Notes Topic 8

Pwm is usually used to control the switching

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Unformatted text preview: rawn on the DC side of the rectifier but it may also be taken before the rectifier over each phase of the generator output. PWM is usually used to control the switching component(s), which are commonly power transistors. This allows for progressive control of the current diversion, so that only the required portion of current may be directed to the dump load keeping the battery at the desired voltage. Westwind use this type of battery charge controller on their range of turbines up to 20kW capacity. 8-12 Figure 8-11 Block diagram of shunt regulation using a resistive load 8.6 Wind and Diesel Hybrid Mini Grids Often electricity utilities deem it uneconomical to extend grids through sparsely populated areas to isolated towns or villages. Therefore many remote communities and cities rely on diesel generating sets for their primary means of electricity generation. A genset is comprised of a stationary diesel engine coupled to a synchronous generator and a governing system to control the voltage and frequency of the generated power. Diesel power stations generally have a number of different capacity gensets that may be operated either individually or in parallel to meet the demand on the local grid. The typical capacity of village or small city scale gensets may be between 100kW-2MW. station and clearly offer small fuel savings hence modest cost reductions. High penetrations, on the other hand, have the potential to offer far greater savings and the promise of dramatic cost reductions, especially in areas of high mean wind speeds. Higher penetrations also require control systems with higher degree of control for both the diesel station and the turbines(s). In periods of high wind, it is theoretically possible for the wind turbine(s) to completely support the load on the grid, and for the diesels to be shut down. Selection of an appropriate capacity genset to meet a load can be a relatively complex task, especially if the load is variable, since gensets only operate efficiently and reliably when relatively well loaded. When the fluctuating power from a wind turbine is input into the system the task becomes even more difficult. Therefore wind turbines that are chosen for modern wind/diesel grids are usually pitch-controlled units where the level of power generated may be closely and accurately controlled. In times where excess wind power is generated some systems use this surplus for other processes such as water desalination, space heating etc. Some sophisticated systems use high-speed flywheels to store surplus energy that may be drawn while spinning reserve is being brought online. Western Power has such a system at Denham on the mid-west Australian coast. Figure 8-13 The working principle of a wind-diesel system Figure 8-12 Installation of a 640kW Diesel Genset (Source: http://www.cummins.com) Fuel costs in remote towns are often relatively high, reflecting the additional transportation costs involved. The relatively high cost of diesel-generated electricity, makes...
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