VaLvE FiRiNg

VaLvE FiRiNg : 

                          The basic valve firing scheme .The valve control generates firing signals. Each thyristor level receives the signal directly from a separate fibre-optic cable. Thus each thyristor level is independent, sharing only a duplicated light source at the group potential.

            The valve control unit also includes many monitoring and protective functions. The return pulse system coupled with short pulse firing scheme is used in present day valve control units. A separate light guide is used to send a return pulse whenever the voltage across a thyristor is sufficient and the power supply unit is charged. If at that time, firing pulse are demanded from the valve control, the light signals are sent to all thyristor control units simultaneously.

           During normal operation, only one set of light pulses are generated in a cycle for each valve, However, during operation at low direct currents, many light pulse are generated due to discontinuous current.

THYRISTOR VALVE

THYRISTOR VALVE 

General   :A thyristor valve is made up of a number of devices connected in series to provide the required voltage rating and also of devices connected in parallel to provide the required current rating. With modern devices of high current ratings, the need for parallel connection does not arise. The number of series connected thyristor is determined  by device ratings, transient overvoltages and protection philosophy.

              The valves are usually placed indoor in a valve hall which is protected against contamination and dust. Valves are generally base mounted in a single, double or quadrivalve configuration. The latter results in a compact valve hall.

            The valves are usually air insulated and cooled using air, water ,oil or freon. The water cooling is normally used in modern converter stations are the power losses are reduced. In a valve, the heat sinks and damping resistors are cooled by the water flowing in ducts. The main objective of the cooling system is to reduce the total thermal resistance for the heat sink. 

GATE DRIVE

A hard drive with sharp rise time is necessary to turn-on the device quickly with large initial mdi/dt. A single short pulse is adequate to turn-on the device,provided the current in the device does not go below the holding current during the conduction period(about 120) of the device. Sometimes, long pulses (or a pulse train lasting the required conduction period) are applied to avoid the blocking of the device due to discontinuous conduction.

             The locus of possible gate trigger points is bounded by two lines A and B. There are also lower and upper limits on direct gate voltage and current. The HVDC valve are functions of the junction temperature. The thyristor in a modern HVDC valve are fired from the optical signals sent from the circuits at the ground potential. These signals in turn are generated from the converter controller. The source of firing energy is provided at each module by a power supply unit which is charged from the forward voltage across the device when it is not conducting. The diagram of typical module with one thyristor per module. The gate pulses are generated in the gating and logic units in response to optical signals,transmitted via fibre-opti light guides separately to each module. The gating and thyristor status are also monitored at the ground level the signals sent via another set of light guides. The breakover diode (BOD) is used to sense overvoltage across the device and protect it by generating pulse. BOD firing is also used as a back-up if the gating and logic unit fails.

AC Voltage grading is provided by resistor RD and capacitor CD in series. This circuit also damps the oscillations produced by step changes in the voltage, which occur during commutation of current from one valve to the next. It also limits the rate of rise of voltage on a late firing module.

   

        The resistor RDC provides for DC voltage grading and this circuit is also used for voltage measurement. The saturable reactor is used to protect the device against di/dt. CH and RH are used as grading circuits for high frequency voltages.

SWITCHING CHARACTERISTICS

SWITCHING CHARACTERISTICS  :

              Turn-on :When a gate drive is applied with the forward voltage above latching voltage (the minimum anode to cathode voltage that will successfully turn-on a thyristor with a given gate drive), turn-on occurs. Becauseof the finite sheet resistance of the p-base region, only those regions of the cathode nearest to the gate are influnced by the gate current. Regenerative switching action is initially restricted to these regions. The establishment of the equilibrium current flow over the cathode area follows by outward spreading froms this conducting plasma, by diffusion. The plasma spreading is relatively slow and occurs with a typical velocity of 0.1mm/. When the area of conduction is small, the voltages across the device is considerable has an upper limit on the di/dt, which in modern amplifying gate thyristor is up to 500 A/. Saturable reactors in series with the thyristor are used to limit di/dt, particularly arising from the discharge of current due to stray capacitances and snubber circuits.

