THYRISTOR VALVE

THYRISTOR VALVE :

              INTRODUCTION :

                                                 HVDC Converters are an assembly of valves which have the propertyof conducting in the forward direction and blocking in the reverse directions. The term 'valve'carried over from the mercury are valve days, is applied even now for thyristor valves which are made up of series and parallel connection of many thyristor cells or devices.

                The major problem with the mercury are valves is the occurrence of are backs (or back fire) which results in the destruction of the recifing property of the valves. Are backs are random phenomena which results in failure to block in the reverse direction. Although the incident of are backs can be reduced by carefully controlling the factors that influence them, complete elimination is impossible and the valve cost is also increased. Furthermore, are backs are non self - clearing and result in line faults which stress transformer windings and anodes in the valve. The maintenance requirements for the valves go up and lead to poor reliability.  

                     Thyristor valves which were developed in the late sixties have eliminated all these problems. They have now completely displaced mercury are valves in HVDC transmission.

                     Thyristors that constitute the valves are also not perfect devices. The major problem is that their ratings cannot be exceed even for short durations. However, there is continuing development in the field of power semiconductors which has brought down the cost while improving the reliability. This chapter reviews the principles of operation, characteristic and control of thyristor devices. Some of the design aspects, protection and testing of thyristor valves for DC transmission applications are also presented

MODERN TRENDS IN DC TRANSMISSION

MODERN TRENDS IN DC TRANSMISSION :

                 The continuing technological developments in the areas of power semiconductor devices,digital electronics, adaptive control, DC transmission.The major contribution of these developments is to reduce the cost of converter stations while improving the reliability and performance.

POWER SEMICONDUCTORS AND VALVES :

           The cost of the converters can come down if the number of devices to be connected in series and parallel can be broughtdown. The size of the devices has gone up to 100 mm (in diameters) and there is no need for parallel connection. The increase in the current rating of the devices has made it possible to provide higher overload capability at reasonable costs and reduce the lower limits on transformer leakage impedance thereby improving the power factor. The voltage ratings are also on the increase. The development of light triggered thyristors should also reduced by the application of zinc oxide gapless arresters and protective firing methods.

           The power rating of thyristors is increased by better cooling methods. Deionized water cooling has now become a standard and results in reduced losses in cooling. Two phase flow using forced vaporization is also being investigated as ameans of reducing thermal resistance between the heat sink and the ambient.

                      As forced commutated converters operating at high voltages are uneconomic, the development of devices that can be turned off by application of a gate signal would be desirable. Gate turn off (GTO) thyristors are already available at 2500 V and  2000A. However, the main disadvantage of GTO's is the large gate current needed to turn them off. MOS (metal oxide semiconductor) controlled thyristor or MCT appears to be a promising technology. An MCT would consists of an MOS integrated circuit created can be switched off by a small gate current. The turn - off time of MCT is also less than one third that of GTOs. However, MCTs are still in the early stages of development.

                The cost of silicon used in the manufacture of power semiconductor devices can be brought down (by 15 to 20 percent) from the use of magnetic CZ (Czochralski) method, instead of the conventional FZ (float zone ) method. Research is also underway in reducing this packaging cost of a device.

1. CoNvErTeR CoNtRoL :

          The development of micro-computer based converter control equipment has now made it possible to design systems with completely redundant converter control with automatic transfer between systems in the case of a malfunction. Not only is the forced outage rate of control equipment reduced but it is also possible to perform scheduled preventive maintenance on the stand -by systems when the converter is in operation. The use of a mini-simulator will make it feasible to check vital control and protection functions.

                  The micro-computer based control also has the flexibility to try adaptive control algorithms or even the use of expert systems for fault diagnosis and protection.

   2.DC Breakers :

                           With the development and testing of prototype DC breakers, it will be possible to go in for tapping an existing DC link or the development of new MTDC systems. Parallel, rather than series operation of converters is likely as it allows certain flexibility in the planned growth of system. The DC breaker ratings  as the control intervenction is expected to limit the fault current.

         The control and protection of MTDC systems is not a straightforward extenction of that used in the two terminal DC systems. The possibility of decentralized control necessitated by communication failure, the coordination of control and protection are some of the issues currently being studied.

    3.CONVERSION OF EXISTING AC LINES

            

               The constraints on RoW are forcing some utilities to look into the operation fo converting existing AC circuits to DC in order to increase the power transfer limit. There could be some operational problems due to electromagnetic induction from AC circuits operating in the same RoW.

