New Technologies in Hvdc Converter Design
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NEW TECHNOLOGIES IN HVDC CONVERTER DESIGN Alf Persson Lennart Carlsson Mikael Åberg ABB Power Systems, Sweden ABB Power Systems, Sweden ABB Power Systems, Sweden 1. SUMMARY HVDC technology took a big step forward around 20 years ago when thyristor valves succeeded the mercury arc valves previously used. The converter station concept introduced at that time, however, has remained practically unchanged since then. Figure 1: Single line diagram of a monopolar station with ® The time has now come for a further major advance in CCC and ConTune AC filter. technology. The introduction of new concepts will change Consequently, many other main circuit components, in whole approach to building an HVDC station. Even though addition to the filter components, such as switching this innovation may not be quite as significant as when equipment, CTs, etc, must be installed. The filters will thyristor valves were introduced, the new features will therefore take up considerable space in a converter sta- greatly improve the operating characteristics of HVDC tion. transmissions and reduce the size and complexity of converter stations. 2.2 De-coupling of filtering and reactive power supply The new generation of converter stations is now likely to include some of the following features: The development of new, effective AC filters, described - a new type of converter circuit, the capacitor commutated in a separate section of this paper, makes it possible to converter (CCC) perform the filtering function through a single filter bank - actively tuned AC filters with small Mvar rating. The new AC filter thus allows de- - air insulated outdoor thyristor valves coupling of the functions of the filtering and the reactive - active DC filters. power generation to a large extent. In this situation, the traditional way of generating the required reactive power Keywords: converter circuit, capacitor commutated would be to install a number of shunt capacitor banks. converter, actively tuned AC filter, outdoor thyristor valve, However, during the last few years, another concept, the active DC filter. capacitor commutated converter, abbreviated CCC, which provides a much more interesting solution, has been studied 2. CAPACITOR COMMUTATED CONVERTERS and developed. 2.1 General 2.3 The CCC concept In a conventional HVDC converter the consumption of The electrical diagram of a CCC is shown in Fig. 2. reactive power is typically around 0.5 p.u. of the active power. This reactive power requirement is in most cases This converter is characterized by the use of commutation fully compensated for locally by installation of shunt AC capacitors inserted in series between the converter trans- filters in the converter station. Requirements for permitted formers and the valve bridge. This circuit has been reactive power unbalance, or AC voltage changes upon proposed in several previous papers, see, for example, Refs. filter switching, in many cases result in splitting of the 1 and 2, as a method of obtaining a self-commutated installed reactive power into several filter/shunt banks. converter. Figure 2: Capacitor Commutated Converter. However, the ABB approach does not aim at achieving a self-commutated converter: instead, it provides reactive power compensation proportional to the load of the converter. The need of switchable shunt capacitor banks Figure 4: Remote single phase to ground fault in the in- for reactive power compensation is thereby eliminated. verter AC network. Since the AC filters are necessary only from the point of view of filtering harmonics, the shunt-connected reactive power generation can be minimized. In the ABB solution 2.5 Improved dynamic stability the size of the commutation capacitor is chosen so that the full load reactive power consumption of the converter is The contribution to the commutation voltage from the compensated by the reactive generation of the small high commutation capacitors results in positive inverter performance AC filter. Fig. 3 compares the reactive power impedance characteristics for an inverter operating at mi- conditions. nimum commutation margin control. An increase in direct current therefore results in a DC voltage increase rather than the opposite, which is the case for conventional inverters with commutation margin control. The dynamic Conventional converter stability of an inverter will thus be dramatically improved with a CCC. CCC Figure 5: Ud/Id characteristics. The improved inverter performance as described above Figure 3: Reactive power conditions for a typical con- results in more economical solutions, particularly for ventional converter and for a CCC. HVDC schemes feeding weak systems and for HVDC sche- mes using very long DC cables. Fig. 6 shows the MAP (Maximum Available Power) curves for a conventional converter and a CCC for SCR = 2. As 2.4 Sturdily constructed and resistant to disturbances can be seen, the CCC is in a very