Proceedings of the 2014 International Conference on Power Systems, Energy, Environment
The efficiency of the active power filters in high
power DC drive systems
Miedzinski B., Kozlowski A., Wosik J., Kalus M.
particularly onerous at the ignition delay angle of the thyristor
up to 900 producing so-called reactive power stroke,
- power factor deterioration (cos ϕ, tan ϕ) at the connection
to the plant what is a commonly used criterion under
calculation of the fee charged for electrical energy delivered
from the producer,
- the occurrence of the so-called commutation kinks in the
supply voltage waveform as a result of cyclical arising in
time,(depending on number of pulses) two phase short- circuits
in rectifier system,
- deformation of the sine wave of supply voltage at the
connection point to the plant,
- generation of electromagnetic disturbances that negatively
affect operation of any other devices powered from the same
network (control and automation equipment, power protection
etc.)
Abstract— Reasons for the current and voltage waveform
distortion in high power DC loads are discussed. The ways to reduce
the negative impact of such customers on supplying network are
presented illustrating them with the results of simulations. Explained
the idea of reactive power compensation and limitation of impact of
the large power load on supplying network when energize heavy DC
drive systems. Simulation results were verified by tests on physical
model and respective conclusions are formulated.
Keywords— DC motor, reactive power compensation, high
harmonics filter, theory of current physical components (CPC).
I. INTRODUCTION
In industry the electric drives of a high power are commonly
used. In cases where precise regulation of rotational speed is
required the DC motors are often used. They provide good
control characteristics and much better energy properties (e.g.
smaller active power loss comparing to this in regulatory
resistors of ring AC motors). Example of such application are
hoisting machinery drives in the mining industry. Supply
systems of these engines require DC voltage source. To the
end of the 60s of last century the Leonard drive system was
dominant. It was composed of three electric machines:
II. WAY TO REDUCE THE NEGATIVE IMPACT OF CONVERTERS ON POWER
SUPPLY
Reduction of voltage waveform distortion ,from the
sinusoidal, and minimizing the content of higher harmonics in
current (and voltage) can be obtained by installing passive
filters of high current harmonics and by feeding power
converters by the phase-shifted voltage values. Depending on
the inverter structure in the current drawn from the network
occur characteristic harmonics “h” related to number of
pulses:
• Synchronous motor for driving DC generator,
• DC generator,
• DC motor as the key element of the system.
sometimes was installed a DC exciter on a common shaft.
Since the late 60s of the last century this system began to
modify replacing 2-machines of DC source (DC generator plus
AC synchronous motor) by static rectifier. Currently are
employed mostly semiconductor, thyristor circuits. However,
they reveal negative impact on the supply network resulting
in:
- increased reactive power consumption drawn by static
converters (rectifiers) from supplying network. It is
h=nk+/- 1, (1)
where: n-number of pulses,
k- 0,1,2,3,…….
For example for 6-pulse rectifier the harmonics are:
h= 5,7,11,13,17,19…………..
whereas, for 12-pulse they are respectively:
h=11,13,23,25,35,37………..
According to the Fourier series representation of periodic
distorted waveform with increasing harmonic number its
amplitude decreases .It is worth to know that inaccurate
activation of the converter components may cause incidence of
other harmonic numbers than resulting from eqn. (1).
In practice the number of pulses of the converter is commonly
increased through the use of transformers with different vector
Miedzinski Bogdan Institute of Innovative Technologies EMAG. Katowice,
Poland (e-mail: bogdan.miedzinski@emag.pl)
Kozlowski Artur Institute of Innovative Technologies EMAG. Katowice,
Poland (e-mail: artur.kozlowski@emag.pl)
Wosik Julian Institute of Innovative Technologies EMAG. Katowice, Poland
(e-mail: julian.wosik@emag.pl)
Kalus Marian Institute of Innovative Technologies EMAG. Katowice, Poland
(e-mail: marian.kalus@emag.pl)
ISBN: 978-1-61804-221-7
74
Proceedings of the 2014 International Conference on Power Systems, Energy, Environment
groups that provide the output voltages shifted in phase. An
example of such a supply system is shown in Fig.1.
Current and voltage waveforms and their harmonics spectra
for this case are illustrated in Fig. 2.
