A 100 MW Converter for the Grimsel 2 Pumped Storage Plant

Pumping efficiency A 100 MW converter for the Grimsel 2 pumped storage plant

HANS SCHLUNEGGER – Hydropower is one of the oldest forms of power generation and also one of the most flexible. Because water can be retained in a reservoir and released when needed, it is ideally suited for meeting demand fluctuations. When the mode is combined with pumped storage (meaning surplus electricity is stored by pumping water into the reservoir), its storage functionality can be expanded and it can furthermore be used in grid control by absorbing or generat ing the energy required to stabilize the grid. To operate more efficiently in pump ing mode, the Grimsel 2 plant of Kraftwerke Oberhasli (KWO) – in the Bernese Oberland region of the Swiss Alps – has taken into service a 100 MW converter supplied by ABB.

Title picture Kraftwerke Oberhasli is a complex of nine power plants located in the Bernese Oberland region of the Swiss Alps. The plants have a total generating capacity of 1,125 MW and an annual production of 2,200 GWh. The lakes have a combined inflow of 700 million m3/year and a storage capacity of 200 million m3.

42 ABB review 2|14 Pumping efficiency 43 more accurately balance supply and At 100 MVA, the Grimsel 2 demand. To do this efficiently, the power needs to be regulated with high preci- installation is the most power- sion. ful drive converter with DC Grimsel 2 link voltage delivered to date. KWO’s Grimsel 2 pumped storage plant in Switzerland ➔ 1 – 2 has four synchronous generators ➔ 3. Until recently, the plant’s ability to regulate power depend- ed on the starting up and shutting down of these individual units. When interme- diate levels of power absorption were required, the difference could only be made up by running an additional unit in generator mode. Pumping and generating at the same time is not only an inefficient use of energy, but also wastes the plant’s most valuable resource: its water.

To regulate the plant’s power uptake more efficiently, KWO decided to equip one of the Grimsel 2 generators with a power-electronic drive. ABB won the or- der for this in 2010, with the installation being commissioned in 2013. umped storage is used in what is called peak shaving: Energy The converter is absorbed by pumping at At 100 MVA, the installation is the most P times of low demand (typically powerful drive converter with DC link at night) and the stored energy is con- voltage delivered to date ➔ 4. It is made verted back to electricity at times of peak of two units of 50 MW, each with its own demand. By mitigating the extreme input and output transformers ➔ 5, and is peaks and troughs of the demand curve, series-connected on both the generator pumped storage decreases reliance on and grid sides. Each of these converters other (often polluting) power sources in turn uses pairs of parallel-connected required to cover peaks. double-phase modules using IGCTs (in-

As well as bridging this classical day to Pumping and generating at night gap, pumped storage can also the same time is not only an help integrate renewables: With inefficient use of energy, but more and more so- also wastes the plant’s most lar and wind power being fed into the valuable resource: its water. grid, pumped storage can compensate for their unpredictability and vari- tegrated gate-commutated thyristors). ability, permitting consumers to continue The overall installation thus features 24 using energy from renewable sources double-phase modules. even when neither the sun is shining nor the wind blowing. The unit’s power is continuously variable between 60 and 100 MW (the lower limit Moreover, moving beyond their strict is defined by risk of cavitation in the mass-storage role, pumped storage pump runner). plants also contribute towards grid control by producing or absorbing the relatively small amounts of power needed to

44 ABB review 2|14 1 Grimsel 2 pumping-storage plant 2 Grimsel 2: the lakes at a glance

Power in turbine operation 4 × 80 MW Height above Volume when Surface area Greatest Name sea level when Power in pump operation 4 × 90 MW full (106 m3) (km2) depth (m) full (m) Average height difference 400 m

Water flow 4 × 22 m3/s Upper reservoir Lake Oberaar 2,303 58 1.47 90

Synchronous speed 750 rpm Lower reservoir Lake Grimsel 1,908 101 2.63 100 Years of construction 1973-80

3 The Grimsel 2 plant has four synchronous generators. 4 The 100 MW converter

Starting system, which adjusts the output of all In turbine operation and in unregulated KWO power plants to comply with the By the end of pumping mode, the machine set is start- loading schedule. The line-side regula- ed up with the turbine ➔ 6. The block tion setpoint is added to the loading March 2014, the transformer is magnetized by the gener- schedule setpoint. The converter power ator and connected to the 220 kV level and speed control are configured in cas- converter had after reaching synchronization condi- cade, the active power being limited by provided 3,500 tions. In regulated pumping mode, the the current pumping head and the maxi- block transformer and the two line-side mum converter power capacity. hours of controlled converter transformers would have to be switched in directly, thereby causing very Reactive power is regulated by a voltage pumping and 850 high inrush current peaks. To avoid this, control loop, set either manually or by hours of phase- the frequency converter DC link is ener- the higher-ranking 220 kV line voltage gized by the start-up transformer through control system. Active power takes pref- synchronous con- the converter diodes on the motor side. erence over reactive power. The transformers are then magnetized by denser operation. the line-side converter and afterwards Operating modes synchronized. The entire starting proce- The following modes can be operated: dure takes only about 10 s, after which – Turbine the machine is accelerated by the con- – Pumping without converter verter to 600 rpm with the watered pump (constant rpm) operating against the closed spherical – Pumping with converter (variable rpm) valve ➔ 6d. After opening the spherical – Phase-angle correction using valve, the speed is adjusted to approxi- converter mately 690 rpm according to the mini- mum power required by the current Turbine operation with converter was not operating head. considered, because adjusting the Fran- cis turbine speed to the relatively small Power control head range would not compensate the The active power is either set manually converter losses. or by the higher-ranking plant control

