In the new VDE-AR-N 4110, grid operators and energy suppliers sometimes place contradictory requirements on the system operator for mixed systems. Christian Wiedemann, Head of Product Management at KBR GmbH, explains the problem.
VDE-AR-N 4110 contains the technical rules for the connection of customer installations to the medium-voltage grid and their operation. Many grid operators have adopted this user rule as their “Technical Connection Conditions” (TAB) and adapted it with their own additions where necessary. A closer look at the “Reactive power” section of the current user rule reveals that some points have been changed compared to the previous version. The background to this is the changed reactive power behavior of modern machines and systems.
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Reference plants
For pure supply systems, the user rule has been adapted as follows:
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Capacitive reactive power consumption is generally not permitted.
- The displacement factor cos phi has increased from inductive 0.90 to 0.95.
- If a subscriber cannot comply with these values, he must install a sufficiently dimensioned system to compensate for the reactive power.
- The measured values are to be determined as 15-minute averages.
In practice, most customer connections are still operated according to the invalid previous version. However, as the technical connection conditions (TAB) are binding, companies are requested in writing by their grid operator to comply with the current specifications in the area of reactive power – because the grids are becoming increasingly capacitive. This is due to the increase in cable capacities and the decline in large power plants that were previously able to provide reactive power.
There are building connections, for example on office buildings, which absorb capacitive reactive power due to their consumers – which is no longer permissible. The displacement factor cos phi of 0.95 is also only partially complied with, if reactive power compensation is installed at all. The reason for this is that some suppliers no longer charge for the additional reactive energy and the compensation systems have been switched off. Other suppliers, on the other hand, continue to charge the reactive energy via the electricity bill, but only when the cos phi has fallen below the monthly average value of 0.9 instead of the currently required value of 0.95.
Another requirement is that the reactive power must be recorded in 15-minute averages as a function of the active power. The energy supplier is currently neither billed nor compensated via the reactive current compensation systems. This is because the cos phi is still billed using the monthly average value. However, the grid operator assesses the power consumption of the customer connections using individual 15-minute average values. It would therefore be necessary to change the reactive power control in the customer system.
Mixing plants
It becomes more complicated and almost impossible to solve in the case of mixed systems consisting of a supply and a generation system:
Here, the requirements of the market roles of grid operator and energy supplier must be fulfilled. Initially, the same requirements for reactive power apply as for pure supply systems. The grid operator specifies one or more methods for providing reactive power to the generation plant at the grid connection point. This can be the reactive power-voltage characteristic Q (U) or the reactive power characteristic in relation to the generated active power Q (P). It should be noted that the generation system must provide the reactive power at the transfer point to the customer system. The energy supplier’s meter is installed there and measures the active and reactive energy for energy consumption and feed-back.
However, there is an undesirable interaction between the existing reactive current compensation of the reference system and the reactive power provision of the generation system: The inductive reactive power consumed by the consumers of the reference system is compensated by the reactive current compensation – as prescribed by VDE AR-N 4110. Regulation is carried out in the same way as for pure reference systems. This prevents the energy supplier from charging for reactive energy.
However, the reactive current compensation also recognizes the reactive power caused by the generation system at its control point and also compensates for this. This means that the dynamic grid support prescribed by the grid operator is no longer required. This interaction continues until the inverter absorbs more reactive power than the reactive current compensation can compensate for, which may be at the expense of active power generation. From a technical point of view, this is a senseless oscillation of the reactive power which places an additional load on both parts of the system. To prevent the oscillation, the control point of the reactive current compensation must be shifted, which is usually impossible in existing switchgear.
Squaring the circle
Another effect: The energy supplier’s meter at the connection point also measures the reactive power consumed by the generating plant and required by the grid operator and charges this to the connection subscriber. Decommissioning the reactive current compensation has the same effect – the additional inductive reactive energy is charged via the electricity bill. In other words: We are talking about squaring the circle here.
VDE-AR-N 4110 goes on to say: “If reactive energy is offset, which is influenced by the generation system, this requires coordination between the grid operator and the system operator. On the one hand, the reactive power mode of the generation plant must not be at the expense of the reference plant and, on the other hand, the reactive power mode of the generation plant must be controllable at the grid connection point.” The role of the energy supplier is not addressed here. The energy supplier receives the metering data from the meter operator on a monthly basis and bills the energy – regardless of who caused the additional reactive energy. There is currently no generally applicable procedure – grid operators, energy suppliers and system operators are on their own in this area of conflict.
A way out of the dilemma
As has been practiced for years in the case of plant transmission with active energy, the reactive energy of the generation plant could be balanced in mixed plants and thus deducted. To do this, meters with 4-quadrant measurement would have to be used in the generation plant instead of the usual 2-quadrant meters. By installing an additional current transformer in the generation system and a summation current transformer, reactive current compensation can also perform this balancing and only compensate for the pure reference reactive power. The reactive power required by the grid operator for grid stability can thus be made available at the grid connection point and the undesirable interaction between the inverter and reactive current compensation is avoided.
Another solution would be intelligent reactive power control for the reference system. The company KBR from Schwabach has made initial approaches to this. In the customer system, the pure reactive power of the reference system could be regulated independently of the system configuration. All in all, generally applicable rules must be defined as to how the reactive power of the supply and generation system can be made available without the connection subscriber being disadvantaged by reactive energy billing.