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filters in the converter station. Requirements for permitted reactive power unbalance, or AC voltage changes upon filter switching, in many cases result in ...

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. The time has now come for a further major advance in technology. The introduction of new concepts will change whole approach to building an HVDC station. Even though this innovation may not be quite as significant as when thyristor valves were introduced, the new features will greatly improve the operating characteristics of HVDC transmissions and reduce the size and complexity of converter stations. The new generation of converter stations is now likely to include some of the following features: - a new type of converter circuit, the capacitor commutated converter (CCC) - actively tuned AC filters - air insulated outdoor thyristor valves - active DC filters.

Figure 1: Single line diagram of a monopolar station with CCC and ConTune® AC filter. Consequently, many other main circuit components, in addition to the filter components, such as switching equipment, CTs, etc, must be installed. The filters will therefore take up considerable space in a converter station. 2.2 De-coupling of filtering and reactive power supply

2. CAPACITOR COMMUTATED CONVERTERS

The development of new, effective AC filters, described in a separate section of this paper, makes it possible to perform the filtering function through a single filter bank with small Mvar rating. The new AC filter thus allows decoupling of the functions of the filtering and the reactive power generation to a large extent. In this situation, the traditional way of generating the required reactive power would be to install a number of shunt capacitor banks. However, during the last few years, another concept, the capacitor commutated converter, abbreviated CCC, which provides a much more interesting solution, has been studied and developed.

2.1 General

2.3 The CCC concept

In a conventional HVDC converter the consumption of reactive power is typically around 0.5 p.u. of the active power. This reactive power requirement is in most cases fully compensated for locally by installation of shunt AC filters in the converter station. Requirements for permitted reactive power unbalance, or AC voltage changes upon filter switching, in many cases result in splitting of the installed reactive power into several filter/shunt banks.

The electrical diagram of a CCC is shown in Fig. 2.

Keywords: converter circuit, capacitor commutated converter, actively tuned AC filter, outdoor thyristor valve, active DC filter.

This converter is characterized by the use of commutation capacitors inserted in series between the converter transformers and the valve bridge. This circuit has been proposed in several previous papers, see, for example, Refs. 1 and 2, as a method of obtaining a self-commutated 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 for reactive power compensation is thereby eliminated. 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 the size of the commutation capacitor is chosen so that the full load reactive power consumption of the converter is compensated by the reactive generation of the small high performance AC filter. Fig. 3 compares the reactive power conditions.

Conventional converter

Figure 4: Remote single phase to ground fault in the inverter AC network.

2.5 Improved dynamic stability The contribution to the commutation voltage from the commutation capacitors results in positive inverter impedance characteristics for an inverter operating at minimum 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 stability of an inverter will thus be dramatically improved with a CCC.

CCC

Figure 5: Ud/Id characteristics.

Figure 3: Reactive power conditions for a typical conventional converter and for a CCC.

2.4 Sturdily constructed and resistant to disturbances The commutation capacitors improve the commutation failure performance of the converter. The capacitors introduce a source of commutation voltage in addition to the AC bus voltage which, if proper control functions are included, can be used to minimize the risk of commutation failures. Typically, a CCC can tolerate a sudden 15-20% voltage drop without developing a commutation failure.

The improved inverter performance as described above results in more economical solutions, particularly for HVDC schemes feeding weak systems and for HVDC schemes 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 can be seen, the CCC is in a very stable situation while the conventional converter is close to the stability limit. The diagrams also show that the load rejection overvoltage which occurs upon pole tripping or commutation failures is reduced from 1.5 to 1.2 p.u. as a result of the small size of the shunt-connected filters for the CCC. The small shunt filters will also reduce the risk of low order harmonic resonances on the AC side.

2.7 Effects on other equipment Introduction of commutation capacitors results in different stresses on the other equipment compared to a conventional HVDC converter. The main influence from the capacitors is a considerable reduction of valve shortcircuit 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.

Figure 6: Maximum power curve for conventional and Capacitor Commutated Converters, SCR = 2, g=17º. 2.6 The capacitor in the CCC concept In principle, it would be possible to locate the capacitors on the AC side of the converter transformers, as proposed in Refs. 3 and 4.

