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Investigation on Reactive Power Support Capability

of PEVs in Distribution Network Operation

Sasan Pirouzi1 Mohammad Amin Latify2 G. Reza Yousefi2

[email protected] [email protected] [email protected] M.Sc. graduate, Department of Electrical and Computer Engineering, Isfahan University of Technology, Isfahan, Iran2 Faculty member, Department of Electrical and Computer Engineering, Isfahan University of Technology, Isfahan, Iran

Abstract According to the environmental and economic

advantages of the Electric Vehicles (EVs), it is predicted that the

penetration of EVs will be increased in the near future. EVs are

fast growing loads in the power systems. Several types of EVs are

invented and commercialized, such as Plug-in Hybrid Electric

Vehicles (PHEVs) and Hybrid Electric Vehicles (HEVs). A PHEV

needs a battery charging plug. The high penetration of the

PHEVs in the Grid has great impact on the network planning

stages, as well as the network operation schemes. Increasing the

number of PHEVs, without an appropriate management, coulddecrease the power quality in distribution networks. In this

paper we present a reactive power management strategy,

considering a high penetration of PHEVs, to tackle with the

negative impact of PHEVs, namely reducing the power losses,

improving the voltage profile and removing line congestion in

distribution network. To do so, at the first step, we obtain a

generic model for

into a distribution network. Then we present a reactive power

management scheme to demonstrate how PHEVs could be used

to control reactive power and how it could minimize the power

losses and improve the voltage profile, as well as removing

congestion in the distribution networks. 33-bus distribution

network is considered to perform the case study. The results are

highly implicate the performance of the proposed method.

Keywords- electric vehicles; reactive power management;

PHEV; loss reduction; voltage profile.

I. I NTRODUCTION

Nowadays, energy consumption and its growth are restrictedwith the environmental concerns, and clean energy productionand efficient consumption are aimed by the regulators, as wellas owners of technologies, all around the world. One of thefast growing technologies is Electric Vehicle (EV). Severaltypes of EVs are in market now, such as Plug-in HybridElectric Vehicles (PHEVs) and Hybrid Electric Vehicles(HEVs). Obviously, a PHEV needs an electricity source for

battery recharging. With increasing the penetration of electricvehicles in distribution networks, the electricity demand will

be increased, and it could be the source of some problems indistribution networks operation, and it should definitely beconsidered in network planning [1]. A high penetration ofPHEVs in a distribution network could lead to increasing thenetwork losses and worsening the buses voltage profile [2].Battery chargers (which are using in PHEVs) are powerelectronic based devices. Thus, dealing with harmonics due toswitching operation of power electronic elements are expected

[3] and single phase chargers worsen the power quality parameters in such distribution systems [4].

The most problems occur in distribution networks, caused by a high penetration of PHEVs, are due to lack of chargingcontrol, as well as absence of a proper coordinatingmechanism between them. Researchers have been paidattention to defects of PHEVs in distribution networks and

presented different strategies to reduce the negative impact of

PHEV in the grid. Active power management of PHEVs isused in [5] to improve the voltage profile along the network,in while the batteries of PHEVs are controlled in such a waythat they should inject the power to the grid on some busesand/or absorb the power on some other buses. In [6], based onthe network level of load and voltage profile, optimum batterycharging time interval is obtained for each incoming PHEVinto the network. A real-time smart load management (RT-SLM) strategy is represented in [7] in which the PHEVs arecharged based on a priority selection over each time interval.The most of articles are focused on active power and lessattention have been made to reactive power management ofPHEVs.

In this paper, we present a reactive power managementstrategy in a distribution network with a high penetration ofPHEVs to tackle with the negative impact of PHEVs into thegrid. We have used the reactive power management to obtainthe optimum voltage profile, as well as gaining the minimum

power losses and increasing the available transmissioncapacity in distribution network. The proposed strategy isimplemented on the 33-bus distribution network and itscapability is appreciated.

