Control of the Power Flow in an Energy System Based on Grid Connected with Photo Voltage Generator

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15 Νοε 2013 (πριν από 3 χρόνια και 7 μήνες)

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Control of the Power Flow in an Energy System Based on Grid
Connected with Photo Voltage Generator

ROZANOV Y.K., KRIUKOV K.V.
Department of electric and electronic apparatus
Moscow Power Engineering Institute (Technical University)
111250, Moscow, Krasnokazarmennaya Street, 14
RUSSIA



Abstract: - Power system on base solar source of energy connected with grid can have many functions using
modern power electronics achievement as full controlled power devices and digital control [1]. Using different
transformations from three coordinate systems to two and p-q theory one can realize different modes of
operating AC/DC converter to control of power flow in the system.

Key words: - photovoltaic energy system, DC/DC converter, AC/DC converter, rectifier, inverter, reactive
power compensator, active filter, power conditioner, digital control system.

1 Introduction
Recently it is increasing an interest to use
renewable energy. There are many renewable
energy sources but solar energy has most popular.
It can be effect of developing power electronics
during last year. There are many new devices based
on which converters, regulators and commutation
switches with many functions can be constructed. It
can be improved photovoltaic systems which can
have many functions such as improving power
quality on load bus, uninterrupted power on it,
active filter of high harmonics current or voltage,
reactive power compensation, changing direction of
power flow (to grid or inversely to accumulator
battery), etc. Using full controlled and fast recovery
devices with microprocessor control we have
possibility to realize all this functions in one energy
system. Digital control of power flows can change
operating mode less than one half-period of power
net’s frequency. Further it can carry out fast
calculation of any parameters of regime. This paper
presents middle power photovoltaic energy system.
It’s shows possibility of power electronics using in
the system.


2 Arrangement and modes of the
system operating
Main components of the system and their
connection are shown on Fig.1.
Photovoltaic cells connected in series and joint in
groups, which connected in parallel. Output of
these modules connected to input of series
connected buck-boost DC-DC converters. The
circuits of converter may be Cuk or it modifications
having good efficiency factor and wide voltage
regulation range. It’s used as maximum-power-
point tracker (MPPT) for the PV modules. Also it’s
regulated battery charger current as photovoltaic
system needed in energy storage unit during
periods of low solar irradiation. From battery side
DC bus connected with DC/AC converter
(converter I on Fig.1) based on full controlled
devices as MOS or IGBT. Also, capacitor
connected in parallel with battery on DC side of
DC/AC converter for shunting high frequency
pulsations. The AC side of the converter connected
with grid. For middle power it is three-phase
converter fulfilled on base of three-phase bridge
topology. It operating with PWM and can provide
current in four quadrants of complex coordinates. It
means that power flow can have any direction.
Other words using PWM we can control phase
current on AC side of the module and change its
direction and character of the power flow.



Fig. 1

Arrangement of the energy system
12th WSEAS International Conference on CIRCUITS, Heraklion, Greece, July 22-24, 2008
ISBN: 978-960-6766-82-4
41
ISSN: 1790-5117
We can regulate voltage on load adding in
system low power (~1/10 S
nom
) AC/DC converter
(converter II on fig.1), which adds small amount of
voltage with corresponding phase angle trough
transformer.
The converter I also can operate as reactive
power generator i.e. source of power with lagging
or lagging current to implement static compensator
and active filter regulating reactive power and
generating high frequency harmonics. It can
compensate current distortion in the grid. It is
important in supplies nonlinear load.
In common case the system can operate in the
following regimes:
1. PV modules generate electric energy, but grid
failure. In this case power provided to vital load.
But control system taking into account the state of
charge of battery for charging it if it’s necessary. In
this case control system operating using algorithm
of operation, which used information about
battery’s state of charge. Control system of DC-DC
also taking into account information about MMP.
Priority in operation determinates by controlling
algorithm. Converter II used as power conditioner
for loads adding in series required value of the
instant voltage level.
2. The grid providing power to all loads including
emergency. Converter I used as reactive power
compensator and active filter of loads. It’s
corrected power factor of loads for grid. Also it can
charge battery operating as rectifier, changing
reference current of the converter’s control system.
The converter II is used as power conditioner for
loads.
3. In case of deficiency of power in PV and grid the
energy system operate as uninterruptible system
from battery.
Power circuit of the control converter I fulfilled
on IGBT transistors. Fairly simple drive circuitry
and constant technology development make
transistors more advantageous than various types of
controlled thyristors up to high power range. It
utilization enables use of medium and high
frequency PWM techniques as well as variable
hysteresis modulation. For the converter II one can
use transistors MOS type as it has small power.


