Current forecasts by the European Wind Association (EWEA) predict that in the next two decades the 2 MW and 3 MW turbines in common use in offshore wind farm projects will largely give way to turbines of 5 MW and above, operating in arrays producing hundreds of MW each. Dave Knapper, general manager, High Voltage Products at ABB UK, reports
This increase in the produced power, the distance from shore, the distance between wind turbines and the number of turbines in a single wind farm now place a growing emphasis on the need to reduce the power losses of the offshore collection grid. So there is a growing trend for the array collection network voltage to evolve from the current medium voltage levels of 33 kV to 72 kV and above.
ABB has responded to this challenge by re-engineering the hybrid PASS M00 multi-function switchgear modules, that cover applications from 66 kV up to 100 KV, to meet the specific needs of wind farm applications. These new PASS M00 modules now offer an increased short-time breaking current rating up to 40 kA, and a maximum continuous current up to 3150 A in a configuration suitable for installation within or attached to large offshore wind turbines.
Hybrid switchgear philosophy
Hybrid switchgear developed for use in traditional onshore substation environments combine conventional air insulated switchgear (AIS) and metal-clad gas insulated (GIS) switchgear. This hybrid solution, as found in ABB's Single-Phase Encapsulated Plug and Switch System or PASS design, uses existing, tried and trusted GIS components together with a conventional and extremely reliable AIS bus to connect the various hybrid modules. All the necessary substation switchgear bay functions, including a circuit breaker, one or more combined disconnector/earthing switches, bushings for connection to single or double busbar systems and a current transformer are integrated in one compact module, eliminating the need for discrete items of equipment for each function.
The specific advantages of hybrid switchgear in a substation application include:
1. Relatively inexpensive but highly reliable AIS bus bar.
2. All live contacts in SF6 - experience has shown that AIS disconnector switch contacts require relatively high levels of maintenance while the experience with GIS is exactly the opposite. The increased level of reliability results in a lower global life cost.
3. Fewer switching elements - use of highly reliable GIS equipment allows the switching elements to be rationalized.
4. Pre-tested in the factory, including seismic tests.
5. Competitive installation costs – time on site minimized; less risk of delay due to adverse site conditions; less demand for skilled resources at site.
6. High degree of factory assembly – higher quality finished bay than when assembled under on-site conditions.
7. Facilitates monitoring / on-line diagnostics – integrated nature of the plant facilitates introduction of electronic monitoring and on-line remote diagnostic analysis.
8. Modularization of the substation – savings during the design and construction stages; minimal variations using standardised components; less risk of design errors; greater confidence in project estimation as costs are predictable.
All together these advantages provide very high reliability and availability of the substation
- drastic reductions in the time needed to install the equipment
- much less space required
- simplified substation layout
- less maintenance required (maintenance on demand)
- very good cost performance for purchasing, maintenance, operation, outage and relocation
- environmentally friendly: recycling / disposal at end of life.
ABB’s PASS switchgear modules comprise these main components.
Since the dielectric strength of the switchgear and the breaking capacity of the SF6 circuit-breaker depend on the density of the SF6 gas, a gas density relay is installed to control gas density and detect leakage.
PASS can be equipped with conventional GIS inductive voltage transformers. In a similar way to current transformers (CTs), several combinations of windings for protection and measurements with different loads are available. A Station Service Inductive Voltage Transformer (SSIVT) can also be provided which combines the normal voltage transformer capability with the possibility to provide auxiliary loads of up to 125kVA.
A rupture diaphragm (rupture disk) is installed for protection against excessive over-pressure due to internal arc faults. When a predetermined over-pressure is reached, the rupture disk will break and relieve the pressure. Deflectors in front of the diaphragms ensure personnel safety.
The compact design of the PASS module is due to the excellent insulation qualities of SF6 gas. Its dielectric strength in a homogeneous field is about 2.5 times greater than that of air at the same temperature and pressure. The design of the live components is such that the distribution field is as homogeneous as possible, allowing the intrinsic strength of the insulating gas to be utilised more efficiently.
PASS is equipped with combined disconnector/earthing switches. The mechanism has a minimal number of mechanical components, it is intrinsically reliable and maintenance-free. All required combinations are possible. In all PASS versions, the position of the combined disconnector/earthing switch is clearly shown by an indicator mechanically coupled to the shaft.
Moreover, visual confirmation can be obtained by means of an inspection window in the enclosure. The disconnector/earthing switch can also be operated manually by means of a crank.
The PASS circuit-breaker is a single-pressure interrupter that operates by means of the well-known self-blast principle. The energy for breaking currents is partly supplied by the arc itself. This reduces the energy the operating mechanism has to supply by about 50 percent compared with a conventional puffer type circuit-breaker.
PASS is equipped with conventional current transformers to meet customer requirements. Several combinations of cores for protection and measurements with different loads are available. Up to five cores can be fitted into the current transformers.
PASS can be provided with either outdoor insulators with maintenance free silicon sheds or gas filled cable sealing end chambers.
The circuit-breaker is fitted with a BLK spring-operated drive, designed to keep the component count to a minimum.
Standard PASS modules fit into a standard shipping container without packaging, so no special arrangements are needed for shipping and transportation. Once on site, only a simple 30° rotation of the outer poles is required for the final layout. For other configurations the modules can be shipped as a complete unit or as individual phase elements depending on the installation arrangement.
The first PASS module, introduced in 1999, was the PASS M0 series rated for voltages up to 170 kV. Since then, PASS M0 units have established a reference list of well over 6,500 bays worldwide.
PASS M00 was introduced in 2003 to meet the specific requirements of the fast expanding markets for 66 kV and 100 kV systems. In 2011, a completely new version was introduced that features many innovative international patents. The most important of these is the rotating breaking chamber, now also applied on the PASS M0. In the rotating breaking chamber, the disconnector and earthing switch moving contacts are installed directly on the circuit-breaker enclosure, which opens and closes the disconnector and earthing switches by turning.
The five-position disconnector, which is integrated in the main chamber, enables the following functions:
• Line disconnection
• Busbar disconnection
• Earthing of the line through the circuit breaker
• Earthing of the busbar through the chamber
The multifunctional PASS M00 module can also integrate current and voltage transformers, surge arresters and protection and control systems. It is available for single and double busbar configurations, has a single and triple pole operating mechanism option and is ideally suited for use as a standard product in the design of new modular substations or for retrofit applications.
PASS M00 for new generation wind farms
PASS M00 is ideally suited to high-voltage (HV) wind farm collection network applications where it offers these advantages:
• Compactness due to full gas insulation
• Fast installation time (no HV test is required on site)
• High reliability obtained through lean design
• Fast repair in case of faults due to modular assembly
• Remote control provided by motor operability
• Easy combination with liquid filled or dry type wind turbine transformers.
A case study of the next generation of offshore wind farms equipped with 5 MW turbines revealed that it was possible to incorporate a complete HV substation within an offshore wind turbine tower through the use of PASS M00 modules.
The key to the use of PASS offshore is it can be cable connected on both sides, and it is possible to have a switching and connection configuration which suits the typical wind turbine string arrangement.