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 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 , 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 , 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 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. harmonics; i.e., the 11th and 13th. The regulator is a PI-regulator and a small standard 6- Filters have two important characteristics: impedance and pulse controlled is used as amplifier to feed the bandwidth. Low impedance is required to ensure that control winding of the reactor. The power needed to feed harmonic voltages have a low magnitude. A certain the control winding is around 1 kW per phase. bandwidth is needed to limit the consequences of filterdetuning.

Detuning of conventional filters is caused by network frequency excursions and component variations, e.g. capacitance changes due to temperature differences.

A filter in which tuning can be adjusted to follow frequency variations and component variations offers several adv- antages: Figure 11: AC filter tuning control. - the filter can be designed with a high Q-factor to pro- vide a low impedance for the harmonics - automatic tuning will ensure that all risks of resonances and current amplification phenomena are eliminated, 3.3 Operational experience implying that the ratings of the AC filter components can be reduced. A test installation of an 11th harmonic ConTune® filter was made in the Lindome station of the 300 MW Konti-Skan 2 ABB has developed and field-tested a new method to HVDC transmission in 1993. The filter has the same achieve continuous automatic tuning of an AC filter. The generated reactive power, 11.6 MVAr at 132 kV, as the concept is based on orthogonal magnetizing of an iron core original filter. Fig. 12 shows the test installation. in the filter reactor. The reactor inductance is controlled by a direct current creating a field perpendicular to the main axis of the reactor.

The permeability of magnetic materials can be changed by applying a transverse DC magnet- ic field. This permeability controlling field has to be oriented perpendicular to the main flux direction and has the effect of lowering the permeability by ”destroying” favourably oriented magnetic domains. A transverse DC field is able to reduce the permeability by several orders of Figure 12: Test installation of a ConTune® filter in magnitude without affecting the Lindome. linearity of the magnetizing pro- Figure 10: cess. Because of the linearity no Variable reactor. additional harmonics are produced. The filter was designed to accommodate frequency variations and component variations that represent the detuning (+2, -3 Hz). A comparison of the performance of the passive and active * Platform with support insulators. tuned filters shows that the 11th harmonic distortion was The platform for a single valve housing or a number of reduced from around 0.026% with the passive filter to aro- valves is of the same design as used for series capacitor und 0.010% with the ConTune® filter with its Q factor of banks. around 200. The converter was in both cases operating * Communication channel. under the same conditions. An important new element needed for the outdoor valve design is a communication channel. It consists of a It should be noted that the original AC filters in Lindome composite for DC application which is used for have a high quality factor for the 11th and 13th filter, Q=65, fibre optics, cooling water and ventilation air between the while a typical value is 30-40. Hence, the distortion with valve housing and earth. the passive filter was already very low. * Valve base electronics. The valve base electronics can be located very close to a The filter performance measured at the test installation single valve or be common to a number of valves. The shows that the ConTune® concept is an appropriate solu- valve control and opto interface are included in the valve tion. The test installation has been in operation now for base electronics. more than two years and operating experience has been * Valve cooling including air-cooled liquid coolers and good. Commercial installation of a ConTune® filter is cooling control. already in progress at the Celilo terminal of the Pacific The most suitable solution as seen today for the valve Intertie. cooling is to have a cooling system serving one pole, i.e., 12 valves. In most cases the cooling system will be a closed single-circuit system with a coolant consisting of a mixture 4. OUTDOOR HVDC VALVE DESIGN of water and glycol for anti-freeze purposes.

