Download The Role of Advance Voltage Source Inverter in Power System

Proc. of Int. Conf. on Advances in Electrical & Electronics 2010
The Role of Advance Voltage Source Inverter in
Power System Transmission line
Mr Rahul Somalwar, Lecturer, Electrical Department, Datta Meghe institute of Engineering Technology & Research,
Wardha
Prof. Moharir, HOD, Electrical Department, Yashwantrao Chauhan College Of Engg. Nagpur
b) At the load end the voltage stability , i,e the voltage of
load end should be maintained constant. The fulfillment of
this condition often results in limitation of transmitted
power to level much below the thermal capacity of such
lines and perhaps even below the available capacity.
With ever increasing the difficulties in obtaining the new
transmission line right now, the present trend to load the
line close to its thermal capacity. Studies indicate that with
utilization of transmission line in interconnected power
system meeting the desired objectives for availability and
operating flexibility need the control of either or both line
impedance and power angle.[1] This is evident due to the
fact that power flow depends on line impedance ,magnitude
of both end voltage and phase difference between phase
voltage. VSI inject a variable voltage to shift phase angle
of transmission line phase voltage. This modifies, the phase
angle difference between the sending and receiving end
voltage of the lines.
Fig 1 shows the schematic diagram. of Static
phase shifter. The SPS input is 3 phase voltage provided by
Excitation transformer ( ET). The output is a 3 phase
voltage injected in the system by series Boosting
transformer( BT ).[2] A converter controls the magnitude
and /or phase angle of the injected voltage . Technical
limitation and merits of SPS primarily depend upon the
characteristics of its converter. Realization of a static
converter with controllable output voltage frequency
require high frequency ( more than 1000 Hz) switching
schemes. Based on the switching loss characteristics of
presently available static switches , such a converter is not
economically attractive choice for power application.[3][4]
We are working on the new technique to construct the
Voltage source inverter by using advance semiconductor
which gives the good result and overcome the drawback of
IGBT and GTO. [5][6]. The converter characteristics
define the ranges that phase angle are controlled.
Abstract - Development of effective ways to utilize
transmission system to the maximum thermal
capabilities has caught much research attention in
resent year. This is one direct outcome of the concept of
flexible A.C. transmission system (FACTS) aspects of
which have become possible due to advances in power
electronics. The Voltage source inverter (VSI) as a
static phase shifter is one of possible way to achieve the
desired effect of FACTS. The role of the Voltage source
inverter in improving the power system stability is
investigated. Voltage source inverter is one of the device
as a static phase shifter (SPS) which improves the
transient stability and damping oscillation. When an
SPS can control both magnitude and phase angle of
injected voltage, it performs the assigned function more
effectively.
A new technique is used to formulate the control
algorithm for the Voltage source inverter. Parameter
uncertainty has been considered in proposed scheme.
Computer simulation shows that introducing phase
angle by Voltage source inverter, the transient stability
can be improved.
The main object of paper is to outline the
various ways of Voltage source inverter as a SPS is
utilize to improve transient stability. The relationship of
Fixed and variable phase shift with transient stability
enhancement has been established. The control
algorithm of Voltage source inverter is conceptualized
to prevent loss of synchronism by extending the
capability of generator to produce sufficient
decelerating energy to counter balance the acceleration
energy gained during fault. Control of time and angle of
swing with role of change of phase shift by SPS is
possible and its applicability with slow and fast acting
SPS has been explored. Work is in progress for
construction of the advance VSI as a SPS. Software
tools used for studies are MATLAB 7.1.
I.
INTRODUCTION
The power that can be transmitted over a long
distance transmission line from the generation end to the
load end is determine by the following condition.
a) At the generation end synchronous stability, so called
first swing stability i,e none of the connected generator
should loose its synchronism, as a result of disturbance on
the connected transmission.
Fig 1
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© 2010 ACEEE
DOI: 02.AEE.2010.01.44
Proc. of Int. Conf. on Advances in Electrical & Electronics 2010
Depending upon the type and the location of an
SPS, it may be utilized for mitigation of small-signal
oscillation and /or enhancement of transient, in addition to
steady state power flow regulation. [13]. Type D phase
shifter is
used as a device ,which can inject a voltage with
controllable phase angle and /or load condition. In case D
both magnitude and phase angle of static converter can
rapidly control. The voltage source inverter (VSI) is
providing independent control over magnitude and /or
phase of its alternating side voltage.[12]
The application of
VSI to inject a phase
quadrature voltage in line yields a new , fast, controllable
phase shifter for active power control. Once a synchronized
VSI is produced it is indeed easy to regulate both the
magnitude and phase angle of the injected voltage to yield
an new controller.
