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Research on Non-Directional Voltage Ride-through Control
Technology of Household PV Grid-Connected Inverter
CHEN Kun, ZHOU You-bin, XU Hua-an, DU Zhen-an, WANG Xiao-kai
State Grid Hubei Electric Power Research Institute, Wuhan 430077, China
I. INTRODUCTION
With recent rapid development and wide application of
photovoltaic power generation, its status improves
greatly. When the grid fails, the routine off-grid
operations on large-scale PV power supply cannot meet
the requirements of normal operation of power system.
The State Grid Corporation of China has made explicit
requirements for this problem in 2011[1]. Therefore, it is
vitally important to improve the ride-through capability
of PV grid-connected power.
The current research on grid-connected inverter
ride-through technology mainly focuses on wind power
low voltage ride-through (LVRT)[2], and the research on
PV grid-connected power ride-through technology also
focuses on LVRT in large-scale PV power plants[3], but
the corresponding high voltage ride-through (HVRT)
technology has not received enough attention. At present,
off-grid operation is applied according to relevant
regulations in China. Unlike wind power, PV
grid-connected power generation application is not
affected by geographical restrictions, and it is mostly in
urban distribution side, so it is often faced with
non-directional voltage fluctuations. As household PV
grid-connected power generation is widely used, once
the power grid suffers failure of sudden voltage rise, the
routine off-grid operation will do harm to the safe and
stable operation of the grid. Therefore, research on PV
grid-connected power HVRT also has great practical
significance.
This paper firstly analyses the current household PV
grid-connected power ride-through requirements in
China and proposes a non-directional ride-through
control strategy of household PV grid-connected inverter
based on voltage feed-forward control. Then, based on
analysis of grid-connected inverter output power control,
this paper has achieved dynamic adjustment of active
power output and reactive power compensation
according to needs, and the role and control principles of
Crowbar circuit in this strategy are described. Finally, the
feasibility and effectiveness of this strategy is verified by
the experiment in which sudden and non-directional
change of grid voltage is simulated.
II. ANALYSIS OF RIDE-THROUGH REQUIREMENT
At present, the State Grid clearly states, PV
grid-connected power should have certain capacity to
withstand voltage anomalies to avoid separation from the
grid, causing grid power loss, as shown in Fig. 1.
电网故障引起电压跌落
1.1
1.0
0.9
光伏电站必须
保持并网运行
U / (p.u.)
Abstract—On account of present situations in which the
government
greatly
promote
household
PV
grid-connected application and the increasing demands
for the control technology of grid-connected inverters,
this paper proposes a non-directional ride-through
control strategy of household PV grid-connected inverter
based on voltage feed-forward control and carries out
research on the active power output and reactive power
compensation. Based on voltage feed-forward control,
this strategy enables instantaneous tracking of grid
voltage to be achieved and suppresses sudden grid
current change caused by PI close-loop control delay and
power
balance
principle
when sudden
and
non-directional change of grid voltage occurs. Combing
research on the active power output and reactive power
compensation, this strategy is able to achieve dynamic
adjustment of active power output and reactive power
compensation according to needs. The feasibility and
effectiveness of this strategy is verified by the
experiment in which sudden and non-directional change
of grid voltage is simulated.
Index Terms—PV; grid-connected inverter; ride-through;
voltage feed-forward.
光伏电站可以从
电网切出
0.2
0
0
1
2
t/s
3
4
Fig. 1 The requirement for low voltage tolerant capacity of
PV power plant
As shown in Fig. 1, during the period when the grid
voltage drops to 20% of rated voltage and recovers
within 3s to 90% of rated voltage, PV power shall
continue to run, so the capacity of PV grid-connected
inverter LVRT directly determines the operating state of
PV grid-connected power and is also related to the
security and stability of the system.
Although China has not yet specific requirements for PV
grid-connected power HVRT, relevant regulations issued
by State Grid [4] can be used for reference, as shown in
Tab. 1.
for actual operation of the PV grid-connected power.
III. THE NON-DIRECTIONAL RIDE-THROUGH
Tab.1 The requirement for response time of
distributed power voltage
Voltage at PCC
CONTROL STRATEGY
Requirements
According to the PV characteristics of PV cells, the DC
bus voltage rise of PV grid-connected system will be
accompanied by reduced power output of PV cells, and
PV cell output power of the open circuit voltage will
drop to zero. Therefore, the restriction on grid-connected
current is generally considered as the key to ride-through
control research, as shown in Fig. 2.
Maximum breaker-separating time
U < 50%UN
≤ 0.2s
Maximum breaker-separating time
50%UN ≤ U < 85%UN
≤ 2.0s
85%UN ≤ U < 110%UN
Continuous operation
(1)UN is
the
Pmax
≤ 2.0s
Maximum breaker-separating time
