Effects of unbalanced Electrical Load (Part:1)

EFFECTS OF UNBALANCED ELECTRICAL LOAD (PART:1)

INTRODUCTION:

Generally, three phase balance is the ideal situation for a power system and quality of delivered Electrical Power. However Voltage unbalance may makes worse effect on Power quality of Electrical Power at distribution level. * The voltages are quite well balanced at the generator and transmission levels. but the voltages at the utilization level can become unbalanced due to the unequal system impedances, the unequal distribution of single phase loads, asymmetrical three-phase equipment and devices (such as three-phase transformers with open star-open delta connections), unbalanced faults, bad connections to electrical connectors. * An excessive level of voltage unbalance can have serious impacts on power quality. In the system the level of current unbalance is several times the level of voltage unbalance. Such an unbalance in the line currents can lead to excessive line losses, losses in the stator and rotor of Motor Malfunctioning of Relay, unsymmetrical measuring of Meters. Voltage unbalance also has an impact on ac variable speed drive systems where the front end converter consists of three-phase rectifier systems * Phase balancing is very important and usable to reduce distribution feeder losses and Improve system stability and security

WHAT IS UNBALANCE VOLTAGE

Any deviation in voltage and current waveform from perfect sinusoidal, in terms of magnitude or phase shift is termed as unbalance * In ideal conditions the phases of power supply are 120 degree apart in terms of phase angle and magnitude of their peaks should be same. On distribution level, the load imperfections cause current unbalance which travel to transformer and cause unbalance in the three phase voltage. Even minor unbalance in the voltage at transformer level disturbs the current waveform significantly on all the loads connected to it * If three phase voltages have the same magnitude and are in exactly 120deg phase displacement, then the Three-phase voltage is called balanced, otherwise, it is unbalanced. * There are no negative- and zero-sequence voltages in a balanced system, only positive-sequence components of balanced three-phase voltage exist. On the contrary, if the system is unbalanced, negative-sequence components or zero-sequence components or both may exist in the system.

CAUSES OF UNBALANCE VOLTAGE

Switching of three phase heavy loads results in current and voltage surges which cause unbalance in the system. * Unequal impedances in the power transmission or distribution system cause differentiating current in three phases. * Any large single phase load, or a number of small loads connected to only one phase cause more current to flow from that particular phase causing voltage drop on line * With continuous operation of motor’s in various environment cause degradation of rotor and stator windings. This degradation is usually different in different phases, affecting both the magnitude and phase angel of current waveform * A three phase equipment such as induction motor and Transformer with unbalance in its windings. If the reactance of three phases is not same, it will result in varying current flowing in three phases and give out system unbalance. * A current leakage from any phase through bearings or motor body provides floating earth at times, causing fluctuating current. * Unbalanced incoming utility supply * Unequal transformer taps settings * Large single phase distribution transformer on the system * Open phase on the primary of a 3 phase transformer on the distribution system * Faults or grounds in the power transformer * Open delta connected transformer banks * A blown fuse on a 3 phase bank of power factor improvement capacitors * Unequal impedance in conductors of power supply wiring * Unbalanced distribution of single phase loads such as lighting * Heavy reactive single phase loads such as welders

HOW TO CALCULATE UNBALANCE

%voltage unbalance= 100x (maximum deviation from average voltage) / (average voltage) * Example: With phase-to-phase voltages of The System is 430V, 435V, and 400V. * The average Voltage=(430+435+400)/3=421V. * The maximum Voltage deviation from Average Voltage=435-421=14V * %voltage unbalance=14×100/421=3.32% * The permissible limit in terms of percentage of negative phase sequence current over positive sequence current is 1.3% ideally but acceptable up to 2%.

EFFECTS OF UNBALANCE VOLTAGE ON SYSTEM AND EQUIPMENT:

The factors for voltage unbalances can be classified into two categories: normal factors and abnormal factors. * Voltage imbalances due to normal factors, such as single-phase loads and three-phase transformer banks with open star-open delta connections, can generally be reduced by properly designing the system and installing suitable equipment and devices. * Abnormal factors include series and s…