P-Delta curve
1.6
P
o
w
e
r
P,
I
n
p
e
r
u
n
I
t
1.4
Pre fault Curve
1.2
Post Fault Curve
1
During Fault
Curve
0.8
0.6
0.4
0.2
0
0
0.5
1
1.5
2
Delta in Degree
2.5
3
3.5
Fig. 2
M d2δ/dt2 = Ps – Pmax Sinθ ---( 1)
δcr= δ1 = P/ 2M ( t1) 2 + δo ------( 2)
where δo = aSin ( Ps/Pmax1), ---------(3)
and δ1 = п – asin( Ps/Pmax3), ---( 4)
δ1 is cleared at t1, it’s a critical clearing angle. At this
stage if A2 ( DEF ) is greater than A1 ( ABCD ) the system
becomes STABLE. For higher values of supply power PS
and t1 the stability is seldom maintain. Even if the stability
is maintained on the fault clearance ,large amplitude
oscillation takes place about the final operating point F.
II. CONTROL ALGORITHM OF STATIC PHASE
SHIFTER
Considering a sample system consisting of a generator.
Connect for an infinite bus bar through the two parallel
transmission line as shown in the Fig 19(a) .in which the
SPS unit is connected with transformer to the generator.
In normal operating condition power transfer
smoothly. When the fault occurred in transmission line, the
power transfer will be reduce. The fault is cleared after t 1
sec by isolating line 2 and after post fault , power carried
by line 1 only. For the single machine infinite bus-bar
system ,this can be conveniently determine by “EQUAL
ARA CRETERIA”.
In power angle curve shown in fig 2, before the fault
occurrence ,the power angle curve is given by
P e1 = P m ax1 S in δ o
During fault ,the power transfer will be
P e2 = P m ax2 Sin δo
After clearing the fault ,the power flow is restored via
healthy line due to which power becomes
P e3 = P m ax 3 S in (δ - α o ) = P m ax 3 S in θ
By using the SWING EQUATION
(
neglecting damping)
Application of equal area criterion to a critically cleared system
Critical clearing angle = 58.2806
1.6
P
o
w
e
r,
p
e
r
u
n
I
t
1.4
1.2
E
D
1
A
Pm
F
0.8
0.6
0.4
C
0.2
0
0
B
20
40
60
80
100
120
Power angle, degree
140
160
180
Fig 3
The fault cleared at δ1 after t1 sec. The machine
swing up and large amplitude oscillation takes place around
the stable point .At quasi stable state point F, dδ/dt
becomes zero but Pe is greater than Ps .To damp out the
oscillation optimally in a dead beat mode at α2 a fixed
phase shift is introduced.
Where α2 = δ2 – asin (( Ps/Pmax3),
By the SPS at time of ts corresponding to α2 stable point of
operation is reached.
For optimal damping of oscillation at the end of the swing
period ts a step change in phase shift α2 – α1 is to be
provided by SPS.
Fig 1(a)
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© 2010 ACEEE
DOI: 02.AEE.2010.01.44
Proc. of Int. Conf. on Advances in Electrical & Electronics 2010
α2 – α1 = п – δ1 ------( 5)
There are two ways to improve the transient stability . By
fixed phase shift and variable phase shift. By introducing
the fixed phase shift α0, increase area A2 ,But area A2
increases in very small amount as compared to variable
phase shift. The step change in phase shift is
α2 – α1 = acos (Ps/Pmax3)
By introducing the α1 and α2 successively the area A2
should be greater than A1 due to which the system
becomes STABLE.
Es = 0.95 + ( j 0.434)* I p.u.
E = 1.023∠ 21.35
Generator internal voltage is the vector sum of terminal
voltage of machine & the voltage necessary to force the
current through the transient reactance =1.125∠32.43
Step 3: power transfer before fault is Pe1 = 1.55 Sin δ
Step 4 : During Double line to Ground fault
Xt = 2.8985 p.u
Power transferred during fault is Pe2 = 0.37Sin δ
Step 5 : Power transferred after fault is Pe3 = 1.11 Sin δ
Step 6 : The system is operating at only single line
III. ANALYSIS : GIVEN DATA
Pe1= 1.55 Sin δ
Pe2 = 0.37 Sin δ
Pe3 = 1.11 Sin δ
Investigation of stability :
Given: Pm1 = 1.55, Pm2 = 0.37, Pm3 =1.11, Ps= 0.833.
