OPTIMAL CURRENT REGULATION STRATEGY FOR THREE-PHASE BACK-TOBACK ACTIVE POWER CONDITIONERS

International Journal of Research in Advanced Technology - IJORAT
Vol. 2, Issue 1, JANUARY 2016
1
All Rights Reserved © 2015 IJORAT
OPTIMAL CURRENT REGULATION
STRATEGY FOR THREE-PHASE BACK-TO-
BACK ACTIVE POWER CONDITIONERS
M. Shunmugavidya
1
, Priyanka Rajan
2
, A.Ravi
3
1
Student, Dept of EEE, FRANCIS XAVIER ENGINEERING COLLEGE, Tamilnadu, India
2
Student, Dept of EEE, FRANCIS XAVIER ENGINEERING COLLEGE, Tamilnadu, India
3
Head of Dept, Dept of EEE, FRANCIS XAVIER ENGINEERING COLLEGE, Tamilnadu, India
Abstract: The objective of this paper is to propose a three phase back-to-back power conditioner with optimal a current
regulation strategy in microgrid. To achieve high stability ,the frequency and the voltage of the microgrid is controlled
by using bidirectional power flow control .The active and reactive power of Active Power Conditioner(APC) is used
here. The dc-link capacitor is the main component of the back-to-back power conditioner for power decoupling and
power flow balancing. Optimal current regulation strategy is developed to improve the power quality and stability of
the micro grids as well as to reduce the dc link capacitance. Under steady state, the optimal ac current regulation is
able to achieve the dc-link voltage regulation and to reduce the injected ac line current variation. Simulation result was
used to demonstrate the feasibility and performance of the proposed active power conditioner.
Keywords: Microgrid, Active Power Conditioner, Bidirectional Power Flow, Line current.
I.INTRODUCTION
Microgrids will play a pivotal role in solving the
energy challenges of the future. Microgrids are
autonomous grids that operate either in parallel to, or
„islanded‟ from, existing utility power grids. They
have the power to efficiently and flexibly meet the
growing energy demand of communities: whether
they are grid-connected or not .Microgrids allow for
fast installation of electricity supply without the need
for expensive transmission infrastructure investments
and the lengthy development approval and
construction process. This will especially empower
remote, non-grid-connected communities around the
world.
To limit the carbon dioxide emission, micro-grid
systems consist of distributed generators (DGs) have
been rapidly developed. The performance of the DGs
will be greatly affected by the unpredictable
environmental conditions, it will create negative
impacts to the power quality of the micro-grid. An
APC is a device connected between two micro-grids,
used to provide the capability to improve the quality
of power on both sides. When fault on one microgrid
is detected, the other microgrid can provide active
and reactive power compensation via the APC.
Hence, the APC becomes a necessary component for
the micro-grid application. Now, power converters
with back-to-back structure are used in many
applications, such as motor drivers, traction power
systems, wind power systems and micro-grid
applications. Therefore, a three-phase back-to-back
APC is utilized to realize active and reactive power
compensation in this project. As soon as a grid fault
occurs, the voltage and the frequency of the system
get disturbed, it will affect the stability of the power
system. However, the frequency and the voltage
magnitude can be compensated by active and reactive
power respectively. Hence, more and more APCs
have been developed with the function of active
power and reactive power compensation.
A ac–ac converter with bidirectional power flow
can be implemented by coupling the dc-link of a
PWM rectifier and a PWM inverter. The dc-link
quantity is then impressed by an energy storage
element that is common to both Stages: a capacitor
CDC for the voltage dc-link back-to-back converter
or an inductor LDC for the current dc-link back-to-
back converter. The PWM rectifier is controlled in
such a manner that a sinusoidal mains current is
drawn, which is in-phase or anti phase with the
corresponding mains line voltage. The
implementation of the V-BBC and C-BBC requires
12 transistors (typically IGBTs) and 12 diodes or 12
reverse conduction IGBTs (RC-IGBTs) for the V-
BBC and 12 reverse blocking IGBTs (RB-IGBTs) for
the C-BBC.
