Optimal Power Factor for the Reactive Load of Small Hydro Power Plants

This article explores the operation algorithm of an excitation regulator of synchronous generators of small Hydro Power Plants (HPPs). The aim is to provide optimum economic parameters, consistent with the dynamically changing requirements of electric supply companies. An integral method for maintaining the power factor has been introduced. The proposed method ensures stable operation of generators in different working modes. It is implemented and the economic effect proved significant. Keywords-small HPP; power factor; reactive load; excitations; AVR


INTRODUCTION
The difference between small and large Hydro Power Plants (HPPs) is the electrical network length.Small HPPs are connected to open radial peripheral junctions in the distribution network, where the impedance can be high with a large resistor and little / ratio.The production of small HPPs is fragmented, seldom centralized and power generation is dependent on water flow.The hourly, daily and monthly power changes will lead to unobstructed reactive power in both network directions.Therefore, fluctuations in the voltage of the busbars are likely to arise in this network part [1][2][3].If these voltage fluctuations are not limited, the power quality requirements delivered to the consumer might be violated.With regard to the work of small-scale generators, connected to a medium voltage grid, a requirement is set.The mode of operation has to restrict the generator's voltage increase above a certain point while generating maximum possible real power [7].Then the excitation system should ensure a mode of operation maintaining the maximum allowed bus voltage of 20kV with restriction of maximum generator voltage, maximum and minimum field current [4].In practice, the structure of excitation regulators (AVR controller) is used (Figure 1).Regulation of excitation is made either by voltage or by power factor (lagging or leading).

1) By voltage
In this mode, the stator voltage of the generator is regulated with a static characteristic.The stable operation depends on the coefficient of the stasis and its sign.

2) By reactive power
When this mode is in use, the generator works parallel with the network at constant reactive power.This is recommended when the generator works for a long time with load close to the nominal one.

3) By power factor
() There is a restriction regarding the mode of operation for small generators connected to a medium voltage grid.It should not allow a power factor under certain value -e.g.0.9.In this case, an excitation system with constant cos () mode of operation is advisable.The set value should be compliant with the given minimal value.

4) By field current
This mode could be performed if the regulation is on the field current.The assigned current value changes according to the power line voltage connecting the plant to the power substation.The set value should consequently be adjusted to the maximum voltage value of the 20kV busbar.Excitation regulator (AVR controller) scheme Assuming that this structure provides reliable operation with a properly-tuned regulative parameter, the question of optimal management, consistent with the economic requirements, remains.The importance of power factor [5] is taken into account.Depending on the registered deviation between the prescribed power factor and the power factor achieved, utility companies apply different criteria and requirements for the mode of operation.In case of deviations of

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