Neutral to earth voltage strongly depends on the neutral current and the relevant circuit impedance, therefore reduction of load unbalance and increasing of neutral path impedance in addition of interval earthed neutral point increasing in system can be reduced it.
A load unbalance is a common feature in a power system, and can be the result of one or more of the following:
-A higher neutral current due to unequally distributed single-phase loads.
-Saturation of power transformers as a result of periodic overloading and load rejections.
-Increased ripples in the rectifier circuits, causing harmonics.
-Malfunctioning of some equipment, possibly because of a fault.
-Oscillating torque in the rotating machines as a result of load variations and harmonics present in the system.
- Feeding non-linear loads such as:
. Induction furnaces
. Arc furnaces and arc welders
. Steel rolling mills
. Large motors with periodic loading
. Thyristor drives
. Railway traction which is mostly through d.c.
. And many loads which may have to be frequently switched
All such loads generate harmonics and cause variations in the fundamental power frequency of the supply system which leads to distortion in the sinusoidal waveform of the voltage. This distortion may affect the quality of the supply system (voltage) beyond desirable limits. A non-sinusoidal and distorted supply system may adversely affect the different loads connected on the system, besides leading to outage of the system itself.
Also medium voltage networks, located between transmission lines and customers at low voltage, need to be as available as possible and as simple as possible to maintain. A vast number of public utilities adopted the concept of isolated network operation i.e. the neutral points of all transformers and eventual generators were held isolated from earth. Along with the rise of neutral to earth voltage due to ground capacitance unbalance of the sound network, rise of the neutral to earth voltage can be the consequence of high voltage (HV) system unbalance, Therefore, neutral to earth voltage can rise from zero value to a considerable value.
The neutral voltage of MV isolated network in normal operation strongly depends on the network structure i.e. the share of overhead lines due to their ground capacitance phase unbalance. In fault condition, the value of the earth-fault current does not depend on fault location and is determined solely by the overall galvanically connected MV network's earth capacitance and leakage. The earth-fault current consists of a relatively large capacitive component and a relatively small active component.