The rest of the paper is organized as follows: in Section II, charger is proposed, its structureand its active and reactive power capability are discussed. Insection III, required data and necessary assumptions forevaluation of the PHEVs effects on a distribution network are

proposed. Section IV is dedicated to the reactive powersupport using PHEVS. Section V is dedicated to thesimulation results, and finally section VI concludes the paper.

II. ELECTRIC VEHICLE CHARGERS

Charger is an interface between the network and vehicle.Basically, it converts AC to DC to charge the battery. Also,the charger could be designed and implemented in such a waythat it would be able to control its active and reactive power[8]. The rest of this section is dedicated to the explanations ofthe mentioned control features of a charger.

2015 23rd Iranian Conference on Electrical Engineering (ICEE)

978-1-4799-1972-7/15/$31.00 c2015 IEEE 1591

7/21/2019 07146473.pdf


A. Structure of the chargers

Different structures have been used for EV chargers. But all ofthem are common in the two main sections, namely AC-DCconverter and DC-AC converter [8]. A charger could bedesigned just to transfer the electricity power from the nidirectionalcharger. Adding to this, if the charger is able to inject the

idirectional [8].

Unidirectional charger is someti -corrected is type of chargers are designedto operate at unity power factor. Bidirectional chargers are Four-quadrant chargers could be transferred between the network and the battery infour quarters of the P-Q system (as discussed in section 2-4).To compare the capabilities of different chargers in reactive

power support, two different types of charchers, namely powerfactor-corrected (unidirectional) charger (Fig. 1), and four-quadrature (bidirectional) charger (Fig. 2) are used and their

bahavior in reactive power support are compared in the testresult section.

Figure 1. Structure of a power factor- corrected (unidirectional) charger [8].

Figure 2. Structure of a Four-quadrature (bidirectional) charger [8].

B. Analysis of charger active and reactive power

As mentioned, focus of this paper is on the bidirectionalcharger structure (Fig. 2) and in this subsection its active andreactive power transfer is analyzed. To do so, the network isconsidered as an ideal voltage source, , with a fixed

magnitude, , and constant frequency, , as presentedin (1):

( ) 2 | | .sin( )net nett V t (1)

The Thévenin equivalent of the bidirectional charger, in point of view, is shown in Fig. 3. It should be noted that is a harmonic distorted voltage. Switching operation ofthe power electronic elements of the charger generatesharmonics. In this study we just considered the mainfrequency and ignored the other relatively small magnitudes ofthe higher harmonics [10-11]. So, the instantaneous Théveninvoltage of the charger could be written as (2):

( ) 2 | | .sin( )ch cht V t (2)

Where is the voltage magnitude of the charger and

is phase difference between and .

Using phasors in steady state condition, network andcharger voltages could be written as (3) and (4), respectively.

( ) | | 0net net

j V (3)

( ) | |ch ch

j V

(4)According to Fig. 3, charger current, , could be formulatedas (5).

( ) ( )( ) | |net ch

line line

line

V j V jI j I

X

(5)

Where, is the line reactance between the network and the

charger. Obviously, and are the magnitude and phaseangle of the , respectively. Resistance of this line isomitted and the corresponding losses is added to the internallosses of the charger, as will be discussed on subsection II.C.

According to Fig. 3, the complex power, delivered by thenetwork to the charger could be written as (6):

*( ). ( )net net line net netS V j I j P jQ (6)

Therefore:

sin..

line

chnet

netX

VVP (7)

)cos..( chnet

line

net

net VVX

VQ (8)

Figure 3. Equivalent circuit of the network and the charger.

C. Active and reactive power losses of the charger

Losses in a charger are include internal active power losses of

the charger, and active and reactive power losses of theconnector between the charger and the network. Internal active

losses are due to diodes and switches that are exist in charger

circuit. The coupling impedance of the charger consists of a

resistance and a reactance. The former causes active power

losses and the latter is a reactive power absorber. So, the total

delivered active power to the charger, , is equal to the

battery power, , and related active losses, . Also,

absorbed reactive power from the network,

, is equal to the

charger reactive power (reactive power in a and b points at


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Fig. 2),

, and reactive absorption in , which is denoted

by

. Losses equations are formulated in (9) and (10).

chloss ba tnet PPP

(9)

chlosschnet QQQ

(10)

Active and reactive power losses of the charger could beexpressed as a function of the absorbed active and reactive

power from the network [11], as illustrated in (11) and (12).

