Download A Novel Improved Performance of Direct Power Control of Unified

A Novel Improved Performance of Direct Power Control
of Unified Power-Flow Controller Fed Induction Drive
System
K.VAJRALABABU
M-tech Student Scholar
Department of Electrical & Electronics Engineering,
Prasad .v Potluri Siddhartha institute of technology,
Vijayawada-520007
M.DEVIKA RANI
Assistant Professor
Department of Electrical & Electronics Engineering,
Prasad .v Potluri Siddhartha institute of technology,
Vijayawada-520007
Abstract— Very Large electric drives and utility
applications require advanced power electronics converter
to meet the high power demands. As a result, power
converter structure has been introduced as an alternative in
high power and medium voltage situations. an electrical
power system is a large interconnected network that
requires a careful design to maintain the system with
continuous power flow operation without any limitations.
Flexible Alternating Current Transmission System (FACTS)
is an application of a power electronics device to control the
power flow and to improve the system stability of a power
system. This controller is used to maximize the power flow
while maintaining stability of the transmission grid. The
direct power control is developed from the instantaneous
power theory. The direct power control is a important
control technique for a unified power flow controller
(UPFC). This controller can be used with a voltage source
converter. A compared to other power flow controllers, this
UPFC provides better results under balanced and
unbalanced conditions. Direct power control is a valuable
control technique for a UPFC, and the presented controller
can be used with any topology of voltage-source converters.
The dynamic analysis of proposed method is evaluated by
using Matlab/Simulink platform and results are conferred.
Energy resources). Unified power-flow controllers
(UPFC) enable the operation of power transmission
networks near their maximum ratings, by enforcing power
flow through well-defined lines [2]–[4]. These days,
UPFCs are one of the most versatile and powerful flexible
ac transmission systems (FACTS) devices.
Index Terms— Direct Power Control, Flexible Ac
Transmission Control (FACTS), Multilevel Converter,
Sliding Mode Control, Unified Power-Flow Controller
(UPFC), Induction Motor Drive.
I. INTRODUCTION
Electric power flow through an alternating
current transmission line is a function of the line
impedance, the magnitudes of the sending-end and
receiving-end voltages, and the phase angle between
these voltages. Essentially, the power flow is
dependent on the voltage across the line impedance.
Electricity market deregulation, together with growing
economic, environmental, and social concerns, has
increased the difficulty to burn fossil fuels, and to obtain
new licenses to build transmission lines (rights-of-way)
and high power facilities. This situation started the
growth of decentralized electricity generation (using
renewable
The existence of a dc capacitor bank originates
additional losses, decreases the converter lifetime, and
increases its weight, cost, and volume. In the last few
decades, an increasing interest in new converter types,
capable of performing the same functions but with
reduced storage needs, has arisen [10]. In the last few
decades, an increasing interest in new converter types, are
performing the same functions but with reduced storage
needs, has arisen. Matrix converter is highly applicable
for ac to ac conversion, allowing bidirectional power
flow, assurance of near sinusoidal input and output
currents, voltages with variable amplitude, and adjustable
power factor [8].
These minimum energy storage ac/ac converters have the
capability to allow independent reactive control on the
UPFC shunt and series converter sides, while
guaranteeing that the active power exchanged on the
UPFC series connection is always supplied/absorbed by
the shunt connection. Unified Power Flow Controller
(UPFC) proposed by Gyugyi. The combination of static
synchronous compensator (STATCOM) and a static series
compensator (SSSC) which are coupled via a common dc
link, to allow bidirectional flow of real power between the
series output terminals of the SSSC and the shunt output
terminals of the STATCOM are controlled to provide
concurrent real and reactive series line compensation
without an external electric energy source. It is the most
versatile FACTS controller for the regulation of voltage
and power flow in a transmission line.
Fig.1. Schematic diagram of UPFC.