   

              An experimental project of converting a single circuit of a double circuit 220kv line is currently under commissioning stage in india.

   4.Operation with Weak AC Systems :

                  The strength of AC systems connected to the terminals of a DC link is measured in terms of short circuit ratio (SCR) which is defined as 

          If SCR is less than 3, the AC System is said to be weak.The conventional constant exinction angle control may not be satisfactory with weak AC system. The recovery of inverters following the clearing of fault in the connected AC system can also be problematic.

              Constant reactive current control or AC voltage control have been suggested to overcome some of the problems of weak AC systems. The use of fst reactive power control at the converter bus by applying static var systems is another alternative. Limiting dynamic overvoltages through  converter control during load rejection is becoming a standard practice.

                   The power modulation techniques used to improve dynamic stability of power systems will have to be modifited in the presence of weak AC systems. Coordinated reactive and active power modulation has been suggested to over come the problems of voltage variations that can limit the effective of power modulation.

 

PLANNING FOR HVDC TRANSMISSION

PLANNING FOR HVDC TRANSMISSION : 

                      The system planner must consider DC alternative in transmission expansion. The factors to be considered are (i) cost (ii)technical performance, and (iii) reliability.

                      Generally, the last two factors are considered as constraints to be met and the minimum cost option is selected among various alternatives that meet the specification on technical performance and reliability.

                     For submarine, cable transmission and interconnecting two systems of different nominal frequencies, the choice of DC is obvious. In other cases, the choice is to based on detailed techno-economic comparison.

                     The consideration in the planning for DC depends on the application. Two applications can be considered as representative.

These are 

1.  Long distance bulk power transmission 
2. Interconnection between two adjacent systems 

      In the first application, the DC and Ac alternatives for the same level of system security and reliability are likely to have the same power carrying capability. Thus the cost comparison would form the basis for the selection of the DC (or AC) alternative, if the requirements regarding technical performance are not critical. 

            In the second Application, Ac interconnection poses several problems in certain cases.For the same level of systems security(and reliability), the required capacity of AC interconnection will be much more than that for DC (even ignoring the beneficial aspects of DC power modulation).Thus the choice for DC interconnection will be based on the following considerations.

1. Small fluctuations in the voltage and frequency do not affect the power flow which can be set at any desired value.
2. The system security can be enhanced by fast control of DC power.

       Having settled on the DC link for interconnection, there are three possible configurations for interconnection. These are :

1. A two terminal transmission where each terminal is located at a suitable place  some where within the network and connected by a DC overhead line or cable.
2. A back to back HVDC station (also called HVDC coupling station)located some where within one of the network and an AC line from the other network to the common station.
3. A back to back station located close to the border between the two systems. This is a special case of the above.

In The choice between the first and second configuration, it is to be noted that converter costs are less for the common coupling station and the AC line costs are greater than the DC line costs. If the distance involved are less than 200km, the second configuration is to be preferred. If the short circuit ratio (SCR) is acceptable, then the third alternative will be the most economic. 

         The specification and design of DC systems require an understanding of the various interactions between the DC and AC systems.The interruption (or reduction) of power in a DC link can occur due to (i) DC line faults (ii)AC system faults.
       The speed of recovery from transient DC lines faults is of concern in maintaining the integrity of the overall system. The power flow and stability studies are used in this context. The recovery of DC link from AC system faults is more complex. The depression of AC voltage at the inverter bus can lead to communication failure and loss of DC power.The DC power output can lead to the reduction of AC voltage and failure of communication (due to corresponding increase in the var demand). An optimum rate of increase in DC power can be determined from stability study. This is influenced by control strategy and system characteristics.

 The following aspects also require a detailed study of the system interactions.

• Var requirements of converter stations 

•  Dynamic overvoltages 

•  Harmonic generation and design of filters 

• Damping of low frequency and subsynchronous torsional osillations 

•  Carrier frequency interference caused by spiky currents in valve (at the beginning of conduction) due to the discharge of stray capacitances and snubber circuits.

      The converter control plays a major role in these interactions and the control strategy should be such as to improve the overall system performance.Digital simulation and HVDC simulators are used for planning and design studies.