),( netnetch

loss QPf P (11)

),( netnetchloss QPgQ

(12)

In this paper, and

are estimated as linear functions

of and

as follows.

netim phnet

r ph

chloss QPP ..

(13)

).( netnet phchloss QPQ (14)

where ,

and

are losses factors of the charger.

D. The capa bility curve of a bidirectional charger

According to (8) and (9), and are controllable

variables. Therefore, by changing and , active andreactive transfer power between the charger and the networkwill be controlled, as summarized in Fig. 4 [8]. As it is shown,the charger could be operated in four modes (areas). In areas Iand IV the charger is charging the battery. So, the chargerabsorbs real power from the network. And in areas II and III,

battery delivers real power to the network. In the reactive point of view, the charger acts as an inductor in areas I and II(absorbs reactive power), and is as capacitor in area III and IV(generates reactive power).

In Fig. 4, the maximum apparent power which could beabsorbed from the network is indicated by a circle. The radiusof this circle could be calculated by (15):

maxmaxmax. linenetnet IVS

(15)

Figure 4. Charger operation modes [8].

Where, and

are the maximum allowed voltage ofthe charger, and the current rating of the interface betweencharger and network, respectively. Therefore, the maximumreactive power should satisfy (16):

22)()(

maxnetnetnet PSQ

(16)

According to Fig. 4, a bidirectional charger could be

treated as a PV bus in the network; because, it is able tomanage its reactive power generation or absorption, and isable to adjust its voltage magnitude. On the other hand, active

power demand (in charging mode) or active power injection tothe grid (discharging mode) is controllable through differentswitching schemes that is not considered in this paper.

III. DATA AND NECESSARY ASSUMPTIONS

Essential data required to determine PHEVs characteristicsand their effects on distribution networks are include plug intime (the , remainedamount of energy in the battery at plug in time, needed time

and energy for a full battery charge, number of PHEVs perhouse, vehicle type and the connection node in the distributionnetwork. These data and assumptions are discussed in thefollowing subsections.

Plug in time

Start charging depends on thevehicle owner . Based on [12], weassumed that the PHEV owners plug in their vehicles whenthey arrive home after their last trip. Fig. 5 shows the

percentage of vehicles versus arrival time at home during anormal weekday of summer [12]. In this paper, we have usedthese data as starting time for PHEVs. It could be seen in Fig.5 that peak arrival time of vehicles occurs between 16:00 to20:00.

Battery state of the charge

The amount of energy released from a battery of a PHEV isdepend on the distance which PHEV is driven and the streets

traffic. A battery has a specific capacity, C in kWh, and itsconsumption energy could be shown by . As a fuel meter incombustion engine vehicles, State of the Charge (SOC) of a

battery could be used as a remained energy in the battery, as itis formulated in (17):

CSOCEc ).( 1 (17)

Figure 5. Percentage of vehicles versus their home arrival time for a regular

weakday of summer [12].


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And SOC could be calculated using (18):

AER for AER

SOC dd

)(1 (18)

Where, is the traveled distance and All Electric Range(AER) is the total distance that could be traveled with a fullcharge battery. could be extracted from such a data

presented in Fig. 6. As it could be seen in Fig. 6, percent of

vehicles and cumulative percent of vehicles are drown versusdriven distance in a regular summer weekday [12].

Figure 6.

Percent of vehicles entered to the network based on driven distanceduring a regular summer weekday [12].