4.1 General 4.3 Operational experience The outdoor air-insulated thyristor valve is a new component, made possible by the development of high A test valve in the Konti-Skan 1 HVDC link has given power . It gives increased flexibility in the sta- operational experience of a valve designed for 275 kV DC tion layout; eliminates the need of a valve hall, including voltage since June 1992. The operation of the test installa- its subsystems; reduces the equipment size; and makes it tion has been very successful, and has provided a basis for easier to upgrade existing stations. Future relocation of an further development. The ongoing development is aiming HVDC station will also be simpler when outdoor HVDC at an outdoor valve design for 500 kV DC voltage. valves are used. 5. ACTIVE DC FILTERS The outdoor valve unit is built as a single valve function; consequently, 12 units are needed for a 12-pulse convertor. 5.1 General Inside the outdoor valve unit, the electrical configuration is of traditional design with air-insulated thyristor modu- Demands regarding permitted interference levels from DC les and reactor modules, and the ambient conditions for lines have become increasingly stringent in recent years. these components being the same as for a valve hall solu- To fulfill these requirements using passive filters a number tion. of large parallel branches are necessary.

4.2 Elements of the outdoor valve A more attractive solution is therefore to use an active DC filter in combination with a small passive DC filter branch. The basic elements of the outdoor valve are: * Valve housing. 5.2 Operating principles The encapsulation of the valve is made of steel or alumi- nium. The insulation medium inside the housing is air at The principle of the active filter is to inject a current via atmospheric pressure. The size of the valve housing has the passive DC filter into the DC circuit as shown in Fig. been chosen to make transportation of a complete and 13. assembled valve possible on roads and railways. The length of the valve housing is a function of the DC voltage for the valve. * Active part with thyristor and reactor modules. The modules are of water-cooled design, similar to the modules used for an indoor installation. Figure 13: Cir- cuit diagram of active DC filter. The current to be injected is formed from the measured As can be seen from the layout, four phases of the harmonics on the DC line. A control system calculates the ConTune® 11th, 13th and passive high pass filters are amplitude and phase angle of a signal that is injected via a included. The fourth phase is added for redundancy reasons power amplifier into the DC circuit to eliminate the in case of a filter outage and can be connected to each of harmonics on the DC line. the three phases .

In Fig. 14 the harmonic content on the DC line is shown 7. CONCLUSIONS for a typical installation, both with and without the active filter in operation. As can be seen from the figure, the active Several new concepts which will result in a new genera- filter reduces the harmonic content considerably. tion of HVDC converter stations have been developed over the past few years. Capacitor commutated converters, actively tuned AC filters and outdoor thyristor valves are three of the most important new features. Active DC filt- ers, optical current transducers, fully computer-based converter controls and deep hole electrodes are other important elements. These technological advances will result in improved operating characteristics, reduced complexity and smaller area requirements for future HVDC converter stations.

8. REFERENCES

Figure 14: Harmonic current content on a typical DC line. 1. Reeve J, Baron JA and Hanley GA, Oct 1968, “A Technical Assessment of Artificial Commutation of HVDC Converters with Series Capacitors”, IEEE Trans. on PAS; 5.3 Operational experience Vol. PAS-87, No. 10, pp. 1830-1840.

The first prototype was commissioned in December 1991. 2. Gole AM and Menzies RW, “Analysis of Certain Aspects Commercial installations have been in operation since of Forced Commutated HVDC Inverters.” autumn 1993 and autumn 1994 for the Skagerrak and Baltic Cable HVDC schemes, respectively. Today active DC filt- 3. Nyati S, Atmuri SR, Gordon D, Koschnik V and Matur ers is a standard solution for HVDC transmissions with RM, April 1988, “Comparison of Voltage Control Devices stringent DC filtering requirements. at HVDC Converter Stations.” IEEE Trans. on Power Delivery; Vol. 3, No. 2, 6. NEW CONVERTER STATION DESIGN 4. Woodford DA, Zheng F, May 1995, “Series Through utilizing the features described in this paper a Compensation of DC Links.”, CIGRE Symposium, Power major impact on the design of converter stations is fore- Electronics in Electric Power Systems, Tokyo. seen. An example of a HVDC converter station for a mono- polar scheme incorporating the features described in this paper is shown in Fig. 15.

Figure 15: Pos- sible layout of converter station with new featu- res included.