Power Quality in Modern Power System
Key words
Flicker; Harmonics; IEEE Std 1159–2019; Power quality; Transients; Voltage fluctu- ations; Voltage sag
As per IEEE standard 1159–2019, power quality problems are classified into the following:
The source voltage can be disturbed and indirectly has an impact on the quality of power.
- Lightning
- Faults
- Non-compatible loads.
- Frequency variations
The source voltage can be disturbed and indirectly has an impact on the quality of power. Generally, power quality problems are divided into two types:
• Voltage-related problems
• Current-related problems
Voltage-related problems include
➡ Sags, swells, blackouts, harmonics, etc.,
Current-related problems include
➡ Electromagnetic field, leakage current, electro-magnetic interference, radio frequency interference, etc.
Transients
Transients are momentary changes in voltage or current or both resulting from switching operation, lightning, faults, etc. The time duration for a transient varies from nanoseconds to milliseconds.
Transient overvoltage is the result of the rapid change of current in an inductive circuit.
External sources
• Lightning strike
• Opening and closing of energized lines
• Breaker opening and closing
• Transients from other users in shared sources
Internal sources
• Switching operation
• High resistance fault
• Photocopy machines
• Welding machines
• Capacitor bank switching
• High-frequency switching in inverter or switch-mode power supply
1. Failures of insulation in inductive loads if the transients exceed the impulsive voltage withstand capability.
2. Failure of equipment due to lightning-induced overvoltage.
3. Failure of semiconductor devices and their associated circuits due to peak inverse voltage.
4. Failure of control wiring in electronic circuits.
To protect against transients, end-users may use transient voltage surge suppressors.
Short duration RMS variation
Variations in RMS voltage lasting from 0.5 cycles to less than or equal to 1 min are classified as short duration RMS variations as per IEEE 1159–2019
➡ Sag : reduction in voltage
➡ Swell : increase in voltage
Voltage Sag
Voltage sags are short duration reductions in voltage magnitude and duration last-ing typically from a few cycles to a few seconds. Definition as per IEEE 1159 is a decrease in RMS voltage between 0.1 and 0.9 pu and a time duration from 0.5 cycles to 1 min.
Voltage sag can be caused by many conditions, a few of which are:
1. Direct online starting of large motors
2. Switching ON large loads in one instance
3. Electrical short circuit
4. Arcing fault in the system
5. Energizing higher-capacity power transformer
6. Switching OFF large reactive power source
7. Operations of surge suppressors in the power system
Voltage sag generally affects the end-user equipment in terms of malfunction of process controllers, programmable logic controllers, adjustable speed drives, robotics in modem industries, reduction in the performance of the motor, flickering in illumination devices, etc.
To protect equipment from interruptions, end-users may use;
- Uninterruptible power supply devices (UPS) and other energy storage systems.
- Active Voltage Conditioners(AVCs)
Voltage Swell
Voltage swells are short duration increases in voltage magnitude lasting typically from a few cycles to a few seconds. Definition as per IEEE 1159 is an increase in RMS voltage between 1.1 and 1.8 pu and a time duration from 0.5 cycles to 1 min. The typical voltage magnitude is between 1.1 and 1.2 pu.
Voltage swell can be caused by many conditions, a few of which are:
1. Switching OFF large motors2. Switching OFF large loads in one instance
3. Electrical short circuit (L-G fault)
4. Switching ON a large reactive power source
Voltage swell generally affects the end-user equipment in terms of malfunction of process controllers, programmable logic controllers, adjustable speed drives,
robotics in modem industries, reduction in the performance of the motor, flickering in illumination, etc.
To protect equipment from interruptions, end-users may use;
- Uninterruptible power supply devices (UPS) and other energy storage systems.
- Active Voltage Conditioners(AVCs)
Voltage Interruption
Momentary interruptions are short duration reductions in voltage magnitude of less than 0.1 pu for less than 1 min time duration. The duration of the momentary interruption is determined by the fault clearing time and breaker closing time.
Interruption can be caused by many conditions, a few of which are:
1. Power system faults (electrical faults)2. Equipment failures
3. Control equipment malfunctions
Production stopped due to interruption, which leads to production losses in manufacturing.
To protect equipment from interruptions, end-users may use;
- Uninterruptible power supply devices (UPS) and other energy storage systems.
- Back-up generators or self-generation equipment is necessary to manage sustained interruptions.
- Other solutions include the use of static transfer switches and dynamic voltage restorers with energy storage.
Long duration RMS variation
Accordingly, reduction in voltage is termed undervoltage, while an increase in the voltage is termed overvoltage.
The absence of voltage is termed a sustained interruption.
Undervoltage
Undervoltages are long duration reductions in voltage magnitude and a duration for more than 1 min. Definition as per IEEE 1159 is a decrease in RMS voltage between 0.8 and 0.9 pu and a time duration for more than 1 min.
Undervoltage can be caused by many conditions, a few of which are:
1. Overload in the circuit2. Higher voltage drop due to undersized cables/conductors
3. Switching ON higher capacity loads continuously
4. Switching OFF large reactive power source
5. Improper tap selection of transformers
6. Unbalanced loading in three-phase, four-wire distribution systems
Constant power load can draw higher current, which leads to higher losses
Reduced performance in equipment like motors and incandescent lamps
To protect equipment from interruptions, end-users may use Automatic Voltage Regulators (AVR) and AVCs (Active Voltage Conditioners).
Overvoltage
Overvoltages are long duration increases in voltage magnitude and a duration for more than 1 min. Definition as per IEEE 1159 is an increase in RMS voltage above 1.1 pu or 110% of its nominal and a time duration for more than 1 min.
Overvoltage can be caused by many conditions, a few of which are:
1. Switching OFF large loads2. Variations in reactive power compensation
3. Incorrect tap settings on transformers
4. Switching OFF higher capacity loads continuously
5. Unbalanced loading in three-phase, four-wire distribution systems
1. Prolonged overvoltage leads to insulation failure
2. Reduced performance of equipment
To protect equipment from interruptions, end-users may use Automatic Voltage Regulators (AVR) and AC Stabilizers.
Unbalance or imbalance
Unbalance or imbalance
IEEE 1159–2019 defines unbalance or imbalance in a three-phase system as the ratio of the magnitude of the negative sequence component to the magnitude of the positive sequence component expressed in percentage. Unbalance is classified into voltage unbalance and current unbalance.
Voltage unbalance is the ratio of the magnitude of the negative sequence component to the magnitude of the positive sequence component expressed in percentage. The expression for voltage unbalance is given in Eq. (1.1):
Unbalance is caused by many conditions, a few of which are :
1. Unbalanced loading among all three phases2. Higher deployment of single-phase loads in three-phase, four-wire distribution systems
3. Unequal distribution impedance among three phases
4. Improper sizing and laying of single cables among three phases
An unbalanced voltage or current affects the end-user equipment in terms of performance. For example:
1. Increased heat in motor windings2. Reduced performance of motors and transformers
3. Higher neutral current in three-phase, four-wire distribution systems
4. Higher voltage total harmonic distortion due to higher impedance in the phase
To protect distribution transformer from unbalance, end-users may use Compensates Capacitive Reactive Power system, derived from Static Var Generator(SVG).
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Waveform distortion
IEEE 1159–2019
defines waveform distortion as a steady-state deviation from ideal sinusoidal characteristics [2,4]. Waveform distortion is classified into the following five types:1. Harmonics
2. Interharmonics
3. DC offset
4. Notching
5. Noise