Download III. UPQC - Academic Science,International Journal of Computer

Power Quality Improvement For
Critical/Unbalanced Loads Using UPQC
Keerti Kulkarni
PG Scholar, Department of Electrical and Electronics
SDM College of Engineering and Technology,
Dhavalgiri, Dharwad-580002. Karnataka. India
Email:[email protected]
Abstract— This paper presents control strategy of three phase four
wire UPQC (unified power quality conditioner) to improve power
quality. A case study at the hospital is taken for the study of power
quality issues. To improve upon certain problems observed, one of
the customer power devices called UPQC is mentioned. UPQC
consist of combine series active filter and shunt active filter. The
former compensates voltage harmonics and the latter will
compensate harmonic currents of non-linear load. The proposed
UPQC system can improve the power quality at the point of common
coupling on distribution system under unbalanced load conditions.
The 3P4W UPQC systems control strategy to balance the unbalanced
load currents is presented in this paper. The neutral current that may
flow towards transformer neutral point is compensated by using a
four-leg voltage source inverter topology for shunt part. Hence, the
series transformer neutral will be at virtual zero potential. A
simulation study of the proposed topology has been carried out using
MATLAB/SIMULINK and the results are presented.
Keywords—Power quality, Total harmonic distortion (THD),
Unbalanced loads, neutral current, filters, UPQC.
I.
INTRODUCTION
The healthcare environment comprises of various combination
of sensitive electronic loads and other commercial loads.
Hence maintaining quality power is essential and critical.
Power quality is the term used for overall voltage quality,
current quality and nature of output waveform which directly
or indirectly affect the loads as well as source [1]. Power
quality problems may arise due to non-linear loads, injection
of harmonics, and interaction between medical equipments.
Common sources of power quality problems found in
hospitals include: inadequate wiring and grounding, high
wattage operating equipment, testing of emergency generators,
physical plant renovation [1]. An attempt was made to identify
the power quality levels to assess the quality of service
received from the utility and locates the areas within the
hospital where problems arising from instability exists or may
develop in the near future. Unbalanced load currents are
common and important problem in 3P4W distribution system.
Some of the custom power devices have come for the rescue.
To provide a balance, distortions free, and constant magnitude
Vinay Janardhan Shetty
Lecturer, Department of Electrical and Electronics
KLS Gogte Institute of Technology,
Belgaum-580008. Karnataka. India
Email:[email protected]
power to sensitive loads and also to restrict the harmonic,
unbalance by the load, UPQC is one of the best solutions [3].
The definition of power quality given in the IEEE std 1100 is
Power quality is the concept of powering and grounding
sensitive equipment in a matter that is suitable to the operation
of that equipment. Power quality is the combination of
voltage quality and current quality. Thus power quality is
concerned with the deviations of voltage and/or current from
the ideal. Voltage disturbances originate in the power network
and potentially affect the customers, whereas current
disturbances originate with a customer and potentially affect
the network.
Description of the hospital system, methodology and
observations of the system is discussed in section II. UPQC
configuration and control algorithm is discussed in section III.
This is followed by simulation results and conclusion in
section IV.
II.
DESCRIPTION OF THE SYSTEM
The hospital complex under study has incoming
supply of 11kV from the utility. Transformer with Δ-Υ
connection and solidly grounded is used to buck the voltage to
440 volts. The critical medical loads are: X-ray, CT, MRI,
ICUs, operation theatres which contains surgical suits etc. The
hospital under study has the following types of power supply.
 Normal Supply (NS) is a direct utility supply
(HESCOM) for non-essential areas.
 Essential Supply (ES) is used for areas of medical
importance that are not critical to patients in the case
