Earthing Effects with Unbalanced Load

This example shows the effects of three different types of earthing connections on network voltages and currents.

In each three-phase network, an AC voltage source drives a resistive load. The load is modeled as subsystem that contains two Variable Resistor blocks from the Simscape™ Electrical™ Passive library.

The earthing connection in each network is modeled using one of these blocks from the Simscape Electrical Connections library: * Grounded Neutral * Floating Neutral * Open Circuit

Initially the load is balanced. At 0.05 s, the resistance on phase-a increases from 1 to 3 Ohms. At 0.07 s, the resistance on phase-c increases from 1 to 1.5 Ohms. The resistance on phase-b does not change during the simulation.

Model

Simulation Results from Simscape Logging

The plots below show the line voltage, phase voltages, and phase currents flowing through an unbalanced three-phase resistive load when it is connected to different earthing options.

Notice that until 0.05 seconds there is no difference between grounded neutral and floating neutral. At 0.05 seconds, the load becomes unbalanced and the currents flowing through it change in both the scenarios. Due to the load being effectively connected to ground, the phase voltage with a grounded neutral remains unaffected throughout the whole simulation.

In the floating neutral scenario, the phase voltage changes significantly as the load becomes unbalanced, accordingly to the different resistance values in each phase. In this case, if a system operator only monitors line-voltages, undesirable overvoltages could occur on individual phases without being observed. This could lead to overheating, accelerated breakdown of insulation, or other system problems.