Comparative analysis of improved high performance direct power control of three phase pwm rectifier

IJRET: International Journal of Research in Engineering and Technology eISSN: 2319-1163 | pISSN: 2321-7308
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Volume: 04 Issue: 03 | Mar-2015, Available @ http://www.ijret.org 394
COMPARATIVE ANALYSIS OF IMPROVED HIGH PERFORMANCE
DIRECT POWER CONTROL OF THREE PHASE PWM RECTIFIER
Hemlata S Pol1
1
Department of Electronics Engineering, Fr Conceicao Rodrigues College of Engineering, Bandra,India
Abstract
Direct Power Control with switching table control produces irregular ripples and variable switching frequency. In the improved
DPC of PWM rectifiers using fractional control period of the active voltage vector and the rest period for the null vector not only
makes it line side inductance independent of the circuit and is very simple to implement due to the duty cycle concept. By
simulation it is identified that the level of distortion as well as regulation of the DC link voltage with control of power factor
improves the overall performance of the drive. The improved Direct Power Control (DPC) PWM methodology is comprehensively
analyzed and studied for three phase rectifiers and compared with the classical DPC with PLL. The MATLAB simulations shows
the effectiveness in obtaining unity power factor and constant dc link voltage control. The power ripples are considerably reduced
and input sinusoidal grid currents are obtained.
Keywords— Direct power control, instantaneous active and reactive power, pulse width modulation, duty cycle, dc
link control, unity power factor
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1. INTRODUCTION
The application of three phase rectifiers in areas like
renewable energy systems as wind turbines and photovoltaic,
FACTS, drives ac and dc, battery storage and micro grid
operations have increased tremendously. There are various
models for power control of PWM rectifier like voltage
oriented control (VOC), voltage based DPC, virtual flux
oriented control (VF-VOC), VF-DPC. Voltage oriented based
control decomposes grid currents into active and reactive
power components separately which through PI controllers
are fed through modulator to synthesize voltage vectors. Fine
tuning of PI is required to achieve steady and dynamic
response.
In DPC for achieving satisfactory performance the switching
frequency required is very high, which adds up to the
hardware burden. Major work has been done to tackle this
problem such as SVM-based DPC, predictive control, fuzzy
logic control and deadbeat control etc. But these methods
further increased the complexity and computational burden.
To address the problem several techniques with new
switching tables were proposed. But they tried to incorporate
accuracy and efficiency by eliminating ac voltage sensors,
which reduced the overall system robustness.
Conclusively the improved DPC that is studied here does not
focus on effectiveness of switching tables rather it focuses on
improving steady performance. The parameter being selected
for this is duty cycle of the voltage vectors. The concept of
duty cycle control is to select the fraction of time for which
the voltage vector will be applied. The existing duty cycle
methods were parameter dependent, which was again
contributing to decreasing robustness and increasing
complexity. The improved method is kept simple by
eliminating the requirement of system parameters. Simulation
results prove that the new improved DPC has high
performance .
Direct power control is high performance instantaneous
power control theory basically similar to direct torque control
in motor drives. It directly selects the desired grid voltage
vector from predefined switching table according to the grid
voltage position or virtual flux position and the errors
between the reference and feedback powers are calculated.
Conventional DPC has a drawback of high power ripples and
variable switching frequency. The improved DPC introduces
the concept of fractional control of duty cycle by active
vectors over the allocated period improves the performance
of the rectifier by making it independent of line inductances
thus robustness is achieved and ripples in power are also
reduced with almost unity power factor operation.
Unique features offered by this improved DPC are:
 Fixed and low switching frequency.
 Sampling frequency for digital implementation is low.
 Parameter independent thus robust.
 Simple and easy control with only two voltage
vectors.
2. PRINCIPLE OF DPC IN THREE PHASE PWM
RECTIFIER
The topology as seen from the fig.1 which is two level can be
mathematically modelled into two phase stationary reference
(α β) frame and with R and L as equivalent series resistance
and choke.

