Effect Of Unbalanced 3-Phase Supply To Laboratory Equipment And Remedy

ABSTRACT

Current and voltage unbalance denigrate laboratory equipment performance. The current unbalance reduces efficiency, productivity and profits at the generation, transmission and distribution of electric energy. Voltage unbalance reduces efficiency, productivity and profits at the consumption/utilization level.

Therefore, the study provides a comprehensive compilation of all possible information on current and voltage unbalance in the electrical laboratory equipment.

This work discussed information on sources of unbalance three phase supplies, its propagation, negative effects upon laboratory equipment and possible remedies are compiled, discussed and analyzed in this thesis.

Due to the economics and efficiency of distribution and load diversity such as three-phase utilization, unbalance current and voltage is an inherent feature in the distribution system. Therefore it has to be mitigated. The thesis discusses methods aimed at reducing the current and voltage unbalance in the distribution system.

 CHAPTER ONE

       1.0                                                      INTRODUCTION

Electronic control systems are the foundation of modern, efficient industrial processes. The reliability and performance of instruments, actuators, sensors, electric motors, and relays can all be affected by the quality of power supplied to them.

Programmable logic controllers (PLC) that unexpectedly experience faults, variable speed drives (VSD) that trip, motors that tend to overheat, as well as erroneous sensor signals can interrupt and shutdown industrial processes. This can lead to product scrapping, major restart delays, customer dissatisfaction, significant costs to business, and the resultant losses.

Any deviation in voltage and current waveform from perfect sinusoidal, in terms of magnitude or phase shift is termed as unbalance. In ideal conditions i.e. with only linear loads connected to the system, 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. Not only in the distribution side but through the transformer, voltage unbalances disturbs the high voltage power system as well.

                       1.1                                            BACKGROUND OF THE STUDY

The economic benefits of energy providers and its users are strongly dependent on the supply reliability, security and efficiency of the power system and consequently, on the supply quality and the loading quality. For instance, negative sequence current increases energy loss at delivery. The negative sequence voltage causes temperature increase of the induction motors. Also, there are other negative effects of the voltage and current asymmetries. Since the voltage and current unbalance phase supply causes various negative effects in power systems, these effects are the subject of our concern and investigations.

The three categories of asymmetry that contribute to the negative effect of unbalance phase supply on the power system are: current unbalance, voltage unbalance and the simultaneous occurrence of both current and voltage asymmetry.

Voltage asymmetry reduces efficiency, productivity and profits at the consumption/utilization level. It contributes to a reverse magnetic field, increases the temperature of windings, reduces output torque and increases the slip of rotating machinery. According to ref. [17] and [18] the effect of voltage unbalance on a three-phase induction motor operating at rated load will cause an increase in losses, increase in the temperature of the windings, reduction of life expectancy and reduce efficiency. For example, according to ref. [17], 1% voltage unbalance phase supply increases motor winding temperature from 120⁰C to 130⁰C with a I2R loss of 33% of the total losses and an efficiency reduction of 0.5%. Furthermore the life expectancy of the windings is reduced from 20 years to 10 years. However as the percent voltage unbalance increase so does the temperature of the motor. For instance at 4% voltage unbalance the winding temperature increase from 120⁰C to 160⁰C with a I2R loss of 40% of the total losses and the  efficiency reduce by 3-4%. At these  values the  life  expectancy  is  further reduced  to  1.25

years. As a consequence of this, motors should be de-rated (larger power rating) to compensate for the extra heating. However, this could increase the difficulty of relay coordination and therefore increase the cost of protection.

Current unbalance means that a negative sequence component occurs in the supply current. Such a component does not contribute to useful energy transmission, but to transmission of energy dissipated in power system equipment in the form of heat. As a result, the current unbalance reduces efficiency, productivity and profits at generation, transmission and distribution of electric energy. Consequently, the capacity of cables, transmission and distribution lines have to be selected based on the level of negative sequence current it will be subjected to during operations. Also the capacity of transformers and the efficiency of motors are reduced. In other words the negative sequence current increases losses in the cables, transmission and distribution lines, transformers and equipment on the power system ref. [14].