               There are three phases of turn-on .The delay time is associated with the establishment of regenerative action in response to the gate current. Its duration depends upon the level of the gate drive. Regeneration is well established during the rise time. The current continues to increase of 10-20% of that predicated for normal conduction using on-state voltage of the thyristor. The spreading phase may last over hundred microseconds.

               Turn-off :  All the three junctions are forward biased during on-state and the base regions contain excess minority and majority charge. This charge must either be swept out by an electric field or decay through regenerative processes within the silicon.

           When the circuit voltage is reversed, the current falls to zero at a certain rate. Once the current reaches zero, the flow reverses,since the minority carrier concentration at the junctions can support this current by diffusion without build-up of depletion layer. The peak value of this reverse  current is reached when the excess hole concentration at the anode junction has fallen to zero. At this time, the voltage across the thyristor reverses with the development of the depletion layer and the voltage across the thyristor reverses with the development of the depletion layer and the current decays in a near exponential manner as a result of charge recombination within the n-base region. The decay of current is dependent on the mean life-time of carriers in the n-base region. Immediately after current zero, a thyristor is unable to support forward voltage. Gradually, the thyristor acquires some forward blocking capability is attained only after a millisecond or so has elapsed from current zero. This characteristic is circuit and temperature dependent 

DEVICE CHARACTERISTICS

DEVICE CHARACTERISTICS :

                      The device can be in one of the three following states :

1. Forward biased and blocking 
2. Forward biased and conducting 
3. Reverse biased and blocking.

               The transition from the first to the second state is called turn-on, while the transition from the second to the third state is called turn-off. The characteristics of the device refer to the parameters of the device both in steady-state and transient conditions(during the transition of state).

Steady state characteristics :

            off-state: The volt-ampere characteristics of the device are during the off-state(both forward and reverse blocking), only a small magnitude of leakage current flows (of the order of 100 mA). The blocking capability with gate open is specified in terms of limiting repetitive peak forward (VDRM) or reverse (VRRM) voltages.

There is also a non-repetitive peak reverse voltage rating (Vrsm) which is specified. The voltage ratings are specified for power frequency (50 or 60 Hz) half-cycle sinusoidal voltages and rated junction temperature of the thyristor(typically 125 C ). The variation of the voltage ratings with junction temperature.

       The behaviour of thyristors under transient voltages is not well understood. However, according to one particular study [10,11], the following conclusions can be drawn:

1. The transient break over voltage of a thyristor is independent of its voltage rating.
2. The forward breakover voltage of a thyristor under a transient voltage may be lower than its voltage rating and decreases with increasing junction temperature. 
3. The instant of forward breakover of a thyristor under slow transients (30600) can be significantly lower compared to those obtained under fast transient voltages(1.2/50s). This behaviour has been attributed to the statistical and formative time lags of avalanche formulation.

         The thyristor capability in the reverse direction is related to the permissible energy losses which in turn is dependent on the variation of the reverse avalanche current with the voltage magnitude of the reverse voltage. The transient voltage blocking capability of a thyristor is perhaps related to the critical power needed to damage the device. Cumulative effects due to transients with less that critical power may result in degradation of the device.

Onstate : There are a number of electrical and thermal parameters that characterize the on-state behaviour. Some of these are as follows:

1. On-state voltage 
2. Mean (average) on-state current ITAV
3. Root mean square value of the on-state current ITRMS
4. Surge (non-repetitive) on-state current (ITSM)
5. Non-repetitive survival rating 
6. Holding curent (IH)
7. Operating temperature range 
8. Junction to case thermal resistance (RTHJC)
9. Contact thermal resistance 

           The on-state voltage is the anode to cathode voltage of a thyristor in the forward conducting state. It is also referred to as the forward voltage drop. This is an important characteristic affecting the power losses during on-state and the parallel operation of thyristors.

           On-state voltage depends upon a number of factors such as the width of various regions, life time and mobility of minority carriers, the physical mechanisms of recombination, etc. The power loss at6 current densities around 100A/Cm (conditions corresponding to maximum contionuous rating) is due to recombinations, whereas at current densities around 1000A/cm (surge conditions), it is mainly due to ohmic heating.

           Silicon wafers of small thickness and long carrier life times give rise to low on-state voltages. However, increasing the carrier life time also increases turn-off time. Trying to optimize both may result in high reverse recovery current.