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The Direct Power Control (DPC) is based on the
instantaneous active and reactive power control loop. There
are no internal current control loop and no PWM modulator
block. The switching states are determined with a switching
table based on the instantaneous errors between the
commanded and estimated values of active and reactive
power.
Fig.1 Three phase rectifier topology [1]
Fig.2 shows the conventional classical configuration of DPC
where the instantaneous active and reactive powers are the
controlled variables, this method is based on switching table
for selecting the desired voltage vector. The drawback of
DPC is the variable switching frequency, system parameters
and the switching table.
The PI controller needs fine tuning and the power ripples
along with harmonic distortion in the input side cannot be
filtered out easily with this basic configuration. In the new
improved DPC with duty cycle adjustment in fractional
control environment not only improves the overall
performance but it reduces the complexity of various system
parameters.
Fig.2 Classical configuration of DPC with PLL
According to the switching table first time as usedby
Noguchi et al for the control scheme as depicted in fig.2. This
method is based on selecting a voltage vector from lookup
table, table 1, according to the errors of active and reactive
powers as well as the angular position of the source voltage
vector.
The drawback of the DPC is variable switching frequency
which depends on the sampling frequency, the switching
table , system parameters, reference values of active and
reactive powers and hysteresis band controllers and the
switching states. This variable frequency introduces
harmonic spectrum in AC line currents and thus the design of
filters become very difficult. In order to attenuate the
harmonics and power ripples large value of sampling
frequency and inductance should be selected this increases
the cost , losses and reduces system dynamics.
There have been many approaches to control these factors as
virtual flux estimaters but PI controller tunning is very
complex and thus it becomes difficult to achieve high
dynamic performance.
.
3. IMPROVED DIRECT POWER CONTROL
WITH FRACTIONAL CONTROL
The improved direct power control method has advantage of:
 Simple algorithm for duty cycle determination
 Independent of line inductance parameter
 Operation at constant frequency simple filter design
 Low switching and sampling frequency
 Calculations are fast and simplified
The equation for grid voltage ‘e’ can be written as :
e = Ri + L
dt
di
+ v (1)
Where v, e, i represent rectifier voltage vector, grid voltage
vector and grid current vector respectively.
Transforming the three-phase model to stationary (α β)
frame ,the complex power S for active power P and reactive
power Q can be given as :
Complex power as per [1]
S= P + jQ = 1.5 ( i*e) (2)
Instantaneous active power p = uα iα + uβ iβ (3)
Instantaneous reactive power q = uβ iα - uα iβ (4)
The equation for grid voltage ‘e ’ is :
e = | e | (5)

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Fig.3 Improved Duty Cycle based DPC [1]
Differentiating e we get :
dt
de
= j w |e| = j we (6) (6)
Differentiating the grid current i :
dt
di
=
L
1
( e – v – R i ) (7)
Now differentiating complex power :
dt
dS
=
L
1
[1.5 ( | e | 2
- v*e ) – (R – j w L).S ] (8)
dt
dP
=
L*2
3
[|e|-Re(v*e) ]- (
L
R
)P–wq (9)
dt
dQ
= -
L*2
3
(Im (v*e) –(
L
R
) Q + w p ) (10)
The expressions can be further simplified as :
dt
dP
=
L*2
3
| e | 2 –
L
Vdc
| e|
L
R
P (11)
dt
dQ
= -
L*2
3
L
Vdc
| e | sin ( ) + w p
(12)
here Vdc is DC-link voltage. The power slopes can be
calculated from eq. 9 and 10 .The rectifier voltage vectors
with 6 sector division are as shown below.
Fig.4 slopes of active power and reactive power verses grid
vector position for various rectifier voltage vectors (assuming
p = 900 W and q = 0) [1]
According to the influences of voltage vectors over active
and reactive power slopes in any sector k can be obtained as
shown in table 1where k is the cycling index .For controlling
the active and reactive power simultaneously we can select
specific set of vectors corresponding to incremental or
decremental nature of P or Q .
Based on the analysis of appropriate set of combinations of
voltage vectors , it is imperative to notice that switching
tables based on grid voltage vectors have some inherent
drawbacks especially in wide power range.
The efforts to improve the selection of appropriate vector is
done as per the following model. The six sectors on
stationary reference frame and the rectifier voltage vectors
are presented in fig.5
Fig. 5 Rectifier voltage vectors and sector division of
DPC.[1]

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For simultaneous control of both active and reactive power
the table. 1 can be summarized . It can be seen that there are
more than two vectors to satisfy a change in power
requirement. For example if both P and Q are increasing then
V0,7 produces least variation on Q, small (first half sector) or
medium (second half sector) variation on P.Vk+3 has the
biggest influence on P and medium on Q. The influence of
Vk+2 is moderate.
Similarly for P increasing and Q decreasing there are
insignificant differences between Vk-1 and Vk-2except that
when transition from sector k to k+1, Vk-2 increases Q rather
than decreasing with a value smaller than that caused by null
vector.
Table 1: Switching table for DPC of PWM rectifier
Sp Sq Selected vector
1 1 V0,7,Vk+2,Vk+3
1 0 Vk-1,Vk-2
0 1 Vk+1
0 0 Vk
Table 2: Comparison of vectors in case of P and Q
increasing[1]
|ΔP| |ΔQ|
V0,7 Small to medium small
Vk+2 Medium to small big
Vk+3 big medium
Table 3: Comparison of vectors in case of P increasing and
Q decreasing [1]
|ΔP| |ΔQ|
Vk-1 big small
Vk-2 small big
One special case of table 1 is selected to obtain active vector
as shown in table 4. Similar results can be obtained from the
other vectors also. Table 4 is used to study the comparison of
DPC models.
Table 4: Active vector selection for the improved DPC with
duty cycle control
Sp Sq Selected vector
1 1 Vk+3
1 0 Vk-1,
0 1 Vk+1
0 0 Vk
Duty cycle is the ratio of the applied duration of the active
vector to the whole period and for duty cycle =1 the
improved DPC will act as STDPC . Since active power is
depending on the fractional control of duty cycle ‘d’ it will
try to improve steady performance of the rectifier .From
equation (6) slopes of the active power for the active vector
s1 and for null vector s2 can be obtained . A typical
waveform employing active as well as null vector is as shown
:
Fig 6 waveform of active power when both active and null
vector is employed for one period [1]
Supposing that P reaches its reference value in a deadbeat
fashion , the equation for P k+1
can be :
= + s1.d.tsp + s2.(1-d).tsp (11)
i.e.
= + s1.d.tsp + s2.(1-d).tsp (12)
from (11) & (12) the optimized duty cycle d can given as :
d = (13)
The above equation for duty cycle requires accurate
knowledge of slopes s1 and s2 of active powers. Unlike
equation (6) where the power slopes were dependant on
information of input inductance and resistance , this
techniques doesn’t require them . Hence trying to maintain
the system simplicity and robustness . By expanding eq (13)
following equation can be obtained :
d =
tspss
Pkef
)21(
Pr


