HVDC stations have a proven high degree of reliability. The 420-kV transmission systems in Austria were first inter-connected to the Czech Grid in 1983 via the Durnrohr HVDC back-to-back station (BBS). In 1993, two additional HVDC stations were commissioned at Etzenricht and Vienna Southeast bridging the transmission systems of Germany and the Czech Republic, as well as Austria and Hungary. These three HVDC links between the UCPTE and CENTREL networks have a nominal capacity of 1750 MW. Because these links are important for energy exchange agreements between countries in Western and Eastern Europe, circuit availability and component reliability were the main objectives considered at the design stage.

The BBS at Etzenricht links the transmission systems of the German utility, Bayernwerk AG (UCPTE network), and the Czech utility CEZ (CENTREL network). Vienna Southeast BBS links the networks of Osterreichische Elektrizitatswirtschafts-AG in Austria (UCPTE network) and the Hungarian utility Magyar Villamos Muvek Rt (MVM) (CENTREL network). As they were in operation for two years their performance can now be compared with the design specification. The two stations are of similar design.

Availability Specification To satisfy the utilities' energy availability specification at minimum capitalized cost, the equipment installed must comprise high reliability components arranged with sufficient redundancy to limit downtimes. Fast fault detection combined with effective repair and maintenance strategies can minimize the duration of the downtime.

Reliability Measures A technically acceptable and economically justifiable high degree of component reliability is not sufficient to satisfy this specification. Planning and design characteristics for HVDC stations have been developed based on practical experience and availability studies. Maintenance strategies were used to determine the effects of component and subsystem reliability. Existing redundancy practice related to the control and protection systems had to be reconsidered because of the change in the late 1980s from analog to digital technology.

The level of planned redundancy of the significant components and systems included in the design of these two HVDC back-to-back stations is listed in Table 2.

Spare Components The large spare parts of the HVDC station at Vienna Southeast were already available from the HVDC station at Durnrohr in Austria, in addition to surplus oil tank smoothing reactors, converter transformers and switchgear components.

The HVDC station at Etzenricht is equipped with an air-core smoothing reactor with two coil sections. Unrestricted operation is possible should a coil fail. A spare single phase converter transformer unit is also available.

The utility and manufacturer (Siemens AG) also established an agreement on the inventory of spare parts for converter valves, converter transformers, ac filters, the thyristor valve cooling system, air-conditioning, auxiliary system and the components for the control and protection systems.

Reliability Performance Tables 3 and 4 include the forced outage statistics for each BBS evaluated in accordance with the CIGRE definitions and illustrate the value of considering in detail all aspects of component reliability at the design stage. Scheduled outages for planned maintenance work involving shutdown of the HVDC system are performed once per year, while work on redundant subsystems can be completed without a shutdown. The maintenance program can be structured to ensure system security is not impaired and the costs involved are minimal. For example, approximately 60% of all maintenance work at Vienna Southeast BBS is undertaken while power transmission remains in service. Also included in the annual maintenance tasks is the replacement of defective redundant components connected in series.

In the first operating year, no planned shutdowns were performed at the Etzenricht BBS. In the second year, station maintenance inspection (280 hr) and planned shutdowns (36 hr) were performed to eliminate faults. At Vienna Southeast BBS, one planned shutdown (280 hr) was undertaken in the second year, with all other work being completed without a circuit outage. These statistics mean that both HVDC systems have fully satisfied the contractual requirements with respect to energy availability during their first two years in operation.

Operating Performance During their first two years of commercial operation the two HVDC stations have completely fulfilled all design expectations as no serious system faults and no component failures of any consequence have occurred. Defective component replacements during this period have totaled 29 for each station, but only two of these defects resulted in a forced outage, both at Etzenricht. System faults resulting in shutdown totaled five at Vienna Southeast and seven at Etzenricht during this operational period.

Subsequent analysis of HVDC system fault reports for Etzenricht revealed that 22 redundancy switchovers occurred in various sections of the installation, seven were due to component failure; the remainder were caused by system faults but power transmission was not disturbed.

One of the main fault causes is the peripheral status of signaling contacts. Regular inspection and maintenance is recommended to ensure reliable signal evaluation and fault identification. Also, experience indicates that contact stability with >60 V dc signaling circuits is better with mechanical status contacts.

The positive results gained with the station open-loop control system and the operating system indicate that there is no need for a redundant configuration for operation of the overall installation. However, the active operating state must be maintained when faults occur in the control and operating systems. Redundancy is justified for digital converter control and digital protection, but the criteria for redundancy switchover for the control system must be supported by theoretical and empirical knowledge.

Summary The design policy on planned redundancy and the agreement on the spare component inventory have proven their worth with respect to the reliability and availability of the two HVDC stations. Comprehensive training of the operating and maintenance personnel at the outset of the project motivated the staff and has had a positive influence on limiting all station downtimes.

The high degree of reliability and availability specified by the utility, which has already been achieved in the first years of operation, is attributable to the technical specification jointly agreed to by the utility and Siemens AG for the economic optimum design. The technical configuration of the HVDC installations at Etzenricht and Vienna Southeast were based on theoretical and empirical knowledge, and their operating performance linking the UCPTE and CENTREL networks is now regarded as an international success.

Editor's Note: This article is based on a paper presented to the IEE Sixth International AC and DC Transmission Conference held at the IEE, Savoy Place, London April 29- May 3, 1996, and published in the IEE Conference Publication Number 423. Publisher Institution of Electrical Engineers, Savoy Place, London WC2R OBL, UK.

K. Liegl is deputy head of the Network Operating and Electrical Engineering Department at Bayernwerk AG, Munich. He received the dipl.-ing degree from the Technical University of Munich and joined Bayernwerk AG in 1976. He has been involved in many projects in electric power engineering, especially power system dynamics, stability, harmonics, superconductivity and HVDC. He was project manager for the HVDC back-to-back station at Etzenricht from 1988 to 1993.

W. Richter joined Verbund-Plan Gesellschaft mbH, Osterreichische Elektrizitatswirtschafts AG in 1964 after graduating in electrical engineering. He is the project manager responsible for HVDC links, SVC and 420 kV GIS and air-insulated switchgear. He was project manager for the HVDC back-to-back stations at Durnrohr and Vienna Southeast.