3 AC/DC converters control system
Recently, more prospective is microprocessor
based control system. Control systems of
converters I and II are similar. Therefore here
converter’s I control is described only. Fig. 2 shows
converter I with control and sensing systems.
Converter’s control provides different modes of
operating including reactive power and distortion
of currents compensation. In this case is rational to


Fig. 2 Converter AC/DC with control and sensing
systems

use the theory of instantaneous power known as the
“Akagi-Nabae theory” [4]. This theory is known as
pq-theory based on Clark transformation, i.e.
abc/αβ. According to it forward abc/αβ
transformation is as follows:


(1)

Matrix (1) coefficient can be chosen (2/3 here.
From (1) the resulting vector is:

(2)

Consequently, reverse Clark transformation is as
follows:

(3)

According pq-theory. One can show real and
imaginary power in αβ frame as follows:

(4)

In [4] corresponds to instantaneous real power as
traditional approach, where q(t) is not the same as
traditional reactive power.
According p and q-theory one can be represented

(5)

12th WSEAS International Conference on CIRCUITS, Heraklion, Greece, July 22-24, 2008
ISBN: 978-960-6766-82-4
42
ISSN: 1790-5117
where
and
– average components that
correspond to fundamental active and reactive
power, consequently;
and
alternative
components, caused by harmonics. Thus, reference
signals for currents
and
from this can be
received.
Reverse Clark transformation results reference
currents three-phase system:


(6)

One of the main advantages of pq-theory is that
there is no need for phase synchronization. Also,
average components p and q can be easily detected
(filtered) by means of low-order low-pass filters.
The latter makes positive impact on system
dynamic and stability.
Though, there is no need in synchronization,
non-synchronized rotating two-axis frame is
necessary Rotating at fundamental frequency it
enables to obtain average components p and q as
constant values. Αβ frame can also be used as
rotating frame in the first place. There is also an
option to use stationary αβ frame and another
separate rotating frame afterwards. The latter way
was chosen for the control system design in the
paper. In the Fig.3 the first part of control system
block diagram is shown. After voltages and
currents abc/αβ transformation in stationary frame
(3/2 symbol in the Fig.3) instantaneous reactive
power is calculated. Then its average detected by
means of LPF. In the other control system channel
active power reference signal component is
obtained from DC bus error by means of PI
regulator. Then reference current components
calculation in αβ frame takes place in both
channels. After that p- and q- reference signal
components are summed up for α and β axis and
taken to transformation in rotating frame.



Fig.3 Reference signal generation (part I)

Part II of the control system block diagram
shown in Figure 4. In it there are two PI regulators
are used to work with error between output current
signal and reference current signal in rotating the
dq-frame. Each PI-regulator operates for one axis:
d or q. Preliminarily, abc/αβ and αβ/dq
transformations applied to convert output signal. So
at the output of PI there is a command signal in dq-
frame. Then reverse αβ/dq and abc/αβ
transformations applied to obtain command signal
that is ready for modulation (see Fig.4).



Fig. 4 Reference signal generation (part II)

As far as active is concerned it is necessary to
have synchronized transformation to preserve
accurate harmonics detection. Synchronization is at
fundamental frequency. Control system channel
responsible for load harmonic detection that utilizes
synchronous abc/dq transformation is presented in
Fig.5.



Fig.5 Load harmonic currents detection channel

As seen from it detected fundamental component
is cancelled out of load current leaving harmonics
only for reference. For synchronization phase-
locked loop (PLL) is commonly used. Another
channel is responsible for DC bus voltage. Since
reference signal contains as control and identical to
the one in converter control system (see Fig.3).
Since reference signal contains as low so high
frequency hysteresis modulation is preferable.
Hysteresis modulation provides good dynamics. In
order to maintain constant switching frequency
variable hysteresis parameter techniques can be
used. Figure 6 shows load and grid currents when
AC/DC converter starts operating.

12th WSEAS International Conference on CIRCUITS, Heraklion, Greece, July 22-24, 2008
ISBN: 978-960-6766-82-4
43
ISSN: 1790-5117


Fig.6 Converter AC/DC as reactive power
compensator (starts operating at 0ms to compensate
inductive load).



4 Conclusion
Recently are actual using alternative sources of
energy. Between them is the solar energy, which
has larger popular. Photovoltaic sources connected
with grid have structure with power electronic
arrangement can be multifunctional without deep
change it. One can control power flow in such
system with different goals make it universal local
station of power energy.
The main part of it structure is power electronics
converter AC/DC fulfilled on base full controlled
devices commutate in four quadrant of complex
plane and digital control system. This converter can
operate modes of rectifier, inverter compensator
reactive power, active filter and so on.
One can make controlling processes simple and
fast calculating in three-phase systems using p-q
theory, abc/αβ and abc/dq transformations, digital
filters, etc. Results of modeling these processes
show and justify it.




References:
[1] J .Dixon, L. Moran, M, Ortuzar, R, Carmi, P.
Barriuso and H. Flores, “Static VAR Compensator
and Active Power Filter with Power Injection
Capability, Using 27-level inverters and
Photovoltaic Cells”, IEEE ISIE 2006, July 9-12,
Montreal, Quebec, Canada, pp.1106-1111.
[2] D. Chang, J. Kin, and S. Sul, “Unified Voltage
Modulation Technique for Real Time Three-Phase
Power Conversion”, IEEE Transactions on
Industry Applications, Vol. 34, №2 1998, 374-378.
[3] H. Akagi “New trends in active filters for power
conditioning”, IEEE Transactions on industry
applications, Vol.32, №6 1996, pp.1312 i322.
[4] K. Huosung, H. Akagi, “The instantaneous power
theory on the rotating p-q-r reference frames,”
Power Electronics and Drive Systems Conference,
1999, pp.422-427.


12th WSEAS International Conference on CIRCUITS, Heraklion, Greece, July 22-24, 2008
ISBN: 978-960-6766-82-4
44
ISSN: 1790-5117