Special Issue: Proceedings of 2nd International Conference on Emerging Trends in Engineering and Management, ICETEM 2013
IMC with HF Power Transformer based on Pulse
Density Modulation
Javad Khodabakhsh
Elnaz Pourmand
Department of Electrical Engineering, Shahr-e-Qods
Branch, Islamic Azad University,
Tehran, Iran
J.Khodabakhsh@gmail.com
Department of Electrical Engineering, South-Tehran
Branch, Islamic Azad University,
Tehran, Iran
El.Pourmand@gmail.com
Abstract—In this paper high frequency transformer link used to
connect two network with different voltage. Matrix converter
control active power flow between networks. Pulse density
modulation technique’s performance evaluated and despite the
need to higher bandwidth in modulator the performance of this
method is good.
in contrast to conventional AC/DC/AC converters. Other
inverter converts DC bus voltage to desired waveform.
Projected scheme used a pulse high frequency link to connect
inverters via a transformer. Figure 1 demonstrates the block
diagram view of overall system and circuit diagram of
inverters.
Keywords-Matrix Converter; Pulse Density Modulation; Active
Power Flow Control; Pulse Width Modulation;
I.
INTRODUCTION
Over the recent years, there has been a shift of focus from
concentrated, non-renewable energy resources to distributed,
renewable energy resources. Proclivity toward these energy
resources poses new technical and economic challenges.
Interconnecting energy converter to grids should meet some
criteria such as grid availability, voltage level compatibility,
and economical efficacy. Energy conversion devices deliver
electrical energy at a specified voltage and current; therefore,
they need to a power conditioner interface that connects them
into grid. Power frequency transformer is a huge and expensive
part of these energy converters. Matrix converters combination
with a high frequency transformer are a typical solution that
extensively use to interface because of minimum energy
storage devices, bidirectional energy flow capability, and low
harmonic injection to network [1], [2]. Matrix converters
classified into two main categories: direct matrix converter
(DMC) and indirect matrix converter (IMC). Matrix converters
have a main drawback that voltage level ratio cannot exceed a
limited range between two sides. Depending on modulation
method, the maximum achievable ratio is 0.866. Adding some
passive series elements. Previous works try to improve this
weakness at DMC [3]. In the IMC works more emphasis on
over modulation technique to increase mentioned ratio [4] and
using high frequency transformer link [5]. Purposed method is
a linear control strategy to active and reactive power control
matrix converter based on pulse density modulation technique.
II.
PURPOSED METHOD
Conventionally an IMC, known as AC/DC/AC converters,
is composed of two inverters that work cascade. Incoming side
absorbs power from grid and converts AC power to DC
waveform as a synchronous rectifier. A virtual DC bus
connects two inverters. Do not needing to massive energy
storage elements in DC bus, matrix converters show superiority
AC Source 1
Inverter
1
Inverter
2
AC Source 2
HF Pulse Transformer
Fig1.a. Overall system block diagram
Fig 1.b. Inverter circuit
High frequency square waveform apply to transformer each
side and control of phase angle between primary and secondary
voltage use for active power flow control. Commonly, carrier
based pulse width modulation method exploited to generate
mentioned high frequency square waveform [5]. This method
is efficient and simple thus its implementation does not need to
a complex hardware platform, but the two key restrictions of
PWM method are as follow: first direct control of pulse width
from zero to one. The problem arises when the pulse width is
less than specified amount the transformer can pass according
to physical restrictions [6]. Practical pulse transformers have a
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International Journal of Advances in Engineering Sciences Vol.3 (3), July, 2013 e-ISSN: 2231-0347 Print-ISSN: 2231-2013
Special Issue: Proceedings of 2nd International Conference on Emerging Trends in Engineering and Management, ICETEM 2013
limited bandwidth; As a result, the narrow pulses cannot go
across real transformers and only cause to losses increasing in
the transformer. Second, PWM method is very sensitive to
noise; therefore any unwanted noise can affect system
operation. To solve the problems, pulse density modulation
(PDM) method can be applied to control signal to obtain pulses
that have width larger than a prefix value, also PDM technique
can be design to be robust to noise.
III.
PULSE DENSITY MODULATION
Pulse Density Modulation is achieved by a unity pulse
width, also the pulses height and a continual time of existence
for the pulses within the switching period. The modulated
parameter is the presence of the pulse. In any time slot it is
evaluated that if the pulse should be exist or not, hence the
density of pulse change from time to time. Regularly, the PDM
modulator, robust to noise, is implemented by a 1 bit digital to
analog converter. The higher order of modulator help to better
noise shaping. In this paper, a first order sigma-delta modulator
used as PDM modulator. Figure 2 illustrates the modulator
block diagram.
Fig.2 Pulse density modulator block diagram
IV.
