AN OPERATIONAL TYPE OF CIRCUIT FOR TCSC THYRISTOR VALAVES VALVES
AND CAPACITOR BANKS.
L FIELD OF INVENTION:
This invention relates to an apparatus for performing operational type tests on Thyristor Controlled Series Capacitor (TCSC) thyristor valves and capacitor banks.
2. BACKGROUND OF THE INVENTION:
400 KV alternating current transmission system is used widely for power transfer. In order to enhance the power transfer capability of these existing transmission sytems, series capacitors are being employed and they are known as Fixed Series Capacitors (FSC) schemes. Thyristor valves in series with reactors are employed in parallel to these capacitors to make the effective series capacitance variable and control the power transfer. This scheme is known as Thyristor Controlled Series Capacitors (TCSC). Thyristor valves and capacitor banks are the important components of TCSC schemes.
Performing operational type tests on TCSC Thyristor valves with voltage and current waveforms as in service condition is difficult. Synthetic test circuits, which are being used for thyristor valves, apply equivalent and not service condition waveforms. This may lead to over or under stress of the components of the thyristor valves. In the case of capacitor banks, as such, there is no suitable practical circuit in use to apply the right operational waveforms. The synthetic circuits are used only for thyristor valves.
The synthetic test circuits are generally consisting of separate voltage source and current source. The test voltage and current waveforms are superimposed from these sources. Voltage source circuit provides blocking voltage to the test valve whereas current source circuit provides the current during the valve conduction.
It is therefore an object of the invention to provide an apparatus to perform operational type tests on Thyristor Controlled Series Capacitor (TCSC) thyristor valves and capacitor bank.
Another object of the invention is to provide an apparatus, which is capable of applying service condition voltage and current waveforms to perform the operational type tests.
Still another object of the invention is to provide an apparatus, which can apply service condition voltage and current waveforms simultaneously, both to the thyristor valves and to the capacitor banks.
Yet another object of the invention is to provide an apparatus which is capable to perform all operational type tests on thyristor valve.
According to the invention there is provided an apparatus for performing operational type'tests simultaneously on thyristor valves; and capacitor banks of a Thyristor Controlled Series Capacitor (TCSC) scheme, the apparatus comprising a test-installation source voltage, a series 'reactor, a capacitor bank in parallel combination with at least one thyristor valve and a reactor connected in series with said at least one thyristor valve, the magnitude of said source voltage being adjusted to force the required source current, the triggering of the thyristor valve is synchronized with the source current, and the source current is adjusted to the line current for which the operational type test being performed.
3. BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS:
Figure-1 is a block diagram of synthetic test circuit and typical voltage and current waveforms generated using synthetic test circuit.
Figure-2 is a set of waveforms generated using the apparatus in accordance with the present invention.
Figure-3 shows the service condition waveforms.
Figure-4 is circuit diagram of typical TCSC installation.
Figure-5 is circuit diagram of invented apparatus.
DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION:
In accordance with the apparatus of the present invention, the Thyristor valve and the capacitor banks form part of the apparatus. The source, the series reactor and the parallel combination of capacitor banks and Thyristor valve in series with another reactor form the apparatus. With proper selection of the parameters of the components of the apparatus the service condition voltage and current waveforms are generated.
The waveforms are as in service condition and are not equivalent ones as in the case of synthetic circuits. The synthetic test circuits are used only for thyristor valves. According to the invention, the thyristor valve and capacitor banks are simultaneously tested with the service condition voltage and current waveforms.
The present invention uses source, which can be a short circuit generator or a variable ac source, feeding a series reactor in series with a parallel combination of capacitor banks and thyristor valve with another series reactor. The thyristor valve and the capacitor banks are the equipment under test and they form part of the test circuit. The circuit generates service condition voltage and current waveforms. Both the thyristor valve and capacitor banks are tested simultaneously.
Figure- 4 shows the circuit diagram of the typical TCSC installation. The thyristor valve is connected across the capacitor banks (C) through series reactors on either side of the thyristor valve. Gl and G2 are the thevenin equivalent sources and Zl and Z2 are the thevenin equivalent impedances of the installation.
In order to produce the service condition stresses on the thyristor valve (T) and capacitor banks (C), one has to build up the circuit as shown in the figure-4 which is not practical due to the large capacity of generator and other circuit elements.
The circuit as shown in Figure-5 has been evolved in the present invention to overcome the problems associated with the prior art.
The functional description of the circuit is as follows:
In figure-5 the circuit components of the invented' apparatus are adjusted considering the number of series connected thyristor functions to be considered for the operational type tests. IEEE standard recommends more than 2 valve functions.
Tl is the Thyristor valve functions considered for the operational type tests. Ls is the source inductahce including the external inductance, if any.
As far as the source voltage is concerned, the test installation source voltage, Vs, is adjusted to pass the required test current.
Vs is the source voltage at required frequency. The magnitude of the Vs is adjuited to force the required source current ls
The triggering of Tl is synchronized with "Is ". "Is" is adjusted to the line current for which the test is to be performed. Adjusting magnitude of Vs for the given firing angle of Tl gives the required current Is.
The apparatus can be used to perform all operational type tests on Thyristor valve.
The values of the circuit components of the apparatus are adjusted and the source voltage Vs can be chosen to a practical low value by suitably choosing Ls according to the sample calculation described hereinafter.
The invented apparatus provides the required test current. The combined voltage and current stresses are as in the service condition. Figure-2 shows the wave forms obtained during the testing using the apparatus of the present invention.
It is also proposed in the invention that the capacitors C shall also form the component of the apparatus so that the capacitors are also subjected to the operational type tests.
Number of thyristor levels in thyristor valve excluding
Redundancy = 13 nos.
TCSC capacitor bank rating = 306 |aF.
Total value of inductance in series with thyristor
Valve (Ltcr) = 4.4 mH.
Steady state line current during one of the operating
Conditions = 500A.
The test circuit parameters are adjusted suitably. The circuit parameters are given below:
Thyristor levels during testing with round off to nearest integer = 3nos.
Equivalent TCSC capacitor = 1000 to 1450 µF.
Equivalent inductance in series with thyristor valve =0.9 to 1.3 mH.
To simulate line current during service condition, series reactor (Ls) in Figure-5 is chosen in such a way that the required current' Is' (= line current) is achieved at low source voltage ' Vs' of the order of 3000 V.

We Claim;
1. An apparatus for performing operational type tests simultaneously on thyristor
valves and capacitor banks of a Thyristor Controlled Series Capacitor (TCSC)
scheme, the device comprising a test-installation source voltage (Vs), a series
factor ( Ls), a capacitor bank ( C ) in parallel combination with at least one thyristor valve ( T1) and a reactor ( L' tcr) connected in series with said at least one thyristor valve (Ti), the magnitude of source ( Vs) being adjusted to force the required source current (Is), is trigerring of the thyristor valve (Ti) is synchronized with the source current (Is) is adjusted to the hire current for which the operational type test being performed.
2. The apparatus as claimed in claim 1, wherein the values of the circuit
components are adjusted according to a number of series-connected thyristor
valves functions being tested.
3. The apparatus for performing operational type test simultaneously on
thyristor valves and capacitors banks of a thyristor controlled series controlled
series capacitors (TCSC) scheme substantially as herein described and
illustrated with reference to the accompanying drawings.