Abstract —To stabilize the PV-connected grid with high penetration factor, the State formulates strict provisions on connecting the PV power stations into the power grid to ensure the safety of power system. On the basis of high-power PV power station LVRT technology, this paper elaborates the research on solving the inverter protection upon low-voltage operation by means of the control strategy under conditions of not adding other hardware to fulfill the safe ride through in the low-voltage zone, and to provide the power grid with reactive support, so as to help the recovery of grid system. Key Words —PV power generation; grid-connected inverter; grid Fault; LVRT

I. INTRODUCTION

The connection of numerous PV power stations into the power grid poses a severe challenge to the safe and stable operation of power system. In the area with large-scale connection of PV power stations into the power grid, when the grid is in fault, taking passive protective decoupling measure would lead to great reduction in the PV power station active output for the sake of the safety of PV system, adding the difficulty in the recovery of the whole power system, and the fault might be even aggravated, causing the decoupling of other units till large-scale power failure. Hence, the new grid connection rules require that the PV power stations must have the active and reactive regulation ability that the conventional power stations possess, and demanded that certain period of uninterrupted grid connection must be maintained upon grid fault to promote the recovery of power system [1-2]. To solve the control strategy of PV inverter during the grid fault, this paper will study the grid-connected control strategy of PV power stations with reference to the single-stage grid-connected inverter, and simulate the LVRT scheme by introducing MATLAB and verify the feasibility and effectiveness of the scheme through experiment based on the platform for 500kW PV inverter experiment.

II. STRUCTURE OF PV GRID-CONNECTED SYSTEM An ideal PV cell may be expressed by the equivalent circuit model of current source parallel diode[2]. This equivalent circuit model may be used to derive the standard equation, thus obtaining the output characteristic curve of PV cell to

simulate the output characteristics of PV array, as shown in Fig.1. The PV system under discussion, adopting the single-stage three-phase grid-connected inverter structure, comprises PV array (formed by several PV cells in serial and parallel connection), inverter, filter and transformer, and its topological structure is shown in Fig.2. In the PV grid-connected control, the algorithm of tracking the maximum power point is adopted to improve the utilization efficiency of PV cells, and proper inverter control strategy is selected to raise the power quality of AC side grid connection. When the power fault leads to the PCC (Point of Common Coupling) voltage sag, the LVRT ability of PV system under such structure will be subject to design and research.

Pmax

P/kW

i/A

Udc/V Fig.1 PV array output characteristic curve

Cdc

PV

Inverter

Linv Lg

Cf

Transformer Grid

Fig.2 Topological Structure Diagram of single-stage Three-phase Grid-

connected PV inverter

III. FAULT RIDE THROUGH REQUIREMENTS ON PV POWER STATION

The LVRT requirements of PV power station in the new grid connection rules are: when different types of faults (including three-phase short circuit, two-phase short circuit and single-phase earth fault) occur in the power system, and the PCC voltage exceeds to the bottom line of the curve, the PV power stations cannot be disconnected from the power grid.

Research on the Control Strategy of PV Grid-connected Inverter upon Grid

Fault Chen Yaai1, Liu Jingdong1, Zhou Jinghua1, Li Jin1

(North China University of Technology · Inverter Technologies Engineering Research Center of Beijing, Shijingshan District, Beijing 100144)

1Chen Yaai, Professor, Research interests include power electrics and electric drives and new energy generation control technology.

1Liu Jingdong, Graduate student, Research interests include new energy grid-connected power generation technology. 1Zhou Jinghua, Associate Professor, Research interests include Multi-level power converter technology and new energy

generation control technology. E-mail: ljd63360@163.com

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2013 International Conference on Electrical Machines and Systems, Oct. 26-29, 2013, Busan, Korea

978-1-4799-1447-0/13/$31.00 ©2013 IEEE

According to the current standard and proposed new standard

[3], the LVRT curve requirement is shown in Fig.3, in which curve 1 is the current standard, and curve 2 is the proposed new standard. During the fault ride through period, the active power for any PV power station which is not disconnected should at least restore to the value before fault at a change rate of 10% rated power per second. In the meantime, for the PV power stations, of which the total installed capacity can reach millions of kW or above, the PV power stations should have the ability of dynamic reactive support during LVRT.