UPFC is the most versatile FACTS controller for the
regulation of voltage and power flow in a transmission
line. The general configuration of UPFC is shown in
above fig.1. It consists of two voltage source converters
(VSC) one shunt connected and the other series
connected. The DC capacitors of the two converters are
connected in parallel. If the switches 1 and 2 are open, the
two converters work as STATCOM and SSSC controlling
the reactive current and reactive voltage injected in shunt
and series respectively in the line [1]. The closing of the
switches 1 and 2 enable the two converters to exchange
real (active) power flows between the two converters. The
active power can be either absorbed or supplied by the
series connected converter [1]-[5]. It is important to note
that whereas there is a closed direct path for the real
power negotiated by the action of series voltage injection
through Converters 1 and 2 back to the line, the
corresponding reactive power exchanged is supplied or
absorbed locally by Converter 2 and therefore does not
have to be transmitted by the line[6]. Thus, Converter 1
can be operated at a unity power factor or be controlled to
have a reactive power exchange with the line independent
of the reactive power exchanged by Converter 2.
Obviously, there can be no reactive power flow through
the UPFC de link.
II. UPFC SERIES CONVERTER MODEL
The series and shunt converter of a UPFC are
HV power electronics. To minimize the voltage stress on
all components while increasing the system voltage level,
multilevel neutral point clamped inverters are a promising
topology. The DPC control method described in this
paper is divided in two parts—a general external part and
an internal topology-specific part. The design principles
for both are explained in detail. The external part is
universal the internal part can easily be adapted to
different topologies of voltage-source converters. In this
paper, a three-level neutral point clamped converter is
used. Other converter topologies use the converter
independent part without further theoretical development.
The converter topology dependent part can be deduced
analogously to the given example.
Fig. 2. Schematic of the equivalent circuit of the UPFC system.
During model construction and controller design, power
sources VS, VR are assumed to be infinite bus. We assume
series transformer inductance and resistance negligible
compared to transmission-line impedance. Under these
assumptions, we can simplify the grid as experienced by
the UPFC to Fig. 2. Sending and receiving end power
sources VS, VR, are connected by transmission line r L.
The total current drawn from the sending end iT consists
of the current flowing through the line iS and the current
exchanged with the shunt converter iP. Shunt transformer
inductance and resistance are represented by LP and rP.
The series inductance and resistance are commonly
accepted as a model for overhead transmission lines of
lengths up to 80 km. The power to be controlled is the
sending end power, formed by the current iS and the
sending end voltage VS. This is the most realistic
implementation for control purposes. The UPFC shunt
converter model is similar and is not described in this
paper; its functions and control are well described in
literature [1], [2] and the performance of the shunt
converter is only of secondary influence on the control
system described in this paper, as demonstrated in
previous work.
Effects of dc bus dynamics are negligible in the control
bandwidth of the power flow. For all simulations and
experiments in this paper, the shunt converter is only used
to satisfy active power flow requirements of the dc bus.
Using the model of Fig. 2, differential equations that
describe the current in three phases can be formulated.
Voltages Vabc=VSabc+VCabc-VRabc are used for notation
simplicity. The differential equations for the UPFC model
are given as
L
(1)
Applying the Clarke and Park transformation results in
differential equations in dq space. Voltages V d=Vsd+VcdVRd and Vq=VSq+VCq-VRq are introduced for notation
simplicity. It is assumed that the pulsation of the grid is
known and varies without discontinuities. Applying the
Laplace transformation and with substitution between the
two dq space transfer functions, (2) is obtained, where
currents isd(s),isq(s) are given in function of voltages
Vd(s) and Vq(s).
(2)
The active and reactive power of the power line is
determined only by the current over the line and the
sending end voltage. Without losing generality of the
solution, we synchronize the Park transformation on VSa,
resulting in Vsq=0. Assuming relative voltage stability,
Vsd(s) =Vsd, VRdq(s) =VRdq .Active and reactive power at
the sending end are calculated as
Fig.3. Schematic of the three-level neutral point clamped converter.
(6)
(3)
Substituting (2) into (3), we receive the transfer functions,
linking PS(s), QS(s) to VS, VR, and VC(s). Both active and
reactive power consists of an uncontrollable constant part,
which is determined by power source voltages V S, VR,
and line impedance L, r and a controllable dynamic part,
determined by converter VC(s) voltage, as made explicit
in
It is interesting to take a further look at the components of
the dynamic part of the active and reactive
power
,
, especially at the response to steps
in series converter injected voltage ,VCd/s, VCq/s. Using
the initial value theorem on (6), we receive
(4)
Splitting in a constant uncontrollable and a dynamic
controllable part results in (5) and (6). For notation
simplicity, VCd(s), VCq(s), are replaced by VCd(s), VCq.
