of DG units have significant problems (Such as ones that work independently from the network have control problems and too much cost) therefore, these factors led to introduction of microgrid concept as a more logical and effective use of the DG (Chowdhury et al., 2009). Unlike power systems that centralized in producing energy, microgrid is near the loads and doesn’t need to high *Corresponding Email Address: [email protected]

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M. Yousefikia, E. Gharibreza, M. Baledi, Academic and Applied Studies Vol.6 (Q.2), 2016, 33-44

voltage power transmission lines with a long distance (Chowdhury et al., 2009). Microgrid is a controlled unit of the main system that responds to major network requirements in a short time and feed local loads with high quality and reliability and reduces network losses (Chowdhury et al., 2009; Ambarnath Banerji et al., 2010). Figure 1 shows the structure of a typical Microgrid. Some of the advantages of a microgrid than a conventional power network can be expressed as follows (Chowdhury et al., 2009): • Generation resources with smaller capacity than large generators in conventional power plants, • Generation of power at distribution voltage level that can be fed directly into the operational distribution network, • Generation resources with small capacity are generally close to consumers, so that the thermal and electrical loads can be fed by appropriate frequency and voltage profiles with negligible losses in transmission line. II. Problem definition Since the structure and the number of DG units in a microgrid is continuously changed according to connectivity status of entire DG units or connection of a new DG unit to it (Chowdhury et al., 2009) In such circumstances, the network equipment withstand ability against short-circuit capacity may be less than the increased short-circuit capacity, that result to reduction of network reliability. Increasing the short circuit level is depends to various number of factors such as type of DG unit, DG distance to fault location and network configuration between the DG and fault location. According to 1547 IEEE standard, DG units should be disconnected from the main network during the fault. This strategy led to a sharp reduction in the reliability of the system. So all the methods that have reduced short-circuit current and prevent the separation of microgrid from the main network (Islanding) are deemed important. Some of the available methods are changing the network configuration or use of additional equipment such as FCL (Moursi et al., 2012). As a result, the most benefits of DG units can be used with prevention of their unnecessary disruption. FCLs responded in less than a power cycle and can limited the instantaneous magnitude of the fault current in a predetermined value during the fault. For this purpose, various technologies of fault current limiter such as superconducting and solid state FCLs have been proposed (Lin et al, 2006; Moursi et al., 2012; Yucheng et al., 2015). When a microgrid is connected to the main network, FCL can be placed on the interface feeder between microgrid and main network. If a fault occurs in the main network, some of the fault current is supplied by microgrid. This will leads to reduced security and power quality of its available loads. But with the installation of the FCL, contribution 34

M. Yousefikia, E. Gharibreza, M. Baledi, Academic and Applied Studies Vol.6 (Q.2), 2016, 33-44

of Fault current provided by microgrid is reduced which causes an increase in security of DG units and power quality of local loads. Finally with doing different scenarios, the impact of FCL on Microgrid have been investigated for occurrence of three-phase fault in different locations of the network (Issarachai et al., 2014; Tummasit et al., 2015)

Figure. 1. The structure of a typical Microgrid III. FCL modeling In this paper, the proposed FCL model of reference 2 has been used to restrict the fault current. Figure 2, indicate FCL circuit structure. Mentioned FCL structure is capable to compensate Voltage sag and reducing the magnitude of fault current during a network fault, and consists of the following two parts: Diode Rectifier Bridge and a small dc limiting reactor (Ldc), a semiconductor switch (IGBT) and a freewheeling diode (D5). Parallel branch consisting of a resistor and an inductor that acts as a compensator. During normal operation mode of network, line current passes from Diode Bridge, dc limiting reactor and semiconductor switch. When a fault occurs in the network and the current is exceeded of a threshold value, the control circuit detects it and turn off semiconductor switch. The diode bridge is out of the circuit and current is transferred to parallel part (Rsh and Lsh). After clearing the fault, semiconductor switch has been turned on and again the current passes among diode bridge and dc reactor. More

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M. Yousefikia, E. Gharibreza, M. Baledi, Academic and Applied Studies Vol.6 (Q.2), 2016, 33-44

details about the structure of the FCL and its control circuit are mentioned by Jafari et al. (2011).

