Lazy feeders

For some years now, one has repeatedly recognised power electronic feeders, such as photovoltaic inverters, wind turbines and battery storage systems, which show a striking similarity in the current and voltage curve shape.

What is desirable and advantageous for purely resistive consumers – namely the pure purchase of active power without fundamental oscillation and harmonic reactive power – has an unfavourable effect on the network feedback in all voltage levels for feeders of electrical energy. Because in the classical sense, the current of the connected loads – and thus the ideal sinusoidal curve shape of the voltage – is the central control parameter. This applies to large synchronous generators in power plants as well as to generators in emergency power systems and the power electronics on the secondary side of UPS systems. A synchronous generator – even a synchronous motor – will react to deviations of the voltage from the sinusoidal form with a correspondingly opposite current feed. Due to this operating principle, in simplified terms, the classic power plant generators always provide the reactive power at the fundamental frequency and at the harmonics that are demanded by the loads. This is at least within the scope of the respective design-related possibilities.

If, however, a modern power electronic feeder largely follows the voltage curve with its current control, it optimises its own frequency-dependent reactive current, but at the same time denies the grid the reactive power required by the consumers. Thus, in the worst case, it even acts as an “amplifier” of the mains feedbacks prevailing in the network. Such effects static in the frequency range and dynamic in the transient range are becoming more and more frequent.

Now, this frequency-dependent reactive power can also be provided by active, passive or hybrid filter systems. However, these possibilities already exist in purely physical terms in the designated feed-in plants. Put simply, all that is missing for this is the corresponding control parameter in the control system of such systems.

This calls for a rethink in approval and standardisation. It is of little help if the relevant standards demand the lowest possible current harmonics from feeders. It would be more helpful to promote such systems that, in addition to supporting the voltage, also participate effectively in maintaining the shape of the sine wave. Just like the good old synchronous generator in the power plant.