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user_guide_cosimulation_details.md

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T&D co-simulation with PSS/E and OpenDSS

This section provides a brief overview of T & D co-simulation with PSSE and OpenDSS as implemented in TDcoSim.

Advantages

The state-of-the-art approach of studying DER impact on the bulk power system entails developing aggregate positive-sequence load and DER model for distribution systems and then applying these models in conventional positive-sequence transmission system simulation software such as PSSE, PSLF, etc.

TDcoSim offers higher degree of fidelity since there is no need to parameterize the aggregate load and DER models for distribution system, which are only approximate representation of actual distribution system behaviors,

TDcoSim also offers time savings by running an integrated T&D co-simulation compared to the approaches of running separate simulations for T&D systems and manually combining the results.

Assumptions in current software version

  1. It is assumed that all the distribution feeders connected to the transmission system load bus have the same characteristics. Hence only one distribution feeder is simulated in an OpenDSS instance. The power output from the single feeder is then multiplied with a scaling factor (calculated automatically by TDcoSim) so that we can match the rated load at the transmission bus with the total load from all the feeders connected to the bus.
  2. No Sub-station is explicity added to the distribution network model by TDcoSim to interface the T bus with the distribution feeder. However if the user provided distribution network model comes with a sub-station, then it is used.

T&D interface

The transmission system simulator (TSS) and distribution system simulator (DSS) are separate programs with their own solution methods. TDcoSim is responsible for exchanging data and synchronizing their runs.

Data exchange

The TSS, PSSE uses positive sequence quantities while the DSS, OpenDSS uses phase quantities. Hence it is necessary to convert positive sequence quantities to equivalent phase quantities and vice-versa. After PSSE completes a solution, it outputs the sequence voltages at the T&D interface. Then (1) is applied to convert the sequence voltages at the boundary bus to phase voltages. Using the phase voltages at the boundary bus, the DSS completes a solution and outputs the phase current injection at the boundary bus, which are expressed in (2).

The phase current injection at the boundary bus is converted to sequence quantities using (3). The current injection at the boundary bus is then used to obtain complex power injection at the boundary bus using (4).

The obtained value of S_TS,+ is used as the total power requirement for the said load bus at the transmission system. It is worth mentioning that the equivalent load that is replaced with the distribution system simulator is modeled as a constant power load with respect to the transmission system simulator. The data exchange across the T&D interface is illustrated in Fig. 1 where the loosely-coupled synchronization protocol is utilized (The synchronization protocols will be introduced in the next section).

loosely coupled protocol

Fig. 1. Loosely couple protocol for data exchange across T&D interface.

Synchronization

Both static and dynamic co-simulations starts with an initialization for both PSSE and OpenDSS software. The T&D interface contains sockets that enable simulators to communicate and exchange data. Specifically, the TSS and the DSS are synchronized through two protocols, namely, loosely-coupled and tightly-coupled protocols.

Loosely-coupled protocol

The loosely-coupled protocol is illustrated in Fig. 2. There is a one-step lag in information exchange between transmission and distribution, but less computation is required.

loosely coupled protocol

Fig. 2. Loosely-couple protocol.

Steady-state T&D co-simulation process

The data exchange protocol for steady-state co-simulation in the current version of the software may be tightly-coupled or loosely-coupled. The initialization steps are:

  1. Input case files (both PSSE and OpenDSS).
  2. Run power flow in PSSE.
  3. Get voltage (p.u.) at load buses from PSSE.
  4. Set OpenDSS VSource voltage to be equal to the voltage from the respective load bus.
  5. Run power flow in OpenDSS, get P and Q requirement for DNetworks.
  6. Scale P & Q using scaling factor and set as input to T bus.

The scaling factor (see assumptions) is calculated by dividing the total load at a transmission system load bus by the aggregated load of one distribution feeder.

At every time step (one half-cycle), the steps 2 to 6 are repeated untill the end of simulation.

Dynamic T&D co-simulation process

The data exchange protocol for dynamic co-simulation in the current version of the software is loosely coupled. The initialization steps are:

  1. Input case files (both PSSE and OpenDSS).
  2. Run power flow in PSSE.
  3. Get voltage (p.u.) at load buses from PSSE.
  4. Set OpenDSS VSource voltage to be equal to the voltage from the respective load bus.
  5. Run power flow in OpenDSS, get P and Q requirement for DNetworks.
  6. Compute difference of P & Q between TNetworkBus and DNetwork.
  7. Calculate compensating shunt value using .
  8. Add and as fixed compensating shunt in PSSE.

At every time step (one half-cycle), the steps 2 to 8 are repeated until the user specified simulation end time is reached.