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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                                                                          Conversion elements                                                                                                     Proposed power plant

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

								
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