Fault Ride Through Effects on Alternators Connected to the Grid

Traditionally, industrial countries have generated most of their electricity in large centralised facilities, such as fossil fuel (coal, gas), nuclear, large solar power plants or hydropower plants. Although these plants have excellent economies of scale, they usually transmit electricity over long distances and negatively affect the environment. More recently, a surge in the concerns over climate change has led to a modification of energy policies so as to facilitate energy to be produced and consumed in an eco‐friendly manner. These changes have brought about an increase in what is now called distributed generation. Distributed generation also seems to fit in well with being able to accommodate a grid architecture with renewables. While distributed generation plants have low maintenance, low pollution and high efficiencies, they have a tendency to make the grid unstable. It is for this reason that a number of grid operators around the world have begun enforcing performance expectations on generating sets. These expectations – called ‘Grid Codes’ exist primarily to ensure stable & continuous operation of power systems. The most challenging aspect of grid codes is the Fault Ride Through / Low Voltage Ride Through requirements. Table 1 and Figure 1 summarise the key requirements of the German grid operator – E.ON [1] and will be the focus of this paper. Low Voltage Ride Through refers to an event when the voltage at the point of common coupling drops below a critical value. Earlier, power plants could disconnect from the grid in the event of a LVRT; but grid codes require that power plants stay connected to the grid for period of time during a LVRT event – without going unstable. The situation is worsened because of the static requirements that grid codes demand. These include continuously operating the power plant, and hence the generating set at an underexcited power factor, while providing rated load at lower than nominal point of common coupling voltage. In this paper, the authors explain the effect of a LVRT on an engine driven alternator connected to the grid and hence explain the challenges that Cummins Generator Technologies as an alternator manufacturer has faced while designing alternators for such applications.