Adjustable speed Asynchronous Machine in Hydro Power Plants and its Advantages for the Electric Grid Stability

Since a long time, it is known that hydraulic machines can work with an increased efficiency at partial load when the speed can be adapted to this partial load. In a standard configuration when the hydraulic machine is coupled to a synchronous machine which is directly connected to the electric grid this speed variation is not possible at all. The use of a static frequency converter which is inserted between the grid and the stator is a well known solution to achieve an adjustable speed machine. Adjustable speed drive systems have already been applied in the past to hydro power plants. Up to now they have mainly been used for start-up purposes of pumps or pump turbines. The used converters were therefore rather small in respect of converter power output (< 30 MVA). When a static frequency converter is used in the stator, this converter has to be sized for the full electrical machine power. The solution is therefore not applicable in an economical way to very big unit outputs (>250 MVA). In the last few years, considerable work has been done in order to develop a system which does not use a big converter and which presents considerable advantages also on the grid’s side. A pilot power plant according to the new ABB-technology has been built and sucessfully commissioned in Spain. This paper contains a description of the doubly fed asynchronous machine with a cylco-converter in the rotor circuit. This configuration allows adjustable speed operation within a certain speed range. The size of the converter needed is much smaller than for a comparable solution with a synchronous machine. The maximum unit power output which may be realized with this system is limited to about 450 MVA. The main part of the paper is dedicated to the behaviour of this adjustable speed generator system in respect of grid stability and grid transients. In a first step the general principles of this adjustable speed solution (called “VARSPEED” at ABB) will be shown. The use of the VARSPEED solution has several advantages which are not only limited to an increased overall efficiency at partial load operation but also: • Possibility of active power control in pumping mode. • Possibility of reactive power control at the interconnection point to the grid. • Possibility of instantaneous power injection into the grid by using the energy stored in the rotating mass. • Reduced abrasion of turbine runners at very silty water conditions. Some of the features mentioned above will be illustrated by an illustrating example. In a second step, the advantages for the electric power grid will be presented by means of a simulation software. The simulations will show the currents, voltages, torques and speed of a VARSPEED installation in the grid during transient and steady state operation. Different kinds of short circuits as well as different load variations in the grid will be applied to the system and its advantageous behaviour will be presented.