A91 Understanding the Growing Concern over Power Quality Part-1

Introduction

In today's digital and connected world, power quality has become very important. It refers to the stability and consistency of the electrical supply, which is crucial for the proper functioning of modern electronic devices and industrial equipment.

Load equipment is more sensitive to power quality variations than equipment applied in the past. Many new devices contain microprocessor-based controls and power electronic devices that are sensitive to many types of disturbances.

The increasing emphasis on overall power system efficiency has resulted in a continued growth in the application of devices such as high-efficiency, adjustable speed motor drives and shunt capacitors for power factor correction to reduce losses. This is resulting in increasing harmonic levels on power system and has many people concerned about the future impact on system capabilities.

There is increased awareness of power quality issues among end users. Utility customers are becoming more informed about issues such as interruptions, sags, and switching transients. As a result, they are challenging utilities to improve the quality of the power delivered.

Many things are now interconnected in a network. Integrated processes mean that failure of any component has much more important consequences.

Power Quality

Power quality is the ability of a power grid to supply power to the consumers efficiently and it also expresses the ability of an equipment to consume the power being supplied to it. In technical terms, power quality is the measure, study and enhancement of sinusoidal waveform at the rated voltage and frequency.

Power quality mainly deals with:

• Continuity of Supply
• Quality of the voltage

The equation of power quality can be written as:?

Power Quality = Voltage Quality + Continuity of Supply

or PQ = VQ + CoS

The power supply can only control the quality of the voltage; it has no control over the currents that particular loads may draw. Therefore, standards in the power quality area are devoted to maintaining the supply voltage within certain limits.

There is a close relationship between voltage and current:

• The current resulting from a short circuit causes the voltage to sag, or disappear completely, as the case may be.
• Currents from lightning strokes passing through the power system cause high impulse voltages that frequently flash over insulation and lead to other phenomena, such as short circuits.
• Distorted currents from harmonic-producing loads also distort the voltages as they pass through the system impedance. Thus a distorted voltage is presented to other users.

Various Definitions of Power Quality Disturbances

General Definition?

ANSI (IEEE) Definition

IEC Definition

Difference between ANSI (IEEE) and IEC Terminology

Categories and Characteristics of Power System Electromagnetic Phenomena

Conducted low-frequency phenomena

• Harmonics, inter-harmonics
• Signal systems (power line carrier)
• Voltage fluctuations (flicker)
• Voltage dips and interruptions
• Voltage imbalance (unbalance)
• Power frequency variations
• Induced low-frequency voltages
• DC in ac networks

Radiated low-frequency phenomena

• Magnetic fields
• Electric fields

Conducted high-frequency phenomena

• Induced continuous wave (CW) voltages or currents
• Unidirectional transients
• Oscillatory transients

Radiated high-frequency phenomena

• Magnetic fields
• Electric fields
• Electromagnetic fields
• Continuous waves
• Transients

Electrostatic discharge phenomena (ESD)

Nuclear electromagnetic pulse (NEMP)

Types of System Disturbances

• Voltage Sags
• Voltage Swells
• Undervoltages
• Overvoltages
• Transients
• Notching
• Noise
• Voltage Flicker
• Interruptions
• Harmonics ?
• Voltage Unbalance
• Power Frequency Variations

VOLTAGE SAGS

Definition

Voltage sag is a decrease in the RMS ac voltage, at the power frequency, lasting less than 2 seconds.

Typical Waveform

Voltage sag usually occurs due to faults in the power system, far away from the bus being considered.

A fault on the line (B-A) will cause a voltage sag at the loads fed from feeder a (notice that the presence of the transformer prevents the voltage at breaker 1 from going to zero.

Voltage Swells

Definition

A voltage swell is a cycle-to-cycle increase in the power line RMS voltage magnitude (fundamental) on any of the phases. Swells typically last between 10 milliseconds and 2 seconds.

Typical Waveform

A voltage swell, in a phase, usually occurs as a result of a line-to-ground fault on another phase, when the system is un-grounded, or resistance grounded.

Undervoltages

Definition

An undervoltage is a decrease in the RMS ac voltage, at the power frequency, lasting more than 2 seconds.

Typical Waveform

This, for example, would occur as result of:

• Starting of large motors
• Severe over-load condition

Overvoltages

Definition

An overvoltage is an increase in the RMS voltage for more than 2 seconds.

Typical Waveform

This could occur due to:

• Over-correction of power factor
• Lightly-loaded transmission lines (capacitive charging current)

Transients ?

Definition

A transient is a sudden change in the voltage or current waveforms which usually lasts less than 1 cycle.

Typical Waveform

Transients can be impulsive transients or oscillatory transients

Impulsive Transients

• Defined as sudden, non-power frequency change in the steady state condition of voltage, current, or both, that is unidirectional in polarity.
• Usually characterized by their rise and decay times (1.2 X 50 μs, 2000 volt impulse transient).
• In most cases, caused by lightning.
• Shape of impulsive transient can be changed quickly by circuit components
• They are generally not conducted far from the source where they enter the power system

Oscillatory Transients

• Sudden, non-power frequency change in the steady-state condition of voltage, current, or both, that includes both positive and negative polarity values.
• An oscillatory transient consists of a voltage or current whose instantaneous value changes polarity rapidly.
• It is described by its spectral content, duration and magnitude
• They can be formed as a result of:

o?? Local system response to an impulsive transient (high frequency)

o?? Back-to-back capacitor switching (medium frequency)

o?? Ferro-resonance and transformer energization

It is evident from the above discussion that power quality plays a crucial role in the operation, performance, and cost of industrial equipment. Poor power quality can lead to equipment malfunctions, reduced efficiency, failures, and increased maintenance costs. Ensuring that the power supply is of high quality and that the equipment is compatible with the delivered power is essential. This not only enhances the reliability and longevity of the equipment but also optimizes overall industrial productivity and reduces operational costs.

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