δ1
A1 = ∫(Ps –Pm2 Sin δ) d δ = 0.3044 ;
δ0
δ2
A2 = ∫(Pm3 Sin δ - Ps ) d δ = 0.1541 ;
δ1
A2 < A1 The system is unstable.
Single machine system supplying bus through two
transformer and double circuit transmission line.
Sending End data: 30 MVA ,3 Phase,60Hz water –wheel
generator
Unsaturated synchronies reactance = Xd = 63.8%
Rated current transient reactance = Xd’ = 25.4%
Negative sequence reactance = X2 =28.99%
Inertia constant ( Kw sec/ Kv) = H= 3 Normal regulator
and excitation system.
A. Transformer:
60 MVA , 3 phase, 60 cycle/sec, Bank connected
at each end of the transmission line.
Reactance = 8%( exciting current is neglected)
Receiving end :Low voltage side of receiver and
transformer connected to an inertia system. Receiver low
voltage bus fixed at 95 % of normal voltage.
Simulation result Fig 4 shows that the system is unstable.
Improvement by Voltage source inverter:
SPS is the static device, which adjust the phase shift as per
the system configuration. To make the system stable
variable phase shift introduce.
B.Transmission Line:
Two circuits in parallel 50 miles long ,10 fit flat
spacing =12.6foot equivalent . Delta spacing conductor are
25000 circular mils copper. The distance between centers is
40 feet & the Conductors are fully transposed .No ground
wire 50 Mw at 100%power factor is delivered to infinite
receiver system . Normal voltage 66 kV.
The Base MVA selected is the MVA of the sending end =
60 MVA
Base current 525Amp , Base voltage = 3.81 kV, Base
Impedance = 72.6 Ohms
Step 1: Impedance of different element in p.u. are X1 & X2
each line equals 39.7 ohms.
Xo =138.2ohms for single line & 108.2ohms for two line in
parallel.
X1 =0.542 p.u. = X2, Xo = 1.90p.u.
Two lines in parallel : Positive and negative sequence
reactance ‘s
X1=X2= 0.275 p.u
Zero sequences Xo=1.49 p.u
Step 2 : TO calculate terminal voltage of sending end
generator .
Load at receiver L.V. bus is 50 MW
At unity power factor = 50/60 = 0.833
The current in the network
I = 0.833 / 0.95 = 0877 p.u.
The terminal voltage at sending end Generator is
Fig 4 Simulation result when Ps =0.91, System is Unstable
STEP 1 : Curve I : – Pm1 Sin δ :
Pm1 = VE / X1
Curve II :- Pm2 Sin δ :
Pm2 = VE /X2
Curve III :- Pm3 Sin θ : where θ = δ – α0
Step 2 :
δ0 = 35.951 , δ1= 63.63 ( At tcr = 0.13) ; δ2 = 186.56
Step 3 : Now. fixed phase shift of α0 is introduce.
α0 = 8.56
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© 2010 ACEEE
DOI: 02.AEE.2010.01.44
Proc. of Int. Conf. on Advances in Electrical & Electronics 2010
Losses occur in all four conditions of operation (on, off,
switching on, switching off). At medium voltage, GTO’s
exhibit very low on-state losses and reasonable turn-off
losses. However, due to switching being nonhomogeneous, external snubber circuits are necessary for
the switching operation. These snubber circuits take up
more than half the volume of the final equipment and
account for much of the design complexity, costs and
losses.[8] The tendency over the years has been for the
designers of all these devices to concentrate mainly on the
power switching itself, so that little attention has been paid
to the complexities involved in real-world applications. The
ideal power switch would switch like an IGBT and conduct
like a GTO thyristor, and it would have the low fabrication
costs and high yields of the GTO thyristors. This is exactly
what the IGCT achieves The IGCT has become the power
semiconductor of choice in Medium Voltage Industrial
Applications. Also in the Energy Management and the
Traction market the versatility of this power switch has
enabled performance improvements and cost savings in a
variety of applications.[10] IGCTs are currently being
applied to such devices as: Medium Voltage Drives
(current and voltage source),Circuit Breakers, Superconducting Magnetic Energy Storage Systems (SMES),
Dynamic Voltage Restorers , STATCOMs, Dynamic
Uninterruptible Power Supplies, Power Conditioners,
Induction Heaters, Traction inverters and choppers.
