Split Model
Performance of Load Flow algorithms are (to varying degrees) dependent on the amount of network that has to be solved. Typically the minimum unit is the electrical islands - all objects that are currently electrically connected to each other without open switches.
To limit the performance impact of the power flow engine, customers that model transmission and sub transmission should consider using model splits. These introduce artificial breaks between sections of the network. At these locations a "virtual load" is introduced on the high side and a "virtual source" is introduced on the low side. For outage management, energization state of the "virtual load" is translated into on/off status for the "virtual source". Similarly for load flow applications, the calculated power flow at the "virtual source" is propagated across as the dynamic load on the "virtual load".
All the NMS applications support seamless integration across model splits.
The advantage of model splits is that they localize the extent of network that needs to be resolved and significantly speed up the applications. The disadvantage is that there is some loss of accuracy in the Load Flow solutions. However, with practical considerations these inaccuracies should be insignificant.
Whilst model splits can be placed at any location, it is recommended that they are placed at the high side of the power transformers in the substations. The reason for selecting this location is:
1. These transformers usually have Load Tap Changing functionality - that way the voltage at the low side bus can be accurately modeled and is somewhat independent of the upstream losses and voltage drops. It often has the benefit of being telemetered and can actually improve power flow accuracy on the distribution feeders.
2. Optimization applications will still be able to consider the LTC as part of the island encompassing the downstream feeders where optimization objects are typically targeted. If the split were below the transformer then it would not form part of the island being optimized.
3. The short-circuit impedances for bus faults are usually well known and readily obtainable from the planning systems. These will be translated into the impedances for the "virtual sources" at the split.