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.

  

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. 

APPLICATION OF DC TRANSMISSION

APPLICATION OF DC TRANSMISSION 

The detailed comparison of AC and DC transmission in terms of economics and technical performance, leads to the following areas of application for DC transmission :

1. Long distance bulk power transmission
2. Underground  or  underwater cables 
3. Asynchronous interconnection of AC systems operating at different frequencies or where independent control of systems is desired 
4. Control and stabilization of power flows in AC ties in an integrated power system .

 The first two applications are dictated primarily by the economic advantages of DC transmission, where the concept of breakeven distance is important. To be realistic, one must also assign a monetary value for the technical advantages of DC (or penalty costs for the drawbacks of AC ). The problem of evaluation of the economic benefits is further complicated by the various alternatives that may be considered in solving problems of AC transmission - phase shifters, static var systems, series capacitors, single pole switching etc.

                  The technical superiority  of DC transmission dictates its use for asynchronous inter connections, even when the transmission distances are negligible. Actually there are many 'back to back' DC links in existence where the rectification and inversion or carried out in the same converter station with no DC lines.The advantage of such DC links liec in the reduction of the overall conversion costs and improving the reliability of DC system.

                The alternative to DC ties may require strengthening existing AC network near the bounder of the two systems. This cost can be prohibitive if the capacity of the tie required is moderate compared to the size of the systems inter connected.

           In large inter connected systems, power flow in AC ties (particularly under disturbance conditions)  Can be uncontrolled and leed to over load and stability problems thus endangering systems security. Strategically placed DC lines can overcome this problems requires detailed study to evaluate the benefits. 

        

             Presently the number of DC lines in a power grid is very small compared to the number of AC lines. this indicates the DC transmission is justified only for specific application. although advances in technology  and introduction of multi - terminal DC (MT DC) systems are expected to increase the scope of applications of DC transmission  it is not anticipated  That AC grid will be replaced by DC power grid in future. there are two major reasons for this.Firstly , the control and protection of MT DC systems is very complex and the inability of voltage transformation in DC network imposes economic penalties. Secondly, the advances in DC technology have resulted in the improvement of the performance  of AC transmission, through introduction of static var systems, static phase shifters, etc.     

   

        The rate of growth of DC transmission was slow in the beginning. in over 16 years, only 6000 MW of DC systems were installed using mercury arc valves. The introduction of thyristor valves overcome some of the problems of system operation mainly due to the arc backs in mercury arc valves. Since then, the rate of growth of DC transmission capacity has reached an average of 2500 MW/years