of supply interruption. The essential supply is backed
up by an emergency generator (Diesel generator).
 Uninterruptible Power Supply (UPS)/stabilizers is
used for operating theatres, patient monitoring and
other equipment that is important to the well being
and safety of patients.
IEEE std-519 establishes harmonic limits on voltage as 5% for
total harmonic distortion and 3% of the fundamental voltage
for any single harmonic. For hospitals and airports Voltage
THD is less than 3% [7]. To define current distortion limits,
IEEE std-519 uses a short circuit ratio to establish a
customer’s size and potential influence on the voltage
distortion of the system.
TABLE 1: MAXIMUM VOLTAGE HARMONIC DISTORTION
For
hospitals/airports)
General
system
Dedicated
system
10%
20%
50%
3%
5%
10%
notch
depth
% THD
voltage
The gate pulses required for converter are generated by the
comparison of a fundamental voltage reference signal with a
high-frequency triangular waveform [6]. Shunt active filter
will reduce the current harmonics and provide reactive power
demand to the load. It has to control the dc link voltage also
which is provided between these two filters. Usually
hysteresis band current controller is employed for pulse
generation.
TABLE 2: MAXIMUM CURRENT HARMONIC DISTORTION
(ISC/IL)
% THD
< 20
5%
20-50
8%
50<100
12%
100-1000
15%
> 1000
20%
Fig.1 Equivalent circuit diagram of upqc
The equations that govern the Fig 1 are:
Where Isc is maximum short circuit current at PCC and IL is
maximum load current at PCC.
For study purpose, few medical loads and few miscellaneous
loads were taken for measurement of electrical parameters
like, % voltage THD, % current THD, current in each phase,
Vrms. The analysis of the system highlighted some aspects.
They were high THD and unbalanced load currents. These
both contribute to high neutral current which will flow back to
transformer neutral resulting in excess heating of the neutral
cable. As the THD increases, the neutral current also
increases.
To reduce the high THD, a passive filter, or an active
filter can be used locally. K-rated transformer can be used.
Sharing of neutral wires is to be reduced, or over sizing of
neutral wires is to be done. Installation of custom power
device like, Unified Power Flow Conditioner which will tackle
almost all the power quality problems can be thought of in the
near future. This is discussed in detail in the following section.
III.
UPQC
The UPQC consists of two three phase inverters
connected in cascade in such a manner that series active filter
is connected in series with the supply voltage through a
transformer shunt active filter is connected in parallel with
the load. The series converter acts as a voltage source to
mitigate voltage distortions. It is used to eliminate supply
voltage flickers or imbalance from the load terminal voltage
and forces the shunt branch to absorb current harmonics
generated by the nonlinear load. As shown in Fig.1 Voltage
injected by series APF must be equal to the difference
between the supply voltage and the ideal load voltage [3].
Control of the series converter output voltage is usually
performed using sinusoidal pulse-width modulation (SPWM).
Vs: Voltage at power supply
VSR: Series-APF for voltage compensation,
VL: Load voltage and ISH: Shunt-APF for current and VSR
compensation.
The source voltage can be expressed as
Vs + VSR = VL
(1)
The current provided by the shunt APF, is the difference
between the input source current and the load current
ISH=IS-IL
(2)
A. Control strategy for series active filter
The control strategy for series active filter is based on unit
vector template. Vsa, Vsb, Vsc are the supply side voltages. As
shown in Fig 2, the injected voltages by the series active filter
are Vinja, Vinjb, Vinjc. These injected voltages will cancel out the
distortions present in the supply voltages and thereby making
the voltages at load side sinusoidal and maintaining
magnitude. Phase locked loop (PLL) is used to achieve
synchronization with the supply voltage. The supply voltages
are sensed and fed to PLL. This will generate two quadrature
unit vectors viz., sinωt and cosωt. The in-phase components
from these unit vectors are used to compute the in-phase
supply voltages which are 1200 displaced. This is given by the
equation

