Recent Advances in Circuits, Systems, Signal Processing and Communications
Impact of High Power DC Drive on Conditions of Operation
of MV Electric Supplying Network
JULIAN WOSIK, MARIAN KALUS, ARTUR KOZLOWSKI, BOGDAN MIEDZINSKI,
MARCIN HABRYCH*
Department Research and Development
Institute of Innovative Technologies EMAG
Str. Leopolda 31, 40-189 Katowice,
POLAND
*Wroclaw University of Technology
Institute of Electric Power Engineering
50-370 Wroclaw, Wybrzeze Wyspianskiego27,
POLAND
wosik@emag.pl; bogdan.miedzinski@pwr.wroc.pl;
Abstract: - The paper discusses the specifics of work of high power DC motors supplied from the MV
network on the example of selected hoist drive in a mine. The key assumptions of operation of such
machine and its way of supplying are considered. Results of simulation studies on the impact of the
converter on medium voltage grid conditions are presented and possibilities of improvement of such
situation are indicated. The effects of the application of the compensation of both passive and active
depending on the location of the compensator unit are discussed.
Key-words: DC high power drive, rectifier, high harmonics, passive LC filter, active power, reactive
power, active power filter, power loss, compensation.
η = ηSM × ηGDC × ηDCM
1. Introduction
where:
ηSM - efficiency of synchronous motor,
ηGDC - efficiency of DC generator,
ηDCM - efficiency of DC motor.
In industrial electrical power installation are used
quite often large drives with a DC motor.
Examples of this are drives of mining hoists. In
the past they were mainly drives operated in
system developed in 1911 by Leonard (Fig. 1). It
consists of three interconnected shaft electrical
machines, namely:
- synchronous motor (SM),
- DC generator (GDC),
- DC motor (DCM).
Development and dissemination of thyristors at
high current levels, at the end of the 60 s of the
last century, made it possible to replace
successfully the two machines DC generating
system (SM+GDC) by static-controlled thyristor
rectifiers (1968) (Fig. 2).
6 kV
6 kV
SM
GDC
LV
DCM
DCM
Fig.2.Simpified scheme of thyristor drive system.
Fig. 1. Diagram of the Leonard drive system
(SM-synchronous
motor
AC,
GDC-DC
generator, DCM- DC motor).
It is characterized by higher (by about 18%)
resultant efficiency. This fact caused the rapid
spread of this type of systems to power DC
motors drive of high power. However, it should
be noted that the power converting systems
drawn, from supplying network, currents
The system together with the DC engine (motor)
runs at 6kV. Resultant efficiency of such system
is a product for all machines:
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(1)
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The safety factor K value can be found from
following formula (5): [2],
significantly distorted from the sine wave.
According to the Fourier representation it can be
expressed in a form:
i( t ) = I1m cos(ω1t + ϕ1 ) +
(2)
+ ∑ I ⋅ cos(ω + ϕ )
h ≥2
hm
ht
1/ 2
2
2

e  I1  h = N q  I h  



(5)
K = 1+
  ⋅ ∑ h   
 1 + e  I  h = 2  I1  
where:
e
- losses due to eddy currents for sine wave
of basic frequency (50 Hz) related to
losses under DC current value equal to
rms of sinusoidal current at reference
temperature,
h
- harmonics order,
I
- rms value of sinusoidal current or for
non-sinusoidal wave but containing all
higher harmonics estimated as bellow:
h
where:
I1m - amplitude of basic harmonic,
ω1 - angular frequency of basic harmonic,
ϕ1 - initial phase of basic harmonic,
Ihm - amplitude of the current harmonic
of h-order,
ωh - angular frequency of h-order harmonic,
ϕh - initial phase of h-th harmonic.
Deformed current flowing through the network
causes deformation of voltage drops that rms
values can be specified from formula:
∆U =
(I1 ⋅ Z1 )2 + (I h ⋅ Z h )2
1/ 2
Ih
q
(3)
where:
Ih - rms of h-th harmonic,
Zh - network impedance for h-th harmonic.
