Three phase voltage source in series with RL branch - the phase to phase voltage (Vrms) is rated voltage x 1.02.

The phase to phase voltage (Vrms) is rated voltage x 1.02 for the Three phase voltage source block in series with RL branch inside Simscape library in Matlab Simulink. I am not able to explain the 1.02 factor. For RMS voltage we usually multiply by sqrt(3). Can the community please help me with this. Thanks and Regards. Aparna.

https://in.mathworks.com/help/releases/R2024b/sps/ug/250-kw-grid-connected-pv-array.html

The question is in relation to the example on 250kW grid connected PV array.

[Aparna Das]

Thank you Shivam Gothi for answering my question.

Answer 1

[Shivam Gothi]

Hello @Aparna Das,

As a summary, the simulink model "250-kW Grid-Connected PV Array" simulates a PV array connected to the grid via three phase converter. In order to solve a power system network with two or more generating sources, it is necessary to make one of the generator busses to be a "slack bus" or "swing bus". The voltage and the electrical angle of this bus needs to be fixed. All the other quantities, such as angles of different busses are calculated with respect to the angle of slack bus.

In this model, the "Three phase voltage source block in series with RL branch" is taken as a "swing bus". This can be verified by navigating to the "load flow" tab found inside the "block parameters" as shown below:

Therefore, we need to provide the bus angle (which is set to zero degrees in "phase angle of phase A (degrees)" as seen in above image) and also the magnitude of bus voltage. The bus voltage is generally expressed in per-unit (PU) system. Here, the voltage of the grid is set to "1.02 PU". Therefore, the field "phase-to-phase voltage (Vrms)" is filled with a value of "1.02*120e3", where "120e3" is the nominal line-line RMS voltage ("Base voltage") and "1.02" denotes the PU value.

Generally, the grid voltage is not fixed. It may vary from its base value, and permissible variation allowed in grid voltage magnitude is roughly 5 % of its rated/base value.

Therefore, in the simulation, you can change the "phase-to-phase voltage (Vrms)" within "1.05*120e3" to "0.95*120e3" in order to simulate a realistic grid. However during under-voltages, the grid voltage drops below the permissible value.

As a conclusion, the "1.02" factor is not a scaling factor to convert the phase voltage to line voltage, but it represents the magnetude of bus voltage is PU (per-unit). In simple words, it says that the magnitude of bus voltage is 2% more than its Base value.

I hope this helps !