Notes Topic 8

The power controller for a typical stand alone

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Unformatted text preview: power controller is needed before connection to the battery bank. The power controller for a typical stand-alone battery-charging turbine consists of a rectifier and a control circuit to regulate the current flow to the battery. A six-diode bridge is normally used to rectify the three phase ‘raw’ AC output of the turbine to DC. The DC current is then directed to the battery bank via a charge controller that prevents the batteries from being overcharged. The most common methods of charge controlling are series and shunt regulation. Since the AC frequency of the generator is electrically ‘isolated’ by the rectifier, full variable speed operation is achieved. The level of complexity of the battery charge controller and the number of additional features is normally reflected by the capacity of the turbine. Some controllers offer features such as: • • • • • Battery temperature compensation- to regulate the charging regime with respect to battery temperature Displays showing voltage, current, RPM etc- to display instantaneous performance Separate automatic over voltage disconnect –to protect batteries in the event of control failure Maximum power point tracking – to electronically optimise the operation point of the generator for maximum output Battery equalisation – a periodic higher voltage charge to initiate gassing and reduce cell stratification 8.5.1 Series Regulation The simplest method of battery charge controlling is series regulation (see Figure 8-10). In this system a semiconductor ‘switch’ or a contactor opens, preventing current flowing to the battery when the battery float voltage is reached. To prevent the contactor ‘chattering’ (repeatedly switching on an off) at the control point, a hysteresis function is normally built into the battery voltage sensing circuitry whenever a relay type switch is used. More advanced systems use the PWM (pulse width modulation) technique. This more sophisticated system uses a control circuit that monitors the battery voltage, and the on-off time of a high frequency semiconductor ‘switch’ (commonly a FET - field effect transistor) is used to control the amount of current flowing to the battery. This allows for an accurately controlled tapered charging regime that reduces current 8-11 8.5.2 Shunt Regulation A superior method of battery charge control is shunt regulation. When the desired battery voltage is reached, switching circuitry ‘shunts’ the turbines current away from the battery and to another load. Commonly a resistive element is used, which dissipates the excess energy as heat. The diverting of power to a resistive load is often termed load dumping (see Figure 8-11). This method of regulation ensures a load is always applied to the rotor, hence reducing the likelihood of the turbine running in an open circuit/ free run condition. By ensuring load is always applied to the turbine, lower rotor speeds are realised, resulting in quieter operation and reduced mechanical wear of the turbine. Generally the dump current is d...
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