The negative sequence current causes voltage unbalance. For instance, the current unbalance caused by very large three-phase loads such as high speed traction systems and AC arc furnace contribute to dissimilar voltage drops on the balanced three-phases of the supply system and consequently, it produces voltage unbalance. For example, a situation is described, where a 350MW steam turbine generator supplies two 60MVA electric arc furnaces (EAF) through a three-mile 230 KV transmission line. As a result, the following sequence of events occurred: the generator had a cracked shaft near the turbine-end coupling, then there was two failures of the rotating portion of the brushless exciter and then while operating close to full load the generator’s exciter–end retaining ring of the rotor failed. This cost the company a significant amount of money and time to repair the generator.

Other negative effects occur at transient unbalance, mainly caused by faults in the power systems. Transient current unbalance occurs due to single-phase – line-to-ground faults and line-to-line faults etc. These are extreme levels of current unbalance that can last for only a few seconds but can lead to system instability and failure if not eliminated in time. Relays and circuit breakers remove the fault current before it exceeds the (iⁿ)²t characteristic of the devices and equipment connected. The operation of re-closers can produce transient unbalance which can result in nuisance tripping of relays. This is because the negative sequence setting has been exceeded due to the transient unbalance.

1.2                                                     AIM OF THE STUDY

Most technicians and plant engineers are aware of the effect of unbalanced voltage or current on laboratory equipment such as induction motors. The motor may generate excessive noise and motor torque and speed are negatively impacted. This study is to study the effect of unbalance three phase power supply and is remedy on electrical laboratory equipments.

1.3                                                PURPOSE OF THE STUDY

The purpose of this work is to study the effect, causes, mitigation of  phase unbalance in the distribution system of three phase power supply which is used in laboratory equipments.

1.4                                             OBJECTIVE OF THE PROJECT

At the end of this work, students involved will be able to understand the following:

1. The meaning of unbalance power supply
2. The effect of unbalance power supply on three phase electrical appliances
3. The remedy to unbalance three phase power supply
4. The sources of voltage and current unbalance
5. The propagation of voltage and current unbalance
6. The level of power unbalance that can be expected in various
   Voltage and current unbalance contribution to harmonic generation
7. Compensation techniques used to mitigate the negative sequence current and voltage that is generated in the power system

1.5                                          CAUSES OF UNBALANCE PHASE

Practical imperfections which can result in unbalances are:-

1. A three phase equipment such as induction motor 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.

– With continuous operation, motor’s physical 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 current leakage from any phase through bearings or motor body provides floating earth at times, causing fluctuating current.

2. 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.
3. Switching of three phase heavy loads results in current and voltage surges which cause unbalance in the system.
4. Unequal impedances in the power transmission or distribution system cause differentiating current in three phases.

1.6                                      EFFECTS OF UNBALANCE OF PHASE

1. The unbalance decreases the motor efficiency by causing extra heating in the motor. Heat generated also effect the equipment life by increasing the operating temperature, which decompose the grease or oil in the bearing and de-rate the motor windings.
2. In induction motors connected to unbalanced supply, the negative sequence currents flow along with positive sequence current resulting in decreased percentage of productive current and poor motor efficiency. Any unbalance above 3% hampers the motor efficiency.
3. Torque (and thus the speed) produced by the motor becomes fluctuating. These sudden changes in torque cause more vibration in the gear box or the equipment connected to it. The vibration and noise produced damages the equipment and also reduces the efficiency of equipment.
4. The variable frequency or speed drives connected to an unbalanced system can trip off. VFD treats high level unbalances as phase fault and can trip on earth fault or missing phase fault.
5. Unbalances cause de-rating of power cables and thus increase I2R losses in the cable. For distribution cables de-rating factor represents the part of total current giving fruitful outcomes.
6. UPS or inverter supplies also perform with poor efficiency and inject more harmonic currents in case of unbalances in the system.
7. Negative phase sequence current flowing due to unbalance can cause faults in the motor, resulting in, tripping or permanent damage of the electrical equipment.