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Power Generation Riddle No.4 - Variable speed generators in Hydro dam schemes
I was wondering, what would the benefits be in implementing variable speed synchronous generators in a hydro generation scheme (not pumped storage), just the standard dam/river scheme? The reason behind this question is that i am conducting a feasibility on implementing these variable speed generators in a current power station, so i am just wondering what the advantages could be?
I would really appreciate in input

Thanks alot for your time

Chris
Author : Chris - From: New Zealand
 
#1
Sat, January 22nd, 2011 - 18:02
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
systems available:
- The system with synchronous machine and static frequency converter (SFC) of the LCI (Load
Commutated Inverter)-type.
- 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.


 
Author : Hamid - From: Iran- Firouzabad in Fars
 
#2
Mon, January 24th, 2011 - 00:04
Thanks so much for your help/response,
i really appreciate it.
I was wondering could you possibly list your references?

Thanks

Chris 
Author : chris - From: New Zealand
 
#3
Mon, January 24th, 2011 - 09:45
You are welcome, the most important of my references is:

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


Daniel Schafer
R&D Manager
ABB Power Generation Ltd.
Dept. KWHT
5242 Birr

Prof. Jean-Jacques Simond
Electrical Engineering Department
Swiss Federal Institute of Technology
DE-LEME
1015 Lausanne
 
Author : Hamid - From: Iran- Firouzabad in Fars
 
#4
Tue, January 25th, 2011 - 04:21
Sorry to be a pain, but i have one final question regarding variable speed generators?

Is it possible to use an already installed, synchronous generator as a variable speed synchronous generator, if you were to connect the existing synchronous generator to a "mini grid" at the power station and do the frequency regulation before it connects to the Main grid.
So basically, the generator used in the variable speed generator setup can be a standard synchronous generator. There are no special features on the generator itself?
Thanks so much, in advance 
Author : chris - From: New Zealand
 
#5
Tue, January 25th, 2011 - 13:04
Dear chris,

It is not any pain, indeed we  appreciate you for new discussions issuance.

Best Regards
Site Manager
Jafarpour 
Author : Jafarpour - From: Iran
 
#6
Tue, January 25th, 2011 - 14:49
Yes it is possible, as is said in my last reply,the use of a static frequency converter (SFC) which is inserted between the grid and the stator is a well known solution to achieve an adjustable speed machine (refer to type D in Figure below) . 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).

 
Author : Hamid - From: Iran- Firouzabad in Fars
 
#7
Thu, April 28th, 2011 - 17:22
dear Chris,
I confirm you the possibility to drive a syn generator with a static converter.
we already did that with synchronous wound salient rotor poles, and with permanent magnet generators in both cases the range of rotation of the turbine is 20-30% (ex. 180-230 rpm, 500-600 rpm, ...)

Agostino 
Author : Valentini Agostino - From: Italy
 
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