CoNvErTeR Transformer

The converter transformer can have different configurations (i) three phase, two winding, (ii) single phase, three winding ,(iii) single phase, two winding. The valves side windings are connected in star and delta with neutral point ungrounded. On the AC side, the transformers are connected in parallel with neutral grounded. The leakage reactance of the transformer is chosen to limit the short circuit currents.One problem that can arise is due to the DC magnetization of the core due to unsymmetric firing of valves. In back to back links, which are designed for low DC voltage levels, an extended delta configuration can result in identical transformers being used in  twelve pulse converter units. This results in the reduction of the spare capacity required. However, the performance of extended delta transformers in practice is still to be tested.

Filters :

          There are three types of filters used :

1. AC filters : These are passive circuits used to provide low impedance,shunt paths for AC harmonic currents. Both tuned and damped filter arrangements are used. 
2. DC Filters : These are similar to AC filters and are used for the filtering of DC harmonics.
3. High frequency (RF/PLC) filters: These are connected between the converter transformer and the station AC bus to suppress any high frequency currents. Some times such filters are provided on high -Voltage DC bus connected between the DC filter and DC line and also on the neutral side.

Reactive Power Source :

               converter stations require reactive power supply that is dependent on the active power loading (about 50 to 60% of the active power). Fortunately, part of this reactive power requirement is provided by AC filters. In addition , shunt (switched) capacitors, synchronous condensors and static var systems are used depending on the speed of control desired.

Smoothing Reactor : 

                                A sufficiently large series reactor is used on DC side to smooth DC current and also for protection. The reactor is designed as a linear reactor and is connected on the line side, netural side or at intermediate location. 

DC Switchgear :

                          This is usually a modified Ac equipment used to interrupt small DC currents (employed as disconnecting switches). DC breaks or metallic return transfer breakers (MRTB) are used, if required for interruption of rated load currents.

           In addition to the equipment described above, AC switchgear and associated equipment for protection and measurement are also part of the converter stat 

CONVERTER UNIT

CONVERTER UNIT

           This usually consists of two three phase converters bridges connected in series to form a 12 pulse converter unit. The total number of valves in such a unit are twelve. The valves can be packaged as signal valve, double valve or quadrivalve arrangements. Each valve is used to switch in a segment of an AC voltage waveform. The converter is fed by converter transformers connected in star/star and star/delta arrangements.

                 The valves are cooled by air,oil,water or freon.Liquid cooling using deionized water is more efficient and results in the reduction of station losses. The ratings of a valve group are limited more by the permissible short circuit currents than steady state load requirements.The design of valves is based on the modular concept where each module contains a limited number of series connected thyristor levels.

                Valve firing signals are generated in the converter control at ground potential and are transmitted to each thyristor in the valve through a fiber optic light guide systems. The light signal received at the thyristor level is converted to an electrical signal using gate drive amplifiers with pulse transformers.

The valves are protected using snubber circuits, protective firing and gapless surge arresters. Some of the details of control and protection of thyristor valves are given.

CoNvErTeR StAtIoN

CoNvErTeR StAtIoN

                              The major components of a HVDC transmission systems are converter stations where conversions from AC to DC (Rectifier station) and from DC to AC (Inverter Station) are performed.  A point to point transmission requires two converter stations. The role of rectifier and inverter stations can be reversed (resulting in power reversals) by suitable converter control.
    
                A typical converter station with two 12 pulse converter units per pole,the various components of a converter station or discussed below. 

DESCRIPTION OF DC TRANSMISSION SYSTEM

DESCRIPTION OF DC TRANSMISSION SYSTEM.

Bipolar link has two conductors, once positive and the other negative.Each may be a double conductor in EHV lines. Each terminal has two sets of converters of identical ratings,in serieson th DC Side. The junction between the two sets of converters is grounded  at one or both ends.Normally,both poles operate at equal currents and hence there is zero ground current flowing under these conditions.

                                 Homopolar link has two or more conductors all having the same polarity (Usually negative) and always operated with ground or metallic return. 

      Because of the desirability of operating a DC link with out ground return, bipolar links are most commonly used. Homopolar link has the advantage of reduced insulation costs, but the disadvantages of earth return outweight the advantages of earth return outweight the advantages. Incidentally, the coronaeffects in a DC line are substantially less with negative polarity of the conductor as compared to the positive polarity. 

                The monopolar operation is used in the first stage of the development of a bipolar line, as the investments on converters can be deferred until the growth of load which requires bipolar operation at double the capacity of a monopolar link.