Application of passive filters of current harmonics results
from the fact that non-linear loading (solid-state converters)
represents a source of current producing high harmonics that
flow into the supply network. So through bypassing of any
non-linear load by passive LC filter with resonance frequency
specified by eqn.(2):
f
110 kV/ 6 kV
P1
P2
Yy0
Dy11
6 kV/ LV
As one can see for frequency less than resonance value the
filter presents capacitive load whereas, for higher-inductive
R2
R1
(2)
1
2π LC
one can limit the amount of current value entering the
network. The characteristics of such a filter is presented in
Fig. 3
P3
6 kV/ LV
rez =
M
DC
Fig. 1 Simplified scheme of the DC drive power supply
XC
XL
Z
or Z = R 2 + (X L − X c )
Z = XL − Xc
2
X L = ωL
XC =
1
ωc
f
fr
a)
Fig. 3 Passive filter characteristics of the frez resonant frequency
b)
respectively. It can therefore ,compensate inductive current
flow for harmonics of number
h < hrez.. Its filtration
efficiency is dependent on accuracy of tuning. However, there
are some factors limiting this efficiency like:-changes in configuration of the supply network,
-change in short-circuit power of the system during the day,
-changing of filter parameters with time due to aging (in
particular, the capacity change).
d)
c)
Current harmonics distributed between the filter and the
supply network is illustrated in Fig. 4
e)
f)
ILh
Ish
IFh
Zsh
g)
ZFh
Load
h)
Fig. 2 (a-h).Current and voltage waveforms and harmonics spectra for
DC drive system as in Fig.1. a,b,c,d –at measurement point P1; e,f,g,h –
at measurement point P2
ISBN: 978-1-61804-221-7
75
Fig.4 Current harmonics distribution depending on the parameters
of the filter and supply network.
Proceedings of the 2014 International Conference on Power Systems, Energy, Environment
Value of current through the network and filter for given
higher harmonics due to non-linear load can be estimated from
eqn.(3)
I Sh = I Lh
z Fh
zSh + z Fh
I Fh = I Lh
z Sh
z Sh + z Fh
(3)
a)
b)
c)
d)
In the case of fine –tuning of the filter to the resonance
frequency occurs:
zFh=0 and IFh=ILh
Such a situation is, however, onerous in practice since the
filter/filters are provided with high harmonics of the network
current due to deformed voltage waveform at the power. It
may than lead to an overloading of the filter and related
thermal destruction of its elements (reactors, capacitors).
Therefore, preventive measure used in practice is to tune the
filter to the frequency slightly lower than the given harmonic
for example, by about 5% thus, to f=0.95frez.. Then, non-zero
value of the impedance of the filter ( z Fh
Fig.6 Current and voltage waveform in network with non-linear
load when apply passive filters (at measurement point P1)
broad spectrum of harmonics, hence their compensation would
require the use of multiple filters or combined filters of a
complex structure. In this case for the filter construction
consideration the generic algorithm can be employed
successfully. In practice however ,it is limited to the
installation of a few filters of a selected harmonics (usually 23) of the highest numbers (amplitude) only to reduce the level
of both current and voltage high harmonics. Reactive power
compensation by means of sectional static capacity banks or
by quasi on-line way using thyristor switches to adjust the
capacity do not allow for perfect compensation of the reactive
power flows in power system with non-linear loads.
Static capacitor banks can be tuned to the needs of the
system with a resolution of the power of the smallest degree of
compensation step. Speed of this way of compensation is
also limited by speed of control unit as well as thyristor
switches of particular sections(switching frequency must be
limited because of durability requirements). The capacity
bank can be overloaded by current harmonics of much
higher frequency as well. However, the highest risk of
damage occurs during the series resonance. In spite of
application of this type of compensation the current
waveform in power supply is far from sinusoidal what can
be compared from Fig. 7
≠ 0 ) effectively
reduces the value of the current flowing into it from the
network .A side effect, however is the deterioration of
filtration efficiency and outflow to the network of respective
amount of current high harmonics generated by non-linear
load(converter). Current and voltage waveforms and their
harmonics spectra when apply perfectly tuned filter for
required harmonic are presented in Fig. 6 as an example.