135%UN ≤ U
Note:
P/kW
Maximum breaker-separating time
110%UN ≤ U < 135%UN
≤ 0.2s
rated
voltage
at
PCC;
(2)The
maximum
0
breaker-separating time refers to the time length from when abnormal
Udc/V
Fig.2 PV curve of Photovoltaic cell
conditions occur to when power supply to the grid stops.
Similar to LVRT, at the moment of sudden grid voltage
rise, the output voltage must be enabled to track the grid
voltage, and the grid-connected current must be
restricted in order to achieve PV grid inverter HVRT.
Therefore, this paper applies double PI close-loop
control of output voltage and grid-connected current as
the basic strategy for the non-directional ride-through of
household PV grid-connected inverter.
Take the single-phase bridge voltage source inverter as
an example. The block diagram of basic control principle
is shown in Fig. 3 where uo, io, ug are PV
grid-connected inverter output voltage, grid-connected
current, grid voltage respectively, Kvp, Kvi are PI
control parameters, Ks is the inverter gain, and Kg is the
grid voltage tracking compensation factor.
The following conclusions can be drawn from Tab. 1:
When the voltage of Point of Common Connection (PCC)
to PV power is within 100% ~ 110%UN, the grid
requires it to run continuously while staying connected to
the grid. If the PV grid-connected inverter does not have
HVRT capacity, the inverter itself will be seriously
damaged;
When the voltage at PCC to PV power reaches 110%UN
and above, the grid allows off-grid operation. If in this
case the PV power can continue to run uninterruptedly
while staying connected to the grid, it can significantly
reduce the number of PV power getting off-grid and
improve the stability of power grid operation.
Thus, the non-directional ride-through capability of PV
grid-connected inverter has great practical significance
Kvp+Kvi/s
1/L1s
Ks
+
+
-
uo
+
1/Cs
1/L2s
+
-
ug
io
Kg
Fig. 3 The basic principle diagram of non-directional voltage ride-through control
A. Instantaneous prediction algorithm
In order to achieve instantaneous tracking of the grid
voltage by the grid-connected inverter, this strategy
controls grid voltage amplitude and phase separately.
For tracking grid voltage amplitude, taking into account
that the PI closed loop is not non-static error tracking,
this strategy adopts instantaneous prediction algorithm [5]
in order to improve the instantaneity in tracking grid
voltage amplitude.
Assume that the sample values for sampling time t1, t2,
t3 are n1, n2, n3, wherein t3-t2=t2-t1=Tc，and Tc is
carrier period. According to Taylor series, the following
formula can be obtained:
f ( x3  t )
 f ( x3 )  f ' ( x3 )t 
1 ''
f ( x3 )t 2   (t 3 )
2
(1)
As Tc is small enough in actual control, the derivative in
Formula (1) be linearized as:
f ' x3 t  n3  n2
(2)
f ( x3 )t  (n3  n2 )  (n2  n1 )  n3  2n2  n1 (3)
''
2
Formula (2) and Formula (3) are placed into Formula (1),
and if ∆t=Tc, then there will be:
operating normally, namely:
f ( x3  Tc )
1
 n3  (n3  n2 )  (n3  2n2  n1 )
2
1
 2(n3  n2 )  (n3  n1 )
2
(4)
As shown in Formula (4), if the grid voltage is sampled
with Tc as the unit and taking ∆t=Tc, in there three
consecutive sampling values can be used to predict the
subsequent sampling value Tc of grid voltage, which will
compensate a Tc for PI closed-loop control delay.
It thus can be seen the compensation time length for PI
closed-loop control delay is determined by ∆t. By setting
∆t according to needs, the instantaneous value of future
grid voltage at ∆t moment can be predicted. However, as
this algorithm is based on linearization of the existing
sample values, the bigger Δt is, the greater the deviation
of the predicted result will be, which is not conducive to
accurate tracking of grid voltage amplitude. Meanwhile,
in order to increase the accuracy of this prediction
algorithm, the number of sampling values and expanded
terms in Taylor series can be increased.
As the grid voltage amplitude and the PV grid-connected
inverter output voltage amplitude have the same
sampling delay, the results of for comparison of both will
not be affected, but the grid voltage sampling delay will
cause judgment error of grid working state by this
algorithm. Therefore, the difference value between the
prediction value of grid voltage amplitude and its rated
value can serve as the criteria for whether the grid is
E (t )  U g (t )  u g (t )
(5)
wherein ug(t) is the instantaneous value of grid voltage at
t moment obtained by the prediction algorithm, and Ug(t)
is the rated instantaneous value of the grid voltage at t
moment obtained by looking up the table.
B. Voltage feed-forward control
Also, as the closed-loop control delay is affected by its
proportional factor and integral factor, it is not enough to
rely solely on the instantaneous prediction algorithm to
compensate. Therefore, sudden change of grid-connected
current caused by PI closed-loop control delay and the
power balance principle at the moment of sudden change
of grid voltage still needs to be taken into account. This
strategy applies feed-forward control [6] at the same time
to suppress sudden change of grid-connected current at
the above-mentioned moment.
As shown in Fig. 3, the transfer function of grid voltage
and grid-connected current is:
I o ( s) ( K g  1) K v K s  L1sCs  1

