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Procedia Computer Science 86 (2016) 309 - 312

2016 International Electrical Engineering Congress, iEECON2016, 2-4 March 2016, Chiang Mai,

Thailand

A Voltage Rise Mitigation Strategy under Voltage Unbalance for a Grid-Connected Photovoltaic System

OENG Lysornga*, SANGWONGWANICH Somboonb

Dept. of Electrical Eng., Faculty of Eng., Chulalongkorn University, Bangkok, Thailand 10330

Abstract

This paper analyzes the effect of unbalanced voltage on voltage rise for a grid-connected photovoltaic (PV) system, and proposes a voltage rise mitigation strategy which injects negative-sequence currents to compensate the unbalanced voltage aiming especially to reduce the voltage rise. The paper also discusses how the averaged powers related to the injected negative-sequence currents are measured by the power meter. Simulation result confirms the superior performance of the proposed strategy over the classical method which suppresses the voltage rise by adjusting the power factor.

© 2016 The Authors. Published by ElsevierB.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/4.0/).

Peer-reviewunderresponsibilityof theOrganizingCommitteeof iEECON2016

Keywords: grid-connected photovoltaic inverter; unbalanced voltage; voltage rise; negative-sequence current injection.

1. Introduction

The occurrence of voltage rise due to the injection of power from a PV system into the power system network is a major problem for high penetration of PV generation. It is also observed that the voltages at the point of common coupling (PCC) are usually unbalanced due to the unbalanced loads in the system. To keep the system in healthy condition, the limits around 5% for voltage rise and 2% for voltage unbalance are required. One popular method to solve the voltage rise problem is by injection of reactive power in positive sequence. However, this is uneconomic because the PV owner may be charged for the injected reactive power. Also, some works have been done on unbalanced voltage compensation by using strategies such as injection of active or reactive power [1-2]. But, their

* Corresponding author. Tel.: (662) 218 6550; fax: (662) 2518991. E-mail address: somboona@chula.ac.th

1877-0509 © 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the Organizing Committee of iEEC0N2016 doi:10.1016/j.procs.2016.05.084

objective is rather to improve the power quality than to solve the voltage rise problem. In general, the voltage rise and voltage unbalance are treated as two unrelated issues.

However, in this paper, it is revealed that voltage unbalance usually contributes significantly to the maximum line-to-line voltage which is the criterion for voltage rise or overvoltage. Therefore, the voltage rise over the limit is in fact caused by two factors; the reverse power flow and the voltage unbalance. With this understanding, it is proposed in this paper that cancellation of voltage unbalance by negative-sequence current injection should be done first to reduce the maximum line-to-line voltage. And if this measure helps to bring the voltage back within the limit, then no further action is needed. Otherwise, additional (positive-sequence) reactive power injection or active power curtailment may be necessary. The advantages of the proposed voltage rise mitigation strategy are three folds. Firstly, the voltage rise is reduced. Secondly, the power quality is improved because the PCC voltage becomes balanced. Thirdly, since the active and reactive powers introduced by the negative-sequence currents are very small, it is more economic compared with the method of reactive power injection. In summary, the aims of this paper are (i) to analyze quantitatively the relationship between the voltage unbalance and the voltage rise, (ii) to discuss the effects of negative-sequence current injection on the power metering, and (iii) to confirm how the proposed strategy which prioritizes the voltage unbalance compensation can solve the voltage rise problem.

2. Relationship between unbalanced voltage and voltage rise

This section analyzes the effect of voltage unbalance based on symmetrical component in order to quantify the relationship between the magnitude of the negative-sequence voltage and the maximum line-to-line voltage defining the voltage rise. Under unbalanced voltage condition, the line-to-line voltages represented by Vab, Vbc and Vca may be represented in terms of symmetrical components as shown in Eq. 1 where V°, V+ and V- are the voltage phasors of zero, positive and negative sequences, respectively and a = ej 2,1/3. For a three-phase three-wire system V0=°, and it can be derived that the magnitudes of the line-to-line voltages Vab, Vbc and Vca are given by Eq. 2. The magnitudes of the three voltages therefore depend on the phase angle a between V+ and V-.

|V ++ V " |v ++ a 2V "

V„ = V ++ aV -

Vmax( LL) — V

+ cos a • V

V - V+

' max(LL) — '

-cos a- V

Fig. 1(a)-(c) illustrate examples when 0<a<60° during which the maximum line-to-line voltage is Vab. By symmetry, the same can be said for -6°° < a < °. We can then show that the maximum line-to-line voltage Vmax(LL) satisfies Eq. 3. Normally, the ratio between negative-sequence and positive-sequence voltages is very small, so Eq. 3 can be approximated by Eq. 4.

