EXPERIMENTAL STUDY OF
THYRISTOR CONTROLLED REACTOR
(TCR) AND GTO CONTROLLED
SERIES CAPACITOR (GCSC)
JYOTI AGRAWAL
Department of Electrical Engineering (M-Tech IV SEM IPS), G.H.Raisoni College Of Engineering, Nagpur Nagpur, India
jyotingp7777@gmail.com
K.D.JOSHI
Department of Electrical Engineering, G.H.Raisoni College Of Engineering, Nagpur Nagpur, India
kdjoshi22@gmail.com Dr. V. K. CHANDRAKAR
GHRIETW, Nagpur Nagpur, India
Abstract:
This paper deals with the simulation of Thyristor controlled reactor (TCR) and GTO Controlled Series Capacitor (GCSC), equipment for controlled series compensation of transmission systems. The paper also presents experimental results of a TCR and GCSC connected to a single-phase system. The experiments are carried out in the FACTS lab of electrical engineering department. The TCR system is simulated using MATLAB and the simulation results are presented. The power and control circuits are simulated. The current drawn by the TCR varies with the variation in the firing angle. Stepped variation of current can be obtained using thyristor switched reactor. The simulation results are compared with the theoretical and practical results. Harmonics and its impact on the system are presented. This paper also presents the GCSC, its main components, principal of operation, typical waveforms and main applications. Duality of the GCSC with the well known thyristor controlled reactor is also discussed in this paper.
Keywords: FACTS; TCR; GCSC; harmonics; single phase power line analyser; MATLAB.
1. Introduction
A
FACTS technology is not a single high-power Controller, but rather a collection of Controller, which can be applied individually or in coordination with others to control one or more of the interrelated system parameters. By means of controlling impedance or phase angle or series injection of appropriate voltage a FACTS Controller can control the power flow as required. This project is based on FACTS technology. TCR is gaining popularity as a method of voltage control. The basic elements of a TCR are a reactor in series with a bidirectional thyristor. This paper focuses on the Thyristor Controlled Reactor. The current in the reactor L can be controlled from a maximum (thyristor valve closed) to zero (thyristor valve open) by the method of firing delay angle control. The TCR produces harmonic currents because thyristors only allow conduction in the reactor for a portion of the cycle.Harmonic current magnitudes vary as the firing angle of the thyristor is varied. Series compensation of power transmission lines is a useful tool to improve the power transfer capability. In power systems where large amounts of power must be transmitted through long transmission lines, sometimes it is necessary to add series compensation, in order to improve system performance. Several FACTS controllers for shunt, series or both shunt and series compensation are now operating in power systems around the world.
block at zero voltage. Hence, the series connection of the GTOs is not difficult. This device continuously regulates the capacitor voltage. By controlling turn-off delay angle γ, one can continuously vary the impedance of the GCSC. The waveforms of the GCSC are similar to those of the well-known thyristor controlled reactor (TCR). The GCSC is the dual circuit of the thyristor controlled reactor (TCR). The use of this analogy simplifies the analysis, modeling and understanding of this equipment. In TCR the control was done by the firing angle and in the GCSC it is done by the blocking angle. The GCSC is the natural solution for series compensation as well as the TCR is the natural solution for shunt compensation. Also, the GCSC may be the simplest and lowest cost solution for line impedance control. This paper presents the waveform of capacitor voltage Vc.
2. Basic operations of TCR and GCSC
2.1. Thyristor Controlled Reactor (TCR)
The circuit of TCR system is shown in fig 1 a. The thyristor-controlled reactor consists of a reactor in series with two parallel inverse thyristors. The two inverse parallel thyristors are gated symmetrically. They control the time for which the reactor conducts and thus control the fundamental component of the current. The thyristors conduct on alternate half-cycles of the supply frequency depending on the firing angle αor conduction angle σ, which is measured from a zero crossing of voltage. The relation between firing angle and conduction angle is given by equation (1):
σ= 2(π−α) (1)
Full conduction is obtained with a firing angle of 0º. Under this condition, the current is reactive and its waveform is purely sinusoidal. There is a partial conduction between 0º and 90º as shown in figure 1b. When the firing angle αincreases from 0º to 90º, the waveform of the current goes away from the original sinusoidal form. For the condition of balanced loading, TCR produces odd harmonics. However, TCR circuits should not be operated at points near resonance as they would generate conditions causing effective harmonic production.
