How to create a three-phase variable load using Simscape

Hi, guys!

I have built a distribution network model in Simulink, where all loads are configured as Three-Phase Parallel RLC Load with the constant PQ mode enabled.

Now I want to replace the load at one of the nodes (purple area) with a variable load whose P and Q values can be modified during the simulation. I initially tried using the Dynamic Load block from Simscape with the External PQ Control option enabled, but this resulted in extremely abnormal voltage waveforms at the bus 23（with significant noise？）. What exactly is causing this issue? Alternatively, are there other methods in Simscape to achieve the functionality I need?

I have also seen others mention the Variable Resistor block, but I have no idea how to connect it into my existing circuit. Or do I need to create a custom block?

bus 23 voltage

dynamic load （positive sequence voltage、P、Q）

Answer 1

[Satyam]

Hi,

You can implement a variable three-phase load in Simscape, but the behavior you observed with the Dynamic Load block usually comes from how the PQ control interacts with the network solver and voltage measurement. You can try these troubleshooting steps to fix the issue:

• The Dynamic Load block with External PQ Control expects stable positive-sequence voltage and properly scaled P/Q inputs. If the P and Q signals change abruptly or are not filtered, the internal control loop may introduce oscillations or noisy voltage at the bus. Adding a small first-order filter or ramp on the P/Q inputs usually stabilizes the response.
• Also verify that the positive-sequence voltage input to the Dynamic Load is measured correctly (typically using a Three-Phase V-I Measurement block followed by sequence extraction if required). Incorrect sequence signals can produce unrealistic waveforms.
• If you simply need a load whose P and Q vary during simulation, another straightforward approach is to keep using the Three-Phase Parallel RLC Load block and programmatically update parameters (R, L, C) using input signals or controlled components so that the equivalent P and Q change with time.
• Another option is to build a controlled impedance load using blocks such as Variable Resistor, Variable Inductor, and Variable Capacitor from Simscape Electrical. These can be connected in a three-phase configuration (e.g., Y-connected per phase) and their control inputs driven by signals derived from the desired P/Q profile. You can refer to the documentation of variable resistor to know more about its usage: https://www.mathworks.com/help/simscape/ref/variableresistor.html