PHEV type

Type of PHEV determines battery capacity, AER and electric

energy consumption per driven kilometer (or mile). Different

types of PHEVs are listed in [8] and [12]. In this paper we

have used [12

IV. R EACTIVE POWER SUPPORT USING PHEVS

The main objective of this paper is to show ability/disability of

PHEVs in reactive management and control in distribution

systems with a high penetration of such type of vehicles. To

do so, we will show how PHEVs could be coordinated andmanaged in such a way that operation of a distributionnetwork could be handled and all operation constraints be

satisfied. Using a dynamic program over 24 hours, we

determine active and reactive dispatch of PHEVs in a

distribution network. We obtain the operation area of a

bidirectional charger, i.e. when a battery should be charged

and when it should be discharged. And also we determine the power factor operation (lead, unity or lag) of the charger. The

method is successfully tested on the 33-bus distribution

network, as reported in the following section.

V. SIMULATION AND RESULTS

A.

Case study

To investigate the penetration impact of PHEVs on the grid,

the radial 33-bus distribution system is considered [13], as

shown in Fig. 7. The network is considered as a residential and

balanced three-phase system. The number of houses on each

bus is illustrated in Fig. 8. Power factor of a residential user is

usually between 0.9 to 0.95 (lag). We have chosen 0.92 (lag)

as an average amount for power factor. Assumed daily load

curve for each house is shown in Fig. 9 (Each time interval in

time axis in Fig. 9 and other figures is equal to 15 minuts. 24

hours is divided into 96 of 15 minutes intervals).A 3.3 kVA unidirectional charger, and a 4.6 kVA

bidirectional charger are used in our numerical and simulation

studies. Loss factors,

and

are considered 0.09,

0.0475 and 0.02, respectively. and charging time are listedin Table I, based on the battery capacity. According to Fig. 5and charging time of PHEVs, the total number of PHEVs that

plugged in into the network will be determined during anytime interval, as demonstrated in Fig. 10.

In this paper, we have assumed that the number of PHEVsare almost equal to the number of houses. Based on this, wedefine three categories as follows:

Group 1: If the number of houses are between zero to 25houses, 21 PHEVs are considered on that bus.

Group 2: If the number of houses are between 26 to 45 houses,30 PHEVs are considered on that bus.

Group 3: If the number of houses are between 46 to 75 houses,60 PHEVs are considered on that bus.

We assumed that he bidirectional charger should havereactive power support, while we expect to have the sameactive power charging current as unidirectional charger. Thatis why the rating of bidirectional charger is chosen larger thanthe unidiractional charger rate. Two specific scenarios areconsidered as follows:

Case I: Study of the distribution network, considering PHEVsarmed with power factor- corrected chargers with the different

level of penetration ( 0chQ ),

Case II: Study of the distribution network, consideringPHEVs armed with bidirectional chargers in capacitor mode

with the different level of penetration ( 0

chQ )

Figure 7. Radial 33-Bus test feeder [13].

Figure 8. Number of houses on each bus in 33-Bus test feeder.


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Case I- Results: Figures 11 and 12 along with Table IIsummarize the results of case I. The daily apparent powerdelivered into the distribution network (through bus 1 in Fig.7) is shown in Fig. 11. It could be seen that with the high

penetration of the PHEVs in the network, peak load of thesystem will be increased. It should be noted that peak of plugin time of the PHEVs were about peak period of the network(Fig. 5 and Fig. 10). So, over the peak time period, some of

the distribution branches could be congested and plug in ofmore PHEVs (or other electric devices) into the network will

be restricted. In this special case (case I) the maximum penetration of the PHEVs into the network is 14.5%. Voltage profile of 33 buses at peak load moment is shown in Fig. 12.As it could be seen in Fig 12, in this moment there are a few

buses with an unacceptable voltage magnitude (under 0.95 perunit).

Figure 9. Daily load curve for each house [12].

TABLE I. CHARGING TIME AND D, BASED ON BATTERY CAPACITY

C C 8 kW 8 kW C 12 kW 15 kW C

Time charging (h) 3 4 6

d AER 0.8 AER 0.75 AER

Figure 10. The total number of PHEVs entered to the network.

Figure 11. Daily apparent power of the distribution network.

At the maximum penetration of PHEVs (14.5%), the

minimum voltage magnitude reaches to 0.9148 per unit at bus

18. Table II compares energy loss for different penetration of

PHEVs. Obviously, with the more penetration of PHEVs the

losses will be more.

