Most hydropower plants employ conventional generators with one generator for each turbine. Generally both the turbines and the generators are designed for a specific site and the turbine and generator speeds are fixed.
More recently variable-speed generators have also started to appear in hydropower applications. These allow an additional degree of flexibility by allowing the turbine speed to be varied in order to operate at the optimum efficiency under differing flow conditions. However variable-speed generators are generally more expensive than their fixed-speed equivalents.
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 successfully commissioned in Spain.
Applying doubly fed asynchronous machine with a cylco-converter in the rotor circuit 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 use of this adjustable speed solution (called “VARSPEED” at ABB) 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.
The Varspeed concept of electrical machines is the optimum solution in the case of large variation in the turbine and pumping heads, enables load control in pumping mode, results in the best overall efficiency and improves the stability in the power system. The reason for these advantages comes from the ability to run the group at the optimum speed (in a given speed range) of the hydraulic machine for all hydraulic conditions. In the case of silt, using Varspeed strongly reduces the abrasion of the turbine runner.
ABB as a supplier for hydro generators has two different
- The system with synchronous machine and static frequency converter (SFC) of the LCI (Load
- The system with a doubly fed wound rotor asynchronous machine fed on the rotor side either by a cycloconverter (DASM-solution) or by a GTO converter. The last solution offers best dynamics and a big speed range.
LCI is one of the earliest inverters developed for variable speed drives. Figure below shows a basic configuration of the LCI-fed SM drive. It is mainly composed of a phase-controlled rectifier and an SCR inverter. The rectifier provides an adjustable dc current id
, which is smoothed by a dc inductor Ld
and then feeds the inverter. Since the SCR device does not have self-extinguishing capability, it can naturally be commutated by load voltage with a leading power factor. The ideal load for the LCI is, therefore, SMs operating at a leading power factor, which can easily be achieved by adjusting rotor field current if
The LCI is unable to use any PWM scheme. Inverter output current iA
is of a quasi-square wave. However, the motor voltage waveform vAB
is close to sinusoidal superimposed with voltage spikes caused by SCR commutations. The motor current contains low-order harmonics, such as the 5th, 7th, 11th, and 13th. These harmonic currents cause torque pulsations, as well as additional power losses in the motor.
The LCI-fed drive features low manufacturing cost and high efficiency due to the use of inexpensive SCR devices and lack of PWM operation. The LCI is suitable for very large drives with a power rating of tens of megawatts, where the initial investment and operating efficiency are of great importance.
However, the input power factor of the drive changes with its operating conditions. In addition, the rectifier input current is highly distorted. In practical applications, the LCI drive is equipped with harmonic filters to reduce line current THD, as shown in Figure. The filters can also serve as a power factor compensator.