110 kV/ 6 kV
P1
6 kV/ LV
6 kV/ LV
F11h
R1
F13h
R2
M
DC
Fig.5 Compensation system of reactive power and high harmonics
of current (11th and 13th) by means of passive LC filters
Because, current of the non-linear load is characterized by a
ISBN: 978-1-61804-221-7
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Proceedings of the 2014 International Conference on Power Systems, Energy, Environment
The use of very fast thyristor switches for each capacity
section or to control inductive current value through the
reactance component of the LC system can improve the power
balance for the required period of time in a statistical sense.
However, the shape of the current flowing through the
reactance element is still highly distorted. Hence, this
compensation system is also non-linear and represents an
additional source of harmonics (current and voltage). For this
reason, it is also unfavorable for cooperating another loads. In
the recent period there is developed a completely new
approach to the problem of reactive power compensation in
circuits with non-linear loads resulting in distorted waveforms
of both current and voltage. This new approach has resulted in
the development of a new power theory enabling
implementation in practice of so-called active compensation
110 kV/ 6 kV
P1
P2
6 kV/ LV
6 kV/ LV
R2
R1
M
DC
III. MODERN METHODS OF REACTIVE POWER COMPENSATION
The problem of reactive power compensation in circuit with
non-linnear loads and therefore, with distorted from sinusoidal
waveforms of voltage and current were of interest to scientists
and engineers from the end of the nineteenth century, i.e.
almost from the beginning of AC application. For many
decades, however, failed to create a coherent theoretical basis
for such compensation, which would allow for construction of
practical compensation systems of high efficiency. In 1971
appeared a concept of the so-called active filters that allow for
high harmonics compensation using keying by means of
semiconductor devices (SCRs). The problem remained to
create a theoretical basis for determining the current reference
(desired quantity) for the compensator and the supply network
as well. The principal difficult was the incidence of various
reasons for current flows( voltage deformation, load nonlinearity) as well as imbalance of the voltage source and
asymmetry of the load. In 1983-84 created two independent
theories of power in circuits with deformed voltage and current
waveforms so-called theory of instantaneous power (IPT) [1],
[2], [3] and the theory of the physical component of the current
(CPC) [3], [4]. The first one is based on an analysis of current
and voltage signals in the time domain and the other on the
analysis of the signals in the frequency domain. IPT output
theory is based on the transformation of phase voltages and
currents in the single phase circuit into the stationary
rectangular system ”p,q” in the α and β axes. However ,this
theory did not allow to compensate the harmonics caused by
distorted supply voltage. Further works on its development led
to the transformation from “p,q” system to rectangular
coordinate system rotating with angular speed equal to this of
phase voltage and current vectors and to its respective
adaptation to the various multi-phase circuits without and with
the neutral conductor [2]. There is also no physical
interpretation of the phenomena of electric energy possible.
Fig.7 Compensation system of reactive power non-linear load by
capacitor bank; a- waveform of voltage and current at measurement
point P1 phase L1; b- waveform of capacitor bank current at
measurement point P2 phase L1
110 kV/ 6 kV
P1
AFF
6 kV/ LV
6 kV/ LV
R2
R1
M
DC
a)
b)
Fig.8 Compensation system of reactive power based on active
power filter; scheme, waveform of voltage and current at
measurement point P1
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Proceedings of the 2014 International Conference on Power Systems, Energy, Environment
The CPC theory does not have these disadvantages.[4]
Developed on its basis a control algorithm for active filter
allows for compensation:
-current higher harmonics,
-reactive power of phase shift,
-currents due to source imbalance,
-so-called scatter current (new physical quantity).
Developed active filter control algorithm (APF) based on the
CPC theory has been verified in simulation studies and
laboratory tests on physical model.
is
R
iL
iF
u
U
i
APF
Load
Fig. 10 Model of system used for the simulation study (control
algorithm APF was developed based on the CPC theory [3])
IV. THE RESULTS OF THE SIMULATION OF REACTIVE POWER COMPENSATION
USING THE ACTIVE POWER FILTER
The simulation tests of the effectiveness of compensation in
circuit with non-sinusoidal currents and voltages and in
unbalanced circuits were conducted on a simulation model
developed in the software package Matlab/Simulink. Model
adopted for the simulation studies is shown in Fig. 8
a)
b)
c)
d)
The results of voltage and current waveforms and their
harmonics spectra after use of the active power filter are
presented in Fig. 8. [5], [6]
110 kV/ 6 kV
P1
AFF
6 kV/ LV
6 kV/ LV
e)
R1
R2
Fig. 11 (a-e) Voltage and current waveforms in system with nonlinear-load and with active power filter,
M
DC
Fig. 9 Model of system used for the simulation study (control
algorithm APF was developed based on the CPC theory [3])
-Application of passive filters neither allow for effective
compensation of high current harmonics nor for compensation
of reactive power flows in circuits with non-sinusoidal voltage
and current waveforms.
