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Special Report | December 2016

Converting to Full-Power

Following a recent visit to ABB´s headquarters in Switzerland, POWER TODAY reports on the world´s largest frequency converter installed by the company at a pump-storage plant in Grimsel 2.

India has its plans in place towards energy-efficiency, and even more, in term of availability and access to power! As Steve Aubert, Product Manager - Hydro Power Converters, ABB, points out, ´The country (India) has plans to increase its renewable energy consumption from the current 36 GW to 175 GW by 2022. While at present, approximately 250 million people in India do not have round-the-clock access, the country is aiming at offering 24x7 power to all households by 2020.´

The year 2016 marks 125 pioneering years for the Switzerland-based power and automation major ABB. The company has been making in India for over six decades. From industrial robots to the first high-voltage direct current transmission and the first LCD watch display, ABB has delivered several firsts through its projects. In fact, on a tour to Switzerland, the POWER TODAY team was witness to many of the company´s delivered projects; one of which was Grimsel 2, installed in a pump storage plant, where ABB has installed the world´s largest frequency converter.

A background
Switzerland-based Kraftwerke Oberhasli KWO operates nine power stations with 26 turbines rated at about 1,125 MW, generating about 2.35 billion kWh of electricity annually - enough to cover the needs of about 1.2 million people. The Grimsel 2 power station was built between 1973 and 1980 and housed four 87 MW turbines. The Oberaarsee, a hydro electric reservoir, lies about 400 m above Lake Grimsel. It supplies water to the power plant´s turbines at a rate of 93 cubic metres per second.

Plan to upgrade
In 2010, KWO aimed to upgrade one of the four power units at Grimsel 2 for variable speed operation. The goal was to increase operational efficiency by improving use of the stored water, mainly by providing energy balancing during pumping mode.

Previously, the Grimsel 2 plant was only able to adjust the amount of absorbed power during pump operation by regulating the number of pumps being used. Each pump was able to operate at full power. Adjustment to the desired power was done through simultaneous generation with other units, thereby consuming valuable stored water from other lakes and generating unnecessary power losses as well.

ABB came on board to install a full power converter system for variable speed operation.

The solution had to fit to the existing powerhouse, and this was a major challenge that ABB faced - to install a full power converter system, especially keeping the electrical machine from the 1980s un¼changed.

This was followed by the decision of installing a 100 MVA frequency converter, the world´s largest of this kind for a variable speed pumped hydro power application.

For smart results!
ABB´s solution consists of a PCS 8000 frequency converter, with an AC 800PEC control unit and step-up/down transformers on the converter grid and machine sides. ABB provided the system and control engineering, installation, and commissioning of the frequency converter.

A smart converter system design enables dramatic improvement in the voltage quality on the stator windings, ensuring smooth operation with the existing unit without excessive stress on the stator windings.

With the full power converter, the frequency on the machine side can be varied, and hence helps control the rotational speed and in turn the power absorbed by the pump. The synchronous machine now operates between 600 and 765 rpm in pump mode, providing up to a maximum of 94 MW of absorbed power. For certain operations, the converter system can be bypassed and run in its original mode at fixed speed.

Switzerland´s first variable speed pump (100 MVA fully rated ´Varspeed´ converter) of this size is located at the Grimsel 2 underground pumped storage power plant. This project denotes the world´s largest ever converter installed at a hydro electric power station; and as a testimony to ABB´s enduring technology, four generators from the BBC era are still operating in the impressive turbine hall today.

Efficient water use
For an efficient functioning, KWO also commissioned an ABB PC8000 static frequency converter. The converter (10x7 m) uses about 1,000 semiconductors and is installed on three separate floors, directly behind the turbine hall. The IGCT-type (Integrated Gate-Commutated Thyristor) power electronics enables operators to adjust the speed of the pump drive between 50 and 105 per cent and thereby control the input power. With the new converter, KWO is able to control the power of one of the four Grimsel 2 machines during the pumping cycle. Previously, power control was possible when turbines were driving the pumps. Currently, Grimsel 2 alone recycles the total water volume of Lake Grimsel five to six times every year.

Overcoming challenges
Space was at a premium inside the installation, because of which the system had to be compact, and has high power density. In addition, the state-of-the-art power electronic component had to be compatible with the existing equipment built in the 80s and adaptable to the installed distributed control and protection systems. The project certainly brought along with it unavoidable challenges, which have been overcome, making for a success story.

- SHRIYAL SETHUMADHAVAN

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