Safe, reliable electricity supplies depend on the circuit breakers that protect power grids in the event of a short circuit. Historically, these circuit breakers, installed in power plants and substations, were air-insulated. Air-insulated switchgear (AIS), depending on the rating, requires a minimum clearance between various active parts and earth - in the order of tens of metres, which means a large area is needed to
accommodate the installation. As an alternative, gas-insulated switchgear (GIS) is available, in which key components such as contacts and conductors are protected with insulating gas. Compactness, reliability and robustness make this a preferred solution where space is a constraint, such as in urban areas or in harsh environmental conditions. Typically, a substation using GIS can be one-tenth the size of a conventional AIS substation. Jon Downs, ABB’s UK General Manager, Utility Substations, explains
GIS technology originated as far back as 1936, when a Freon-GIS assembly, rated at 33 kV, was demonstrated in the United States. Later, in the mid-1950s, sulphur hexafluoride gas (SF6) was discovered, a gas that has excellent insulating and arc-extinguishing properties. By the mid-1960s GIS was sufficiently well developed to be commercially viable and appealing to a broader market.
In 1966, ABB’s first installation was a 170 kV GIS underground substation for Zürich’s city centre. In 1976, the first 500 kV GIS was delivered to Claireville, Canada. With the installation of the first 800 kV GIS in South Africa in 1986, the company took GIS to the ultra-high-voltage (UHV) level and this ‘Alpha substation’ has been in operation for more than 20 years without any failures or un- planned interruptions.
To date, ABB has over 200,000 bays of high voltage GIS around the world. The 550 kV GIS at the hydroelectric power plant in Itaipu, Brazil is one of the world’s largest installations, with 51 bays. This was overtaken in 2008 by the GIS inside the Three Gorges Dam in China, which has 73 bays.
One of the most recent challenges for ABB, also in China, has been the development of a GIS installation for a UHV rating of 1,100 kV, 63 kA, 6,000 A, which is the most compact GIS ever produced for this voltage.
The focus of the company’s research and development in GIS has not only been in increasing operating voltages, but also in improving the performance and compactness of existing equipment. Most recently, this has seen the ELK-3 series for 420 kV completely upgraded to create a new generation design that breaks several performance barriers.
High performance in a compact design
The compactness of the new ELK-3 GIS, which is 8,100 mm in length, 2,160 mm wide and 3,800 mm in height makes it ideal for installations with constrained space requirements where a traditional GIS with the same 420 kV, 5000 A and 63 kA ratings would not fit.
The new design reduces the switchgear volume by up to 33% compared to its predecessor, resulting in a considerably smaller footprint and reducing infrastructure such as land and building costs by 40%.
Due to the ELK-3’s compact and advanced design, it can be factory assembled and fully tested, and shipped as one bay with all secondary cabling and local control cubicle in a container and on low bed trailers instead of many assembly units, delivering a remarkable 40% reduction in installation and commissioning time compared to competing switchgears
In addition to time and cost efficiencies, the compact ELK-3 is also very environmentally friendly. Because of its smaller size, the amount of SF6 insulating gas required is about 40% less than other switchgear.
The reduced amount of sealing, flange connections and support structures makes efficient use of resources by reducing thermal losses and infrastructure investments. The use of less packing material and streamlined transportation also improves its carbon footprint.
Together with the integrated local control cubicle front panel, the drive cubicle for all drives for disconnector and earthing switches provides easy access for operation of the equipment. The drives carry position indicators, hand crank access and padlocks at the operator corridor.
Gas density information can be supplied through a combined density sensor/monitor and indicated on the front panel. All instrument transformers are wired to the control cubicle and have no additional interfaces in-between. A service platform above the bays gives convenient access to viewports and gas handling equipment.
A full set of standardised connection elements allows for all possible configurations and building optimisation, including integration into existing buildings. The reduction in the number of parts to connect and specific flange protection against environmental elements ensures highly reliable gas insulation.
IEC 61850 ensures future-proof interoperability
To ensure an interoperable and future-proof substation, the ELK-3 has been designed to incorporate the core values of the IEC 61850 standard. The standard defines strict rules for realising interoperability between functions and devices used for protection, monitoring, control and automation in substations independent of the manufacturer. This enables utilities to effectively future-proof their high voltage substations.
At the core of the ELK-3 switchgear system is an advanced double-motion puffer circuit-breaker with one chamber and an optimised and compact layout that minimises the volume of insulating gas required by 20%.
Each circuit-breaker comprises three single-phase breaker poles. Each pole consists of the spring operating mechanism, the interrupter column with one interrupting chamber and the enclosure with the basic support structure. During overhauls, the interrupter unit can be removed easily from the enclosure and replaced by a new unit.
The circuit-breaker uses the double-motion puffer operating principle. At high currents, the pressure inside the puffer increases rapidly due to rising temperatures. This closes the puffer valve. The energy now needed to extinguish the arc is produced by the arc itself. At low currents, the puffer valve remains open and the circuit-breaker acts as a puffer breaker.
Double-motion puffer interrupters require far less operating energy than single- motion puffer interrupters. Arcing contacts on both sides of the breaker are coupled by a gear and move in opposite directions (double-motion), which minimises mechanical reaction forces and further reduces the necessary drive energy.
The circuit-beaker uses a spring operating mechanism for single and three pole operation. The mechanism combines the non-wearing properties of a hydraulic system with the robustness of mechanical spring operating mechanisms.
The spring operating mechanism is composed of a charging system, energy storage with a disk spring stack, and independent actuator piston and control valves, auxiliary switches and position indicators. The hydraulic energy control system is integrated within a compact and sealed block that does not require any external piping. All the components are easily accessible for maintenance and repair.
Scotland to benefit from the first of the new generation
The new generation ELK-3 GIS found its first UK customer in SP Energy Networks. In October 2012, the Scottish Power subsidiary placed an order for 15 bays, which will help integrate the growing levels of wind power production in southern Scotland. This is ABB’s first major 400 kV GIS project in Scotland for around 20 years.
Planned to go into operation in 2013-15, the plant represents an investment of around £16m and will be installed at two brand new substations at Hunterston and Wishaw and also as an upgrade to an existing third substation at Torness.