Due to the dc-link energy storage element, there is
an advantage that both converter stages are, to a large
extent, decoupled regarding their control for a typical
sizing of the energy storage. Furthermore, a constant
mains-independent input quantity exists for the PWM

2
inverter stage, which results in a high utilization of
the converter‟s power capability. On the other hand,
the dc-link energy storage element can have a
relatively large physical volume compared with the
total converter volume, and when electrolytic
capacitors are used for the dc-link of the V-BBC, the
service lifetime of the converter can potentially be
reduced.
II. PROPOSED SYSTEM
A. CIRCUIT DIAGRAM
The circuit diagram and control blocks of the
proposed three-phase back-to-back APC are shown in
Fig 1. It consists of two bidirectional ac-dc/dc-ac
converters. As soon as the voltage or frequency
disturbance on the weak micro-grid side is detected,
the converter connected to the weak grid will operate
as a dc-ac inverter and supply the demanded active
and reactive power from the dc-link to the disturbed
micro-grid. In the meantime, the active and reactive
power commands will be transferred to the active and
reactive current command, Id,cmd and Iq,cmd. The
Proportional-Integral (PI) controllers of the current
commands are designed in the conventional fashion.
Fig 1 Conceptual Diagram of Micro-grid
with back-to-back APC
Meanwhile, for power flow balancing, the
converter on the opposite side of the dc link will
operates as an ac-dc rectifier. For simple control, the
Direct-Quadrature Transformation method is used in
this paper. The three-phase voltages and the input
currents are converted into the directed axis voltage
and current components, Vd and Id, and the
quadrature axis voltage and current, Vq and Iq. Thus,
the complex power of the three-phase
rectifier/inverter can be written as:
𝑠 =
3
2
𝑉𝑑 𝐼 𝑑 + 𝑉𝑞 𝐼𝑞 + 𝑗 𝑉𝑞 𝐼 𝑑 − 𝑉𝑑 𝐼𝑞 (1)
Fig 2 Circuit Diagram of back-to-back APC
The circuit diagram of back-to-back APC is shown
in the Fig 2. A phase-locked loop or phase lock loop
(PLL) generates an output signal whose phase is
related to the phase of an input signal. Here it is used
to fix the voltage value of q-axis as zero. Therefore,
the active and reactive power command can be:
𝑃𝑐𝑚𝑑 =
3
2
𝑉𝑑 𝐼𝑑 (2)
𝑄𝑐𝑚𝑑 = −
3
2
𝑉𝑑 𝐼𝑞 (3)
Fig 3 Conceptual Diagram of Droop Control
Strategy
As soon as the frequency or voltage disturbance of
the micro-grid occurs, the proposed APC should be
able to transfer appropriate active/reactive power to
the weak micro-grid. The droop control is commonly
adopted for power quality control because of its
simplicity. The conceptual diagrams of the droop
control strategy are shown in Fig. 3. The

3
mathematical equations of the frequency and voltage
regulation via the active and reactive power
compensation can be expressed as:
𝑓𝑚𝑎𝑥 − 𝑓𝑚𝑖𝑛 = −2 × 𝑚 𝑝 × 𝑃𝑚𝑎𝑥 (4)
𝑉𝑚𝑎𝑥 − 𝑉 𝑚𝑖𝑛 = −2 × 𝑚 𝑄 × 𝑄 𝑚𝑎𝑥 (5)
where 𝑓𝑚𝑎𝑥 ,𝑓𝑚𝑖𝑛 ,𝑉𝑚𝑎𝑥and 𝑉𝑚𝑖𝑛 are the
maximum and minimum frequency and voltage of the
micro-grid, respectively. 𝑃𝑚𝑎𝑥 and 𝑄𝑚𝑎𝑥 represent
the maximum active and reactive of the APC. The
slope of f-P and V-Q droop characteristics are
represented as 𝑚𝑃 and 𝑚𝑄 respectively.
B. POWER CONDITIONER
A power conditioner (also known as a line
conditioner or power line conditioner) is a device
intended to improve the quality of the power that is
delivered to electrical load equipment. While there is
no official definition of a power conditioner, the term
most often refers to a device that acts in one or more
ways to deliver a voltage of the proper level and
characteristics to enable load equipment to function
properly. In some uses, power conditioner refers to a
voltage regulator with at least one other function to
improve power quality (e.g. power factor correction,
noise suppression, transient impulse protection,
etc.).The terms "power conditioning" and "power
conditioner" can be misleading, as the word "power"
here refers to the electricity generally rather than the
more technical electric power. Conditioners
specifically work to smooth the sinusoidal ac wave
form and maintain a constant voltage over varying
loads.