          On-state curent ratings are determined by the junction temperature which must be kept below the value necessary to ensure that it can block the recovery voltage after a worst case credible overcurrent. The temperature build-up in a thyristor valve.(The failure of a thyristor following an overcurrent is essentially a high temperature voltage failure produced by intense local heating induced by excessive leakage current at high voltage).

               The surge current capability of a thyristor is based on its filamentation temperature at which mesoplasmas are formed. However, the requirement to have surge suppression capability (voltage blocking following the surge current) will result in the operation of the thyristor at reduced junction temperature. The maximum junction temperature attained due to the cumulative effects caused by the passages of repeated current surges should be below the thyristor filamentation temperature.

              The holding current IHis defined as the minimum current required to maintain the thyristor in the on-state. It is the forward current below which it will cease to conduct . As the forward current is reduced, the turn-off occurs when recombination causes the minority carrier level to fall below the base-region doping level. The holding current reduces with increase in junction temperature.  

   

PRINCIPAL OF OPERATION

PRINCIPAL OF OPERATION : 

                                 The principle of operation of thyristors can be explained by the two transistor analogy. Here a thyristor is replaced by a PNP and NPN transistor connected in regenerative feedback. If the gate current Ig is injected into the transistor T2, its collector current Ic2 amplifies the collector current Ic1 of of transistor T1. This in turn reinforces the gate current IG. Eventually, T1 and T2 go into complete saturation and all the junctions become forward biased. 

      

               A silicon transistor has the property thatὰ1 and ὰ2 are the common- base current gains and ICBO1 and ICBO2 are the common base leakage currents of  T1 and T2 respectively.

       

     A silicon  transistor has the property that ὰ is very low at low emitter current and rise rapidly as the emitter current builds up. When the device is off, Ig =0, and Ia will be the leakage current. If it is possible to raise the emitter currents of T1 and T2, such that (ὰ1 +ὰ2) apporaches unity, then the device triggers into saturation. There are several means of achieving this :

1. Injection of gate curent(normal turn -on)
2. By increasing the forward voltage above a limit, Vbo called break-over voltage. In this case, the minority-carrier leakage current at middle junction increases due to avalanche effect.
3. By increasing the anode voltage at a rate such that the depletion layer capacitance at the middle junction will create a displacement current(dv/dt turn-on).
4. At a high enough junction temperature, the leakage current increases and casues a turn-on.
5. Direct irradiation of light on silicon creates electron-hole pairs, which under the influence of electric field result in a current to trigger the thyristor.

           Triggering the device into saturation is called turn-on.Controlled turn-on without damaging the device is only feasible through gated turn-on. The device remains in a conducting state untill the current is maintained by the circuit action, above the holding current. During this period, the gate has no control on the conduction. The turn-off process which results in the device regaining its blocking state achieved either by:(i)line communication or (ii)forced communication.

             In both cases, the circuit voltage source is reversed which in turn will drive the current to zero. After a time lapse of tq, the turn-off time, the voltage can be reversed again, when the device regains its blocking state.

THYRISTOR DEVICE

DESCRIPTION :

                          Thyristor is now defined as a generic as a generic term applicable to the whole range of four layer (PNPN) semiconductor switches. It is also known commercially as silicon controlled rectifier (SCR). The structure of a thyristor with the three terminals and its electrical symbol are device can carry current only in one direction from anode to cathode and the instant of initiation of conduction can be controlled by the gate. 

          The voltage rating of a thyristor (the ability to withstand, when turned off) is now in the range of 5 kv while the current rating has gone up to 3000A. While the current ratings available now are adequate, thus avoiding the ne necessity of parallel connection, the voltage rating is insufficient to make up a high voltage valve. Thus,series connection of devices to make up a thyristor valve is necessary and introduces some problems that have to be considered in the design of valves and protection.

              Increasing the voltage rating of a thyristor is feasible, but it is at the cost of increased losses,turn-off times and reduced peak allowable junction temperatures. The capability of a thyristor to with stand high voltages critically depends on the quality of silicon crystal from which the device is made the more uniform the crystal using low energy neutron or gamma rays results in light, precisely controlled doping.