Fig.3 LVRT Ability Requirement of PV power stations

IV. LVRT FULFILLMENT On condition of stable grid voltage, the DC power output by the PV array should be transformed via the inverter control into the high-quality three-phase AC power with stable current, good sine degree and low harmonic content. Besides, the power factor should meet the grid connection requirement. To fulfill this objective, the dual closed loop control with DC side voltage outer loop and grid-connected current inner loop is usually adopted [4]. The grid fault will cause the balanced or unbalanced sag of PCC voltage. It may be known from the power balance relation that the voltage sag will lead to the increase of the grid-connected current, and when it exceeds the rated value, it might cause over-current, endangering the system safety; with regard to voltage unbalanced sag, a series of problems, such as inaccurate phase locking caused by the disordered phase-locked signal and the fluctuation of d, q-axle reference current values, may happen. These problems would lead to the distortion of grid-connected current, affecting the uninterrupted grid connection of PV power stations; thus, the redesign of phase lock and positive and negative sequence separation technique are introduced.

A. Design of phase locked loop The unbalanced grid voltage sag or frequency change will lead to such problems as phase-locked angle deviation or poor sine degree of phase-locked signal. As the quality of phase-locked signal directly influences the waveform output quality of grid-connected current, the phase locking problem upon asymmetric grid fault may be solved through the grid voltage positive sequence component phase locking[5-6]. According to the symmetrical component method, the unbalanced three phase voltage may be decomposed into balanced positive sequence component and negative sequence component. The positive sequence component is taken as the input signal of phase locked loop, thus getting the accurate phase angle of positive sequence component upon unbalanced grid voltage.

B. Positive and negative sequence separation technique To suppress the negative sequence component of grid-connected current and obtain the correct phase-locked signal, the positive sequence component should be extracted from the unbalanced grid voltage in the control. Since the d, q-axle components under the rotating coordinate system may be acquired from the unbalanced three phase voltage through coordinate transformation, and the equation after its transformation may be

⎥⎦

⎤⎢⎣

⎡⎥⎦⎤

⎢⎣⎡

−−−−−+⎥

⎦

⎤⎢⎣

⎡=⎥⎦

⎤⎢⎣⎡

nq

nd

pq

pd

q

d

)2cos()2sin()2sin()2cos(

ee

ee

ee

θθθθ

(1)

It can be seen from equation (1) that the d, q-axle components of grid side voltage consist of direct current component and double frequency component. The extraction mode of double frequency notch filter may be used to filter the negative sequence double frequency component on the right side and get the positive sequence component. Its functional block diagram is shown in Fig.4; the same mode can be applied to the extract negative sequence component.

du

qu

pdupqu

paupbupcu

θ

au

bucu

Fig.4 Positive Sequence Extraction Method Based on Double Frequency

Notch Filter

C. LVRT control strategy 1) CONTROL PRINCIPLE

During LVRT, to effectively ensure the stable grid-connected current and achieve three-phase balance, the control strategy of suppressing the negative sequence current may be adopted [7-8]. This control algorithm maintains the three-phase balance by regulating the AC side negative sequence current component to be 0, and ensures the AC side only contains the positive sequence current component. The control block diagram is shown in Fig.5, in which the computational equation of d, q reference current values of current inner loop is

⎥⎦⎤

⎢⎣⎡⎥⎦

⎤⎢⎣

⎡ −+

=⎥⎦

⎤⎢⎣

⎡

0

0pd

pq

pq

pd

2pq

2pd

*q

*d

])()[(32

QP

eeee

eeii

(2)

