This paper presents a configuration and a control strategy for dynamic voltage restorer (DVR). In order to compensate the voltage of each phase separately, a closedloop PI control law in the dq reference frame is proposed. The proposed method provides a fast response and effective sag compensation capabilities. In addition, in order to detect voltage sag, a linear Kalman filter is employed toestimate threephase voltages. By using Kalman filter, the voltage sag can be detected faster than other conventional methods. Therefore DVR can compensate voltage sag quickly and accurately. The obtained results that are simulated in Matlab Simulink indicate that the proposed method can mitigate the balanced and unbalanced voltage sag types efficiently in the distribution networks.
Trang 1IEEE PEDS 2015, Sydney, Australia
9 – 12 June 2015
A Control Strategy for Dynamic Voltage Restorer
University of Science and Technology – The University of Danang Quynhon University
dtviet@ac.udn.vn, huuhieu019@yahoo.com, nglehoa79@gmail.com nmkhoaqnu@gmail.com
Abstract-This paper presents a configuration and a control
strategy for dynamic voltage restorer (DVR) In order to
compensate the voltage of each phase separately, a closed-loop
PI control law in the d-q reference frame is proposed The
proposed method provides a fast response and effective sag
compensation capabilities In addition, in order to detect voltage
sag, a linear Kalman filter is employed to estimate three-phase
voltages By using Kalman filter, the voltage sag can be detected
faster than other conventional methods Therefore DVR can
compensate voltage sag quickly and accurately The obtained
results that are simulated in Matlab/ Simulink indicate that the
proposed method can mitigate the balanced and unbalanced
voltage sag types efficiently in the distribution networks
I INTRODUCTION
According to the IEEE defined standard (IEEE Std
1159-1995), a voltage sag is defined as: “an rms variation with a
magnitude between 10% and 90% of nominal voltage and
duration between 0.5 cycles and one minute” An example of
voltage sag event is shown in Fig 1 Voltage sag is usually
caused by faults such as short circuit, heavy load
switching,… Voltage sags are the most important power
quality problem facing industrial customers today [1]
Equipment used in many modern industrial plants (process
controllers, programmable logic controllers, adjustable speed
drives) actually become more sensitive to voltage sags as the
complexity of the equipment increases and the equipment is
interconnected in sophisticated processes [1] Even relays and
contactors in motor starters can be sensitive to voltage sags,
resulting in shut down of the process
The use of custom power devices is one of the most
efficient methods to mitigate voltage sag There are many
custom power devices Each of which has its own benefits
and limitations [5] Among the several novel custom power
devices, DVR is now becoming more established in industry
to mitigate impact of voltage disturbances on sensitive loads
DVR is a series-connected device designed to maintain a
constant RMS voltage across a sensitive load [2], [4] The
basic principle of DVR is to inject a voltage of required
magnitude and frequency, so that it can restore the load
voltage to the desired amplitude and waveform even when the
source voltage is unbalanced or distorted Generally, it
employs a gate turn off thyristor solid state power electronic
switches in a pulse width modulated (PWM) inverter
structure DVR can generate or absorb independently
controllable real and reactive power at the load side In other
words, DVR is made of a solid state DC to AC switching power converter that injects a set of three phase AC output voltages in series and synchronism with the distribution line voltages
Fig 1 A voltage sag event.
In this paper, a new configuration and control strategy for DVR is proposed Its idea is based on the minimization of the energy supplied from DVR irrespective of the balance of the three-phase voltages supplied to the load In the control strategy of DVR, close-loop with PI control based on d-q reference frame in order to generate reference voltage and Kalman filter is used to detect volage sag quickly and accurately
II APROPOSED METHOD OF DVRCONTROL
A Configuration of DVR
In this study, a proposed configuration of DVR includes control unit, energy storage unit, voltage source converter (VSC), LC filter and series transformer as shown in Fig 2
Fig 2 Configuration of DVR
Trang 2B Control Strategy of DVR
Control strategy is one of the most importance units of
DVR, it consists of voltage sag detection unit in order to
switch between standby and active modes accurately, and to
generate reference voltage based on Kalman filter to
compensate voltage sag in supply voltage A detail of the
proposed control strategy for DVR is presented as follows
1 Phase Locked Loop (PLL):
The PLL is a widely used mechanism which tries to track
the phase of the incoming signal Synchronizing with the
signal can be achieved by comparing the phase difference
between the incoming signal and reference signal generated
from a voltage-controlled oscillator via a phase detector and
through a loop filter A phase locked loop is shown in Fig 3
Where γ is an instant angle of the supply voltage and θ is a
angle of PLL The PLL tracks the positive sequence
component of the supply voltage and PLL angle is used to
transform from α-β reference frame to d-q reference frame
and vice versa
Fig 3 A PLL to synchronize DVR to supply voltage.
