3. Short Duration Voltage Variation:
- When the rms value of voltage deviates for duration less than 1 minute, it is termed as short duration voltage variation.
- This category falls in the IEC (International Electrotechnical Commission) category of voltage dip and short interruption, each type of variation can be designated as Instantaneous, Momentary or Temporary or depends on its duration.
- Short duration voltage variation is caused by fault condition, the energization of Heavy load which requires high starting current or intermittent losses connection in power wiring depending upon the fault location of system condition.
- The fault can cause either temporary voltage drop, voltage rise, a complete loss of voltage.
- Short duration voltage variations are Interruptions, Voltage Sag, and Voltage Swell.
a. Interruptions:
- It occurs when supply voltage or load current decreases to less than 0.1pu for a time period not exceeding 1minute.
- They are mostly caused by power system faults, equipment failure.
b. Voltage Sag or Dip:
- It is a disturbance where the rms value of the line voltage is reduced between 0.1 to 0.9pu for a period of duration between 0.5 cycle to 1minute.
- They are caused by system faults, energizing of heavy load, and starting of large motor.
c. Voltage Swell:
- It is an increase in rms voltage between 1.1 to 1.8pu for a duration from 0.5 cycle to 1minute.
- They are caused by SLG faults, switching off large load, energizing large capacitor bank.
4. Voltage Imbalance or Voltage Unbalance:
- It is sometime defined as the maximum deviation from the average of the 3-phase voltage or current, divided by the average of 3-phase voltage or current expressed in percentage.
- Voltage imbalance is the measure of voltage differences between the phases of a three-phase system.
- It is mainly caused by big single-phase loads.
5. Waveform distortion:
- A steady-state deviation from an ideal sine wave of power frequency principally characterized by the spectral content of the deviation.
The various form of waveform distortion:
a. DC Offset:
- The presence of a dc voltage or current in an ac power system is termed as DC Offset.
- It may saturate the transformer core causing additional heating and transformer life.
- DC may also cause electrolytic erosion of grounding electrodes and other connectors.
b. Harmonics:
- A sinusoidal voltage or current having frequencies that are integral multiple of the frequency at which the supply is designed to operate (termed as the fundamental frequency usually 50Hz to 60Hz).
- They are caused by non-linear loads.
- Total harmonic distortion is generally used to measure the effective value of harmonic distortion.
- Total Harmonic distortion is the ratio of the rms of the harmonic content to the rms value of the fundamental quantity and is expressed as the percentage of Fundamental.
c. Inter Harmonics:
- A sinusoidal voltage or current having frequencies that are not integral multiple of the frequency at which the supply is designed to operate (termed as the fundamental frequency usually 50Hz to 60Hz).
- They are mostly caused by static frequency converter, cycloconverter, induction furnaces, arcing devices, etc.
d. Notching:
- Notching is the periodic voltage disturbance caused by the normal operation of power electronic devices when phase is commutated from 1-phase to another.
e. Noise:
- It is defined as unwanted electrical signal with broad band spectral content lower than 200KHz superimposed upon the power system voltage or current in phase conductor, or found on neutral conductors or signal lines.
- They are generally caused by power electronic devices, control circuits, arcing equipment, solid state rectifier, etc.
- This problem can be reduced by using filters, isolation transformer, line conditioner.
6. Power Frequency Variation:
- Power frequency variations are a deviation from the nominal supply frequency (i.e., 50 to 60 Hz).
- It is mainly caused by an imbalance between generation and load which happens due to fault in bulk power transmission system.
- Now a days, in modern interconnected power system, this problem is rare.
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