Where, P0 and Q0 respectively stand for the average active power and reactive power output by the grid-connected inverter. Considering the actual conditions, the inverter is connected into the power grid with the unit power factor, and now the reactive power is 0. Thus, in the program design, the power calculation may be removed, while the outer loop directly sets the positive sequence d axle current instruction value. After grid voltage sags, reactive compensation is realized through reactive positive sequence q axle current setting, i.e. the voltage outer loop may adopt the outer loop setting in Fig.5. During LVRT, the amplitude of grid-connected current should be limited to an extent to protect the grid-connected inverter devices. When it reaches the amplitude limit, the

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power output by the inverter will decrease to maintain the power balance, and the outer loop setting will be adjusted accordingly. Considering the output characteristic curve of PV panel shown in Fig.1, the voltage of DC bus will rise while the output power will decrease, till the open circuit voltage. The

output power is 0 now, thus, during PV LVRT, it would not cause the over-voltage of DC bus. And this is also the advantage of PV power station compared with the wind power generation.

Separating Positive and

Negative Sequence

SPWM

dcU

abce

L

gLpqeθ

fC

ω0s1

++Limit

[0,2π]

C_abci

pqe

Positive sequence current

regulator

PIdcU*

dcU +-MPPTdci

Negative sequence current

regulator

Calculation of Active and

Reactive Power

+

pd_refipq_refi

nq_refind_refi

pde

nqen

de

ndi n

qi

pdi p

qi

+

nqende

pde

ndi nqi

pdipqi

abci

pde

PI Separating Positive and

Negative Sequence

Fig.5 LVRT control principle Diagram Hence, during LVRT, the PV array will not operate at the maximum power point, and the MPPT control part may be disconnected. It should pay attention to the relation between DC bus voltage and output current during the fault ride through period when adopting outer loop control mode. To prevent the unmatched output setting, this paper adopts the disconnection of the outer voltage loop after detection of voltage sag, making the control system work in the constant current output mode; as to the requirements on reactive output in the grid connection rules, now the set value of active current should be decreased, and dynamic modification of the set reactive current should be adopted during fault to fulfill the output of reactive current.

2) REACTIVE SUPPORT The new PV grid connection rules provide that the PV system must have the ability of dynamic reactive support during LVRT, and the reactive current support should be related to the amplitude of grid voltage sag. The reactive support curve is shown in Fig.6. Thus, based on this, the setting of reactive power Q (or iq) can be adjusted by predicating the amplitude of voltage sag. The reactive current value, injected by PV power stations into the power grid and the value of which depends on the required amplitude limiting requirements of the total output current, can be modified in real time. With regard to the requirement of real-time reactive current output, assuming that the maximum reactive power is injected into the grid side, the maximum set active current cannot exceed the limit imax, which shall meet

2*

q2

max*d )()( iii −≤

(3).

The control strategy designed in this paper fulfills LVRT during the fault ride through by means of the grid connection mode which focuses on reactive current and it is supported by the active current output.

Rea

ctiv

e C

urre

nt S

uppo

rt（pu

）

Fig.6 Reactive Support Curve

3) ACTIVE RECOVERY When the grid fault is removed, and grid voltage resumes to 0.9pu, now through adjusting the setting value of d-axle current and clearing the q-axle current, the voltage outer loop is connected with the set voltage resuming to the value before fault at a slope from the current bus value; then, the MPPT function is initiated, and the LVRT state ends.

V. SIMULATION, EXPERIMENT AND RESULT ANALYSIS To meet the needs of research subject, the simulation research on the 500kW PV inverter is carried out firstly. Parameters are as follows: PV capacity: 500kW; PV array open circuit voltage: 720V; DC bus capacitor: 6600�F; filter inductance: 0.17mH, 0.05mH (grid side); filter capacitor 200�F; grid side rated current 1500A, and permissible maximum current 1650A.

