Alf Persson

NEW TECHNOLOGIES IN HVDC CONVERTER DESIGN

ABB Power Systems, Sweden

Consequently, many other main circuit components, inaddition to the filter components, such as switchingequipment, CTs, etc, must be installed. The filters willtherefore take up considerable space in a converter sta-tion.

2.2 De-coupling of filtering and reactive powersupply

The development of new, effective AC filters, describedin a separate section of this paper, makes it possible toperform the filtering function through a single filter bankwith small Mvar rating. The new AC filter thus allows de-coupling of the functions of the filtering and the reactivepower generation to a large extent. In this situation, thetraditional way of generating the required reactive powerwould be to install a number of shunt capacitor banks.However, during the last few years, another concept, thecapacitor commutated converter, abbreviated CCC, whichprovides a much more interesting solution, has been studiedand developed.

2.3 The CCC concept

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

This converter is characterized by the use of commutationcapacitors inserted in series between the converter trans-formers and the valve bridge. This circuit has beenproposed in several previous papers, see, for example, Refs.1 and 2, as a method of obtaining a self-commutatedconverter.

Lennart Carlsson

ABB Power Systems, Sweden

Mikael Åberg

1. SUMMARY

HVDC technology took a big step forward around 20 yearsago when thyristor valves succeeded the mercury arc valvespreviously used. The converter station concept introducedat that time, however, has remained practically unchangedsince then.

The time has now come for a further major advance intechnology. The introduction of new concepts will changewhole approach to building an HVDC station. Even thoughthis innovation may not be quite as significant as whenthyristor valves were introduced, the new features willgreatly improve the operating characteristics of HVDCtransmissions and reduce the size and complexity ofconverter stations.

The new generation of converter stations is now likely toinclude 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.

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

2. CAPACITOR COMMUTATED CONVERTERS

2.1 General

In a conventional HVDC converter the consumption ofreactive power is typically around 0.5 p.u. of the activepower. This reactive power requirement is in most casesfully compensated for locally by installation of shunt ACfilters in the converter station. Requirements for permittedreactive power unbalance, or AC voltage changes uponfilter switching, in many cases result in splitting of theinstalled reactive power into several filter/shunt banks.

Figure 1: Single line diagram of a monopolar station withCCC and ConTune® AC filter.

ABB Power Systems, Sweden

2.5 Improved dynamic stability

The contribution to the commutation voltage from thecommutation capacitors results in positive inverterimpedance characteristics for an inverter operating at mi-nimum commutation margin control. An increase in directcurrent therefore results in a DC voltage increase ratherthan the opposite, which is the case for conventionalinverters with commutation margin control. The dynamicstability of an inverter will thus be dramatically improvedwith a CCC.

Figure 2: Capacitor Commutated Converter.

However, the ABB approach does not aim at achieving aself-commutated converter: instead, it provides reactivepower compensation proportional to the load of theconverter. The need of switchable shunt capacitor banksfor reactive power compensation is thereby eliminated.Since the AC filters are necessary only from the point ofview of filtering harmonics, the shunt-connected reactivepower generation can be minimized. In the ABB solutionthe size of the commutation capacitor is chosen so that thefull load reactive power consumption of the converter iscompensated by the reactive generation of the small highperformance AC filter. Fig. 3 compares the reactive powerconditions.

Figure 3: Reactive power conditions for a typical con-ventional converter and for a CCC.

CCC

Conventional converter

2.4 Sturdily constructed and resistant to disturbances

The commutation capacitors improve the commutationfailure performance of the converter. The capacitorsintroduce a source of commutation voltage in addition tothe AC bus voltage which, if proper control functions areincluded, can be used to minimize the risk of commutationfailures. Typically, a CCC can tolerate a sudden 15-20%voltage drop without developing a commutation failure.

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

Figure 5: Ud/Id characteristics.

The improved inverter performance as described aboveresults in more economical solutions, particularly forHVDC schemes feeding weak systems and for HVDC sche-mes using very long DC cables.

Fig. 6 shows the MAP (Maximum Available Power) curvesfor a conventional converter and a CCC for SCR = 2. Ascan be seen, the CCC is in a very stable situation while theconventional converter is close to the stability limit.

The diagrams also show that the load rejection overvoltagewhich occurs upon pole tripping or commutation failuresis reduced from 1.5 to 1.2 p.u. as a result of the small sizeof the shunt-connected filters for the CCC. The small shuntfilters will also reduce the risk of low order harmonicresonances on the AC side.

2.6 The capacitor in the CCC concept

In principle, it would be possible to locate the capacitorson the AC side of the converter transformers, as proposedin Refs. 3 and 4.

However, it was deemed that it would not be possible tocompletely avoid ferro-resonance problems and certainother drawbacks using this concept. The location of thecapacitors between the converter transformers and thevalve bridge results in full control of the capacitor currentsand complete elimination of the risk of ferro-resonance.