Figure. 2. proposed FCL topology (Jafari et al., 2011) IV. Case Study System Studied power system consists of distribution network which a microgrid is connected to it. Microgrid has two DG unit, one of which is a wind farm with 10 MW power and another is a synchronous diesel generator with 5 MW power. Consumption Power of microgrid internal loads is 12MW industrial load. The structure of the studied power network and microgrid is shown in Figure 3. More information about the studied network is available (UmerA.Khan et al., 2011) .One the most important index in power quality assessment is voltage sag (Wenyong et al., 2016). To prevent voltage sag jump during a fault, a proper solution is introducing large limiting impedance (FCL). One concern in this regard is to find the perfect place to locate the FCL. Since the right place for FCL is on feeders with DG or in PCC, different scenarios have been investigated by putting FCL in two different locations. Intended Places for FCL in this simulation are: on feeder with CHP (location 1 in figure. 3) In microgrid connection point to the main network (PCC) (location 2 in figure. 3)

Figure. 3. Single-line diagram of the studied Power system 36

M. Yousefikia, E. Gharibreza, M. Baledi, Academic and Applied Studies Vol.6 (Q.2), 2016, 33-44

In this Section, the analytical analysis of voltage sag is discussed in various situations. At the first step, the positive-sequence equivalent circuit of proposed system in the normal condition is shown in figure. 4. The simple voltage divider method is applied to calculate the voltage sag.

Figure. 4. Positive-sequence equivalent circuit of the case study system in the normal condition In the normal state, the voltage magnitude in the PCC can be expressed as (1): ( )

= ̅

̅

( )

( )

+ (̅

)

Where,

( )

̅

( )

ZA= ((ZA2+ZCHP) || ZLA) + ZA1

Voltage phasor of the PCC in the normal state; Phasor of source voltage; Phasor of source impedance; impedance of feeders in the normal condition; 37

(1)

M. Yousefikia, E. Gharibreza, M. Baledi, Academic and Applied Studies Vol.6 (Q.2), 2016, 33-44

ZB= (Z + Z )

ZC = ((ZC2+ZWIND) || ZLC) + ZC1 (ZA||ZB||ZC) condition, i.e. ̅

( )

=

equivalent impedance of parallel feeders in the normal

In the normal state, ̅ ( ) is greater than (̅ ) . So the PCC voltage is almost equal to the source voltage. At the second step By Putting the FCL in proposed areas and apply a three-phase fault at two different points specified in figure 3 (on CHP feeder (F1), at the end of load feeder (F2)), Voltage sag equations are derived. The positive-sequence equivalent circuit of proposed system with fault in F1 and FCL in locations 1 & 2 is shown in figure. 5.

a

b

c

Figure. 5. Positive-sequence equivalent circuit of the case study system with fault in F1, a: Without FCL, b: FCL at location1, c: FCL at location2 ( )

=

( )

( )

( )

In the F1 fault condition, the voltage of PCC can be expressed as (2): (2) 38

M. Yousefikia, E. Gharibreza, M. Baledi, Academic and Applied Studies Vol.6 (Q.2), 2016, 33-44

Where, Voltage phasor of the PCC during fault;

( )

ZA = ZA1 + ZF1 ZC = ZC1+ ZC2+ZWIND ̅

( )

=

(ZA ||ZC)

If the FCL is located in location 1 , ZA is changed as (3): ZA = ZA1 + ZFCL+ ZF1

(3)

And also by setting the FCL in location 2, ZS is changed as (4): ̅

( )

=

̅

( )

+ ZFCL

(4)

The positive-sequence equivalent circuit of proposed system with fault in F2 and FCL at locations 1 & 2 is shown in figure. 6.

a

b

c

Figure. 6. Positive-sequence equivalent circuit of the case study system with fault in F2 , a: Without FCL, b: FCL at location1, c: FCL at location2 39

M. Yousefikia, E. Gharibreza, M. Baledi, Academic and Applied Studies Vol.6 (Q.2), 2016, 33-44

( )

=

( )

( )

( )

In the F2 fault condition, the voltage of PCC can be expressed as (5): (5)

Where, Voltage phasor of the PCC during fault;

( )

ZA = ZA1 + ZA2 + ZCHP ZB = ZB1 + ZF2 ZC = ZC1+ ZC2+ZWIND ̅

( )

=

(ZA || ZB || ZC)

If the FCL is located in location 1, ZA is changed as (6): ZA = ZA1 + ZFCL+ ZA2 + ZCHP

(6)

And also by setting the FCL in location 2, ZS is changed as (7):

̅

( )

=

̅

( )

+ ZFCL

(7)

