Power converters for pumped storage hydro power plants
A while ago we started a series on hydro power generation. Today we invite you to learn about how power converters for hydro applications make the pumped hydro storage more efficient and more attractive while supporting the stability of the power grid and allowing the integration of other renewable energy sources as part of power generation mix of the future.
Pump hydro storage power plants enjoy renaissance in Europe after couple of years of stagnation [1]. Almost all countries worldwide increase their installed capacity of renewable energy, particularly wind and solar. This is surely good news for the environment. However, it also demands larger storage capabilities. Pumped hydro storage power plants are proven to be the best and most efficient large scale energy storages – “large batteries” in the power grids. Let us have a closer look how modern pumped hydro storage power plants are being operated and how they can benefit from power converters.
Fundamental role of pumped storage power plants
Pumped storage power plants are the most popular, most practical and most efficient large-scale energy storage plants [2]. They allow to pump water up when there is excess of power in the grid and generate hydroelectric power when the demand is high. One solution is to have separate pump system and generation system. There is a mechanical string that always operates in the regime motor-pump while other string runs in turbine-generator mode. These two systems normally do not operate at the same time. Such solution is called a ternary unit. Another solution is to use a reversible turbine. Such set then allows the operation as turbine-generator as well as pump-motor.
As the pumped hydro storage power plant shall help balancing the power generation and consumption, there is a need to control the power [3], [4]. In fact, this is rather complex task and most pumped hydro storage power plants are being controlled centrally by the grid operators. In short, it is not needed to always pump with maximum active power. Moreover, it is of advantage to adjust the speed in turbine mode. And here comes the power electronic frequency converter into the picture. It is basically large to very large variable frequency drive (VFD) that allows the speed control (and therefore flow and power control) of the pump/turbine.
The variable speed control of such pump uses the exact same principles as industrial pumps with VFDs use. However, instead of controlling the flow the aim is to control the active power and therefore support a stable grid frequency (P/f control). By adjusting the speed of the turbine the system can be operated at its optimum load point, independent on the nominal speed. It basically means that the motor-pump or generator-turbine system reaches better efficiency and supports the grid with bi-directional controllable active power. The “hydro storage battery” shall be operated as efficient as possible.
Role of pumped storage power plants in modern power grid
Future power grid will become more complex and more challenging to control. With integration of renewable power there is larger amount of small/medium size generation units. Moreover, the power generation becomes less predictable.
Modern pumped storage hydro power plants help to solve such challenges and contribute to grid stability. With variable speed feature the plant is not just a generation unit or a consumer but can better adjust the active power and help controlling the grid frequency. By injecting or consuming the reactive power the pumped storage plants also help to control the grid voltage at the point of common coupling (PCC). As the power grid becomes more dynamic, the start-up time of a pumped storage plant is getting more important. Variable speed allows expansion of the storage capacity (wider range of the head) making more projects commercially viable. Finally, variable speed enables better turnaround efficiency of the plant and further reduces the environmental impact. There are several pumped storage power plants with variable speed technology under construction and even more of them are under evaluation/in a planning phase.
Now we come to the core: what are the power converter technologies used in pumped hydro storage plants?
1. VFD soft starter
A soft starter is used to start the machine and synchronize it to the grid. The starting power depends on the load (watered or de-watered pump runner) and on the system losses to be covered. Once the conditions for synchronization are met, the machine is transferred to the grid and operates as direct on-line with fix speed. VFD soft starter is a power electronic based converter that allows frequency-controlled start (not to be mistaken with a soft starter that only reduces voltage for starting). The fix-speed machines in hydroelectric power generation are typically synchronous machines and therefore the load-commutated inverter (LCI) is a very good fit, both economically as well as technically. However, remember that this solution only solves the starting problem. It does not allow speed control in normal operation.
2. AC excitation (doubly fed)
Another power electronic converter is so called AC excitation. This solution already allows not only starting, but also a speed control in certain range (e.g. ±10%). Unlike LCI, AC excitation only works with asynchronous (induction) machine. In fact, it is a special type called doubly fed induction machine. The stator is directly connected to the grid of a fix frequency while the rotor is supplied from a frequency converter with a variable frequency. This is the slip frequency. AC excitation converter is bi-directional allowing power flow from the grid to the rotor or vice versa feeding power back into the grid. The advantage of AC excitation is the dimensioning of the converter. The power supplied to or from the rotor corresponds roughly the stator power multiplied by the slip. Therefore, a system with ± 10% speed variation results in a converter sized for approx. 10% of the machine power rating. That rule of thumb is valid for steady state operation. There are certain fault conditions (low voltage ride through) that require the AC excitation to be dimensioned for larger power in order to withstand those conditions. Despite that fact, the power rating of AC excitation is still much smaller than the power rating of the electric machine. This is a good argument from cost and efficiency perspective.
3. Full converter (fully fed)
Finally, the third solution is a ‘full converter’. The terminology differs a bit. Alternative names are fully fed solution or a converter fed synchronous machine (CFSM) [5]. This power converter transfers the full power of the machine. Meaning it is a large unit in terms of MVA. Naturally, it is a bi-directional unit supplying power from the grid to the stator (motor-pump mode) or from the stator into the grid (turbine-generator mode). The machine is of synchronous type with static electrical excitation, normally realized with brushes and sliprings (DC excitation). From converter perspective it is the most expensive solution with regards to initial investment. However, it allows unparalleled flexibility of operation and extensive grid support. This solution features full grid compliance allowing to manage faults with fault ride through function. The converter can control the pump (resp. turbine) across a wide speed range.
There are two main converter platforms used for CFSM. Both of them are based on VSI technology. Especially the latter one opens up new possibilities. We will discuss them in more detail in a follow-up article (Part 2).
Pros and cons of each solution
As usual, various technical solutions have their advantages and drawbacks. They are related to the performance, footprint, investment cost etc. In our next article we will compare above three technologies from various perspectives. Stay tuned!
References
[1] The rise of variable speed Pump Storage Plant, Supergrid Institute, Link to article
[2] Valavi and A. Nysveen, “Variable-Speed Operation of Hydropower Plants: A look at the past, present and future”, IEEE Industry Application Magazine, September/October 2018
[3] Pumped storage hydro power plants – Introduction, https://mb-drive-services.com/pumped-storage-hydro-power-plants/, July 2020
[4] Pumped storage power plant Dlouhe Strane, https://mb-drive-services.com/pspp_dlouhe_strane/, September 2020
[5] Variable speed drive for converter fed synchronous machine, https://new.abb.com/power-converters-inverters/energy-storage-grid-stabilization/converters-for-pumped-storage-plants/pcs-8000-variable-speed-converter/variable-speed-drive-for-converter-fed-synchronous-machine