2 Using Kalman Filter to Detect Voltage Sag
The Kalman filtering has been recognized as a powerful
state estimation technique [3] It is a model based on the
optimal estimator with minimum error covariance Given
observed data, a Kalman filter is described by a set of
dynamic process (i.e., state) equations and a set of
measurement (i.e., observation) equations as follows:
k k k k
k k k k
where xk is the state vector and zk is the measurement at time
index k; Φk is the state transition matrix; Hk is the
measurement matrix; and wk and vk are the model and
measurement errors, respectively The model structure
required by the Kalman filter is flexible to allow the
measured signal to be represented in many ways based on
different assumptions
To overcome the disadvantages of the conventional
methods for detecting voltage sag based on d-q frame, an
algorithm to detect voltage sag based on Kalman filter [3] is
employed in this paper In this method, three Kalman filters
are used to track three-phase voltages, so that both balanced
and unbalanced voltage sags can be detected
Fig 4 shows a single phase diagram to detect voltage sag
by Kalman filter, where the input of Kalman filter is
one-phase voltage signal v(t), the outputs of Kalman filter consists
of the estimated phase angle θ(t) and the estimated voltage
magnitude Vm(t) To detect voltage sag, the estimated voltage
magnitude Vm(t) is compared to the reference voltage
Vref=1(pu) and then compared by hysteresis comparator to generate voltage sag detection signal
Fig 4 Voltage sag detection using Kalman filter
Fig 5 A balanced voltage sag detection; a) A 15% balanced voltage sag; b) Conventional dq-based method; c) Kalman filter-based method
Fig 6 An unbalanced voltage sag detection; a) A 15% unbalanced voltage sag; b) Conventional dq-based method; c) Kalman filter-based method
Fig 5 shows the capacity of recognising the 15% balanced voltage sag starting at the time of 0.2 second and 0.3 second
In this case, both methods can recognise voltage sag however
Trang 3Kalman filter-based method is clearly better than
conventional dq-based method as the time for recognising is
shorter
However, in term of 15% unbalanced voltage sag, these
methods give completely different results which are showed
in Fig 6 It can be seen that conventional d-q method (Fig
6.b) cannot recognise the 15% unbalanced voltage sag
(“shallow” voltage sag) but Kalman filter method (Fig 6.c)
3 Control Methodology
A space vector control has been applied to generate the
reference voltage as shown in Fig 7 Where VSabc is the
supply voltage; VLabc is the load voltage; θPLL is phase angle
of PLL; Standby/Active is signal of standby or active mode
from Kalman filter algorithm VL.ref(dq) is reference voltage in
d-q set (VL.refd=1pu; VL.ref.q=0) G1÷G12 is pulse signal of
IGBTs 1 to 12
Space vector control is applied to control DVR, therefore
three-phase voltage are converted to d-q reference frame in
(2):
Fig 7 Control strategy of DVR
2
a d
b q
c
V V
V V
V
(2)
The DVR measures the three-phase supply voltages and
three-phase load voltages, then transform the voltages to the
dq-system The PLL uses the three-phase supply voltages and
generates the filtered PLL angle The angle of the PLL (θPLL)
is used in the transformation to the rotating dq-system The
DVR controls the load voltages to be in-phase with the supply
voltages by setting the d-reference voltage to 1pu and the
q-reference voltage to zero The error between the actual supply
voltages and the wanted load voltages is calculated and is
feedforward to PI control and the PWM signals to the three
full bridges is generated During normal supply voltages the
DVR is in standby mode, and the DVR-reference voltages are set to zero
III RESULTS AND DISCUSSIONS
A Simulation Model on Matlab/Simulink
DVR is modeled and simulated using Matlab/ Simulink software The power circuit and control system model of DVR is shown in Fig 8 and Fig 9, respectively In these diagrams, the load is supplied by a supply source that may appear voltage sag DVR's power circuit (including the serial transformers, filters, inverter) is connected between the source and the load The parameters of this system are chosen
as shown in Table I
Fig 8 Simulation model of DVR on Matlab/Simulink
Fig 9 Control system for DVR
TABLE I
PARAMETERS OF THE S IMULATION M ODEL
Supply source 22 kV; 50 Hz Distribution transformer 100 kVA; 22/0,4 kV;
Δ/Y 0 -11 Load 48 Ω Series transformer Ratio 1:1; 1 kVA