A key component in a CCC is thecommutation capacitor. The steadystate operating voltage of thecommutation capacitor is defined bythe direct current. The capacitors mustbe protected against overvoltages byparallel ZnO varistors. The voltagestresses on the capacitors, as well asthe energy requirements made of theparallel varistors, are relatively lowcompared to the installed capacity,and consequently the commutationcapacitors can be of compact design.Figs. 7 and 8 show a typical layoutfor a commutation capacitor with itsvaristors, and the voltage of thecommutation capacitor in normal ope-ration.

Figure 6: Maximum power curve for conventional andCapacitor Commutated Converters, SCR = 2, g=17º.

Figure 8: Commutation capacitor voltage.

Figure 7:CommutationCapacitor.

2.7 Effects on other equipment

Introduction of commutation capacitors results in diffe-rent stresses on the other equipment compared to aconventional HVDC converter. The main influence fromthe capacitors is a considerable reduction of valve short-circuit currents. This is due to the voltage drop across thecommutation capacitor varistors. On the other hand, asomewhat higher peak voltage across the valve, as well ashigher extinction voltage steps, will be obtained comparedto conventional HVDC.

The voltage contribution from the commutation capacitorswill support the commutation of the direct current fromone valve to another; i.e., the overlap angle will be reducedcompared to a conventional HVDC converter. The reducedoverlap angle will result in somewhat higher AC harmoniccurrents and the reduced overlap will, in combination withthe higher extinction voltage step, give somewhat increasedgeneration of harmonics on the DC side compared toconventional HVDC. The increased harmonic productionof a CCC is of the order of 20 % and can be coped with byusing high performance filters on both the AC and DCsides.

Figure 9: Valve short circuit current.

With the location of the commutation capacitors on thevalve side of the converter transformer, the rating of theconverter transformer can be reduced by reducing the no-minal phase-phase voltage on the valve side; i.e., thereactive power flow through the transformer is minimized.

2.8 Impact on station design

The elimination of switched reactive power compensationequipment will simplify the AC switchyard and minimizethe number of circuit-breakers needed, which will reducethe 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 di-gital simulation programs and in the HVDC simulator overthe last few years. Design rules for the CCC have beendeveloped and verification of the CCC concept in a highpower test circuit will be finalized at the beginning of 1996.

The conventional filter reactor design has been modifiedby inserting a core and a control winding. A DC current inthe control winding affects the permeability of the coreand thus changes the inductance of the reactor. Nomechanically moving parts are needed. Fig. 10 shows thebasic 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 ofthe harmonic is used as an input signal to control tuning.

The regulator is a PI-regulator and a small standard 6-pulse controlled rectifier is used as amplifier to feed thecontrol winding of the reactor. The power needed to feedthe control winding is around 1 kW per phase.

3.3 Operational experience

A test installation of an 11th harmonic ConTune® filter wasmade in the Lindome station of the 300 MW Konti-Skan 2HVDC transmission in 1993. The filter has the samegenerated reactive power, 11.6 MVAr at 132 kV, as theoriginal filter. Fig. 12 shows the test installation.

The filter was designed to accommodate frequencyvariations and component variations that represent thedetuning (+2, -3 Hz).

3. CONTINUOUSLY TUNED AC FILTERS

3.1 General

HVDC converters produce current harmonics on the ACside and voltage harmonics on the DC side. For a 12-pulseconverter, AC-side harmonics of the order 12n±1 arecreated. A typical filter set-up consists of 11/13 and HP24filters.

To obtain good performance, low impedance tuned filtersoften need to be provided for the lowest characteristicharmonics; i.e., the 11th and 13th.

Filters have two important characteristics: impedance andbandwidth. Low impedance is required to ensure thatharmonic voltages have a low magnitude. A certainbandwidth is needed to limit the consequences offilterdetuning.

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

A filter in which tuning can be adjusted to follow frequencyvariations and component variations offers several adv-antages:- 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,implying that the ratings of the AC filter componentscan be reduced.

ABB has developed and field-tested a new method toachieve continuous automatic tuning of an AC filter. Theconcept is based on orthogonal magnetizing of an iron corein the filter reactor. The reactor inductance is controlledby a direct current creating a field perpendicular to themain axis of the reactor.

The permeability of magneticmaterials can be changed byapplying a transverse DC magnet-ic field. This permeabilitycontrolling field has to be orientedperpendicular to the main fluxdirection and has the effect oflowering the permeability by”destroying” favourably orientedmagnetic domains. A transverseDC field is able to reduce thepermeability by several orders ofmagnitude without affecting thelinearity of the magnetizing pro-cess. Because of the linearity noadditional harmonics are produced.

Figure 10:Variable reactor.

Figure 11: AC filter tuning control.

Figure 12: Test installation of a ConTune® filter inLindome.

