The Impact of Distributed Energy Resources (DERs) on Automatic Voltage Control and OLTC Assets
Fundamentals Ltd
Total Voltage Control - An ecosystem of solutions to deliver end-to-end voltage compliance for a low carbon grid.
The dynamics of the electricity grid have changed significantly over the years due to increasing numbers of DER (distributed energy resource) connections.
Whereas the grid was designed and built for uni-directional power flows from bulk generation sites at the ‘top’ of the network down to consumers at the ‘bottom’, we now have bi-directional flows dependent on the output of DER. This will vary dynamically between two extremes: low DER output/high consumer load, and high DER output/low consumer load where power flows from the ‘bottom’ to the ‘top’ (reverse power flow). An example of how dynamic the grid can be is shown in the data below for a twin-transformer substation which has a large solar farm connected downstream on one of the outgoing 33 kV feeders. Both transformers can be seen to experience reverse power flows at times of high solar output over the course of a sunny day. The effect of cloud cover is clearly shown.
As a result of dynamically varying magnitudes of load seen by the transformers, on-load tapchangers (OLTCs) can be operated more frequently by existing automatic voltage control (AVC) schemes. The amount of switched power during an OLTC varies with the amount of power flowing through the transformer, an example below shows two different transition impedance values and the resulting switched power. Network owners/operators can expect to see more operations and differences in contact wear patterns.
Some OLTCs have reduced switching capacities for reverse power flow and network owners/operators are understandably concerned about this, often imposing operational restrictions or even very costly transformer/OLTC replacements without knowing the true capability of the existing units.
We help utilities determine a safe level of reverse power capability for OLTCs which can reduce the need for or even eliminate restrictions and replacements.
The reverse power capability can be calculated from the application data, e.g. nameplate information from the transformer and OLTC. Additionally, if the OLTC is controlled by a sufficiently advanced relay (such as the SuperTAPP SG), reverse power blocking can be applied in settings to match the calculated capability. An example OLTC power capability assessment is shown in the graph below.
To conclude, it is possible to obviate severe operational constraints and even expensive OLTC replacements by a simple calculation based on application data.
Please contact Fundamentals for help and advice.