U g ( s) L2 s(1  K v K s )  L1sL2 sCs
(6)
wherein Kv=Kvp+Kvi/s.
According to the design principles of feed-forward
control, the block diagram after adding the feed-forward
control is shown in Fig. 4, in which
G1(s)=(Kg-1)KvKs-L1sCs-1,
G2(s)=1/(L2s(1+KvKs)+L1sL2sCs)。
Kg
G1(s)
G3(s)
+
-
+
Kvp + Kvi /s
Iref1
Kip
+
+
Ks
1/L1s
1/Cs
uo
+
G2(s)
io
Grid
+
-
-
+
+
ug
-
Fig. 4 The block diagram based on voltage feed-forward control
To simplify deduction, when Kg=1, the following can be
obtained from Fig. 4:
I o ( s) G1 ( s)G2 ( s)(1  L1sCs  G3 ( s) K s )

U g ( s)
L1s  1  G2 ( s) K v K s  L1sG2 ( s)
218V with a drop rate of about 30%, and the simulation
results are shown in Fig. 5.
(7)
As shown in Formula (7), to completely eliminate
disturbances of grid voltage to grid-connected current,
there is:
G3 ( s ) 
 1  L1sCs
Ks
(8)
Formula (8) is the transfer function of feed-forward
control of grid-connected current.
In order to verify the effectiveness of the feed-forward
control, LVRT is taken as example in simulation. Grid
voltage drop is simulated at 0.3s, the PV grid-connected
inverter LVRT, the peak voltage value falls from 311V to
(a)
(b)
Fig. 5 Simulation results
Fig. 5(a) shows that, before adopting the voltage
feed-forward control, sudden current change occurs at
the moment of sudden voltage change; Fig. 5(b) shows
that, after adopting the feed-forward control, sudden
current change is effectively suppressed at the moment
of sudden voltage change.
namely the amplitude difference.
From Formula (9) and Formula (10), the following can
be obtained:
V. RESEARCH ON ACTIVE POWER OUTPUT AND
REACTIVE POWER COMPENSATION CONTROL
2
A. Analysis of active power output and reactive power
compensation control
If PV grid-connected system is equivalent to a voltage
source, there will be the equivalent circuit of PV
grid-connected system as shown in Fig. 6, in which L is
the joint inductance between PV grid-connected inverter
and the grid, R is the equivalent internal resistance of PV
grid-connected system, Uo∠ δ is the output voltage of PV
grid-connected inverter, and Ug∠ 0° is the grid voltage.
L
U
 U gU o 
  P 2  g  Q 
 
X
 X 
2
2
U
 U gU o 

  P 2  g
X
 X 
2
(11)
As shown in Formula (11), when P=0, the output or
absorbed reactive power reaches maximum, and it is
related to inverter output voltage, voltage at PCC, filter
inductor and the active power output then. It is restricted
by the rated capacity limits of PV grid-connected
inverter.
And the steady state vector correlation on the AC side of
grid-connected inverter is shown in Fig. 7

