The first 380 MWe single shaft Siemens V94.3A based, standardised combined cycle power plant to reach full power commercial operation is feeding the network at Otahuhu near Auckland in New Zealand. Extensive modularisation has been used in its construction. There will also be great interest in its initial operating experience. Staff report

The 380 MWe single shaft combined-cycle power plant supplied by Siemens under a turnkey contract to Contact Energy in Otahuhu, New Zealand, is now operating under fully commercial conditions. Siemens says this is the largest single shaft combined cycle unit currently in commercial operation.

Contact Energy Limited is one of the largest power producers serving the competitive power market in New Zealand and operates a number of other power plants. The gas fired GUD plant will be erected directly adjacent to the site of the existing Otahuhu gas turbine power plant in the south of Auckland.

On-site work had begun in May 1997. The number of personnel working on the construction site peaked at around 500. The power plant was handed over to the operator in December 1999. The gas turbine, the steam turbine, the generator and the I&C equipment were supplied by Siemens, but a major share of the other supplies has been provided by partner companies in New Zealand.

Operated as a base-load generating facility, Otahuhu meets around 9 per cent of New Zealand’s power demand. Because of the immediate vicinity of the site to the capital city – Auckland – special attention was paid to making the plant’s architecture blend in well with the natural surroundings.

To date, remote power plants have met the major part of the power demand for the Auckland region. With construction of the Otahuhu plant, more electricity will be generated in the immediate vicinity of the loads and the transmission losses previously incurred will be cut by around 50 MW.

Natural gas from the Maui fields in Taranaki, in which Contact has a holding of over 40 per cent, will be used as fuel. Due to its favourable fuel consumption and exhaust emission rates, the new power plant will contribute significantly towards reducing pollutant and CO2 emissions.

Reference power plant

The combined cycle block in Otahuhu was built as a state-of-the-art reference power plant based on a standardised modular design with proven features and will be equipped with highly efficient technology. The central feature is a 380 MW GUD1S.94.3A single-shaft turbine-generator.

The main advantages claimed for the design are high combined cycle efficiency of about 58 per cent, the modular design, the experience accumulated with reference plants, flexible operation and the straightforward plant configuration.

Despite the enormous distances the key components have to travel, very short construction times are possible with the aid of a modularised power plant design and advanced project management tools. The existing infrastructure can be exploited to a large extent for grid connection and cooling water intake.

The single-shaft concept

The Siemens single shaft concept with the gas and steam turbine arranged on one common shaft offers the advantages of:

• Low specific investment cost.

• Highest thermal efficiency.

• Short project and installation time.

• High operating flexibility (e.g. short start-up times and part load performance).

• Low operating and maintenance costs.

• Limited visual impact.

Standard plant sections up to the entire power island and package-engineered power plant sections were developed to match specific applications. A triple pressure reheat cycle was selected to achieve the highest possible efficiency level.

The entire power module, including the heat-recovery boiler, is assembled on a flat foundation slab. A high-bay building for the turbines and generator features a heavy-duty crane to install all major equipment, and support service and maintenance.

The common hydrogen-cooled generator is solidly coupled to the gas turbine. A synchronous clutch is used for the steam turbine connection to the generator to provide maximal operating flexibility. The two-casing steam turbine features an HP barrel-type turbine with full-arc admission and a combined IP/LP turbine with an axial exhaust to the condenser. In developing this concept, particular emphasis was placed on the following items:

• Standard rotating equipment: gas and steam turbines, generator and clutch between steam turbine and generator.

• Use of prefabricated and pretested electrical and power control modules.

• Provision of short start-up times with full-load steam bypass and auxiliary boiler for steam turbine sealing and cooling.

• Reliability, availability and maintainability.

The maintainability of the gas turbine is helped by the following features: l All moving blades are individually replaceable without lifting the rotor.

• Stationary blade carriers are removable without lifting the rotor.

• Alignment of the compressor blade carrier is possible without opening casing.

• Rear bearing inspection/repair is possible without opening exhaust-gas section.

• Minor inspection is possible through the spacious annular combustion chamber without burner removal.

• Lay-down areas are provided beneath the overhead travelling crane.

The I&C concept

The instrumentation & control system for the Otahuhu GUD power plant is equipped with Teleperm XP, a state-of-the-art process control system. This system ensures user-friendly, economic and reliable plant operation through the integration of the following subsystems:

• Automation.

• Process control and information.

• Software engineering.

• Instrumentation & control system fault diagnostics.

• SINEC bus system.

The entire control room and the I&C systems are prefabricated and have undergone systems testing in Germany. Provisional settings and quality assurance enable on-site commissioning in a very short time. An engineering system is used to configure all the subsystems and to generate the associated software.

The electrical concept

The generator feeds into the grid via the main transformer and the 220 kV substation. A step-down auxiliary transformer taps the auxiliary power from the generator busbars to the 6.6 kV switchgear. In addition, the plant is provided with a power supply from the 33/22 kV system via a start-up transformer.

Two separate auxiliary power supplies are provided for the turbine-generator and for the peripheral equipment. Battery backed 220 V DC systems are supplied from the 400 V system via battery chargers for the I&C levels.

A static frequency converter is being used for motoring the generator and accelerating the gas turbine. For start-up and standstill phases the 220 kV breaker is kept open and the auxiliary power is supplied to the 6.6 kV switchboards from the 33/22 kV substation.