Sl.No	HVDC - Systems	Transmission distance KM			Rated VoltagekVxNo.of circuits	Nominalcapacity MW	Max continuouscapasityMW	Commissioning date	Comments
		overhead line	cable	Total
A	Mercury-arc-Valve Systems
1	Gtland-Swedish Mainland	0	96	96	150	30	30	1954/70
2	Cross Channel 1 (GB-F)	0	65	65	±100	160	160	1961
3	Volgograd-Donbass (SU)	470	0	470	±400	720	720	1962-65
4	Konti-skan (DK-S)	95	85	180	250	250	275	1965
5	Sakuma (J)	_	_	_	125×2	300	300	1965
6	New Zeland (NZ)	570	39	609	±250	600	600	1965
7	Sardinia-Italian Mainland	292	121	413	200	200	200	1967
8	Vacounver pole I (CDN)	totla 41	total 33	74	±260	312	312	1968/69
9	pacific Intertie (US)	1362	0	1362	±400	1600	1600	1970
10	Nelson River Bipole I (CDN)	890	0	890	±450	1620	1669	1973-77
11	King snorth (GB)	0	82	82	±266	640	640	1974
B	Thyristor Valve Systems
12	Eel River (CDN)	_	_	_	80×2	320	350	1972	Asynchronous Tie
13	Skagerrak (DK-N)	113	127	240	±250	500	510	1976/77
14	David A.Hamil (USA)	_	_	_	50	100	110	1977	Asynchronous Tie
15	Cabora Bassa - Apollo (MOC-ZA)	1414	0	1414	±533	1920	1920	1977/79
16	Vancouver Pole 2 (CDN)	total 41	total  33	74	-280	370	476	1977/79
17	Square Butte (US)	749	0	749	±250	500	550	1977
18	Shin-Shinano (J)	_	_	_	125×2	300	300	1977	50/60 Hz  Tie
19	Nelson River Bipole 2 (CDN)	930	0	930	±250	900	1000	1978
20	Cu (Underwood Minneapolis)(us)	710	0	910	±400	1000	1100	1979
21	Hokkaido-Honshu (J)	124	44	158	250	300	300	1979/80
22	Asaray (PY-BR)	_	_	_	26	50	_	1981	50/60 Hz  Tie
23	EPRI Compact Station (USA)	_	0.6	0.6	100/400	100	_	1981
24	Vyborg (USSR-Finland)	_	_	_	±85×3	170	_	1982	Asynchronous Tie
25	Inga Shaba (ZAIRE)	1700	0	1700	±500	560	_	1982
26	Dumrohr (A)	_	_	_	±145	550	633	1983	Asynchronous Tie
27	Gotland 2-Swdish Mainland	7	91	98	150	130	165	1983
28	Eddy Co. (USA)	_	_	_	82	200		1983	Asynchronous Tie
29	Itaipu (BR)	783/806	0	783/806	±300	1575		1984
30	Chateauguary (CDN)	_	_	_	140	1000		1984	Asynchronous Tie
31	Itaipu(BR)	783/806	0	783/806	±600×2	6300		1985-87
32	Oklaunion (US)	_	_	_	82	200		1984	Asynchronous Tie
33	Pacifik Intertie	_	_	_	±500	400		1985
34	Wien Sud-Ost (A)	_	_	_	145	550		1987	Asynchronous Tie
35	Corsica Tap (F)	_	_	_	200	50		1986
36	Greece-Bulgaria	_	_	_	NA	300			Asynchronous Tie
37	Madawaska (CDN)	_	_	_	144	350		1985	Asynchronous Tie
38	Miles City (US)	_	_	_	82	200		1985	Asynchronous Tie
39	Walker Co. (US)	256	0	256	±400	500-1500		1985	Asynchronous Tie
40	Cross Channel 2 (GB-F)	0	72	72	±270×2	2000		1985/86
41	Kanti-Skan2(DKS)	95	85	160	250	270		1988/89
42	Ekibastus-Centre (USSR)	2400	0	2400	±250	6000		1985-88
43	Store Baelt (DK)	35	30	55	280	350		1989-90
44	Skagerrak (DK-N)	113	127	240	300	320		1988-89
45	Intermountain (US)	794	0	794	±500	1600		1987
46	Liberty Mead (US)	400	0	400	±364/±500	1600/2200		1989-90
47	Nelson River Biipole 3	930	0	930	±500	2000		1992/97
48	Chicoasen (MEX)	720	0	720	±500	900/1800		1985/90
49	Yukatan-Mexico City
50	Quebec-New England	175/375		175/375	±450	690/2070		1986/92
51	Des Cantons-Camerford	175		175	±450	690		1986
52	Sidney (US)	_	_	_		200		1986	Asynchronous Tie
53	Black Water (US)	_	_	_	56	200		1985	Asynchronous Tie
54	Highgate (US)	_	_	_	56	200		1985	Asynchronous Tie
55	SACOI-2 (Italy)				200	300		1989
56	Pacific IntertieII (US)				±500	1100
57	Gezhouba-Nan Qiao (China)	1080	_	1080	±500	1200		1987-91
58	Rihand-delhi (India)	1000	_	1000	±500	1000		1987
59	Uruguaiana (BR-Argentina)					50		1986/87	Asynchronous Tie
60	Camerford-Sandy Pond	200				1400		1990
61	vvVindhyachaI (India)	_	_	_	70	250×2		1988	Asynchronous Tie
62	Gotland 3-Swdish Mainland	_	98	98	150	130	165	1987
63	South finland East Sweden	35	185	220	350	420		1989/90