In the three-phase fault condition (that is a balanced fault), ̅ ( ) will be small. Consequently, a comparison of equations shows that the voltage sag jump occur in the fault interval. A practical solution to control this voltage jumping in the power system is using FCL. Simulation results in the next section indicate the right place of FCL according to equations derived above. V. Simulation and numerical results By Putting the FCL in mentioned areas and apply a three-phase fault for 0.15 seconds at two different points specified in figure. 3 ;on CHP feeder (F1) and at the end of load feeder (F2), microgrid behavior has been investigated with and without FCL. According to the obtained results, FCL is able to decrease the fault current amplitude provided by Microgrid for three-phase fault in two different points of network. This will be subject to increase security of DG units in Microgrid and its reliability to feed the loads. One of the very important impacts of FCL on Microgrid is in the Voltage Sag compensation at the PCC and at the Microgrid’s load, when a fault occurs. Due to FCL inherent properties, it can lead to reduce voltage sag at the PCC and also from the perspective of the microgrid internal loads that eventually would increase power quality and reliability of the microgrid. So the voltage sag, as one of the most important power quality index that can affect the system characteristics, will improve with presence of FCL. Furthermore the amount of transmitted power at PCC point will 40

M. Yousefikia, E. Gharibreza, M. Baledi, Academic and Applied Studies Vol.6 (Q.2), 2016, 33-44

improve during the fault by placing the FCL. In the following, different scenarios have been investigated to validate FCL efficiency in security increasing and power quality development as a part of smart grids (MaZhengbo and Chen Bo, 2011). Modeling of FCL and desired network has been done by Matlab software.

A. Fault on CHP feeder (F1) In this part, microgrid behavior has been studied by applying a three phase fault as an exception fault for the microgrid and its loads over a period of 0.15 sec on CHP feeder (F1 in figure.3). Comparison between voltage sag at PCC and fault current at different locations (location 1, 2 figure.3) and results are shown in Figures 7-A and 7–B respectively. In this scenario, when the FCL is located at PCC, the amplitude of fault current decreases in compare with the time that there is no FCL in the network or it is located at the CHP feeder. This subject is Due to placing FCL at the PCC which increases the impedance and eventually results to limiting the amount of fault current. In this situation, the voltage sag at the PCC point and the amplitude of fault current is extremely reduced than when the FCL is not available in the network or placed in CHP feeder. In this case unlike the other conditions, Microgrid transmitted power is not zero during the fault as long as the FCL is located at the PCC which increases the security of DG units in the microgrid.

4

x 10

1500

FCL at PCC Without FCL FCL at substation FCL at CHP feeder Without FCLFCLat PCC

2

Without FCL

FCLat FCL at PCC subsation FCL at CHPPCC feeder FCLat Without FCL

1000 500 0 Current(A)

Voltage(v)

1.5

1

-500 -1000 -1500

0.5

-2000

0 0.35

0.4

0.45

0.5

Time(sec)

0.55

0.6

-2500 0

0.65

Figure. 7-A. Voltage sag at PCC

0.1

0.2

0.3

0.4

0.5

Time(sec)

0.6

0.7

0.8

Figure. 7-B. Fault current

41

0.9

1

M. Yousefikia, E. Gharibreza, M. Baledi, Academic and Applied Studies Vol.6 (Q.2), 2016, 33-44

B. Fault at the end of load feeder (f 2) In this part, microgrid behavior has been studied by applying a three phase fault over a period of 0.15 sec at the end of load feeder (F1 in figure.3). Comparison between voltage sag at the PCC and fault current has been done by placing FCL at different locations (locations 1&2 figure.3 ) and results are shown in figures 8-A and 8-B respectively.

4

x 10

2

Voltage(V)

1.5 without FCL at PCCfcl FCL feeder FclatatCHP substation

1

Without FCL Fcl at pcc

0.5

0 0.35

0.4

0.45

0.5

0.55

Time(sec)

0.6

0.65

Figure. 8-A. voltage sag at the PCC 1500 1000

Fcl at PCC FCL at PCC Fclfeeder at substation FCL at CHP Without Without FCL fcl

500

Current(A)

0 -500 -1000 -1500 -2000 -2500 0.35

0.4

0.45

0.5

Time(sec)

0.55

0.6

0.65

0.7

Figure. 8-B. Fault current According to results, the amplitude of the fault current decreases in the case that FCL is located at the PCC (location 2 figure.3) rather than its absence. This is because of impedance increasing between Normal generation resources on the network and fault location. So the amplitude of fault current is reduced if the FCL is located at the PCC than when the FCL is not available in the network or placed in CHP feeder. This occurs because the high percentage of fault current is supplied by existing power plants in the grid. So the voltage sag at the PCC and the amplitude of fault current is decreased 42