Uo∠δ
Ug∠0°
uo


uL
io
R


Fig. 6 The equivalent circuit of PV grid-connected system
Q
U oU g
X
U oU g
X

uo

uL

ug
(b)


ug
uo


io
ug

s i n
cos 

uL
(a)
The formula of output power from the PV
grid-connected inverter to the grid can be obtained from
Fig. 6 as follows:
P
ug
io
(9)
U g2
io
X

uL
(c)
(d)
Fig. 7 AC steady state vector of inverter
(10)
in there X=jωL (ignoring R).
As shown in Formula (9) and Formula (10), the active
power output can be controlled by adjusting the phase
difference between the output voltage and the grid
voltage, i.e. the power angle δ; once the power angle δ is
determined, directional control of the reactive power
output can be achieved by adjusting between Uo and Ug ,

uo
As shown in Fig. 7, the ratio between the active and
reactive power of the inverter output can be controlled by
controlling the phase difference between the
grid-connected current and grid voltage, i.e. the power
factor angle θ.
In summary, combining Fig. 4, there is the control block
diagram of this strategy, as shown in Fig. 8.
Kg
G1(s)
G3(s)
+
Kvp + Kvi /s
-
-
+
Iref1
Kip
+
+
+
+
Ks
1/L1s
1/Cs
uo
+
io
G2(s)
Grid
+
-
ug
-
φ
+
-
δg
Fig. 8 The control block diagram of PV grid-connected inverter
For detection of the grid voltage phase signal, this
strategy applies hardware phase zero-crossing detection
circuits. Because delays in the phase zero-crossing
detection of grid-connected current and grid voltage both
have the same length of time, the detection delay does
not affect the detection accuracy of the power factor
angle.
B. Analysis of Crowbar circuit control
As shown in Fig. 2, no matter how the voltage drops, the
PV array output voltage cannot exceed the open circuit
voltage, which means dominant advantages of protection.
Based on this principle, Crowbar circuit is not generally
adopted in the current design of PV grid-connected
system.
However, during the voltage drop, depth of drop may
change. Once the depth changes from deep to shallow,
the PV cells do not have the reverse features, i.e. the
output voltage is automatically lowered to ensure that the
output power increases, and therefore, in order to meet
the requirements of State Grid, this paper chooses to
keep the Crowbar circuit to guarantee the adjustment
capacity of reactive power during grid voltage drop.
By detecting the output DC voltage of the PV array, this
strategy determines whether the DC voltage exceeds a
preset maximum volume, issue a command about
whether to start Crowbar circuit and get PWM output [7]
by PI control. The control block diagram is shown in Fig.
9.
Upv
-
Upv*
光伏阵列输出电压
波动判断
PI
PWM
Crowbar电路
Tab.2 Component parameters
Three-phase voltage
regulator
Three-phase rectifier
75A/1600V
IGBT
100A/1200V
Filter capacitor
3300μF/450VDC
In order to better verify the control effect of this strategy,
this experiment has designed inverter load connection
circuit as shown in Fig. 11, wherein L1 is filter inductor
10.7mH (measured), L2 is connection inductance 1.0mH
(measured), C is the filter capacitor 2.2μF, R1 and R2 are
the voltage dividing resistors, and K is the switch.
Short-circuiting operation R1 is performed.
Y
Ug是否正常

L1
Fig. 9 Control block diagram of Crowbar
In summary, Formula (5) is taken as operation criteria for
the state of grid. Then, when the grid operates normally,
the PV grid-connected inverter runs in inverter operation
mode with unity power factor; when the grid encounters
temporary fault, the PV grid-connected inverter controls
the output power control according to needs in order to
help restore the grid voltage; based on detection of the
DC bus voltage, Crowbar circuit is controlled to ensure
that the PV array works in the vicinity of the actual
output power of the grid-connected inverter. The flow
chart of control program is shown in Fig. 10.
380VAC/0~430VAC/4A/50Hz/3KVA
uo
L2
R1

io
C
R2
K
GRID
本实验电网
电压采样点
Fig. 11 Load connection circuit
In this experiment, the voltage loaded on R2 is used as
the grid voltage sample values. Sudden change of grid
voltage is simulated by switching K. Take basic control
parameters as shown in Tab. 3. The experimental results
are as follows:
Tab.3 Control parameters
跟踪Ug，单位功率因数
逆变运行
carrier wave ratio N=256; carrier cycle Tc=78.125μs
返回
N
PI Control parameters Kvp=10, Kvi=0.1(1/s, Kip=1
Y
低电压穿越？
N
Crowbar工作？
N
按预设发出无功
返回
tracking control feedback factor Kg=1.02
Y
PWM控制


uo
ug
Y
高电压穿越？
按预设吸收无功
返回
u(140V/格)
N
过流保护
N
返回
t(10ms/格)
(a)
Fig. 10 Program flow chart



ug
uo
ug
u(140V/格)
TMS320F28XXX is applied as the control core and
single-phase bridge voltage source inverter is taken as
example. The experiment is carried out by
short-circuiting the resistive load to simulate the sudden
and non-directional change of grid voltage. The main
power electronic devices and their parameters chosen in
the experiment are shown in Tab. 2.