A. Simulation and analysis of PCC voltage sag fault The system simulation model is built under the MATLAB environment. The simulation research on the voltage balanced and unbalanced sag of PCC is implemented. In the simulation experiment, the controlled voltage source is used to replace the power grid. Its advantages are that it may simulate the amplitude, phase, phase angle variation and adjustment of switching time of the three phase voltage. When the power grid has the three phase voltage balanced

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sag (the three-phase voltage sags to 20%pu), as shown in Fig.7 (a), the proposed control algorithm is applied. When the fault takes place, the grid-connected current decreases, and now the DC bus voltage increases to maintain the power balance (see Fig.7 (b)); the grid-connected current waveform during fault is shown in Fig.7(c). It may be seen from the figures that during the fault ride through, the DC bus and grid-connected current can fulfill safe ride through according to the set target.

t/s

Fault time eabc Fault time

Fig.7(a)Grid-voltage sags waveform Fig.8(a)Grid-voltage sags waveform

t/s

t/s

Fault time Udc Fault time

t/s

Fig.7(b) DC bus voltage waveform Fig8.(b) DC bus voltage waveform

t/s

Fault time iabc Fault time

t/s Fig.7(c) grid-connected current waveform Fig.8(c) grid-connected current waveform

Fig.7 grid-connected current Waveform Under three-phase balanced sag Fig.8 grid-connected current Waveform Under three-phase Unbalanced sag

In case of unbalanced grid voltage sag, this paper takes the three-phase unbalanced sag as an example in the simulation verification. The set PCC voltage sag waveform is shown in Fig.8 (a). Now, the DC bus voltage and grid-connected current waveforms are shown in Fig.8 (b) and (c). It may be known from the above figures that the DC bus will have double frequency fluctuation. Under the parameter conditions set in the paper, the grid-connected current approaches the three-phase balance, and keeps high sine degree, conforming to the theoretical design effect. The simulation results above show that as to the depth of PCC voltage and unbalanced sag conditions, this scheme can fulfill uninterrupted grid connection and provide reactive support when it is above the curve 1 in line with the grid connection rules.

B. Experiment and analysis of PCC voltage sag fault Under the equal conditions, experiments are made on the basis of the 500kW experiment platform. Fig.9 shows the grid line voltage eab, DC bus voltage Udc and grid-connected current ic waveforms upon 20% of three-phase sag under light load 140kW; Fig.10 shows the experiment waveform upon 20% of single-phase sag under full-load 500kW.

t/s

eab

Udc

ic

(a) Instant waveform upon 20% of Light Load three-phase sag

t/s

eab

Udc ic

(b) Instant waveform upon 20% of Light Load three-phase resumption

Fig.9 Light Load three-phase 20% sag waveform Analysis

t/s

eab

Udc

ic

Fig.10 Heavy Load Single-phase 20% sag waveform Analysis

Fig.9 (a) shows the instant waveform when sagging, and Fig.9 (b) indicates the resumption instant waveform after sagging. It may be known from figures that upon the voltage sag due to the grid fault, the grid-connected current will fulfill dynamic switching according to the set curve of control strategy, and the grid-connected current will decrease, and send reactive power; the DC bus voltage quickly rises to the open circuit voltage; it will operate at this stage till the recovery from the fault. When the voltage resumes, the active power will recover according to the designed state. After the whole state ends, the MPPT function resumes. Fig.10 is the full-load single-phase sag waveform. It is found through experiments comparison that the experiment result is consistent with the simulation conclusions, indicating that the proposed control strategy can fulfill safe ride through during the grid fault.

VI. CONCLUSIONS Upon the grid fault, the PV grid-connected inverter adopts the fault ride through control strategy proposed in this paper, and the grid-connected current will maintain three-phase balanced output. According to the requirements of the new PV grid-connection rules, it should keep uninterrupted grid connection on condition of protecting the system safety and ensure the grid-connected current with low THD; this control mode makes that the PV grid-connected inverter possesses the LVRT ability, and can adapt to uninterrupted grid connection upon the grid voltage fault under the extreme conditions, thus meeting the provisions on connecting the PV power stations

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into the power grid.

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