+
tpss
tsps
)21(
2


(14)
Considering the denominator to be constant the parameter
dependence is eliminated in first term , whereas second term
is complex and parameter dependent term. Here the
numerator s2 is active power slope caused by the null vector
which has small but constant influence over the reactive
power. Hence the first term can reflects the regulation of
active power whereas the second reflects regulation of
reactive power .
Thus the final expression for this algorithm can be given as :
d = |
Cp
Pkef Pr
| + |
Cq
QkQref 
| (15)

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Based on the variations in Cp and Cq the accuracy of the duty
cycle d influence the steady performance and dynamic
response only partly , but it would not cause much influence
on the stability of system, because the mechanism of STDPC
still works to ensure the system is stable. DPC has high
performance control logic for PWM rectifier based on
instantaneous power theory which was first proposed by
Akaji in his paper.
Extensive simulations showed that to achieve good steady
state and dynamic performance
Cp = Cq =
L
Vdc
e peak
tsp (16)
Where e peak
is the peak phase value of grid voltage. The
basic principle of DPC is similar to direct torque control
(DTC) in motor drives it directly selects the desired voltage
vector from a switching table which is already defined ,
according to the grid voltage position (or virtual ﬂux
position) and the errors between the feedback value and
reference active/reactive power value. In voltage oriented
control the internal current loop is eliminated in DPC. As a
result, DPC features very quick dynamic response with
simple structure.
4. SIMULATION RESULTS
A digital computer simulation model is developed in
MATLAB/SIMULINK platform to verify the effectiveness of
the control method under steady state conditions. The
waveforms obtained confirm the improvement in the DPC
with minimum distortion and less harmonic noises ie. THD .
The simulations are tested for two different sampling
frequency of 20 KHz and 40 KHz and the effect of variation
in the result can be observed significantly.
The system parameters used for simulations are Line
resistance R = 0.3 Ω, L = 10mH, Vdc = 300V, DC bus
capacitor = 2350μ F, active power constant gain Cp =183.7
W, Reactive power constant gain Cq = 183.7 Var, fs = 50
Hz, tsp = 50μ sec
Fig 7 Simulink block for improved DPC

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Fig 8 simulation of DPC with 20 Khz sampling frequency Fig 9 simulation of DPC with 40 Khz sampling frequency

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Fig. 10 simulation of DPC with PLL at 20 KHz
Fig. 11 simulation of DPC with PLL at 40 KHz

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Table 5: THD i for different values of sampling frequency
(Q ref = 0, P ref = 900 W)
Configuration
Sampling frequency
Classical
DPC with
PLL
THD %
Improved
DPC
THD%
DPC1 20KHz 5.58 5.27
DPC2 40 KHz 8.77 8.46
5. CONCLUSION
A DPC with simple approach of with duty cycle control was
simulated and studied for harmonic distortion, power factor,
switching losses and sinusoidal input ripple. It is found that
using Direct Power Control and its controlled algorithm the
performance of the rectifier greatly improves due to less
harmonic distortion, good power factor, less switching loss
and sinusoidal input current waveform. The main advantage
of using duty cycle in DPC is that the rectifier parameters are
independent of the control and only a fraction of
controlperiod is used. Also the methodology used in DPC is
of instantaneous active and reactive power theory which are
regulated separately so the control does not require inner
current loop as required for voltage oriented control thus
giving high dynamic performance.
REFERENCES
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, IEEE TRANSACTIONS ON POWER ELECTRONICS,
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ELECTRONICS, VOL. 28, NO. 11, NOVEMBER 2013
[3]. “ Predictive Direct Power Control of Three-Phase
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Detection”PanfiloR.MartinezRodriguez,Member,IEEE,Gerar
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[7]. Simple Direct Power Control of Three-Phase PWM
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Comparative analysis of improved high performance direct power control of three phase pwm rectifier

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Comparative analysis of improved high performance direct power control of three phase pwm rectifier