V„b > V++ V- cos00 = V++ V- Vab ^ V* +

Vbc > V*+ V- cos2400 = V*- 0.5V' Vbc > V+ +

Vca > V+ + V~ cos1200 = V*- 0.5K" Vca > V* +

" cos300 = V- + 0.86K-- cos 2700 = V-" cos1500 = V0.86K-

Vab > V+ + V~ cos600 = V* + 0.5V-Vb > V++ V- cos3000 = V++ 0.5V~

3300 = V * + 0.86K-2100 = V0.86K-

Fig. 1. Phasor diagram showing magnitudes of line-to-line voltages under voltage unbalance for various a.

180° = V

When the phase angle a rotates further than 60o, the maximum line-to-line voltage will change from Vab to be Vbc or Vca instead. However, similar relations (Eqs. 3-4) are still valid. From this investigation, it can be concluded that under any unbalanced condition, the maximum line-to-line voltage (Vmax(LL)) is given by:

Vmax(LL) = V+ + cos«• V~ ^ V^ll) >V+ + kV for 0<a<600&k = 0.5 (5)

According to Eq. 5, it can be said that the voltage rise is caused by both positive and negative-sequence voltages. The maximum line-to-line voltage is always increased by the negative-sequence voltage by at least a factor of 0.5. As by the standard, the unbalanced voltage could reach 2% in the system, and this means that it will cause at least 1% of voltage rise which could not be neglected. Therefore, compensation of voltage unbalance by suppression

of negative-sequence voltage will help reducing overvoltage significantly, especially when the PCC voltage is heavily unbalanced.

3. Effects of negative-sequence currents to the power metering

Since the injection of negative-sequence currents to compensate voltage unbalance may introduce additional power flow to the PV system and lead to extra charge by the utility, it is thus necessary to clarify this effect and compare with the conventional reactive power injection method. There are two main methods for calculation of the reactive power, i.e. Time Shift and Power Triangle methods. In this paper the Time Shift method is adopted because it has been more frequently used in commercial metering [3]. Its principle is similar to active power calculation except that the phase of the voltage waveform is shifted by 900 before multiplying with the current waveform as shown in Eq. 6, where QTS is the total reactive power and T is period of fundamental frequency.

qts =l t j0 Vk r + 4 (i)dt (6)

k=a,b,c t V 4 J

If we decompose the voltage and current into positive and negative components as shown in Eq. 7., where the superscript +, - denotes the positive and negative-sequence components, respectively, then it can be derived that the total reactive power as measured by the power meter will be the summation of positive and negative-sequence reactive power, Q+ and Q- as shown in Eq. 8.

v* (t) = v+k (t)+v; (t)

X T fc

t + — I il (t)dt = 0

4 (t ) = i+t (t )+ik (t )

t+ T ) i1 (t )dt = 0

••• qts=.Z ^^jOv;^+4 i (?)di^^jOv.fi+4 h aw=e++Q

In general, Q+ because considering the same amount of active and reactive currents in positive and

negative sequences, the negative-sequence voltage is very small compared to the positive one. So, the compensation of unbalanced voltage not only reduces the voltage rise but also is more economic because the PV owner will be charged less due to the small amount of QTS.

4. Unbalanced voltage compensation

PV system is connected to the power system through a current-source inverter as shown in Fig. 2(a). The inverter feeds power from the PV panels to the network by injection of positive-sequence current, and compensates the voltage unbalance by injection of negative-sequence current. The negative-sequence voltage extraction scheme and the unbalanced voltage controller are shown in Fig. 2(b), and 2(c), respectively.

Negati ve sequence dq Voltage

Volt age ext raction

Current Source Inverter

4MWofPV Generator

Current Source

dq+/ abc 11:/

/flliC /dq-

Transformation indoublefrequency _ Q

Fig. 2: Unbalanced voltage compensation (a) PV system; (b) Negative-sequence voltage extraction; (c) Unbalanced voltage compensator

5. Simulation Results

The proposed voltage rise mitigation strategy is applied to a system which consists of a 4 MW PV farm connected to a 100 km-long distribution line using the simulation platform of MATLAB/ Simulink. Line impedance is assumed to be equal to 0.16+j0.33O./km . 6MVA three-phase load with 0.85 power factor lagging and three other single-phase loads are connected to the distribution line as depicted in Fig. 3.

