Thursday, 21 February 2019

Effects Arises due to Voltage Unbalance

Unbalanced systems indicate the existence of a negative sequence that is harmful to all poly phase loads, especially three-phase induction machines.

The main effect of voltage unbalance is motor damage from excessive heat. Voltage unbalance can create a current unbalance 6 to 10 times the magnitude of voltage unbalance. In turn, current unbalance produces heat in the motor windings that degrades motor insulation causing cumulative and permanent damage to the motor. 

This scenario would result to expensive facility downtime due to motor failures.

The graph below shows the relationship between voltage unbalance and temperature rise, which approximately increases by twice the square of the percent of voltage unbalance.

  1. Increased current loading and losses in the network.
  2. With equal load power the phase currents can attain 2 to 3 times the value, the losses 2 to 6 times the value. It is then only possible to load lines and transformers with half or one third of their rated power.
  3. Increased losses and vibration moments in electrical machinery.
  4. The field built up by the negative sequence component of the currents runs against the phase sequence of the rotor and therefore induces currents in it, which lead to increased thermal loading.
  5. Rectifiers and inverters react to unbalance in the power supply with uncharacteristic harmonic currents.
  6. In three-phase systems with star connection, current flows through the neutral conductor.

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