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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

16 Š 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)

17 © 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

18 © 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.

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20 © 2010 ACEEE DOI: 02.AEE.2010.01.44

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