30km_llna2

1MVA, PF=0.85

30km_llne3

1MVA, PF=0.85

10km_Une4 PCC

4MVA, PF=0.85

Fig. 3: Power system configuration used in the simulation (PV generation is connected at the end of the line).

The simulation results in Fig. 4 illustrate the PCC voltages and inverter currents before and after compensation by two different strategies, i.e. PF control and negative-sequence voltage control. The injection of power from PV generator and unbalanced load give rise to 5% of voltage rise over the 22 kV rated voltage and 1.6% unbalance voltage.

In Fig. 4(b), the power factor of the inverter is set to 0.98 lagging. The voltage rise has been reduced from 5% overvoltage to 0.6% under voltage. On the contrary, the unbalance voltage has been increased from 1.6% to 2.3%, which is over the allowable limit. Fig. 4(c) shows the performance of negative-sequence current injection. The voltage rise has been reduced from 5% to 4.3%, which is not as good as the PF control technique, but still it is in the allowable limit. On the other hand, the unbalanced voltage has been reduced from 1.6% to 0.18%, which could not be achieved by PF control technique.

>2.4r §2.3-

□l 2.

No compensation

No compensatoin

KmmmiMTk

> 2.3 0) i?2.2 Î5

> 2.1 O

4 4.41 4.42 4.43 4.44 4.45 4.46 4.47 4.48 4.49 4.5 Time (s) (a) No compensation

x 104 Compensation by PF regulation

eœeœe5e0

4.4 4.41 4.42 4.43 4.44 4.45 4.46 4.47 4.48 4.49 4.5 Time (s)

(d) No compensation

Compensation by PF regulation

j '80[±i±i±:l±:l±j

4.4 4.41 4.42 4.43 4.44 4.45 4.46 4.47 4.48 4.49 4.5 Time (s)

(b) PF control

> 2.2 O

1 A A | 1

4.4 4.41 4.42 4.43 4.44 4.45 4.46 4.47 4.48 4.49 4.5 Time (s)

(e) PF control

Compensation by negative sequence control __

amm/iama \\k

4.42 4.43

4.44 4.45 4.46 4.47 4.48 4.49 Time (s)

Negative-sequence voltage control

4.4 4.41 4.42 4.43

4.44 4.45 4.46 4.47 4.48 4.49 Time (s)

(f) Negative-sequence voltage control

Fig. 4: Simulation results showing PCC voltages (a)-(c) and inverter currents (e)-(f)

It can be concluded that the injection of negative-sequence current can reduce around 1.4% of unbalanced voltage and 0.7% of voltage rise, which equals to 50% of voltage unbalance as predicted by Eq. 5. In Figs. 4(e) and 4(f), the current injected by inverter in the case of negative sequence control equals almost to that of PF control. So, injection of negative-sequence current brings the PCC voltage back to the limit and becomes balanced with less active and reactive power.

6. Conclusions

4 4.41

This paper analytically reveals that at least 50% of the unbalanced voltage contributes to the voltage rise, and proposes that the voltage rise should be mitigated firstly by unbalance compensation through injection of negative-sequence currents. The effects of this technique on the power metering are also discussed, and it is clear that negative-sequence currents cause very small amount of both active and reactive powers. Finally, the correctness of the analytical results and the feasibility of the proposed strategy are confirmed by simulation.

References

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Inverters", IET Pow. Elec., 2014, Vol. 7, Iss. 5, pp. 1055-1063.

2. M. Savaghebi, J. M. Guerrero, A. Jalilian and J. C. Vasquez: "Secondary Control for Voltage Unbalance Compensation in an Islanded

Microgrid", In Proceedings of the 2nd IEEE Inter. Con. on Smart Grid Com., SmartGridComm 2011. (pp. 499-504).

3. U. Stephen, C. Vincent and K. Venkat: "Implementation of a Three-Phase Electronic Watt- Hour Meter Using the MSP430F471xx", TEXAS

INSTRUMENTS, SLAA409A, JUN. 2009.