uo
u(140V/格)
V. EXPERIMENT
t(25ms/格)
(b)
t(10ms/格)
(c)


uo
ug
u(140V/格)

ug
u(140V/格)

uo
t(25ms/格)
(d)
the premise of ensuring the active power output, reactive
power is sending to the grid system or absorbed
according to power system instructions for purpose of
making full use of the inverter capacity.
REFERENCES
t(10ms/格)
(e)
Fig. 12 Experiment results
As shown in Fig. 12(a), the simulation grid has normal
power supply and runs in inverter operation mode which
is close to unity power factor.
As shown in Fig. 12(b) and (c), voltage of simulation
grid drops, and PV grid-connected inverter is riding
through with low voltage. Low-voltage ride-through of
grid voltage amplitude is achieved within two frequency
cycles. Switch from inverter operation mode with unity
power factor to pure capacitance operation mode is
achieved within four frequency cycles.
As shown in Fig. 12(d) and (e), voltage of simulation
grid rises, and PV grid-connected inverter is riding
through with high voltage. High-voltage ride-through of
grid voltage amplitude is achieved within two frequency
cycles. Switch from inverter operation mode with unity
power factor to pure capacitance operation mode is
achieved within four frequency cycles.
[1] State Grid. Q/GDW617 2011Technical Requirements for PV
Power Plants Connection to the Grid[S]. Beijing: State Grid,
2011.
[2] Jesus Lopez, Eugenio Gubia, Eneko Olea, Josu Ruiz, et al. Ride
Through of Wind Turbines With Doubly Fed Induction Generator
Under Symmetrical Voltage Dips[J]. IEEE transactions on
industrial electronics, Vol. 56, No. 10, 2009.
[3] Yin B, Oruganti R, Panda S K. An output-power-control strategy
for a three-phase PWM rectifier under unbalanced supply
conditions[J]. IEEE Transactions on Industrial Electronics, 2008,
55(5): 2140-2150.
[4] State Grid. Q/GDW480 2010 Technical Regulations of
Distributed Power Connection to the Grid[S]. Beijing: State Grid,
2010.
[5] MA Zhao-biao, HUI Jing, PAN Jian. Study on Photovoltaic
Grid-connected Inverter based on Repetitive-PI Control[J].
POWER ELECTRONICS, 2008, 43(3): 25-27.
[6] ZHOU Bin. LVRT of Photovoltaic Inverter Based on Feedforward
Control[J]. POWER ELECTRONICS, 2013, 47(8): 49-51.
[7] AN Zhi-long. Research on Photovoltaic grid-connected control
strategy and low voltage ride-through control[D]. NCEPU,
master's thesis, 2012.
VI. CONCLUSIONS
Based on analyzing the necessity of research on high and
low voltage ride-through technology, this paper proposes
a non-directional ride-though control strategy of
household PV grid-connected inverter based on voltage
feed-forward control:
(1) An instantaneous prediction algorithm is adopted for
tracking the grid voltage amplitude signal, which, to
some extent, compensates PI closed loop control delay;
voltage feed-forward control is applied to effectively
suppress sudden change of grid-connected current caused
by sudden change of grid voltage.
(2) The direction of reactive power is controlled by
controlling the inverter output voltage amplitude. The
ration between active and reactive output is controlled by
controlling the phase difference between the
grid-connected current and grid voltage, i.e. the power
factor angle.
(3) Necessity of Crowbar circuit application in this
strategy is explained based on the PV curve of PV cells.
The experiment is carried out by simulating the sudden
and non-directional change of grid voltage. Experimental
results have verified the feasibility and effectiveness of
the strategy as a practical control method.
In actual situations, the grid-connected inverter is
generally not working in full capacity, so when the grid
voltage is normal, this strategy can also be adopted. On
CHEN Kun was born in the city of Wuhan, Hubei Province, China in
1986. He earned the Ph. D in Power Electronics in Wuhan University in
2014. His major field of study is inverter control, PV grid-connected
control, DC transmission control and protection.
E-mail：[email protected]