As a result the voltage waveform at “transitive
points” of busbar in particular substations is also
distorted. Since, ”sensitive receivers” are
connected (powered) to the “transitive”,
(common), points therefore, their operation under
non-sinusoidal conditions constitutes a risk for
them. An additional negative effect is due to
increased active power losses in electric devices
what, in turn, reduces their ampacity comparing
to rated sinusoidal supply. Examples of these are
power transformers. Power that can be loaded the
transformer under distorted current must be
respectively reduced to avoid overheating and
related potential failure. For example, for oilimmersed transformers this reduced power value
can be estimated from form:
S
Snh = n
(4)
K
where:
Snh - authorized apparent power under distorted
currents flow,
Sn
-rated power of transformer (for sinusoidal
current),
K - safety (reduction) coefficient.
ISBN: 978-960-474-359-9
 h=N 
I =  ∑ I 2h 
 h =1 
- rms current harmonics of h-order,
- exponent, that value experimentally
estimated is equal respectively:
q= 1.7 for transformers with round or
rectangular cross section of wirings,
q=1.5 for transformers with foil winding
at low voltage side.
As a result permissible load may be lower than
the rated power of up to several tens of percent.
2. Way to Reduce the Impact of
Converting Units for MV Power
Supply Network
To power a DC load (DC motor) can be used
simple 6-pulse bridge rectifiers. The shape of the
supply is then significantly distorted. Therefore,
in order to reduce the defomations commonly are
used two transformers (operating in parallel) of
different vector group value what results in
α=300 phase shift between voltage vectors. As a
result the number of pulses of the rectifier is
increased and the deformation of the current
waveform drawn from the network is respectively
smaller. Simplified scheme of such a system is
shown in Fig. 3.
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Recent Advances in Circuits, Systems, Signal Processing and Communications
6 kV
-
TR1
TR2
6/0,44 kV
1,6 MVA
Dy 11
6/0,44 kV
1,6 MVA
Dd 0
R1
The analysis was performed for an angle
modulation equal to α=600 (rectifier operation;
the cage is moving up). To evaluate the efficiency
of the high harmonics compensation such
methods were compared:
- application of respectively selected passive
filters (LC type) of 11th and 13th harmonics
connected to the 6kV substation busbar
(Fig. 4),
- connection of active power filter (APF), to
the 6 kV substation busbar, controlled by an
algorithm developed on the basis of the
Current Physical Components (CPC) theory
[3], [4], [5].
R2
M
2 MW
Fig. 3. Schematic of power supply of DC motor
by means of controlled rectifier.
.
Depending on the number of pulses of the
rectifier system applied the current waveform in
the network contains high harmonics of h number
equal to:
h=kn+/- 1
(6)
where:
n - number of pulses,
k=1, 2, 3, …..
3. The Results of Simulation Study
Waveforms of current and voltage in the supply
network, active, reactive and apparent power,
power factor (cos α) as well as high harmonics
spectra of voltages and currents are presented for
example in Fig. 5-Fig. 7. (The simulation model
was based on MATLAB/SIMULINK package
[6]).
For the system presented in Fig. 3 were carried
out respective simulation studies regarding the
current and voltage waveforms at various points
of the system (Fig.4). These quantities are shown
for example in Fig. 5. The study was performed
for a simulation model developed using
MATLAB/Simulink package.
The cage hoist drive motor and transformers
data were as follows:
TR
110 kV/6 kV
5 h, 11 h
6 kV
Drive DC motor:
- (type PW-124/02,DFME WROCLAW)
separately excited,
- rated power Sn =2000kW,
- rated voltage Un = 870V.
MDC
Fig.4. Compensation system of reactive power
and high harmonics of current (11th and 13th) by
means of passive LC filters.
Converter supplying transformers: TR1 and TR2
(type RESIBLOC, ABB):
- rated power Sn = 1600 kVA,
- rated voltage Un = 6/0.44 kV,
ISBN: 978-960-474-359-9
rated current In =154/2099A,
vector group of the transformer TR1 was
Dyn-11 whereas, this of TR2 was Dd-0
respectively.