Figure 12. Voltage profile at the peak load moment.

TABLE II. ENERGY LOSS FOR EACH PENETRATION OF PHEVS.

No PHEVs 10% PHEVs 14.5% PHEVs

11337.1 kWh 11644.1 kWh 11782.5 kWh

(a)

(b)

(c)

Figure 13. Comparion of case I and case II for 14.5% penetration of PHEVs,

(a): Daily apparent power of the distribution network, (b): Voltage profile at peak load period, (c): Network active losses.

0 10 20 30 40 50 60 70 80 90 1000.5

1

1.5

2

2.5

3

3.5x 10

-3

time (15 min)

L o a d P o w e r ( p u )

Active Power

Apparent Power

0 10 20 30 40 50 60 70 80 90 1001

1.5

2

2.5

3

3.5

4

4.5

time (15 min)

A p p a r e n t P o w e r ( p u )

NO- PHEVs

10% PHEVs

14.5% PHEVs

S max

0 5 10 15 20 25 30 350.9

0.92

0.94

0.96

0.98

1

Bus

V o

l t a g e ( p u )

NO- PHEVs

10% PHEVs

14.5% PHEVs

V min

0 10 20 30 40 50 60 70 80 90 1001

1.5

2

2.5

3

3.5

4

4.5

Time (15 min)


NO-PHEVs

14.5% PHEVs for Case I

14.5% PHEVS for Case II

S max

0 5 10 15 20 25 30 350.9

0.92

0.94

0.96

0.98

1

Bus

V o l t a g e ( p u )

NO PHEVs


14.5% PHEVs for Case II

V min

0 10 20 30 40 50 60 70 80 90 1000.06

0.08

0.1

0.12

0.14

0.16

0.18

0.2

0.22

Time (15 min)

A c t i v e P o w e r L o

s s ( p u )




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Case II- Results: The same procedure as case I is done, andthe results are presents in Fig. 13-a to 13-c and Table IIIconsidering a 14.5% as the PHEVs penetration. In this case,

bidirectional charger is operated at its maximum apparent power, and also it is in the capacitive mode while charging the battery (mode IV in Fig. 4). It could be seen that operationindices are improved. According to Table III, apparent powerat peak load with a 14.5% as the PHEVs penetration, is

reduced by 2.83% comparing to the results of case I (using power factor-corrected chargers). In case 2, the maximumPHEVs penetration reaches to 28.8% (a 98.62% increasecomparing with case I). The daily apparent power and voltage

profile (for the peak load moment) are illustrated in Fig. 14and 15, respectively.

TABLE III. PERCENT IMPROVEMENT OF NETWORK PARAMETERS IN CASE

II FOR PENETRATION OF EVS 14.5%.

Network parameter

Energy loss(kWh)

Peak Powerloss (pu)

Minimumvoltage

(pu)

Linecapacity

(pu)

Case I 11782.5 0.2176 0.9148 4.2

Case II 11544.7 0.2087 0.9185 4.081

Percentimprovement 2.02% 4.09% 0.4% 2.83%

Figure 14. Daily apparent power of the distribution network, case II.

Figure 15. Voltage profile at the peak load moment, Case II.

I. CONCLUSION

In this paper we presented impact of PHEVs penetration intothe voltage profile, losses and congestion of the distributionnetworks. Two different types of chargers are used anddiscussed in this paper, namely power factor-correctedchargers and bidirectional chargers. The results show that

bidirectional chargers are more successful to control reactive power and voltage profile in the network. Obviously, thelower losses and higher available transfer capability willresult, if PHEVs are equipped with bidirectional chargers arethan power factor- corrected chargers.

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0 10 20 30 40 50 60 70 80 90 1001

1.5

2

2.5

3

3.5

4

4.5

Time (15 min)


NO- PHEVs

28.8% PHEVS for Case II

S max

0 5 10 15 20 25 30 350.9

0.92

0.94

0.96

0.98

1

Bus

V o l t a g e ( p u )

No- PHEVs


V min


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