(7)
It is clear that only VCd (t) affects the derivative
d
instantaneously, and only VCq (t) affects the
derivative d
instantaneously.
III.THREE-LEVEL NEUTRAL POINT CLAMPED
CONVERTER
(5)
A schematic of a three-level neutral point
clamped converter is given in Fig. 3. This topology and its
mathematical model have been diligently described in.
Each leg k of the converter consists of four switching
components Sk1, Sk2, Sk3, Sk4, and two diodes Dk1and Dk2.
The diodes Dk1, Dk2, clamp the voltages of the connections
between Sk1, Sk2, and Sk3, Sk4, respectively, to the neutral
point, between capacitors C1,C2.There are three possible
switching combinations for each leg k, thus three voltages
umk. The three levels for voltages umk produce five
different converter phase-output voltages Uk .The upper
and lower leg currents Ik, or their respective sum i, can
be described in function of the output line currents ik .The
system state variables are the line currents i1, i2, i3, and the
capacitor voltages UC1,UC2. This system has the dc-bus
current i0 and the equivalent load source voltages Ueqk as
inputs. Under the assumption that the converter output
voltages Uk are connected to an req, Leq system with a
sinusoidal voltage source ueq with isolated neutral, as in
Fig. 3, we can write the equations for the three-phase
currents i1,i2 ,i3 as in
(8)
The capacitor voltages UC1, UC2 are influenced by the sum
of the upper and lower leg currents i,
and the input
current
, as in
(9)
From the restrictions on the states of the switching
devices in each leg of the converter, we can define the
ternary variable
, representing the switching state of
the entire leg, as
(13)
With this variable
, and the derived variables
and , straightforward equations can be found for the
description of the other variables in the system.
Combining the equations of the system dynamics (8) and
(9), the complete system equation is (14),
where
,
,
are aiding functions
describing the precise dynamics in function of the
switching state. It is important to realize that this system
equation is not constant, nor continuous.
(14)
(10)
To simplify notation, combinations of this variable
and are introduced
, ,
(11)
(12)
Fig. 4.Vector arrangement in five levels in , for three-level threephase converter. (a) Five levels in , . (b) Five levels in , .
If we assume the voltage balance of the capacitors C1, C2,
the 27 possible combinations of leg switching state
variables
,
, lead to 27 sets of phase voltages
U1,U2,U3 and 27 voltage vectors after Clark
transformation to
-space. The 27 voltage vectors can
be divided in 24 active vectors and 3 null vectors. The 24
active vectors form 18 unique vectors; 12 vectors form 6
redundant pairs.
TABLE I
OUTPUT VOLTAGE VECTORS
Vector
C . d(
-
1
2
1
1
1
1
1
0
0
3
1
1
-1
0
4
1
0
-1
5
1
0
0
6
1
0
1
7
1
-1
1
8
1
-1
0
9
1
-1
-1
10
0
-1
-1
11
0
-1
0
12
0
-1
1
13
0
0
1
14
15
0
0
0
0
0
-1
16
0
1
-1
17
0
1
0
18
0
1
1
19
-1
1
1
20
-1
1
0
21
-1
1
-1
22
-1
0
-1
23
-1
0
0
24
-1
0
1
25
-1
-1
1
26
-1
-1
0
27
-1
-1
-1
)/dt
.
0
sign of the derivative of the voltage unbalance UC1-UC2,
the sign of the instantaneous active power P will be used.
Since UDC will always be positive, the sign of P depends
only on the sign of
. Assuming perfect voltage
balance, the instantaneous outgoing power P of the
converter is given by the internal product of the switching
state variables
and outgoing line currents
scaled
by the capacitor voltage by
(16)
0
IV. MATLAB/SIMULINK RESULTS
0
Here simulation is carried out in several different cases, in
that 1). Proposed Three level NPC Based DPFC, 2).
Proposed Three level NPC Based DPFC with Induction
Machine Drive.