The current in the reactor can be expressed as
iL(t)=V/ωL (sin ωt- sin α) (2)
The term (V/ωL) sinα is an α dependent constant.
Fig 1.a. Circuit of TCR
V is the input AC source. TCR is realized using inductor and anti-parallel switch.
2.2. GTO Controlled Series Capacitor (GCSC)
The circuit of GCSC system is shown in fig 2 a. It consists of a fixed capacitor in parallel with a bidirectional switch made up of a pair of GTO thyristor. In contrast to a thyristor, a GTO thyristor can be turned off upon command. A GCSC is a dual of a TCR which is connected across a voltage source which is assumed to be sinusoidal. The objective of the GCSC scheme is to control the voltage Vc across the capacitor at a given line current i. When the GTO valve is closed the voltage across the capacitor is zero, and when the valve is open, the voltage across the capacitor is maximum. For controlling the capacitor voltage, the closing and the opening of the valve is carried out in each half-cycle in synchronism with the ac system frequency. If the GTO’s are kept turned-on all the time, the capacitor C is bypassed and it does not present any compensation effect. However, if the GTO’ are turned-off once per cycle, at a given blocking angle γ counted from the zero-crossings of the line current, the capacitor turns alternately on and off, in series with the transmission line, and a voltage Vc appears. The level of series compensation is given by the fundamental component of the capacitor voltage Vc. This level may be varied by controlling the blocking angle γ of the semiconductor switches. When the opening of the valve is delayed by the angle γ with respect to the crest of the line current, the capacitor voltage can be expressed with a defined line current, given by equation (3)
i(t) = I/ωC (sin ωt-sin γ) (3)
The term I/ωCsin γ is simply a γ dependent constant by which the sinusoidal voltage obtained at γ = 0. The turn-off delay angle γ defines the prevailing blocking angle given by equation (4)
ζ = π - 2 γ (4)
As the turn-off delay angle γ increases, the correspondingly increasing offset results in the reduction of the blocking angle ζ of the valve, and the consequent reduction of the capacitor voltage. At the maximum delay of γ = π/2, the offset also reaches its maximum of I/ωC at which both the blocking angle and the capacitor voltage becomes zero.
Fig 2 a. Circuit of GTO-Controlled Series Capacitor
3. Simulation Results for the TCR
The simulation was done by writing the program in Matlab version 7.2 and the results are presented here. The ILF(α), normalized to the maximum current ILFmax is plotted against delay angle α as shown in Fig 3.
The amplitude ILF(α) of the fundamental reactor current can be expressed as
ILF(α) =V/ωL (1-2α/π - sin 2α/π) (5)
3.1. Description of Single Phase Power Line Analyser
Single phase power line analyser consists of input AC source, transmission line model, static var compensator configuration like thyristor switched capacitor (TSC), thyristor controlled reactor (TCR), thyristor switched series capacitor (TSSC) and GTO controlled series capacitor (GCSC) and display meter section. Static var configuration model set up had provided the variac for controlling the voltage of the thyristorized controller. The variac output voltage can be adjusted by changing the increment and decrement switch which is given to the thyristorized and GTO controller. SCR pulse controller is placed to give firing pulse for the thyristor which is connected through the 9 pin pulse connector. Each and every controller function can be analysed individually, for the firing pulses that are taken from the digital pulse controller. Some functional keys are provided in the digital pulse controller which is explained below.
-
Selection and increment function- Selection and decrement function
- Cursor movement
- Enter key
- Reset
Transmission line model is connected with TCR. The single phase resistive load of 220.6 Ω is connected with transmission line output terminals. Input voltage can be varied by using the increment and decrement voltage adjustment Knob. Firing pulses can be varied from the digital controller.