AC power, as delivered, is contaminated with
noise. Additional noise is added by your everyday
appliances, computer power supplies, etc. Without
exception, this degrades the performance of your
sensitive audio / video components. Also, electrical
surges and spikes have becomes common-place,
occurring on a daily basis. These and other harmful
AC events put valuable equipment at risk. A good
power conditioner filters and cleans incoming AC
power and dramatically improves your equipment's
performance. Audio sounds better and your picture
looks cleaner. It will increase the longevity of your
connected components since contaminated AC add
wear and tear to power supplies and other internal
circuits. A good power conditioner protects your
equipment from damaging AC events such as surges,
spikes, lightning and high voltage. An AC power
conditioner is the typical power conditioner that
provides "clean" AC power to sensitive electrical
equipment. Usually this is used for home or office
applications and has up to 10 or more receptacles or
outlets and commonly provides surge protection as
well as noise filtering.
Power line conditioners take in power and modify
it based on the requirements of the machinery to
which they are connected. Attributes to be
conditioned are measured with various devices, such
as, Phasor measurement units. Voltage spikes are
most common during electrical storms or
malfunctions in the main power lines. The surge
protector stops the flow of electricity from reaching a
machine by shutting off the power source.
The term "Power Conditioning" has been difficult
to define historically. However, with the advances in
power technology and recognition by IEEE, NEMA,
and other standards organizations, a new actual
engineering definition has now been developed and
accepted to provide an accurate depiction of this
definition. "Power Conditioning" is the ability to
filter the AC line signal provided by the power
company. "Power Regulation" is the ability to take a
signal from the local power company, turn it into a
DC signal that will run an oscillator, which generates
a single frequency sine wave, determined by the local
area needs, is fed to the input stage of power
amplifier, and is then output as specified as the ideal
voltage present at any standard wall outlet.
A micro-grid is different from a main grid system,
which can be considered as an unlimited power so
that load variations do not affect the stability of the
system. On the contrary, in a micro-grid, large and
sudden changes in the load may result in voltage
transient of large magnitudes in the AC bus.
Moreover, the proliferation of switching power
converters and nonlinear loads with large rated power
can increase the contamination level in voltage and
current waveforms in a micro-grid, forcing to
improve the compensation characteristics required to
satisfy more stringent harmonics standards. The APC
has proved to be an important alternative to
compensate current and voltage disturbances in
power distribution systems.
C. DESIGN OF POWER CONDITIONER
A good quality power conditioner is designed with
internal filter banks to isolate the individual power
outlets or receptacles on the power conditioner. This
eliminates interference or "cross-talk" between
components. If the application will be a home theater
system, the noise suppression rating listed in the
technical specifications of the power conditioner will

4
be very important. This rating is expressed in
decibels (db). The power conditioner will also have a
"joule" rating. A joule is a measurement of energy or
heat required to sustain one watt for one second,
known as a watt second. Since electrical surges are
momentary spikes, the joule rating indicates how
much electrical energy the suppressor can absorb at
once before becoming damaged itself.
D. USES OF POWER CONDITIONER
Power conditioners vary in function and size,
generally according to their use. Some power
conditioners provide minimal voltage regulation
while others protect against six or more power
quality problems. Units may be small enough to
mount on a printed circuit board or large enough to
protect an entire factory. Small power conditioners
are rated in volt-amperes (VA) while larger units are
rated in kilovolt-amperes (kVA). Ideally electric
power would be supplied as a sine wave with the
amplitude and frequency given by national standards
(in the case of mains) or system specifications (in the
case of a power feed not directly attached to the
mains) with an impedance of zero ohms at all
frequencies.
E. BACK-TO-BACK-CONVERTER
The main circuit of the back-to-back converter is
composed of renewable source, load, and switching
devices. The switching devices or IGBT are divided
into two sets that are equal to 24 pieces. The 12
switching devices will be operated as a rectifier, and
the 12 switching devices will be performed as an
inverter. It also has C1and C2, which is called DC-
link, between the rectifier and the inverter. Fig 4
represents the back-to-back converter.
