Download A Novel DFACTS Device for the Improvement of Power B.Y.Bagde

A Novel DFACTS Device for the Improvement of Power
Quality of the Supply
P. M. Meshram1 B.Y.Bagde2 R.N.Nagpure3
1,2 & 3 Sr. Lecturers, Dept. of Electrical Engg., Yeshwantrao Chavan College of Engg., Nagpur, INDIA,
1. [email protected], 2. [email protected], 3. [email protected]
ABSTRACT
The proliferations of the non-linear devices cannot
be restricted at transmission and distribution level
because of their compactness and power handling
capacity but they also draw non-linear current and hence
degrade the power quality. The different non-linear loads
at the distribution side are adjustable speed drives,
fluorescent lighting and personal computers (PC’s),
television sets, refrigerators etc.
In this paper we propose the controller at the
distribution side, i.e., between the utility and the
customer for the improvement of the quality of supply
and therefore it is called DFACTS device. The concept is
analyzed and completely simulated for different types of
loads i.e. linear balanced; linear unbalanced; non-linear
balanced and non-linear unbalanced.
Keywords: DFACTS, FACTS, power quality & Voltage
source converters (VSC)
concept of DFACTS device uses two Voltage Source
Converters (VSC’s) utilizing Pulse Width Modulation
(PWM) technique where inherent characteristics of
reducing the lower order harmonics is used for the
domestic, i.e., at the distribution side for improvement of
quality of the power supply.
The proposed concept has been simulated for
different types of loads and balanced as well as
unbalanced supply voltages.
2. PRINCIPLE OF OPERATION
The two- level, six-pulse voltage source converter
(VSC) is shown in Fig. 1.
Let
Es1
Fundamental component of the ac bus
voltage.
Ec1
Fundamental component of VSC s output
voltage.
δ
Angle between Es1 and Ec2.
X
Converter reactance.
P
Active power
Q
Reactive power
1. INTRODUCTION
For a given transmission line, three key parameters
determine power flow: terminal bus voltages, line
impedence, and the relative phase angle between the
sending and receiving end. To modify these parameters,
a variety of mechanically switched devices like shunt
connected capacitors and phase sifting transformers are
used but none responds quickly enough to changing
conditions to provide real-time flow control. Each of
these conventional power controllers has a conceptual
equivalent based on power electronics. In addition, with
advanced thyristor technology, novel controllers that
have no single conventional analog have been developed
and are termed as FACTS devices [1]. There are several
FACTS controllers, namely, static var compensator
(SVC), thyristor controlled phase angle regulator,
thyristor controlled phase angle regulator (TCPAR),
static compensator (STATCOM), unified power flow
controller (UPFC), etc. [2-3].
The above FACTS devices are used for maintaining
i.e. either supporting or the preventing from rising the
voltage means supplying or absorbing the reactive power
but not exclusively for the improvement of quality of the
supply. This proposed DFACTS device is exclusively for
the improvement of the quality of the supply. The new
Idc
Es1∠δ
L,R
Esa
L,R
Esb
L,R
Esc
Ec1∠ 0
S1
S2
S3
I1
I2
Vdc
C
I3
S1'
S'2
S'3
Fig. 1: Power Circuit of a Three Phase VSC.
P=
E s1 E c1 sin δ
X
Q=
E s1 ( E c1 cos δ
X
(1)
E s1 )
(2)
From (1), the active power flowing over the VSC is
primarily determined by δ . The flow of it is determined
by the relative position between Es1 and Ec1. When Ec1
lags behind Es1, the VSC functions as a rectifier and
absorbs real power from the left ac network. When Ec1
leads Es1, the VSC works as an inverter and gives real
power to the left ac system. Similarly from (2) the
reactive power is controlled by Ec1.
Three basic control modes for the VSC are (a)
Constant dc voltage control, (b) Constant dc current
control, (c) Constant ac voltage control for the passive
load, mode (c) is used and one side of the system should
adopt constant dc control method.
Ec2∠0
2C2
R2
Es2∠δ2
Qload
L2
Edc
2C2
ZL
Filte
Fig. 3: Load Side of the Device.
From Fig. 3, we have
3. SYSTEM DESCRIPTION
The DFACTS device consists of two controllers,
i.e., constant dc voltage controller and ac voltage
controller. The constant dc voltage controller maintains
constant dc voltage while ac voltage controller maintains
the required rms value of the voltage at the load side.
The two-voltage source converters are shown connected
back-to-back in the Fig 2. Ps1 and Qs1 are the active and
reactive power supplied; Pc1 and Qc1 are the active and
reactive power absorbed by the VSC. Es1 and Es2 are
the peak-to-peak voltage of the supply and load side.
Ps1 Q s1
M2
ES 2 =
ZL
EC 2
Z2 + ZL
Z1
E c 2 ∠ 0 E s2 ∠ δ 2
V SC
E dc
V SC
Z2
IL
I
ZL
2
(0 ≤
Edc
M 2 ≤ 1)
(3)
(4)
From (3) and (4) we get
ES 2 =
Pc1 Q c1
E s1∠ δ1 E c1∠ 0
A ctiv e
N etw o rk
EC 2 =
M2
ZL
Edc ( 0 ≤ M 2 ≤ 1 )
2 Z2 + ZL
ZL
∂E S 2
1
Edc ; 0
=
∂M 2
2 Z2 + ZL
(5)
(6)
where:
E S 2 load voltage.
E S 2 VSC output voltage at the load side.
DC
C o n tro ller
at
S tatio n 1
AC
C o n tro ller
at
S tatio n 2
Fig.2: Back-to-Back VSCs
M2
Modulation index at the load side.