3.2. Front panel diagram of TCR
Using the front panel diagram that is single phase power line Analyser as shown in fig 4, the results obtained on Digital Oscilloscope are presented here. The current through TCR with α=0º is shown in fig 5a. The through TCR with α=70º is shown in fig 5b.
Fig 4. Front Panel Diagram of TCR
Fig 5.b. Waveform of current in TCR with α=70º Fig 5.a. Waveform of current in TCR with α=0º
4. Simulation Results for the GCSC
4.1. Front panel diagram of GCSC
Using the front panel diagram of GCSC that is single phase power line Analyser as shown in fig 6, the results obtained on Digital Oscilloscope are presented here. The voltage across the capacitor at different ON and OFF angle are shown in Fig 7.
Fig 6. Front Panel Diagram of GCSC
Fig7a. Waveform of voltage across capacitor Vc at Fig7b. Waveform of voltage across capacitor Vc at αON = 0ºαOFF = 180º αON = 10ºαOFF = 170º
Fig 7a shows that the GTO’s are turned ON at αON = 0ºandturned OFF at αOFF = 180º that is the GTO’s are kept turned-on all the time, the capacitor C is bypassed and it does not present any compensation effect whereas when the opening of the valve is delayed by the angle γ and the GTO’s are turned ON at αON = 10ºandturned OFF at αOFF = 170º the waveform of voltage across capacitor is shown in Fig 7b
5. Harmonics in TCR
Harmonics that arise from the interaction of thyristor controlled reactors (TCRs) and power systems can sometimes cause stability problems. The stability problems are hard to analyse since the harmonics are affected by the power system. TCR in addition to the wanted fundamental current also generates harmonics. For identical positive and negative half–cycles, only odd harmonics are generated. The amplitudes of these are a function of angle α , as expressed by the equation 6
I
Ln(
α
)=V4/
ω
L
π
{(sin
α
cos(n
α
)-ncos
α
sin(n
α
))/(n(n^2-1))} (6)
The simulation was done by writing the program in Matlab version 7.2 and the results are presented here. The amplitude variation of the harmonics, expressed as percent of the maximum fundamental current is shown plotted against α in Fig 8
Fig 8. Amplitudes of the harmonic components in the current of the TCR versus delay angle α
6. Effect of TCR on Transmission voltage
Test results for with and without TCR are recorded in Table1 and Table2. Here Vs stands for Sending end voltage, VR stands for Receiving end voltage, VM for Midpoint voltage, IL for Load current. Table2 shows the
values of current and voltage at different firing angles. By definition reactors (inductors) absorbs reactive power when connected to an ac power source. The main objective of applying reactive shunt compensation in a transmission system is to increase the transmittable power. This is required to improve the steady-state transmission characteristics as well as the stability of the system. Fast reactive compensation can increase the performance of line. TCR being a power electronics based device, can provide variable reactive power in fraction of a second. Here from the test results tabulated below we can observe that when TCR is ON as shown in Table2 (reading 1) the TCR conducts current over full 180º resulting in maximum inductive var output absorbed by the fully conducting reactor. With the increase in firing angle VM and VR goes on increasing. When
the thyristor is off i.e. firing angle is 90º VM and VR is maximum as shown in Table2 (reading 5). A basic var
arrangement consisting of a fixed capacitor with a thyristor controlled reactor (FC-TCR) is generally used. The constant capacitive var generation (Qc) of the fixed capacitor is opposed by the variable var absorption (QL) of
the thyristor controlled reactor, to yield the total var output (Q) required. At the maximum capacitive var output, the thyristor controlled reactor is off (α=90º). To decrease the capacitive output, the current in the reactor is increased by decreasing the firing angle α. At zero var output, the capacitive and the inductive currents become equal and thus the capacitive and inductive vars cancels out. With further decrease of angle α, the inductive current becomes larger than the capacitive current, resulting in the net inductive var output.