1
Ua  
Ub   - 1/2
  
Uc  
- 1/2




3
2 

3
2 
0
sin  
cos  


(3)
To get reference voltages, the unit vectors Ua, Ub, Uc as
obtained above will be multiplied by the desired peak value at
the PCC voltage.
V * la
Ua 
V * lb  V* Ub 
lm


 
V * lc 
Uc 
(4)
The reference voltages obtained above and sensed load
voltages Vla, Vlb, Vlc are given to hysteresis controller. The
output of the hysteresis is given to the gate signals of the
series active filter, and thereby the voltage is injected through
injection transformer.
Instantaneous reactive power is:
qlabc= Vlabc_α*ilabc_α-Vlabc_β*ilabc_β
(8)
Reactive power is taken as zero because utility should
not provide load reactive power. As the load is unbalanced,
each phase may not carry equal currents, and therefore the
power too. In order to balance the power from utility side, total
active power is redistributed among the three phases.
Hence power per phase is
Ps/phase= {( Pla+ Plb + Plc )/ 3}
(9)
The reference currents are obtained by the inverse of
equation (6) as below:
i * sa _  
Vla _  Vla _    ps / phase  pdc / phase
i * sb _   = inv  Vlb _  Vla _   * 0
 (10)



 

Pdc/phase is the constant dc-link voltage to overcome the
UPQC losses. Hence reference current for phase a is:
=
……
.
……………… (11)
Fig 2: Control scheme for series active filter[4]
The reference neutral current can be obtained by adding all the
sensed load currents.
B.
Control strategy for shunt active filter
The control scheme for the shunt active filter as shown in
fig.3 is based on P-Q theory (Instantaneous reactive power
theory). According to this theory, a single phase system can be
defined as a pseudo-two phase system by giving 900 lead or lag
i.e, each phase voltage and current of the original three phase
system can be considered as three independent two phase
systems. These two phase system can be represented in α-β coordinates. The actual load current and load voltages are
considered as α-axis quantities, whereas lead/lag voltages and
currents are considered as β-axis co-ordinates. The reference
voltages extracted for series active filter are used instead of
actual load voltages, as P-Q theory gives poor results under
unbalanced load voltages. Equations governing this theory:
iln=ila+ilb+ilc
(12)
and compensating neutral current is
ish_n=-iln
(13)
Using PI controller, the dc link is maintained. The dc link
voltage is given by
………….. (14)
For phase a, the load voltage and current in α-β coordinates are:
Vla _   Vlm sin( t ) 
Vlb _   = Vlm cos(t )

 

ila _  
ilb _   =


………… (5)
il a (t  l )

ilb((t  l )   / 2) ………(6)


Similarly for phase b, with 1200 phase shift and for phase c, 1200 phase shift, equations are formed.
Instantaneous active power is:
plabc= Vlabc_α*ilabc_α+Vlabc_β*ilabc_β …………… (7)
Fig 3: Control scheme for shunt active filter [5]
I. SIMULATION AND RESULTS
The proposed control scheme has been simulated in
MATLAB/SIMULINK. The system is modeled in
SIMULINK with system frequency 50 Hz; taking unbalance
load condition prevalent in the hospital is as below:
Selected signal: 25 cycles. FFT window (in red): 2 cycles
100
50
0
-50
-100
0.3
0.35
0.4
0.45
0.5
Time (s)
Fundamental (50Hz) = 113.1 , THD= 0.85%
0.6
Mag (% of Fundamental)
0.5
0.4
0.3
0.2
0.1
0
0
2
4
6
8
10
Harmonic order
12
14
16
18
20
Fig 6: balanced source current and THD with UPQC
Fig 7: non zero neutral current flowing towards transformer neutral without
UPQC.
Fig 4: SIMULINK model of UPQC
Fig 8: compensating neutral current provided by UPQC
Selected signal: 50 cycles. FFT window (in red): 2 cycles
50
0
Fig 9: reduced neutral current towards transformer with UPQC
-50
0.8
0.85
0.9
0.95
1
Time (s)
II. CONCLUSION
Fundamental (50Hz) = 53.58 , THD= 8.24%
7
Mag (% of Fundamental)
6
5
4
3
2
1
0
0
2
4
6
8
10
Harmonic order
12
14
16
Fig 5: unbalanced source current and THD without UPQC
18
20
From the results we can observe that, source current THD
without UPQC is 8.24%, while with UPQC, it has reduced
considerably to 0.89%, which is within IEEE-519 standards
mentioned during the case study of hospital analysis of
power quality. Albeit the load is unbalanced, the source
current is not getting affected with UPQC. Another
observation is reduction of neutral current with the aid of
UPQC. The UPQC provides a compensating neutral current
in order to negate the neutral current, and thereby reducing
the unnecessary effect on neutral cable and transformer. A
custom power device like UPQC can be installed at the PCC
panel in order to overcome the problems encountered in the
hospital.
ACKNOWLEDGMENT
The authors wish to acknowledge the constant support from
the SDM hospital administration, where the study was
undertaken.
REFERENCES
[1] Keerti Kulkarni, (2014), “Power Quality Issues In
Healthcare Centre”, “International Journal of Current
Engineering and Technology”, Vol.4, No.3.
[2] Yash Pal, A. Swarup, and Bhim Singh, (2012), “A Novel
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[3] Vinod Khadkikar, A. Chandra, A. O. Barry, T. D. Nguyen
(2005), “Steady State Power Flow Analysis of Unified Power
Quality Conditioner (UPQC)”, IEEE, 0-7803-9419-4/05.
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“Unified Power Quality Conditioner for Power Quality
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