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Recent Advances in Circuits, Systems, Signal Processing and Communications
a)
f)
b)
g)
c)
h)
d)
i)
e)
j)
Fig. 5 Current and voltage waveforms in
supplying network (6 kV) of a DC motor before
compensation.(a-voltage, b-harmonics spectrum
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Recent Advances in Circuits, Systems, Signal Processing and Communications
of voltage, c-enlargement of voltage spectrum, dphase current waveforms, e-current harmonics
spectrum, f-enlargement of current spectrum, gpower factor (cos ϕ), h-active power, i-reactive
power, j-apparent power).
e)
a)
f)
b)
g)
c)
h)
d)
i)
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Recent Advances in Circuits, Systems, Signal Processing and Communications
j)
b)
Fig. 6. Results of compensation of high
harmonics of current (11th and 13th) by means of
passive LC filters, (a-voltage waveform, bharmonics spectrum of voltage, c-enlargement of
voltage spectrum, d-load current, e-harmonics
spectrum of current, f-enlargement of current
spectrum, g-power factor (cos ϕ) value, h-active
power, i-reactive power, j-apparent power.)
c)
TR
110 kV/6 kV
d)
APF
6 kV
e)
DCM
Fig. 7. Compensation system of reactive power
and high harmonics of current by means of active
power filter (APF).
f)
a)
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Recent Advances in Circuits, Systems, Signal Processing and Communications
g)
obtain effective compensation of both reactive
power and high harmonics level it is necessary to
use active power filters (APF).
4. Conclusions
Use of effective high current harmonics
compensation can significantly reduce the content
of high harmonics of both voltage and current
waveforms in supplying network of MV.
Passive filters (LC) however, allows only for
slight improvement of power factor value.
Although, they visibly limit the value of reactive
power but don’t reduce it completely. The current
waveform is however, improved.
Application of active power filters (APF)
controlled by algorithm developed on basis of
CPC theory allows effectively:
- ensuring a sinusoidal waveform of current
provided from the supplying MV network,
- providing the highest power factor value
(cos α equal to 1),
- almost complete elimination of the reactive
power flow in supplying network (apparent
power=active power).
h)
i)
References:
[1] Strzelecki R., Supronowicz H.: Power factor in
Supplying Systems and Methods of its
Improvement, Oficyna Wydawnicza Politechniki
Warszawskiej, Warszawa 2002 (in polish),
[2] EN 464-3:2007 Three phase oil-immersed
distribution transformers 50 Hz, from 50 kVA, to
2500 kVA with highest voltage equipment not
exceeding 36 kV – Part 3: Determination of the
power sating loaded with non-sinusoidal
currents.
[3] Czarnecki L. S.: Orthogonal Decomposition of
the Current in Free Phase Non-linear
Asymmetrical Circuit with Nonsinusoidal
Voltage, IEEE Trans. on IM, Vol-37, No 1, pp
30-34.
j)
Fig. 8. Results of compensation of high
harmonics of currents and reactive power by
means of active power filter (APF) controlled
according to CPC theory (a-current waveforms
generated by APF system, b-voltage waveforms,
c-high harmonics spectrum of voltage, denlargement of voltage spectrum, e-current
waveforms, f-current high harmonics spectrum,
g-enlargement of current spectrum, h-power
factor (cos ϕ), i-active power, j- reactive power,
j-apparent power).
From the investigated results (confirmed by
experiment Fig. 6, Fig. 8) one can see that to
ISBN: 978-960-474-359-9
[4] Wosik J., Kalus M., Kozlowski A., Miedzinski B,
Habrych M.: The Efficiency of Reactive Power Nonlinear Loads. Electronics in Electrotechnika. Vol. 19,
No 7, 213 pp. 29-32.
[5] Wosik J., Kalus M., Kozłowski A., Miedzinski B.:
Improvement of the Electric Energy Quality by Use of
the Active Power Filters. Proceedings of International
Conference on Renewable Energies and Power
Quality ICREPQ 13. Bilbao, 20-23 March 2013.
[6] Piatap. Getting started with Matlab 7. Oxford
University Press Inc. 2006.
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