ZL
load impedance.
Edc
dc voltage
Z2
Impedance between VSC inverter and load.
Eq (6) shows that there is a linear relationship between
E S 2 and M 2 , so pure PI controller is adequate to meet
the control equipment.
4. DCVOLTAGE CONTROLLER DESIGN
The complete mathematical analysis and block
diagram of the dc voltage controller is given [4]. The
block diagram of the same is realized in MATLAB 6.5
E s2 ref
∆E 2
PI
F req u en cy
S ig n al
G en eratin g
F irin g
P u lse
F irin g
P u lse
5. AC CONTROLLER DESIGN
For ac voltage control station, assume the dc voltage
utilization ratio of the adopted PWM method is 1 and the
modulation index is M 2 0 ≤ M 2 ≤ 1 .
C alcu latio n
of
F irin g
P u lse
E 2
L o ad
S id e
E 2b
V o ltag es
E 2
Fig.4: AC Controller
6. SIMULATION RESULTS
Load
Voltage
Load
Voltage
Load
Voltage
Load
Voltage
Fig.5: For Linear Balanced and Unbalanced, NonLinear Balanced & Unbalanced Loads.
Load
Current
Load
Current
Load
Current
Load
Current
Fig.8: For Linear Balanced and Unbalanced, NonLinear Balanced and Unbalanced Loads.
Supply
Voltage
Supply
Voltage
Supply
Voltage
Supply
Voltage
Fig.6: For Linear Balanced and Unbalanced Loads
Fig.9: For Non-Linear Balanced and Unbalanced Loads
Supply
Current
Supply
Current
Supply
Current
Supply
Current
Fig.7: For Linear Balanced and Unbalanced Loads
Fig.9: For Non-Linear Balanced and Unbalanced Loads
The rated parameters of the system are as follows:
Es1=11kV (peak to peak); R1=0.002ohm, L1=20 µ H,
The DC voltage is 500V. The rated capacity of each
VSC is 500kW and the switching frequency is 2 kHz.
Increasing the modulation index with increasing DC
voltage could vary the load voltage magnitude. The
following different loads are considered.
(1) Linear balanced load
La =100 kW, 40 kVAR;
Lb = 100 kW, 40 kVAR;
Lc = 100 kW, 40 kVAR
(2) Linear unbalanced load
La = 50 kW, 30 kVAR;
Lb = 100 kW, 30 kVAR;
Lc = 50 kW, 50 kVAR;
(3) Non-linear balanced load
La = Lb = Lc = 500 kW.
(4) Non-linear unbalanced load
La = 167 kW;
Lb = 167 kW;
Lc = 100 kW.
where La, Lb and Lc are the loads connected to a, b and
c phases.
Simulation results for the different loads as well as
balanced as well as unbalanced supply voltages are
shown in the Fig 5 and Fig 6.
Following conclusions can be made from the
Table 1 and Table 2: (a) Supply side voltage and current
distortions are well within the limits though the loads are
made deliberately distorted for the balanced supply
voltages. (b) Though supply is made unbalanced and the
loads are of different types, supply side voltage and
current distortions are well within the limits. (c) The full
load capacity is 500kW and it is fully applied in the case
of non-linear balanced load.
Table 1: Total Harmonic Distortion (THD) in the Load
and Supply for Balanced Supply Voltages.
%
THD
in
↓
Linear
Balanced
load
Linear
unbalanced
load
Nonlinear
balanced
load
Non-linear
unbalanced
load
Load
voltage
1.87
3.44
9.26
32.53
Load
current
1.86
2.97
24.24
55.19
Supply
voltage
0.6
0.37
0.48
0.60
Supply
current
0.61
2.72
0.49
0.38
Table 2: Total Harmonic Distortion (THD) in the Load
and Supply for Unbalanced Supply Voltages.
%
THD
in
↓
Linear
Balanced
load
Linear
unbalanced
load
Nonlinear
balanced
load
Non-linear
unbalanced
load
Load
voltage
7.67
3.44
9.13
32.25
Load
current
7.23
2.97
24.67
58.15
Supply
voltage
0.6
0.37
0.54
0.49
Supply
current
0.92
2.72
0.49
0.40
7. CONCLUSIONS
The power quality at the supply side is maintained
though loads are deliberately made distorted and follows
the IEEE 519 1992 standards. Therefore this DFACTS
device could be used at the distribution side i.e. between
utility and the customer. This device could be called
series-series controller. In the case 3 i.e. for non linear
balanced load the full capacity of load was applied and it
still shows the distortions on the supply side are well
within the standards. Therefore whatever the impurities
on the load side are not transferred on the supply side
irrespective loads natures.
The characteristics of the power quality
improvement of the proposed DFACTS device is also
hold good for the balanced as well as un-balanced supply
voltages. And there is no rigmarole method of
identifying the harmonics to reduce those and
subsequently reducing the distortions i.e. for
improvement of quality of the supply.
8. REFERENCES
[1] Karl E. Stahkopf and Mark R. Wilhem “Tighter
Controls for Busier Systems” IEEE Spectrum, April
1997 pp48-52.
[2] L. Gyugui, “ Unified power flow for flexible AC
transmission system” IEE Proceeding, PartC, Vol.
139, No. 4 July 1992,pp323-332.
[3] N. G. Hingorani and and L.Gyugui, Understanding
FACTS, IEEE press, New York,1999.
[4] Blasko.V. and Agirman I, “Modellinng and Control
of Three-Phase Regenerative AC-DC Converters”
IEEE2001 pp2235-2240.