-Compensation based on stationary
capacity banks
employment is not suitable for circuits with non-linear voltage
and current waveforms due to high risk of ferro/resonance.
-Installing of lag compensators with thyristor switches
however, allows you to improve the balance of power and
increase power factor value cos ϕ (tan ϕ) but introduces to the
system distorted currents.-Reactive power compensation by
means of active power filter (APF) based on the control
algorithm developed on the basis of theory of physical current
components allow for successful compensation of reactive
power flow of phase shift and in addition to:
-elimination of current harmonics in the supply network,
-load symmetrization (in case of unbalanced load),
V. THE RESULTS OF THE EFFECTIVENESS OF REACTIVE POWER
COMPENSATION FOR THE PHYSICAL MODEL
To conduct laboratory tests a network model composed of
power source with non-linear load RL (6th pulse controlled
rectifier) has been developed and assembled. Active power
filter of 10kVA, operated in open control system, was
connected as in Fig.10. The resulting voltage and current
waveforms and their harmonics spectra for characteristic point
of the system are presented in Fig.11
ISBN: 978-1-61804-221-7
VI. CONCLUSIONS
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Proceedings of the 2014 International Conference on Power Systems, Energy, Environment
-elimination of the scatter current occurring for deformed
source voltage and when the load impedance is frequency
dependent.
REFERENCES
[1] H. Akagi, Y. Kanazawa, A. Nabae” „Generalized theory of the
instantaneous reactive power in three phase circuits”; Proceedings of the
Int. Power Electr. Conf (JIEE IPEC), pp 1375-1386, Tokyo/Japan 1983
[2] A. Ferrero, G. Superti-Furga: “A new approach to the definition of
power components in three-phase systems under nonsinusoidal conditions”.
IEEE Trans. Instrum. Meas., Vol40, pp. 568-577, June 1991
[3] L.S. Czarnecki: “Dynamic, power quality oriented approach to theory
and compensation of asymmetrical system under nonsinusoidal
conditions”, Europ. Trans. Electr. Power, 5, pp.347-358, ETEP199
[4] L.S. Czarnecki: “Power in Electrical Circuits under nonsinusoidal
conditions Wafeform Currents and Voltages”, (in polish), Oficyna
Wydawnicza Politechniki Warszawskiej, Warszawa 2005
[5] J. Wosik, M.Kalus, A.Kozlowski, B.Miedzinski, M.Habrych: „The
Efficiency of Reactive Power Compensation of High Power Nonlinear
Loads” Elektronika Ir Elektrotechnika, vol.19, No7, pp.29-32,2013
[6] J.Wosik, M.Kalus, A.Kozlowski, B.Miedzinski” “Improvement of the
Electric Energy Quality by use of active Power Filters” Proceedings of
ICREPQ’13, Bilbao, March 2013.
B.Miedzinski (Prof.)– bogdan.miedzinski@emag.pl – Institute of
Innovative Technologies EMAG, Katowice Poland. Area of his
activity are: contact materials, power theory, research of signals by
using Rogowsky coil, switches.
A.Kozlowski (PhD)– artur.kozlowski@emag.pl - Institute of
Innovative Technologies EMAG, Katowice Poland. Area of his
activity are: contact materials, switches, power quality, power
electronics, flameproof power supply devices. Head of R&D
Department.
J.Wosik (PhD)– julian.wosik@emag.pl - Institute of Innovative
Technologies EMAG, Katowice Poland. Area of his activity are:
dispatching systems, theory of power, power quality, construction
flameproof devices, compensation systems of reactive power and
active power filters.
M.Kalus (PhD) – marian.kalus@emag.pl - Institute of Innovative
Technologies EMAG, Katowice Poland. Area of his activity are:
drive AC and DC systems and simulation research, converters power.
ISBN: 978-1-61804-221-7
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