Pumping efficiency 45 New pumped 5 Schematic of the converter Breaker Block transformer Bypass switch

storage plants with M

13.5 kV 50 Hz direct operation (750 rpm) pump turbines are 220 kV Grid-side Double- DC link Double- Machine-side transformers phase limitation phase transformers almost exclusively 2 × 50 MVA modules modules 2 × 50 MVA equipped for variable-speed 50 Hz operation, using 46 – 51 Hz converter double-fed operation (690 asynchronous – 765 rpm) motors according 50 Hz to the state of Charging transformer technology. Converter

6 The turbine and its connections 7 The machine-room cavern, with location of the transformers and converter.

a b Transformers

c c

c c e d h e d g f

a Pressure shaft to/ d Spherical valve 333 Turbine from Lake Oberaar e Throttle operation b Pressure shaft to/ f Turbine 777 Pumping Cooling units Converter from Lake Grimsel g Pump operation c Connection pipes h Generator / motor

Project execution the existing plant cooling water network The converter delivery and erection is also installed. schedule was adjusted to the refurbish- ing work taking place at the same time. Operating experience This covered: overhaul of the hydraulic The installation has been in service since machinery, spherical valve and butterfly May 2013. By the end of March 2014 it valve, renewal of the instrumentation had provided 3,500 hours of controlled and control systems, replacement of the pumping and 850 hours of phase-syn- turbine governor and excitation equip- chronous condenser operation ➔ 9. ment, and replacement of the 220 kV switchgear. Looking ahead New pumped storage plants with pump Sufficient space was available in the turbines are almost exclusively equipped existing cavern for the entire converter for variable-speed operation, using dou- system ➔ 7 – 8. The four transformers were ble-fed asynchronous motors according installed at machine room floor elevation to the state of technology. There are seri- and the converter units in the basement, ous drawbacks to this type of motor, where the cooling system connected to however: The complex rotor is subject to

46 ABB review 2|14 8 The 100 MW converter could be accommodated in the basement of the machine-room cavern. Despite progress with other storage technologies, pumped storage remains the only mature and affordable means of energy storage suitable for grid regulation.

9 The converter permits accurate regulation of the plant’s power.

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0 Power (MW) -100

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-300 22:00 00:00 02:00 04:00 06:00 08:00 10:00 12:00 14:00 16:00 18:00 20:00 22:00 00:00 02:00 04:00 06:00 08:00

19.10.13 20.10.13 21.10.13 Time

Schedule Power KWO Regulation

Schedule + tertiary Power Grimsel 2

Hans Schlunegger design limits that restrict speed increase the only mature and affordable means Kraftwerke Oberhasli AG to comply with the optimal speed of the of energy storage suitable for grid regu- Innertkirchen, Switzerland pump-turbine. Furthermore, the starting lation, and thus has an important role [email protected] procedure is much more complicated, to play in the generation landscape of sometimes even requiring pump-turbine tomorrow. The new converter at Lake For ABB-related inquiries, please contact Fritz dewatering, and compliance with Grid Grimsel is a contribution to the Swiss Wittwer ([email protected]) or Steve Aubert ([email protected]). Code requirements is more difficult. For Energy Strategy for 2050, which seeks these reasons the trend in future will be to assure the continuity of the Swiss Further reading more toward synchronous machines with energy supply while increasing the use G. Sydnor, R. Bhatia, H. Krattiger, J. Mylius, full-size converters. Fitting an existing of renewables. D. Schafer,” Fifteen Years of Operation at NASA’s pumped storage plant with a full-size National Transonic Facility with the World’s largest converter therefore offers an ideal way This article is adapted from “100 MW-Vollumrichter adjustable Speed Drive,“ NASA archive. of testing this future-oriented technology im Pumpspeicherwerk Grimsel 2” published in elec- J. Hell, M. Egretzberger, R. Schürhuber, A. Lechner, trosuisse Bulletin 3/2014. in actual practice. Experience so far with Y. Vaillant, “Full size converter solutions for pumped storage plants, A promising new technology“, the full-size converter at Grimsel 2 Hydro2012, Bilbao, Spain, 2012. pumped storage plant is very promising. H. Schlunegger, A. Thöni, “100 MW Full-Size Despite progress with other storage Converter in the Grimsel 2 Pumped Storage Plant,“ technologies, pumped storage remains Hydro 2013.

Pumping efficiency 47