0
0
Case 1: Proposed three level NPC Based DPFC
0
0
0
0
0
The 3 null vectors also form only 1 unique vector. This
results in 19 different voltage vectors. To simplify the
vector selection, the 27 vectors are grouped into 5 levels
in the , and dimension, based on their component in
this dimension. The levels and vector grouping are
represented in Fig. 4. Each combination of levels ,
corresponds to one unique voltage vector. Assuming
that the capacitors C1 and C2 have equal capacity and
using the relation of the three line currents i1+i2+i3=0, the
dynamics of the voltage balance UC1-UC2 can be derived
from (14), leading to
(15)
In Table I, the effect of the output voltage vectors on the
capacitor voltage balance is listed. Comparing these
values with those of
for the values of the
redundant vectors, given in bold, they depend on the same
currents and except for the sign, are equal. To know the
Fig.5 Matlab/Simulink Model of Proposed Three level NPC Based
DPFC
Fig.5 shows the Matlab/Simulink Model of Proposed
Three level NPC Based DPFC using Matlab/Simulink
platform.
(a)
(b)
Fig.6. UPFC series converter controlling power flow
under balanced conditions, 2.5-s view during stepwise
changes of active and reactive power flow reference (a)
and (b) Active & Reactive power, Three Phase Currents.
Fig.8. UPFC series converter controlling power flow,
comparison between controllers Simulation under
unbalanced conditions, 70% single-phase voltage sag.
Case 2: Proposed Three level NPC Based DPFC with
Induction Machine Drive
(a)
Fig.9 Matlab/Simulink Model of Proposed Three level NPC Based
DPFC with Induction Machine Drive
(b)
Fig. 7. UPFC series converter controlling the power flow
under balanced conditions, 250-ms view during stepwise
change of active and reactive power flow reference of
active power & reactive power, currents and unit voltage
values.
Fig.10 Stator Currents, Speed, Electromagnetic Torque
Fig.10 shows the Stator Currents, Speed, and
Electromagnetic Torque of Proposed Three level NPC
Based DPFC with Induction Machine Drive.
[6] L.Liu, P.Zhu,Y.Kang, and J.Chen, “Power-flow control performance
analysis of a unified power-flow controller in a novel control scheme,”
IEEE Trans. Power Del., vol. 22, no. 3, pp. 1613–1619, Jul. 2007.
[7] S. Ray and G. Venayagamoorthy, “Wide-area signal-based optimal
neuro controller for a upfc,” IEEE Trans. Power Del., vol. 23, no. 3, pp.
1597–1605, Jul. 2008.
[8] H. Fujita, Y. Watanabe, and H. Akagi, “Control and analysis of a
unified power flow controller,”IEEE Trans. Power Electron., vol. 14, no.
6, pp. 1021–1027, Nov. 1999.
[9] J. Guo, M. Crow, and J. Sarangapani, “An improved upfc control for
oscillation damping, ”IEEE Trans. Power Syst., vol. 24, no. 1, pp. 288–
296, Feb. 2009.
[10] M. Zarghami, M. Crow, J. Sarangapani, Y. Liu, and S. Atcitty, “A
novel approach to interarea oscillation damping by unified power flow
controllers utilizing ultra capacitors, ”IEEE Trans. Power Syst., vol. 25,
no. 1, pp. 404–412, Feb. 2010.
Fig.11 3-Level Output Voltage
Fig.11 shows the 3-Level Output Voltage of Proposed
Three level NPC Based DPFC with Induction Machine
Drive.
V.CONCLUSION
However, the extensive use of power electronics
based equipment with pulse width modulated variable
speed drives are increasingly applied in many new
industrial applications that require superior performance.
The DPC technique was applied to a UPFC to control the
power flow on a transmission line. The technique has
been described in detail and applied to a three-level NPC
converter. The main benefits of the control technique are
fast dynamic control behavior with no cross coupling or
overshoot, with a simple controller, independent of nodal
voltage changes. The realization was demonstrated by
simulation and experimental results on a scaled model of
a transmission line. The controller was compared to two
other controllers under balanced and unbalanced
conditions, and demonstrated better performance, with
shorter settling times, no overshoot, and indifference to
voltage Unbalance. By the use of UPQC attain constant
regularized power and attain the load condition with
respect to good stability factor.
REFERENCES
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transmission control,” IEEE Trans. Power Del., vol. 10, no. 2, pp.
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