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then the FCL is located at PCC while the amount of transmitted power at PCC is increased. VI. Conclusions This paper presents a novel method to increase security and improve power quality of microgrids that be connected to the distribution network. A complete power system with a connected microgrid has been simulated in Matlab/Simulink environment. Microgrid behavior was evaluated by applying a three phase fault across the network by putting the limiting fault current (FCL) in sensitive parts of the grid. According to the obtained results, with microgrids connection to distribution network, FCLs can be used to avoid increasing fault current amplitude, increasing security of DG units and improvement of microgrid loads power quality. So by the precise survey in this paper it is clear that FCL arrangement at the PCC was more effective in enhancing the security and improving the power quality of microgrid than when the FCL is placed in CHP feeder. Acknowledgements We would like to thank Khorramshahr University of marine science and technology for supporting this work under research grant contract No.66. References Ambarnath Banerji, Sujit K. Biswas, and Bhim Singh (2010), Enhancing Quality of Power to Sensitive Loads with Microgrid, IEEE transaction. Chen Bo, Cheng Luwen, Wu Zhigan, Lin Lingxu, Wang Ru, Chen Zen (2011), Study On the Reliability of Integration between Smart Micro-grid and Distribution Network, Supported by The national high technology research and development program (863 Program) 2011—Research On Self-healing Control Technology of Smart Distribution Network (GRANT NO. 2011AA05A114)) ,IEEE. Issarachai Ngamroo; Tanapon Karaipoom (2014), Improving Low-Voltage RideThrough Performance and Alleviating Power Fluctuation of DFIG Wind Turbine in DC Microgrid by Optimal SMES With Fault Current Limiting Function, IEEE Transactions on Applied Superconductivity, vol (2), pp 15-24. Lin Ye, M. Majoros, T. Coombs, A. M. Campbell (2006), System Studies of the Super conducting Fault Current Limiter in Electrical Distribution Grid, IEEE. MaZhengbo, Li Linchuan, and Dong Tuo (2011), Application of a Combined System to Enhance Power Quality in an Island Microgrid, IEEE.

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M. S. El Moursi, RoshdyHegazy (2012), Novel Technique for Reducing the High Fault Currents and Enhancing the Security of ADWEA Power System, IEEE Transactions On Power Systems. M. Jafari, S. B. Naderi, M. TarafdarHagh, M. Abapour, S. H. Hosseini (2011), Voltage Sag Compensation of Point of Common Coupling (PCC) Using Fault Current Limiter, IEEE Transactions On Power Delivery, vol (26). N. Tummasit; S. Premrudeepreechacharn; N. Tantichayakorn (2015), Adaptive overcurrent protection considering critical clearing time for amicrogrid system Smart Grid Technologies - Asia (ISGT ASIA), IEEE Innovative, pp 1-6. S. Chowdhury, S.P.Chowdhury and P.Crossley(2009), Microgrids and Active Distrbution Networks, IET Renewable Energy Series 6. Teymoor Ghanbari; Ebrahim Farjah (2013), Unidirectional Fault Current Limiter: An Efficient Interface Between the Microgrid and Main Network, IEEE Transactions on Power Systems, vol ( 28) , pp 1591-1598. UmerA. Khan,J. K.Seong,S.H.Lee, S.H.Lim (2011), Feasibility Analysis of the positioning of superconducting fault current Limiters for the smart grid Application using simulink and simpowersystem, IEEE Transactions of superconductivity,vol (21). UmerA.Khan, J. K.Seong, S.H.Lee, S.H.Lim (2011), Feasibility analysis of the application and positioningof DC HTS FCL in a DC microgrid through modeling and simulation using Simulink and SimPowerSystem, Elsevier. Umer A.Khan,J. K.Seong,S.H.Lee, S.H.Lim (2011), Application and Positioning Analysis of a Resistive Type Superconducting Fault Current Limiter in AC and DC Microgrids using Simulink and SimPowerSystem, International Conference on Electric Power Equipment - Switching Technology - Xi'an – China. Wenyong Guo; Liye Xiao; Shaotao Dai (2016), Fault current limiter-battery energy storage system for the doubly-fed induction generator: analysis and experimental verification, IET Generation, Transmission & Distribution, vol( 10), pp 653-660. Yucheng Zhang, Roger A. Dougal, State of the Art of Fault Current Limiter and Their Applications in Smart Grid , This work was supported by the US National Science Foundation under grant 0652271 and by the Office of Naval Research under grant # N00014.

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