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 overlap angle will result in somewhat higher AC harmonic currents and the reduced overlap will, in combination with the higher extinction voltage step, give somewhat increased generation of harmonics on the DC side compared to conventional HVDC. The increased harmonic production of a CCC is of the order of 20 % and can be coped with by using high performance filters on both the AC and DC 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.

Figure 7: Commutation Capacitor.

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 parallel ZnO varistors. The voltage stresses on the capacitors, as well as the energy requirements made of the parallel varistors, are relatively low compared to the installed capacity, and consequently the commutation capacitors can be of compact design. Figs. 7 and 8 show a typical layout for a commutation capacitor with its varistors, and the voltage of the commutation capacitor in normal operation.

Figure 8: Commutation capacitor voltage.

Figure 9: Valve short circuit current. With the location of the commutation capacitors on the valve side of the converter transformer, the rating of the converter transformer can be reduced by reducing the nominal phase-phase voltage on the valve side; i.e., the reactive power flow through the transformer is minimized. 2.8 Impact on station design The elimination of switched reactive power compensation equipment will simplify the AC switchyard and minimize the number of circuit-breakers needed, which will reduce the area required for an HVDC station built with CCC. 2.9 CCC - a fully developed concept The CCC concept has been thoroughly studied in both digital 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 power test circuit will be finalized at the beginning of 1996.

3. CONTINUOUSLY TUNED AC FILTERS 3.1 General HVDC converters produce current harmonics on the AC side and voltage harmonics on the DC side. For a 12-pulse converter, AC-side harmonics of the order 12n±1 are created. A typical filter set-up consists of 11/13 and HP24 filters. To obtain good performance, low impedance tuned filters often need to be provided for the lowest characteristic harmonics; i.e., the 11th and 13th. Filters have two important characteristics: impedance and bandwidth. Low impedance is required to ensure that harmonic voltages have a low magnitude. A certain bandwidth is needed to limit the consequences of filterdetuning.

The conventional filter reactor design has been modified by inserting a core and a control winding. A DC current in the control winding affects the permeability of the core and thus changes the inductance of the reactor. No mechanically moving parts are needed. Fig. 10 shows the basic design of the reactor. 3.2. Control of tuned AC filters A simplified diagram of the filter control is shown in Fig. 11. The phase angle between the voltage and current of the harmonic is used as an input signal to control tuning. The regulator is a PI-regulator and a small standard 6pulse controlled rectifier is used as amplifier to feed the control winding of the reactor. The power needed to feed the control winding is around 1 kW per phase.

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 advantages: - the filter can be designed with a high Q-factor to provide a low impedance for the harmonics - automatic tuning will ensure that all risks of resonances and current amplification phenomena are eliminated, implying that the ratings of the AC filter components can be reduced. ABB has developed and field-tested a new method to achieve continuous automatic tuning of an AC filter. The concept is based on orthogonal magnetizing of an iron core in the filter reactor. The reactor inductance is controlled by a direct current creating a field perpendicular to the main axis of the reactor.

Figure 10: Variable reactor.

The permeability of magnetic materials can be changed by applying a transverse DC magnetic 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 magnitude without affecting the linearity of the magnetizing process. Because of the linearity no additional harmonics are produced.

Figure 11: AC filter tuning control.

3.3 Operational experience A test installation of an 11th harmonic ConTune® filter was made in the Lindome station of the 300 MW Konti-Skan 2 HVDC transmission in 1993. The filter has the same generated reactive power, 11.6 MVAr at 132 kV, as the original filter. Fig. 12 shows the test installation.

Figure 12: Test installation of a ConTune® filter in Lindome. 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 tuned filters shows that the 11th harmonic distortion was reduced from around 0.026% with the passive filter to around 0.010% with the ConTune® filter with its Q factor of around 200. The converter was in both cases operating under the same conditions. It should be noted that the original AC filters in Lindome have a high quality factor for the 11th and 13th filter, Q=65, while a typical value is 30-40. Hence, the distortion with the passive filter was already very low. The filter performance measured at the test installation shows that the ConTune® concept is an appropriate solution. The test installation has been in operation now for more than two years and operating experience has been good. Commercial installation of a ConTune® filter is already in progress at the Celilo terminal of the Pacific Intertie.