Fig 4 Back-to-Back Converter
It consists simply of a force-commutated rectifier
and a force-commutated inverter connected with a
common dc-link. The properties of this combination
are well known; the line-side converter may be
operated to give sinusoidal line currents, for
sinusoidal currents, the dc-link voltage must be
higher than the peak main voltage, the dc-link voltage
is regulated by controlling the power flow to the ac
grid and, finally, the inverter operates on the boosted
dc-link, making it possible to increase the output
power of a connected machine over its rated power.
An important property of the back-to-back converter
is the possibility of fast control of the power flow. By
controlling the power flow to the grid, the dc-link
voltage can be held constant. The presence of a fast
control loop for the dc-link voltage makes it possible
to reduce the size of the dc-link capacitor, without
affecting inverter performance. In fact, the capacitor
can be made small enough to be implemented with
plastic film capacitors.
Fluctuations in the load cannot be smoothed in the
converter, but must be accommodated by other
means. One alternative is to simply transfer such
fluctuations to the power grid, but this may re-
introduce the line-current harmonics the back-to-back
converter is supposed to eliminate. However, load
fluctuations will be random and thus relatively
harmless compared to the in-phase harmonics
generated by diode rectifiers. Another alternative is
to use the load itself. In a typical drive, the
mechanical energy stored in the drive is several
orders of magnitude larger than the electrical energy
stored in the DC-link capacitor in a back-to-back
converter. If the application does not need servo-class
performance, there is no reason why the rotational
speed cannot be allowed to fluctuate slightly. A pump
drive, for instance, may be perfectly satisfactory if
the speed regulation performance is comparable with
a directly connected induction motor.
The smallest feasible capacitor, chosen on the
basis of switch-frequent voltage ripple, is too small to
absorb (within voltage limits) even the
[electromagnetic] energy stored in the main flux of a
connected electrical drive. This places high demands
on the controller which must be absolutely reliable. If
the controller fails, the stored energy may raise the
voltage of the DC-link beyond acceptable levels
(enough to break the rectifier and/or the inverter).
This may also result from circuit-breaker tripping. To
avoid DC-link overvoltage resulting from e.g
controller failure, also the back-to-back converter
must have a voltage limiting device. This can consist
of a traditional brake chopper. However, the average
power rating needed is much smaller than for a
conventional converter, although the peak power
rating would probably be more than the rated power
of the converter3. The chopper must be independent
of the converter controller to be operational in event

5
of a controller failure. Preferably, the chopper should
operate directly from the DC-link voltage.
F. OPTIMAl CURRENT REGULATION
STRATEGY
The conceptual diagram of the dc-link voltage and
ac-line current regulation strategy is shown in Fig 5.
At first, the reference voltage Vdc,high and Vdc,low
should be determined. If the dc-link voltage is greater
than the high voltage bound, Vdc,high, the output
current command, Id,ref, should be decreased with a
constant value, ΔId. If the dc-link voltage is lower
than the low voltage bound, Vdc,low, the output
current command, Id,ref, should be increased with a
constant value, ΔId. Otherwise, the current reference