LC filter 18 μF; 10 mH Voltage source converter H-bridge converter
DC source voltage 150 Vdc Switching frequency 10 kHz Sampling time 25 s
B Simulation Results
1 A Balanced Voltage Sag
We suppose that a balanced three-phase voltage sag in the source with 50% magnitude starts at 0.05s and ends at 0.15s The supply voltages are shown in Fig 10(a) and Fig 10(b) When t<0.05s, there is no voltage sag in the system, therefore the DVR is set in the standby mode In this mode, the voltage
on DVR (Fig 10.c, d) is the voltage drop in the serial transformer At t=0.05s the voltage sag appears in the supply
Trang 4voltage, then Kalman filter will generate voltage sag
detection signal after a delay time (Fig 10.g, h) Therefore
DVR will change its operation mode to the active mode to
compensate the missing voltage at the load side so that load
voltages are maintained at the rated voltage (Fig 10.e, f)
Fig 10 A balanced voltage sag; a-b) Supply voltage; c-d) DVR voltage; e-f)
Load voltage; g-h) Signal of voltage sag detection of phase A by Kalman
filter
2 An Unbalanced Voltage Sag
In this case, we assume the voltage sag appears in
one-phase of 22kV source side with 50% magnitude starts at
t=0.05s and ends at t=0.15s The 22/0.4 kV distribution
transformer structured connection windings is Δ/Y0-11
therefore phase A and phase B are in low voltage while phase
C is still in normal voltage (Fig 11.a, b) In this case, Kalman
filter will detect voltage sag on phase A and phase B Then
DVR will change its operation mode to the active mode in
order to maintain the load voltage at the nominal voltage
In both cases of the balanced and unbalanced voltage sag
are mentioned above Using fast Fourier transform to analysis
the frequency spectrum of the compensated voltage signals at
load side by DVR for a period of one cycle of the load voltage signal The results of the frequency spectrum of phase
A load voltage when balanced voltage sag occurs at source side is THD=1.17%, as shown in Fig 12 Fig 13 shows the result of frequency spectrum of the phase A load voltage when unbalanced voltage sag occurs at supply side is THD=1.34% In both cases showed the second order frequency components appear in load voltage due to switching IGBT valves in the voltage source inverter but this value is not significant
Fig 11 An unbalanced voltage sag; a-b) Supply voltage; c-d) DVR voltage; e-f) Load voltage; g-h) Signal of voltage sag detection of phase A by Kalman filter
IV CONCLUSION
Kalman filter is used to quickly and accurately detect voltage sag compared with dq transformation traditional method used in many other studies In this paper, authors use three Kalman filter to detect the three-phase voltage sag individually Another advantage of the proposed DVR configuration that is capable of saving energy in compensation mode by just compensates phases that appear
Trang 5voltage sag In standby mode, using inverter’s IGBT to
bypass circuit the secondary side of the serial transformer so
no need to use additional bypass switches
The results were verified in simulation model in
Matlab/Simulink The balanced voltage sag and unbalanced
voltage sag were simulated in this paper to examine the
performance of proposed DVR The simulation results show
that the proposed DVR ensure to compensate voltage sag to
maintain the load voltage at rated voltage
Fig 12 Frequency contents of phase A during balanced voltage sag
ACKNOWLEDGMENT
The authors would like to thank National Foundation for
Science and Technology Development of Vietnam and
Quynhon University for the financial support to carry out this
work
Fig 13 Frequency contents of phase A during unbalanced voltage sag
REFERENCES
[1] M.H Bollen, Understanding Power Quality Problems: Voltage Sags
and Interruptions, New York: IEEE Press, Vol 1, 2000
[2] J.G Nielsen and F Blaabjerg, “A detailed comparison of system
topologies for dynamic voltage restorers,” IEEE Transactions on
Industry Applications, vol 41, no 5, pp.1272-1280, 2005
[3] M.H.Bollen andI.Y.H.Gu, Signal Processing Of Power Quality
Disturbances, Wiley- IEEE Press, 2006
[4] F BadrkhaniAjaei, S.Afsharnia, A Kahrobaeian, and S Farhangi, “A fast and effective control scheme for the dynamic voltage restorer,”
IEEE Transactions on Power Delivery, vol 26, no 4, pp
2398-2406,2011
[5] C Benachaiba and B Ferdi, “Voltage quality improvement using
DVR,” Electrical Power Quality and Utilisation, Journal,vol XIV, no
1, 2008