Step 4 : For variable phase shift , α0 = - 26.37 It is
negative as this form of control is contrary to the basic
concept of operation of SPS for stability enhancement.
θ = δ - α0 = 63.63 +26.37 = 90 deg .
θ is kept constant because it is required to make dδ / dt
equal to dα / dt at θ = 90 deg to obtain minimum value of
setting time is ts ;
To enhance transient stability and optimal damping further
we introduce
α2 = δ2 – δ1 + α0
α1 = 64.13, α2 = 99.07 :
Settling time ts = t1 Pm3/ ( Pm3 – Ps) = 0.7215 sec
As the rate of change of phase shift is calculated dδ / dt = 427.07
Investigation of stability
δ1
A1 = ∫ (Ps –Pm2 Sin δ) d δ = 0.3044 ;
δ0
δ2
A2 = ∫(Pm3 Sin δ - Ps ) d δ = 0.4395
δ1
A1 = 0.3044
; A2 = 0.4395
Since A2 > A1 , The system is stable.
CONCLUSION
To investigate the effectiveness of the SPS under
different fault condition the equal area criteria is applied
and simulation is conducted.
The result of control algorithm is simple and straight
forward that
• Without SPS the power system loses synchronism
at particular shaft power Ps
• With SPS the power system is stabilized at
particular shaft power Ps.
Fig 5 Simulation result at Ps =0.91,When introducing the Phase angle by
VSI ,System is Stable
Utility of Voltage source inverter for transmission
stability enhancement and damping of large oscillation has
been elaborated. Expression for fixed phase shift, step
change in phase shift and rate of change of phase shift in
term of system parameters, power flow and rate of change
of rotor angle variation has been obtained. Classical model
of power system has been used for getting the expression
and their computations.
The stability investigation has been carried out and
the system is found to be stable after introduction of phase
shift by SPS. The investigation has been done by analytical
,graphical & numerical methods. Earlier we have kept θ
constant, for dδ/dt equal dα /dt to obatian minimum
settling time [7]. As we have high dα /dt = 427.07 o/sec
,the system is stable, but for slower rate of phase shift the
system may not be able to regain stability. The value of dα
/dt must be achieved in a particular moderate range.
Advance Voltage source Inverter
From the very beginning, the development of
power semiconductors was nothing more than a search for
the ideal switch. The lowest on-state and commutation
losses, the highest possible commutation frequency and a
simple drive circuit. Power silicon switches have increased
steadily in complexity and capability.[6][9] The first
silicon-controlled rectifiers could switch power off only at
the end of an AC cycle. From the transistor and Darlington
to the IGBT, low-voltage applications have benefited all
the way along while the medium-voltage user could only
look on — GTO’s and more GTO’s, nothing else . The
introduction of IGBTs brought faster switching, but at
present their switching losses are acceptable only at low
voltage levels. GTO thyristors consist of thousands of
individual switching elements fabricated on a silicon wafer.
19
© 2010 ACEEE
DOI: 02.AEE.2010.01.44
Proc. of Int. Conf. on Advances in Electrical & Electronics 2010
The scheme of the introduction of the phase shift
after the fault clearance is verified by a computer program
and the system which was earlier unstable was found to be
stable after the phase shift α is introduced by SPS. The
new technique is suggested in this paper is advanced static
phase shifter in which new semiconductor IGCT is used
instead of GTO or IGBT for designing the voltage source
inverter. The paper analyzed the advantages of IGCT over
other power electronics switches. The application of this
switch is studied by using voltage source inverter. Work is
in progress for investigation of comparative study of VSI
using IGBT & IGCT. MATLAB –7.1 simulation software
used for observing the wave forms.
Transient stability analysis is used to investigate
the stability of power system under sudden and large
disturbances, and plays an important role in planning and
operation of the power system. The transient stability
analysis is performed by combining a solution of the
algebraic equations describing the network with numerical
solution of the differential equations. Although significant
improvements have been made in the application of
numerical and computational methods to the transient
stability calculation, the computational demands are rising
rapidly at the same time. Therefore there is a continual
search for faster and accurate solutions to the transient
stability problem.
[5]
[6]
[7]
[8]
[9]
[10]
[11]
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