should remain unchanged. The output current
command, Id,ref, will be changed at the beginning of
the ac mains cycle with the help of using PLL
It can be expressed as the following equations:
If 𝑉𝑑𝑐(𝑛) > 𝑉𝑑𝑐,𝑕𝑖𝑔𝑕 ,
then 𝐼𝑑,𝑟𝑒𝑓 (𝑛) = 𝐼𝑑,𝑟𝑒𝑓 (𝑛−1) − ∆𝐼𝑑 (6)
Else if 𝑉𝑑𝑐(𝑛) < 𝑉𝑑𝑐,𝑙𝑜𝑤 ,
then 𝐼𝑑,𝑟𝑒𝑓 (𝑛) = 𝐼𝑑,𝑟𝑒𝑓 (𝑛−1) + ∆𝐼𝑑 (7)
Otherwise, 𝐼𝑑,𝑟𝑒𝑓 (𝑛) = 𝐼𝑑,𝑟𝑒𝑓 (𝑛−1) (8)
Fig 5 Conceptual diagram of current regulation
strategy
It is very simple method of control but selecting a
proper current command is difficult for different
power levels. If we select a small ΔId value, the
response of dc link regulation will be slow. If we
select a large ΔId value, it will increase the regulation
response but the large ac current variation will bring
a negative impact to the power quality. The optimal
ac line current regulation strategy integrated with the
voltage trend method is proposed in this paper to
reduce the change of input current and to achieve dc
link regulation,
To avoid rapid dc-link voltage change, the current
change command should be as small. Eventually, the
optimal ac line current adjustment value, ΔId,opt, for
the high/low dc-link voltage bound can be expressed
as:
∆𝐼𝑑,𝑜𝑝𝑡 −𝐻 =
𝐶 𝑑𝑐
3×𝑇𝑎𝑐 ×𝑉 𝑑
[𝑉𝑑𝑐,𝑕𝑖𝑔𝑕
2
− 2𝑉𝑑𝑐(𝑛)
2
+ 𝑉𝑑𝑐(𝑛−1)
2
]
(12) (9)
∆𝐼𝑑,𝑜𝑝𝑡 −𝐿 =
𝐶 𝑑𝑐
3×𝑇𝑎𝑐 ×𝑉 𝑑
[𝑉𝑑𝑐,𝑙𝑜𝑤
2
− 2𝑉𝑑𝑐(𝑛)
2
+ 𝑉𝑑𝑐(𝑛−1)
2
]
(13) (10)
The optimal current regulation strategy can be
expressed as:
If 𝑉𝑑𝑐(𝑛) > 𝑉𝑑𝑐,𝑕𝑖𝑔𝑕 and ∆𝑉𝑑𝑐(𝑛−1) > 0,
then 𝐼𝑑,𝑟𝑒𝑓 (𝑛) = 𝐼𝑑,𝑟𝑒𝑓 (𝑛−1) − ∆𝐼𝑑,𝑜𝑝𝑡 −𝐻 (11)
Else if 𝑉𝑑𝑐(𝑛) < 𝑉𝑑𝑐,𝑙𝑜𝑤 and ∆𝑉𝑑𝑐(𝑛−1) < 0
then 𝐼𝑑,𝑟𝑒𝑓 (𝑛) = 𝐼𝑑,𝑟𝑒𝑓 (𝑛−1) + ∆𝐼𝑑,𝑜𝑝𝑡 −𝐿 (12)
Otherwise, 𝐼𝑑,𝑟𝑒𝑓 (𝑛) = 𝐼𝑑,𝑟𝑒𝑓 (𝑛−1) (13)
G. SIMULATION OF PROPOSED SYSTEM
Fig 6 simulink model of proposed system
The simulink model of the proposed system is
shown in the Fig 6. It represents the back-to-back
converter connected between two microgrids.

6
H. PHASE LOCKED LOOP
A phase-locked loop (PLL) is an electronic circuit
with a voltage- or current-driven oscillator that is
constantly adjusted to match in phase (and thus lock
on) the frequency of an input signal. In addition to
stabilizing a particular communications channel
(keeping it set to a particular frequency), a PLL can
be used to generate a signal, modulate or demodulate
a signal, reconstitute a signal with less noise, or
multiply or divide a frequency. PLLs are frequently
used in wireless communication, particularly where
signals are carried using frequency modulation (FM)
or phase modulation (PM). PLLs can also be used in
amplitude modulation (AM). PLLs are more
commonly used for digital data transmission, but can
also be designed for analog information. Phase-
locked loop devices are more commonly
manufactured as integrated circuits (ICs) although
discrete circuits are used for microwave.
A PLL consists of a voltage-controlled oscillator
(VCO) that is tuned using a special semiconductor
diode called a varactor. The VCO is initially tuned to
a frequency close to the desired receiving or
transmitting frequency. A circuit called a phase
comparator causes the VCO to seek and lock onto the
desired frequency, based on the output of a crystal-
controlled reference oscillator. This works by means
of a feedback scheme. If the VCO frequency departs
from the selected crystal reference frequency, the
phase comparator produces an error voltage that is
applied to the varactor, bringing the VCO back to the
reference frequency. The PLL, VCO, reference
oscillator, and phase comparator together comprise a
frequency synthesizer. Wireless equipment that uses
this type of frequency control is said to be frequency-
synthesized.
