24-01-2013, 11:20 AM
Autonomous micro hydro power plant with induction generator
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ABSTRACT
Autonomous micro hydro power plants (MHPP) are a reliable solution for supplying small power
consumers in areas located far from the distribution grid. When an induction generator (IG) is used in
such a power plant, voltage and frequency need to be stabilized. This paper presents a single control
structure that ensures both the voltage and frequency regulation of an isolated induction generator (IG).
The control structure consists in a voltage source inverter (VSI) with a dump load (DL) circuit on its DC
side. The VSI operates at constant frequency, thus stabilizing the IG frequency also. For voltage regulation
two cascaded regulators are used, which have as reference the line voltage and the VSI DC voltage,
respectively. Simulations and experiments are carried out in order to investigate the reliability of such
configuration when supplying static and dynamic loads.
Introduction
Nowadays, the utilization of renewable energy sources (RES) to
match the increasing electric energy demand is determined by
environmental concerns, fossil fuels depletionandalsobyeconomical
and social aspects. In Romania, energy production from renewable
sources is based mainly on hydro power plants, as the hydrographic
network offers a large unexploited potential for micro and small
plants [1].Accordingto thenationalhydroelectric company, themicro
hydro power plants (MHPP) are those facilities with installed powers
up to 1MW. They can operate both grid-connected or in islanded
mode.
In this paper, the case of an autonomous MHPP is considered. For
remote areas situated at a considerable distance from the distribution
network, MHPP with installed powers of tens of kW are
a feasible solution for energy supply. The most suitable turbine for
such a configuration is BankieMichell. Although its efficiency is
lower than the one of other turbines, it remains the same for a wide
range of heads and flows; usually the head varies between 1 and
200 m, while the flow between 20l/s and 10 m3/s. Thus, this type of
turbine is the best choice for MHPP that do not require significant
flow regulation. As for the generator, the induction generator (IG)
seems more suitable than the synchronous one, due to the following
advantages: price, robustness, simpler starting and control.
System configuration
The proposed control topology consists in a Voltage Source
Inverter (VSI) in combination with a Dump Load (DL) circuit, as
results from Fig. 1. This combination ensures with both voltage and
frequency regulation. The VSI is a three-phase PWM inverter with
six transistors. Its control requires the generation of six PWM
pulses, which drive the transistor bridge. It is connected at the IG
leads through an inductor placed on each phase. The VSI operates at
constant synchronous frequency (fn ¼50 Hz), maintaining the IG
frequency constant, excepting the start-up [10].
The dump load connected to the VSI DC side will be controlled
so that the voltage across the CDC capacitor remains at a constant
level, maintaining the system voltage in a standard variation range.
Thus, the difference between the power delivered by the IG and
the loads demand will circulate through the VSI towards the CDC
capacitor, which acts as a short-time energy storage element. The
DC voltage variation ratio depends on the capacitance value and on
the amount of power transferred from the IG towards the capacitor.
The capacitor value plays a very important role during transitory
regimes, when it has to handle large amounts of energy (in or out).
System modeling
The hydraulic turbine model
The performances of a hydraulic turbine can be analyzed starting
from its main characteristics, such as torque vs. speed, power vs.
speed, efficiency vs. speed. In Fig. 4, the torque vs. speed characteristic
of a hydraulic turbine, for different admission degrees is
considered.
It can be seen that, for the same admission degree, the torque
decreases linearly with the rotational speed. The maximum torque
corresponds to a null speed, thus results the high starting torque of
hydraulic turbines. The speed corresponding to the point where the
curve cuts the horizontal axis is called runaway speed.
Simulations and experimental results
The reliability of the proposed control system is tested through
a series of simulations using the Matlab/Simulink environment.
With the help of SimPowerSystems library, the main components
were implemented: the IG, the hydraulic turbine emulator, the
capacitor bank, the loads, the VSIþ DL control system; also, a
measurement subsystem was added.
Conclusions
This paper proposes a control system for an autonomous micro
hydro power plant equipped with induction generator. The control
system contains a voltage source inverter (VSI) to stabilize the
frequency and a dump load (DL) to deal with voltage regulation.
The proposed control topology was simulated using the Matlab/
Simulink environment. Also, an experimental test bench was
build. The specific conclusions drawn from this paper work are:
- The proposed single control topology is simple and effective, as
it doesn’t require complex data acquisition nor computational
resources;
- The two main regulators react promptly and bring the main
parameters (voltage and frequency) to their nominal values in
case of transitory regimes;
- In addition to Ref. [15], the contribution of this paper is that,
highly inductive loads suchas inductionmotors canbeconnected
in the systemwithout affective the energy quality parameters;
- Single phase loads could be connected without creating
stability issues by using special designed circuits like the ones
presented by the authors in Ref. [16].