An PID controller employ to control of active power flow
through converter. Control variable is phase difference between
reference waveform of two inverters. The output of controller
feed to PDM modulator and switching signal generate
according to these waveform and conditions for matrix
converter operation [1]. In order to implement above
conditions used to proposed method in [5]. The pulse minimum
width of PDM modulator to be assumed as 3 msec control
system block diagram demonstrate on Figure 4
Fig.4.a Matrix converter control system block diagram
Fig.4.b. Matrix converter output side duty cycle control
ACTIVE POWER FLOW CONTROL
Main functionality of matrix converter introduced in this paper
is active power flow control. Inverters change input low
frequency voltage to pulse high frequency voltage. Voltage at
two sides of transformer can be expressed by
According to Fourier series first term is first harmonic of high
frequency pulse injected to transformer. High frequency
transformer model as leakage inductance
Fig.3 IMC high frequency link model
Active power flow via transformer can be calculated by
Because of limited bandwidth of transformer higher order
harmonic can be ignored.
V. CASE STUDY
In many cases need to connect two separate network
together with the different voltage. In case study a bidirectional
LV energy source/absorber such as wind turbine assumed.
Wind turbine can be used to store energy in a fly wheel in the
off load time and return energy to system in peak time. To
control system a bidirectional active power regulator is
necessary. The fuel cell is connected through an IMC with high
frequency transformer link. Transformer is designed to operate
at 10 kHz; therefore power density is very higher than power
transformers. To simulate system wind turbine assumed as a
three phase voltage source and MV network modeled as a
infinite bus. Inverter switches assumed ideal bidirectional
switch to eliminate the commutation phenomenon and more
concentration on purposed method. The model developed in
MATLAB/SIMULINK. The following is the simulation results
of the circuit demonstrate in figure 1. The simulation
conditions are follows, a 100-kHz and, medium voltage
network as input network is 6.6 kV and low voltage is
sinusoidal voltage then the output voltage is 50 Hz, 400V, and
the load transfer control range 1 kW. To evaluate the
performance of the system, three methods are considered:
Power flow from MV source to LV source, Island operation,
Power flow from LV source to MV source. A case study of
this situation one step function with power 1kw is selected. The
frequency of PDM modulator was chosen 108 kHz.
Figure 5 illustrate the per unit voltage of transformer two
windings in the situation that IMC isolate networks. As shown
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International Journal of Advances in Engineering Sciences Vol.3 (3), July, 2013 e-ISSN: 2231-0347 Print-ISSN: 2231-2013
Special Issue: Proceedings of 2nd International Conference on Emerging Trends in Engineering and Management, ICETEM 2013
in figure 5 primary and secondary voltages are in phase and
zero active power flow through IMC.
Fig. 7. System performance in island mode command
2.
Fig. 5 Per unit voltages at island mode
Figure 6 illustrate the voltage in the transformer primary and
secondary side in the situation power flow from MV network
toward LV network
Isolated network inject active power to main network
At this case, at the first two network operate as isolate
networks then control command force IMC to power flow from
LV to MV network. Figure 8 illustrates system operation
Fig. 8. System performance in power flow from LV network to MV
network command
Fig. 6 Per unit voltages at conduction mode
Phase shift in figure 6 shows the power flow through IMC and
high frequency transformer. Figure 5 and 6 illustrate system
operation condition in steady state. To evaluate the system
operation in transient mode a step command applied to system
input. Three different scenario is selected to show system
performance.
1.
3.
Isolated network absorb active power from main
network
At this case, at the first two network operate as isolate
networks then control command force IMC to power flow from
MV to LV network. Figure 9 illustrates system operation
IMC isolate networks
At the first situation power flowed from LV to MV and control
input change to isolate networks. Figure 7 illustrates system
operation.
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International Journal of Advances in Engineering Sciences Vol.3 (3), July, 2013 e-ISSN: 2231-0347 Print-ISSN: 2231-2013
Special Issue: Proceedings of 2nd International Conference on Emerging Trends in Engineering and Management, ICETEM 2013
performance in PDM case. The weak point of PDM technique
is higher band width need and necessity to complex hardware
in implementation in order to PWM case.
REFERENCES
[1]
[2]
[3]
[4]
Fig. 9. System performance in power flow from MV network to LV
network command
As shown in figure 4 through 6 a 100W command applied
to the system, and the parameters is adjusted that matrix
converter power flow settle at new set point in less than 2
power cycle. In figure 4 matrix converter isolate two network.
In figure 5 and 6 isolated network let to active power flow in
two directions.
VI.
[5]
[6]
[7]
CONCLUSION
[8]
Implementing indirect matrix converter with high
frequency transformer link, PDM method used as modulation
technique to improving system performance instead of PWM
method. The simulation result illustrate the system
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International Journal of Advances in Engineering Sciences Vol.3 (3), July, 2013 e-ISSN: 2231-0347 Print-ISSN: 2231-2013