Classic reactive current compensation, in which capacitors are used to compensate for the inductive reactive current, has been state of the art in trade and industry for decades. The compensation of reactive current in mixed systems is new and therefore topical. A certain amount of expert knowledge is required to comply with the applicable requirements of grid operators and energy suppliers. This is because the technical implementation that needs to be mastered is complex.
Supply, production and mixing system
We are all familiar with the classic supply system. This is a system in which the electrical energy is drawn exclusively from the grid of the grid operator. The technical connection conditions (TAB) of the grid operator clearly describe the reactive power behavior of this system. The reactive current is compensated here with a classic compensation system. The measuring point (installation location of the current transformer) for the regulation is always located in the feed-in field of the low-voltage distribution system.
[Reference system with reactive current compensation]
Generating plants have been known for a number of years. These are systems in which energy is generated and fed into the energy grid (e.g. hydropower, biogas, wind power or PV systems). These are power plants that have to provide reactive power for static voltage maintenance from a total active power of 135 kW. This is intended to keep slow voltage changes in the distribution grid within acceptable limits. The reactive power required for this (inductive and capacitive) is usually provided by the inverters or generators of the generation plants. The grid operator specifies one of four methods for this:
- Reactive power voltage characteristic Q(U)
- Reactive power characteristic curve as a function of the active power Q(P)
- Reactive power with voltage limiting function
- Displacement factor cosφ
The reactive power must be made available at the connection point of the generating plant to the grid operator’s distribution grid. Certified EZA or parking controllers must be used for this purpose.
[Generating plant]
Many mixed systems are currently being created due to the increased retrofitting of buildings with generation systems.
These consist of a supply system and one or more generation system(s). In this case, the same requirements apply to the reactive current as for individual supply and generation systems – all requirements together. A few special features must therefore be observed with regard to reactive current compensation. Otherwise, one compensation measure cancels out the other, which would make the energy grid more unstable.
[Mixing system with reactive current compensation for the reference system]
In mixed systems, the reactive current of the reference system (and only this) must be further compensated. While the cosφ in low-voltage systems must be between 0.9 ind. and 0.9 kap., the limits for medium-voltage systems are between 0.95 ind. and 1. At the same time, the generating system must contribute to the static voltage stability of the grid by providing inductive or capacitive reactive power at the grid connection point. This is stated in the VDE user rule and in the TAB of the grid operators.
Reactive current compensation in mixing systems
As we can see from Figure [Mixing system with reactive current compensation for the reference system], the control of the reactive current compensation of the reference system also detects the reactive current of the generating system due to the installation location of the current transformer and attempts to compensate for this as well. This would cancel out the static voltage maintenance, which must not happen under any circumstances. The solution is to record the static reactive current of the generating system(s) and to consider it correctly by the reactive power controller of the reference system. Theoretically, this can be achieved using a simple current transformer in the generating system and a summation current transformer. In practice, there are usually several generating plants, which are also located far away from the low-voltage distribution board. This results in long distances for the current transformer cables, which leads to “overloading” and therefore to high measurement errors.
[Mixing system with correct reactive current compensation of the reference system]
The key to the solution is the intelligent and modular multicomp-D6 reactive power controller from KBR. With the help of the multimess D4 measuring sensors, which are installed in the individual generation systems, it can continuously calculate the required reactive current of the reference system and compensate for it in accordance with the standards.
In addition, the inductive or capacitive reactive power provided by the generation systems is available at the connection point to the grid operator’s grid and can contribute to voltage stability. Communication with the reactive power controller takes place via an RS 485 field bus.
Our sales engineers will also be happy to help you with complex designs of reactive current compensation measures and special solutions – via web meeting or on site.