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Various energy sources generate electrical energy in a form that cannot simply be injected into the electricity network, including photovoltaic panels, microturbines, batteries and fuel cells. These sources generate either DC voltage or AC voltage with variable frequency and/or an amplitude or voltage not compliant with the electricity network. To connect such sources to the electricity network nevertheless, a power electronic inverter or transformer must be used.
The transformer converts a DC voltage into an AC voltage with adjustable frequency and amplitude. With suitable control, the transformer can ensure the local source supports the electricity supply network and actively assists in controlling voltage amplitude and frequency.
At present, “energy sources connected to the electricity network” may not act in voltage or frequency control on the network, and they must even be disconnected from the network in the case of network failures or problems. This is not a problem in a network where only a very small percentage of the power generated is supplied by local sources and the rest is supplied by large power stations. Future perspectives for the electricity sector do, however, foresee a strong increase in the number of local sources, often of a renewable nature. If a large share of the energy sources is connected locally (or distributed), it may appear beneficial or even necessary to involve these sources in controlling the mains voltage and possibly frequency.
Local energy sources connected to the network with a transformer currently act as power sources. The mains voltage is regarded as a constant, and the transformer injects a certain power into the network (depending on how much power the primary energy source - sun, wind, etc. - is generating at the time). So, a power source only injects power into the network and is not involved in supporting the network: the transformer will not adapt its output power based on the value of the network voltage amplitude or frequency, but only based on the available primary energy. It is possible to offer a form of network support with these types of sources. This is usually done through a central controller that adapts the reference values for the output current of the different converters, for example to force up the local voltage amplitude a little. This requires external communication between the different converters.
As mentioned earlier, it may be advisable in future to involve local energy sources in network control. An example of a control strategy for power electronic converters, allowing involvement in voltage and frequency control, is droop control. This method is already used for controlling the power produced by the large central generators on the network.
Droop control can be explained as follows. The electricity network has a nominal frequency of 50Hz. When a large load is added to the network, the sources connected to the network must generate more power. This does not take place immediately, and the difference between the required and generated power is temporarily accommodated by the kinetic energy of the central generators (which work with synchronous generators). The rotor of a synchronous generator has a certain mass, and when this rotates at a certain speed (for example 1,500 rpm for a synchronous machine with two pole pairs connected to a 50Hz network), the rotor has a certain kinetic energy determined as I.ω2, with I the moment of inertia in kg.m2 and ω the rotational speed in rad/s. When the power required by the consumers is greater than the power generated by the sources, a part of the kinetic energy of the rotating generators is used to compensate for this difference and the generators will run more slowly. As a result, the frequency of the mains voltage will also drop.
Figure 1: Principle of frequency droop control
To counteract frequency fluctuations on the network, droop control is used with a number of large central generators. These generators adapt their power output based on the frequency according to the diagram shown in Figure 1. When, for example, the frequency is lower than 50Hz, the generators generate more to eliminate the imbalance between the power generated and consumed on the network. This takes place using the active power, as the transmission network has a mainly inductive network impedance. On such a network, the frequency is particularly influenced by the active power and the voltage amplitude by the reactive power.
Just as for the frequency, droop control can also therefore be developed for the voltage amplitude: if the voltage is too low, the reactive power generated is adapted so the voltage drop is counteracted.
Such droop control can also be used for controlling a power electronic transformer. This will then adapt the active or reactive power exchanged with the network based on the value of the voltage amplitude or frequency that the transformer detects. Account must be taken of the fact that small-scale local sources are connected to the low or medium voltage network where the network impedance is mainly resistive. As a result, the voltage amplitude will be primarily influenced by the active power, and the frequency by the reactive power.
An important advantage of droop control is that this can be simply applied in parallel for a number of sources. Indeed, no external communication is required between the different network-connected converters - the converters base their control action (the adaptation of their output power) on the measurement of the frequency and voltage amplitude.
Different strategies exist to have a transformer exchange a certain required active and reactive power with the network. A reference can be used for the output current or output voltage of the transformer. A mathematical model is often drawn up of the system transformer network (with or without output filter) with which the output voltage the transformer must supply can be calculated to exchange a certain current (or therefore a certain power) with the network. This can be done quite simply with PI controllers, for example. Another option is to develop an extensive model in which an estimate is made of what the situation of the network will be during the next sample period, and on this basis the optimal reference value for the output voltage (or current) of the transformer is determined. A compromise must always be sought between the quality of the control (fast control of voltage, current and power to their desired values, robustness of the control - if the parameters vary from their set values) and the arithmetic complexity of the control algorithm (which determines the load for the computer, microcontroller or DSP).
Article courtesy: Electrical engineering department (ESAT)-ELECTA, KU Leuven
Author: Tom Loix