Static Capacitor Exciter Stand-Alone IG for Pumping Systems

Bearing in mind the energy storage capacity or water pumping in a reservoir for later use appears to be the most appropriate ways to employ wind energy, which has a supply that depends on time, by day & season. As variable speed is useful, to tap most of the wind energy from cut-in to cut-off wind speeds, the frequency of the voltage produced by the IG varies noticeably, however the ratio V/f does not vary as that much. For induction-motor-driven pumps, such a situation is satisfactory. As a result, the frequency of the IG should not to be controlled at induction motor terminals. For providing controlled voltage at various speed levels, the single value capacitor needed to self- excite the IGs have to be varied. A static capacitor exciter (SCE shown in fig below) performs just that.

The SCE handles only the reactive power requirements of both the IG and IM by means of adequate control [15,16].

For self-excitation, it is necessary that the magnetic flux in the IG remains above a certain level, & the output voltage proportional to IG speed must be generating a good approximation to that. If speed feedback does not exist, a proportional to frequency will execute the work.

A vector control system who holds both the capacitor voltage & the generator voltage proportional to speed (or frequency) is revealed in fig five.21

The stator flux in the IG is close-loop controlled (instead imposed only) through the generator voltage amplitude regulator along the id channel. The D-C capacitor voltage is PI controlled as well for corresponding the generator voltage relative to speed.

 

When the magnetization current in the machine is kept constant through V/f control, there will be inverse variation in the value of the capacitor and speed by an equivalent value. The IG stator flux is, thus, constant over extensive range of speeds. The induction motor flux should also be kept constant with that of generator speed (V/f control).

Typical behavior of such a system [16] is illustrated in Fig 5.22, where the speed is ramped from 1200 to 1800 rpm & back. The IG & IM flux & capacitor voltage V_dc are recorded.

As predicted, the response is smooth & stable. The centrifugal characteristics are such that the pumping flow rate Q varies proportionally with wind speed, until the highest power ceiling of the stall-regulated wind-turbine is achieved.