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Innovative application of induction and synchronous
generators in autonomous small hydropower plant
Prof. M.T.E. Kahn
NeuroMorphoGenics/NMG Africa
Abstract
Hydropower is the most important of the renewable for
electrical power generation. The potential of small
hydropower has not been exploited due to economies of
scale of large plants and cost of the equivalent fossil fuel
powered plants. The current principal elements of energy
conversion are hydraulic turbines and synchronous
generators. Effective exploitation of a hydropower requires
two or more generating units to be employed in a plant. In
this study, a two unit small hydropower plant is proposed, a
synchronous generator driven by a governor regulated
turbine and an induction generator coupled with
unregulated hydraulic turbine. The synchronous generator
determines the generation voltage and power frequency,
also it adjusts its output accordingly to plant load
requirements. Induction generator power output is constant
and proportional to slip. Static compensator incorporated
generates reactive power to meet magnetizing power
requirements of the induction generator and part of that
demanded by the load. The proposed techniques have
succeeded to exclude a governor and the of synchronizing
equipment. Implementation of the proposed plants can help
electrifying communities in off grid remote areas with hydro
potential at a relatively low cost.
Introduction
Hydropower, large and small remains by far the most
important of the renewable source for electrical power
generation worldwide. Because of economies of scale of
large hydro schemes and relatively low investment costs of
equivalent fossil fuel powered plants, small hydropower
potential (not exceeding 10MW) is under exploited. Power
in small hydropower plant is generated by natural run-off
without flow regulation therefore generation capacity varies
according to weather conditions.
The effective exploitation of available generation capacity
of a plant is effected by applying two or more generating
units even of different types and capacities.
Achievements in research and development on machines
and power electronics show possibilities of implementing
competitive small hydropower plants at low costs. The
introduction of cheap induction generators in some micro
hydro plants and the development and operation of static
VAR sources with power solid state controls has enhanced
the opportunity for use of large scale induction generators.
This study proposes the implementation of cheaper small
hydropower plants employing the large scale induction
generators and synchronous generators simultaneously.
Proposed power plant
Conversion elements
Hydro energy is based on the flow of naturally circulating water and its drop from
a higher level to a lower land surface. Conversion of the energy to electricity
takes place in electro-mechanical generating units consisting of hydraulic
turbines and electric generators.
The turbines convert water power into mechanical power through the rate of
change of angular momentum of the water. The effect of this change in angular
momentum is to induce torque on the shaft on the runner coupled to the
generator. The speed of rotation is the rate at which this angular momentum is
changed and if the driven is synchronous generator, determines the frequency
of power generated. The general formula for hydro system power output is
Schematic diagram of the
plant
Pm gqh
For frequency regulation, the turbines are individually equipped with governors. The function of a governor is to measure the
rotational speed of the generator; to compare it to the reference value (50Hz) and based on the error signal, to instruct the
hydraulic actuator to open or close wicket gates or needles which control water flow into the turbine.
Turbine-governor characteristics
Governors are designed to operate as proportional control systems
characterized by a wide spread droop of 4% across the operational
range. There is an inbuilt facility within a governor that allows an
operator to adjust the set point frequency.
For line A, the set point is
1.04 p.u and the frequency
will be 1.00 p.u when the
increased to rated generator
power. Changing the set
point to 1.02 p.u moves the
line to B. Set point
adjustment
allows
the
operator to decide on how
the demand is shared by the
generators on the system.
Induction generator
Induction generators commonly used are induction
machines with cage rotors; the attractive features of these
machines are simplicity and ruggedness in construction and
hence low cost.
As a generator, the induction machine is driven by a prime
mover at a speed above the synchronous speed. Part of the
rotor shaft power is transferred across the air gap and
delivered to the system as generated power.
Frequency/power characteristics
of turbine-governor
Equivalent circuit of an induction generator
Synchronous generator
A single synchronous generator supplying power to
an impedance load acts as a voltage source whose
frequency is determined by its prime mover speed.
In power system modelling, a synchronous
generator is represented by its direct axis
reactance x in series with a constant voltage E
Ignoring rotor saliency effects, the power transferred by the
generator to external system is determined by the power angle
EVt
characteristic as
P
sin 
x
The active power is limited by the capability of the prime mover.
When the active power loading and voltage are fixed, the allowable
reactive power loading is limited by either armature or field heating.
For small slip, the power transferred across the air gap can
be estimated as
2
V s
Pg 
rr
Where
jxm
V  Vt  I o (rs  jxs )  Vt
rs  jx11
x11  xs  xm
and Io is zero-load exciting current
The exciting current absorbed from the system
Im
Ea

jxm
Equivalent diagram of the
plant
The plant consists of a synchronous generator driven by a
governor regulated hydraulic turbine operating in parallel
with an induction generator coupled to uncontrolled
hydraulic turbine supplying a constant mechanical power.
The incorporated static compensator supply reactive
power to meet the magnetizing power requirement of the
induction generator and part of that demanded by load so
as the synchronous generator is not over loaded
Conclusions
The preliminary investigation indicates the possibility of
implementing small hydropower plant at low cost by
employing synchronous-induction generator combination
whose operation requires neither synchronization nor
governor for the hydraulic turbine driving the induction
generator. In addition, the combination offer chances of
lowering capacities of switchgears and sizes of current
carrying equipment of the plant due to short circuit
characteristics of the induction generator.
Ruggedness of induction generator and its governor less
turbine prime mover further reduces maintenance cost of
the generating unit. The generation of reactive power
generated by STATCOM change the main role of the
synchronous generator in the plant to determinant of
generation frequency and the common bus voltage.
The application of induction generators in micro
hydropower plants has been widely reported; therefore
the function ability of the proposed small hydropower
plants cannot be doubted. However, further work on
modelling of the plant and simulation of its operation in
both normal and faulty conditions has to be undertaken to
determine capacity of switchgears and sizes of
equipment required in a specific plant and their cost
implications.
Implementation of the proposed autonomous plants can
help electrifying communities in off grid remote areas with
hydro potential at a relatively low cost.