* Platform with support insulators.The platform for a single valve housing or a number ofvalves is of the same design as used for series capacitorbanks.* Communication channel.An important new element needed for the outdoor valvedesign is a communication channel. It consists of acomposite insulator for DC application which is used forfibre optics, cooling water and ventilation air between thevalve housing and earth.* Valve base electronics.The valve base electronics can be located very close to asingle valve or be common to a number of valves. Thevalve control and opto interface are included in the valvebase electronics.* Valve cooling including air-cooled liquid coolers andcooling control.The most suitable solution as seen today for the valvecooling is to have a cooling system serving one pole, i.e.,12 valves. In most cases the cooling system will be a closedsingle-circuit system with a coolant consisting of a mixtureof water and glycol for anti-freeze purposes.

4.3 Operational experience

A test valve in the Konti-Skan 1 HVDC link has givenoperational experience of a valve designed for 275 kV DCvoltage since June 1992. The operation of the test installa-tion has been very successful, and has provided a basis forfurther development. The ongoing development is aimingat an outdoor valve design for 500 kV DC voltage.

5. ACTIVE DC FILTERS

5.1 General

Demands regarding permitted interference levels from DClines have become increasingly stringent in recent years.To fulfill these requirements using passive filters a numberof large parallel branches are necessary.

A more attractive solution is therefore to use an active DCfilter in combination with a small passive DC filter branch.

5.2 Operating principles

The principle of the active filter is to inject a current viathe passive DC filter into the DC circuit as shown in Fig.13.

A comparison of the performance of the passive and activetuned filters shows that the 11th harmonic distortion wasreduced from around 0.026% with the passive filter to aro-und 0.010% with the ConTune® filter with its Q factor ofaround 200. The converter was in both cases operatingunder the same conditions.

It should be noted that the original AC filters in Lindomehave a high quality factor for the 11th and 13th filter, Q=65,while a typical value is 30-40. Hence, the distortion withthe passive filter was already very low.

The filter performance measured at the test installationshows that the ConTune® concept is an appropriate solu-tion. The test installation has been in operation now formore than two years and operating experience has beengood. Commercial installation of a ConTune® filter isalready in progress at the Celilo terminal of the PacificIntertie.

4. OUTDOOR HVDC VALVE DESIGN

4.1 General

The outdoor air-insulated thyristor valve is a newcomponent, made possible by the development of highpower thyristors. It gives increased flexibility in the sta-tion layout; eliminates the need of a valve hall, includingits subsystems; reduces the equipment size; and makes iteasier to upgrade existing stations. Future relocation of anHVDC station will also be simpler when outdoor HVDCvalves are used.

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 configurationis of traditional design with air-insulated thyristor modu-les and reactor modules, and the ambient conditions forthese components being the same as for a valve hall solu-tion.

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 alumi-nium. The insulation medium inside the housing is air atatmospheric pressure. The size of the valve housing hasbeen chosen to make transportation of a complete andassembled valve possible on roads and railways. The lengthof the valve housing is a function of the DC voltage for thevalve.* Active part with thyristor and reactor modules.The modules are of water-cooled design, similar to themodules used for an indoor installation. Figure 13: Cir-

cuit diagram ofactive DC filter.

The current to be injected is formed from the measuredharmonics on the DC line. A control system calculates theamplitude and phase angle of a signal that is injected via apower amplifier into the DC circuit to eliminate theharmonics on the DC line.

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

5.3 Operational experience

The first prototype was commissioned in December 1991.Commercial installations have been in operation sinceautumn 1993 and autumn 1994 for the Skagerrak and BalticCable HVDC schemes, respectively. Today active DC filt-ers is a standard solution for HVDC transmissions withstringent DC filtering requirements.

6. NEW CONVERTER STATION DESIGN

Through utilizing the features described in this paper amajor impact on the design of converter stations is fore-seen. An example of a HVDC converter station for a mono-polar scheme incorporating the features described in thispaper is shown in Fig. 15.

Figure 14: Harmonic current content on a typical DC line.

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

7. CONCLUSIONS

Several new concepts which will result in a new genera-tion of HVDC converter stations have been developed overthe past few years. Capacitor commutated converters,actively tuned AC filters and outdoor thyristor valves arethree of the most important new features. Active DC filt-ers, optical current transducers, fully computer-basedconverter controls and deep hole electrodes are otherimportant elements. These technological advances willresult in improved operating characteristics, reducedcomplexity and smaller area requirements for future HVDCconverter stations.

8. REFERENCES

1. Reeve J, Baron JA and Hanley GA, Oct 1968, “ATechnical Assessment of Artificial Commutation of HVDCConverters with Series Capacitors”, IEEE Trans. on PAS;Vol. PAS-87, No. 10, pp. 1830-1840.

2. Gole AM and Menzies RW, “Analysis of Certain Aspectsof Forced Commutated HVDC Inverters.”

3. Nyati S, Atmuri SR, Gordon D, Koschnik V and MaturRM, April 1988, “Comparison of Voltage Control Devicesat HVDC Converter Stations.” IEEE Trans. on PowerDelivery; Vol. 3, No. 2,

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

Figure 15: Pos-sible layout ofconverter stationwith new featu-res included.