4. OUTDOOR HVDC VALVE DESIGN

* Platform with support insulators. The platform for a single valve housing or a number of valves is of the same design as used for series capacitor banks. * Communication channel. An important new element needed for the outdoor valve design is a communication channel. It consists of a composite insulator for DC application which is used for fibre optics, cooling water and ventilation air between the valve housing and earth. * Valve base electronics. The valve base electronics can be located very close to a single valve or be common to a number of valves. The valve control and opto interface are included in the valve base electronics. * Valve cooling including air-cooled liquid coolers and cooling control. The most suitable solution as seen today for the valve 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 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 power thyristors. It gives increased flexibility in the station layout; eliminates the need of a valve hall, including its subsystems; reduces the equipment size; and makes it easier to upgrade existing stations. Future relocation of an HVDC station will also be simpler when outdoor HVDC valves are used.

A test valve in the Konti-Skan 1 HVDC link has given operational experience of a valve designed for 275 kV DC voltage since June 1992. The operation of the test installation has been very successful, and has provided a basis for further development. The ongoing development is aiming at an outdoor valve design for 500 kV DC voltage. 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. Inside the outdoor valve unit, the electrical configuration is of traditional design with air-insulated thyristor modules and reactor modules, and the ambient conditions for these components being the same as for a valve hall solution. 4.2 Elements of the outdoor valve The basic elements of the outdoor valve are: * Valve housing. The encapsulation of the valve is made of steel or aluminium. The insulation medium inside the housing is air at atmospheric pressure. The size of the valve housing has been chosen to make transportation of a complete and 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.

5.1 General Demands regarding permitted interference levels from DC lines have become increasingly stringent in recent years. To fulfill these requirements using passive filters a number of large parallel branches are necessary. A more attractive solution is therefore to use an active DC filter in combination with a small passive DC filter branch. 5.2 Operating principles The principle of the active filter is to inject a current via the passive DC filter into the DC circuit as shown in Fig. 13.

Figure 13: Circuit diagram of active DC filter.

The current to be injected is formed from the measured harmonics on the DC line. A control system calculates the amplitude and phase angle of a signal that is injected via a power amplifier into the DC circuit to eliminate the harmonics on the DC line.

As can be seen from the layout, four phases of the ConTune® 11th, 13th and passive high pass filters are included. The fourth phase is added for redundancy reasons in case of a filter outage and can be connected to each of the three phases .

In Fig. 14 the harmonic content on the DC line is shown for a typical installation, both with and without the active filter in operation. As can be seen from the figure, the active filter reduces the harmonic content considerably.

7. CONCLUSIONS Several new concepts which will result in a new generation 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 filters, 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.

5.3 Operational experience The first prototype was commissioned in December 1991. Commercial installations have been in operation since autumn 1993 and autumn 1994 for the Skagerrak and Baltic Cable HVDC schemes, respectively. Today active DC filters is a standard solution for HVDC transmissions with stringent DC filtering requirements.

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; Vol. PAS-87, No. 10, pp. 1830-1840. 2. Gole AM and Menzies RW, “Analysis of Certain Aspects of Forced Commutated HVDC Inverters.” 3. Nyati S, Atmuri SR, Gordon D, Koschnik V and Matur RM, April 1988, “Comparison of Voltage Control Devices at HVDC Converter Stations.” IEEE Trans. on Power Delivery; Vol. 3, No. 2,

6. NEW CONVERTER STATION DESIGN Through utilizing the features described in this paper a major impact on the design of converter stations is foreseen. An example of a HVDC converter station for a monopolar scheme incorporating the features described in this paper is shown in Fig. 15.

Figure 15: Possible layout of converter station with new features included.

4. Woodford DA, Zheng F, May 1995, “Series Compensation of DC Links.”, CIGRE Symposium, Power Electronics in Electric Power Systems, Tokyo.

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