stable situation while the conventional converter is close to the stability limit. The commutation capacitors improve the commutation failure performance of the converter. The capacitors The diagrams also show that the load rejection overvoltage introduce a source of commutation voltage in addition to which occurs upon pole tripping or commutation failures the AC bus voltage which, if proper control functions are is reduced from 1.5 to 1.2 p.u. as a result of the small size included, can be used to minimize the risk of commutation of the shunt-connected filters for the CCC. The small shunt failures. Typically, a CCC can tolerate a sudden 15-20% filters will also reduce the risk of low order harmonic voltage drop without developing a commutation failure. resonances on the AC side. 2.7 Effects on other equipment Introduction of commutation capacitors results in diffe- rent stresses on the other equipment compared to a conventional HVDC converter. The main influence from the capacitors is a considerable reduction of valve short- circuit currents. This is due to the voltage drop across the commutation capacitor varistors. On the other hand, a somewhat higher peak voltage across the valve, as well as higher extinction voltage steps, will be obtained compared to conventional HVDC. The voltage contribution from the commutation capacitors will support the commutation of the direct current from one valve to another; i.e., the overlap angle will be reduced compared to a conventional HVDC converter. The reduced Figure 6: Maximum power curve for conventional and overlap angle will result in somewhat higher AC harmonic Capacitor Commutated Converters, SCR = 2, g=17º. currents and the reduced overlap will, in combination with the higher extinction voltage step, give somewhat increased 2.6 The capacitor in the CCC concept generation of harmonics on the DC side compared to conventional HVDC. The increased harmonic production In principle, it would be possible to locate the capacitors of a CCC is of the order of 20 % and can be coped with by on the AC side of the converter transformers, as proposed using high performance filters on both the AC and DC in Refs. 3 and 4. sides. However, it was deemed that it would not be possible to completely avoid ferro-resonance problems and certain other drawbacks using this concept. The location of the capacitors between the converter transformers and the valve bridge results in full control of the capacitor currents and complete elimination of the risk of ferro-resonance. A key component in a CCC is the commutation capacitor. The steady state operating voltage of the commutation capacitor is defined by the direct current. The capacitors must be protected against overvoltages by Figure 9: Valve short circuit current. parallel ZnO varistors. The voltage stresses on the capacitors, as well as With the location of the commutation capacitors on the the energy requirements made of the valve side of the converter transformer, the rating of the parallel varistors, are relatively low converter transformer can be reduced by reducing the no- compared to the installed capacity, minal phase-phase voltage on the valve side; i.e., the and consequently the commutation reactive power flow through the transformer is minimized. capacitors can be of compact design. Figs. 7 and 8 show a typical layout 2.8 Impact on station design for a commutation capacitor with its varistors, and the voltage of the The elimination of switched reactive power compensation commutation capacitor in normal ope- equipment will simplify the AC switchyard and minimize ration. the number of circuit-breakers needed, which will reduce Figure 7: the area required for an HVDC station built with CCC. Commutation Capacitor. 2.9 CCC - a fully developed concept The CCC concept has been thoroughly studied in both di- gital simulation programs and in the HVDC simulator over the last few years. Design rules for the CCC have been developed and verification of the CCC concept in a high Figure 8: Commutation capacitor voltage. power test circuit will be finalized at the beginning of 1996. 3. CONTINUOUSLY TUNED AC FILTERS The conventional filter reactor design has been modified by inserting a core and a control winding. A DC current in 3.1 General the control winding affects the permeability of the core and thus changes the inductance of the reactor. No HVDC converters produce current harmonics on the AC mechanically moving parts are needed. Fig. 10 shows the side and voltage harmonics on the DC side. For a 12-pulse basic design of the reactor. converter, AC-side harmonics of the order 12n±1 are created. A typical filter set-up consists of 11/13 and HP24 3.2. Control of tuned AC filters filters. A simplified diagram of the filter control is shown in Fig. To obtain good performance, low impedance tuned filters 11. The phase angle between the voltage and current of often need to be provided for the lowest characteristic the harmonic is used as an input signal to control tuning.