Table 1. Test results for without TCR
Sr.no Without TCR
Vs (Volts) VM (volts) VR (Volts) IL (amp)
Table 2. Test results for with TCR
Sr.no With TCR
Vs (Volts) VM (Volts) VR (Volts) Firing angle α (º) IL (amp)
1. 100 18 15 10 0.03
2. 100 33 36 30 0.06
3. 100 52 56 50 0.07
4. 100 68 69 70 0.09
5. 100 80 74 90 0.1
7. Operating V-I Area of TCR
A practical TCR can be operated anywhere in a defined V-I area, the boundaries of which are determined by its maximum attainable admittance, voltage and currents ratings as illustrated in Fig 9. The admittance as a function of angle α can be written directly from
BL(α) = 1/ (ωL){1-((2α)/π)-(Sin2α/π)} (7)
The test results for variation in voltage and current with fixed firing angle is recorded in Table 3. Using these test results V-I area for TCR is obtained in the Matlab for two different firing angle i.e. at α=80º and α=65º as shown in fig 9.
0.02 0.022 0.024 0.026 0.028 0.03 0.032 0.034 0.036 0.038 0.04 20 25 30 35 40 45 IL VL alpha=65(deg) alpha=80(deg)
Fig 9. Operating V-I area of the TCR
Table 3. Test results for variation in voltage and current with fixed firing angle
Sr.no Firing angle α (º) VTCR (Volts) ITCR (Amp)
1. 0
25 0.04 35 0.08 50 0.13 84 0.17
2. 65
35 0.02 65 0.03 98 0.04
3. 80
45 0.01 68 0.03 98 0.05
4. 90
45 0.01 70 0.02 85 0.06
8. Duality with the thyristor controlled reactor
waveform of the GCSC is similar to current waveforms of the TCR. Table1 shows a comparison between the dual characteristics of both equipment.
Table 4. Dual characteristics of the GCSC and the TCR
Gate Controlled Series Capacitor (GCSC) Thyristor Controlled Reactor (TCR)
1) Semiconductor switches in parallel with a capacitor. 1) Semiconductor switches in series with a reactor.
2) Supplied by a current source. 2) Supplied by a voltage source.
3) Series voltage Vc at a given line current is Controlled.
3) Shunt current at a given line voltage is controlled
4) Connected in series with the transmission line. 4) Connected in shunt with the transmission line. 5) Voltage controlled by switches blocking angle. 5) Current controlled by switches firing angle. 6) Switches fire and block with zero voltage. 6) Switches fire and block with zero current.
9. Prospective Applications
The GCSC could be typically used in applications where a TCSC is used today, mainly in the control of power flow and damping of power oscillations. The GCSC may operate with an open loop configuration, where it would simply control its reactance, or in closed loop, controlling power flow or current in the line, or maintaining a constant compensation voltage. Power Oscillation Damping scheme may also be easily attainable with the GCSC.
10.Conclusion
The characteristic performance of thyristor controlled reactor and GTO Controlled Series Capacitor circuit has been studied using single phase power line analyser and Matlab version 7.2. The performance of TCR was found to be highly dependent on the Firing angle of the TCR. It was found that as the firing angle increases, results in the consequent reduction of the reactor current. TCR has the ability to ensure a continuous and fast reactive power and voltage control which can increase the performance of the system such as control of transients over voltages at power frequency, preventing of voltage collapse, increase in transient stability and decrease in system. This paper also presented a equipment for controllable series compensation of transmission lines: Gate Controlled Series Capacitor (GCSC). Emphasis is given to the fact that the GCSC is the dual device of the Thyristor Controlled Reactor (TCR). GCSC is more compact, with lesser passive component: it does not need reactors and its capacitor bank is much smaller. The waveform obtained on the digital oscilloscope demonstrate the operating principles of the GCSC. The performance of GCSC was found to be highly dependent on the turn-off delay angle of the GCSC. It was found that as the turn-off delay angle γ increases, the correspondingly increasing offset results in the reduction of the blocking angle ζ of the valve, and the consequent reduction of the capacitor voltage. GCSC can be used effectively to provide a fast and continuous compensation of the line impedance.
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