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


Daniel Schafer
Prof. Jean-Jacques Simond
R&D Manager
Electrical Engineering Department
ABB Power Generation Ltd.
Swiss Federal Institute of Technology
Dept. KWHT
DE-LEME
5242 Birr
1015 Lausanne
Switzerland


Summary
Since a long time, it is known that hydraulic machines
can work with an increased electricity-tool.com/doc/108448-Adjustable-speed-Asynchronous-Machine-in-Hydro-Power-Plants-and-its-/' >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 grids 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 electricity-tool.com/doc/108448-Adjustable-speed-Asynchronous-Machine-in-Hydro-Power-Plants-and-its-/' >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.
1. Variable speed for large motor-generators
As briefly explained, 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
electricity-tool.com/doc/108448-Adjustable-speed-Asynchronous-Machine-in-Hydro-Power-Plants-and-its-/' >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.
[
4
]
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.
In the following we will consider only the DASM-
solution which is perfectly suitable for the application
we are looking at. A single line diagram of this
configuration is given below:

Figure 1. Diagram of a VARSPEED installation

This adjustable speed system, also known as sub- and
supersynchronous cascade uses a doubly fed slip ring
rotor asynchronous machine (DASM). In this
application the stator of the DASM is directly connected
to the network, while the rotor is fed by a three-phase,
sinusoidal current of slip frequency, which enables to
run the unit in a given speed range around the
synchronous speed.
For large Varspeed units, the asynchronous machine is
the critical component and therefore we will mainly focus
on it.
1.1 Design of a Doubly Fed Asynchronous Machine for
Varspeed
The structural design of the wound rotor asychronous
machines compared to a conventional salient pole
synchronous machine is different only on the rotor side.
All the static components such as stator core, stator
frame, stator winding, bearings, bearing bracket etc. are
for both of the same design concept. Due to the small air
gap of asynchronous machines, ABBs structural design
concept with oblique elements is of great advantage
especially with respect to ensuring roundness and
concentricity of stator and rotor.
The rotor is equipped with a conventional 3-phase
winding which is fed via 3 slip-rings by a 3-phase
current system at the actual slip frequency. The design
has to fulfill the following requirements: The rotor rim with the 3-phase winding must be
designed to meet minimum iron losses considering
the slip frequency, and must be in accordance with
the specified limits such as temperature rise and
mechanical stress. The rim must be equipped with radial air ducts for the
cooling air. The overhang of the rotor winding must be
supported properly for all mechanical and thermal
conditions.
For asynchronous machines, up to a bore diameter of
about 3 m the rotor winding overhang is normally
supported by a prestressed bandage of stainless steel
wire. The fitting of this bandage is very difficult due to
the required thermo -mechanical process and in case of a
break-down of bars in the rotor winding the whole
bandage must be removed. Furthermore the cooling of
the overhang is strongly reduced.
For bigger machines, a new end winding support
concept based on retaining bolts is applied.
To minimize the iron losses, the rotor rim consists of
high strength steel sheets with a thickness of less than 1
mm , which are stacked to form the rotor rim. The
laminated ring has radial air ducts and is fixed onto the
hub by means of special wedges.
The wedges are arranged around the circumference in
such a way that concentric radial movements of the rotor
rim, resulting from the centrifugal force, are possible. On
the outer periphery of the rim there are slots into which a
3-phase winding is placed, which in turn is secured
against forces acting in the radial direction by non
magnetic slot wedges.
The following figure shows the supporting and pressing
system for the rotor end winding.
Rotor core
End turns
Retaining bolts
Rotor slot

Figure 2. Supporting and pressing system of the end
winding
The supporting ring is assembled by stacking 2 to 6 mm
thick sheet - steel segments to form single rings which
are pressed together using prestressed shear-pressing
bolts, resulting in a homogeneous ring. The single rings
are separated from each other by the use of specially
designed spacers. The cooling air flows between the
individual rings to the winding overhang. Around the
outer circumference, threaded templates are slid into
axial T-shaped slots.
Radial bolts are passed through the spaces between the
upper and lower winding bars and screwed into the
threaded templates. Insulating pads under the bolt
heads guarantee an even press