Since a PLL requires a certain amount of time to
lock on the frequency of an incoming signal, the
intelligence on the signal (voice, video, or data)
can be obtained directly from the waveform of the
measured error voltage, which will reflect exactly the
modulated information on the signal.
Fig 7 Block Diagram of PLL
The block diagram of PLL is shown in the Fig 7. A
phase detector compares two input signals and
produces an error signal which is proportional to their
phase difference. The error signal is then low-pass
filtered and used to drive a VCO which creates an
output phase. The output is fed through an optional
divider back to the input of the system, producing a
negative feedback loop. If the output phase drifts, the
error signal will increase, driving the VCO phase in
the opposite direction so as to reduce the error. Thus
the output phase is locked to the phase at the other
input. This input is called the reference.
I. PI CONTROLLER
PI controller will eliminate forced oscillations and
steady state error resulting in operation of on-off
controller and P controller respectively. However,
introducing integral mode has a negative effect on
speed of the response and overall stability of the
system. Thus, PI controller will not increase the
speed of response. It can be expected since PI
controller does not have means to predict what will
happen with the error in near future. This problem
can be solved by introducing derivative mode which
has ability to predict what will happen with the error
in near future and thus to decrease a reaction time of
the controller. PI controllers are very often used in
industry, especially when speed of the response is not
an issue.
J. SIMULATION RESULTS
Fig 8 Input current of back-to-back APC
The input current of back-to-back active power
conditioner is shown in the Fig 8. The input current
variation with the proposed strategy is relatively
smaller without bringing negative impact of power
quality of ac mains.
Fig 9 output current of back-to-back APC

7
The output current of back-to-back active power
conditioner is shown in the Fig 9. Here the x axis
denotes the time and the y axis denotes the current
value.
Fig 10 Input voltage of back-to-back APC
The input voltage of back-to-back active power
conditioner is shown in the Fig 10. Depending upon
the grid voltage, the reactive power is injected or
absorbed.
Fig 11 Output voltage of back-to-back APC
The output voltage of back-to-back active power
conditioner is shown in the Fig 11. The output
voltage is lesser than the input voltage. The output is
fed to the weak grid.
Fig 12 DC link voltage of back-to-back APC
The DC link voltage of back-to-back active power
conditioner is shown in the Fig 12. This figure shows
the charging and discharging of capacitor. By using
proposed strategy the voltage reaches the hysteresis
band is minimum.
In order to remain the power flow balance, the dc-
link voltage is regulated between the hysteresis
bound by the input current, of the front-end
converter. As soon as the high voltage bound is
detected, the input current reference, Id,ref, will be
reduced. Similarly, if the low voltage bound is
detected, the input current will be increased.
Fig 13 Input power of back-to-back APC
The input power of back-to-back active power
conditioner is shown in the Fig 13. Here the x axis
represents time and the y axis represents power
values. Depending upon the frequency APC will
inject or absorb the active power.
Fig 14 PWM generation
The pwm generation for the converters is shown in
the Fig 14. The generated pulses are given to the
switches in the converter. Here we use 6 switch
converters.
III CONCLUSION
This paper presents a three-phase back-to-back
APC with optimal current regulation strategy for
micro-grid applications. To achieve high stability, the
frequency and the voltage of the microgrid is
controlled by using bidirectional power flow control.
The demanded active and reactive power of the APC
via bi-directional power flow control provides the
ability to improve the power quality and stability of
micro grids. Optimal current regulation method for
the APC is proposed in this paper. . When fault on
one microgrid is detected, the other microgrid can
provide active and reactive power compensation via
the APC. Hence, the APC becomes a necessary
component for the micro-grid application Under
steady-state, the optimal ac line current regulation
method is able to regulate the dc-link voltage and the

8
injected ac line current variation will be minimized.
By using the optimal current regulation strategies, the
required dc-link capacitance of the back-to-back APC
can be reduced.
ACKNOWLEDGMENT
First of all we would like to thank the almighty for
giving me sound health throughout my paper work. This
research was supported/partially supported by our college.
We thank our staffs from our department who provided
insight and expertise that greatly assisted the research,
although they may not agree with all of the
interpretations/conclusions of this paper.
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