POWER QUALITY

The electric power network has undergone several modifications from the time of its invention. The modern electric power network has many challenges that should be met in order to deliver qualitative power in a reliable manner. There are many factors both internal and external that affect the quality and quantity of power that is being delivered. This chapter discusses the different power quality problems, their causes and consequences.

Power Quality :

The quality of electric power delivered is characterized by two factors namely-“continuity” of supply and the “quality” of voltage. As indicated by IEEE standard 1100, Power Quality is characterized as-

"The idea of controlling and establishing the touchy supplies in a manner that is suitable for the operation of the gear."

Power quality Problems

There are many reasons by which the power quality is affected. The occurrence of such problems in the power system network is almost indispensable. Therefore, to maintain the quality of power care must be taken that suitable devices are kept in operation to prevent the consequences of these problems. Here an overview of different power quality problems with their causes and consequences is presented.

 

Interruptions:

It is the failure in the continuity of supply for a period of time. Here the supply signal (voltage or current) may be close to zero. This is defined by IEC (International Electro technical Committee) as “lower than 1% of the declared value” and by the IEEE (IEEE Std. 1159:1995) as “lower than 10%”. Based on the time period of the interruption, these are classified into two types -

A.  Short Interruption:

If the duration for which the interruption occurs is of few mille seconds then it is called as short interruption.

Causes:

The causes of these interruptions are-

• Opening of an Automatic Re-closure

• Lightening stroke or Insulation Flash over

Consequences:

• The data storage system gets affected

• There may be malfunction of sensitive devices like- PLC’s, ASD’s

B.     Long Interruptions:

If  the  duration  for  which  the  interruption  occur  is  large  ranging  from  few  mile

seconds to several seconds then it is noticed as long interruption.

Causes:

The causes of these interruptions are-

·         Faults in power system

·         Human error

·         Improper functioning of protective equipment

Consequences:

This type of interruption leads to the stoppage of power completely for a period of time      until the fault is cleared.

 

Waveform Distortion:

The power system network tries to generate and transmit sinusoidal voltage and current signals. But the sinusoidal nature is not maintained and distortions occur in the signal. The cause of waveform distortions are -

·     DC Offset: The DC voltage which is present in the signal is known as DC offset. Due to the presence of DC offset, the signal shifts by certain level from its actual reference level.

·     Harmonics: These are voltage and current signals at frequencies which are integral multiples of the fundamental frequency. These are caused due to the presence of non-linear loads in the power system network.

·      Inter Harmonics: These are the harmonics at frequencies which are not the integral multiples of fundamental frequency.

·     Notching: This is a periodic disturbance caused by the transfer of current from one phase to another during the commutation of a power electronic device.

·       Noise: This is caused by the presence of unwanted signals. Noise is caused due to interference with communication networks.

 

Frequency Variations:

The electric power network is designed to operate at a specified value (50 Hz) of frequency. The frequency of the framework is identified with the rotational rate of the generators in the system. The frequency variations are caused if there is any imbalance in the supply and demand. Large variations in the frequency are caused due to the failure of a generator or sudden switching of loads.

Transients:

The transients are the momentary changes in voltage and current signals in the power system over a short period of time. These transients are categorized into two types-impulsive, oscillatory. The impulsive transients are unidirectional whereas the oscillatory transients have swings with rapid change of polarity.

Causes:

There are many causes due to which transients are produced in the power system. They are- Arcing between the contacts of the switches, sudden switching of loads, poor or loose connection, lightening stroke.

Voltage Sag:

The voltage sag is defined as the dip in the voltage level by 10% to 90% for a period of half cycle or more.

Causes:

The causes of voltage sag are-starting of an electric motor, which draws more current, faults in the power system sudden increase in the load connected to the system

 

Voltage Swell:

Voltage swell is defined as the rise in the voltage beyond the normal value by 10% to 80% for a period of half cycle or more.  

Causes:

De-energization of large load, Energization of a capacitor bank , Abrupt interruption of current Change in ground reference on ungrounded phases.

Voltage Unbalance:

The unbalance in the voltage is defined as the situation where the magnitudes and phase angles between the voltage signals of different phases are not equal.

Causes: Presence of large single-phase loads faults arising in the system.