A92 Understanding the Growing Concern over Power Quality Part-2

This article continues from "A91 Understanding the Growing Concern over Power Quality Part-1." New readers seeking a better understanding of the subject are encouraged to read the preceding article.

The power quality is the set of parameters defining the properties of the power supply as delivered to the user in normal operating conditions, in terms of the continuity of voltage and voltage characteristics.

In the previous article (A91), the following system disturbances were discussed:

• Voltage Sags
• Voltage Swells
• Undervoltages
• Overvoltages and
• Transients

In this article, we will discuss the following system disturbances:

• Notching
• Noise
• Voltage Flicker
• Interruptions
• Harmonics
• Voltage Unbalance and
• Power Frequency Variations

Notching

Definition

Notching is a repetitive sub-cycle disturbance in the voltage waveform caused by the overlap in the conduction of power electronics in rectifiers or adjustable speed drives (ASDs).

Typical Waveform

Rectifier Operation

Example:

IEEE 1100-1992 (Emerald Book) Case History:

• New 1000 hp solid-state dc drive installed in a manufacturing plant
• Problems developed in the existing electronic equipment connected to the same 480 V system
• These operational problems were attributed to line-voltage notching surge

There are several types of noise, for example:

• Common-mode noise
• Transverse mode noise

Voltage Flicker

Definition

A variation of input voltage sufficient in duration to allow visual observation of a change in electric light source intensity. (IEEE Std. 1100-1992 Emerald Book)

Typical Waveform

Interruptions

Definition

An interruption or an outage occurs when voltage at supply terminals is close to zero or a complete power loss. Based on the duration, interruptions are subdivided into:

1. Sustained interruptions, which are terminated through manual restoration or replacement
2. Temporary interruptions, which lasts less than two minutes and terminated through automatic restoration.
3. Momentary interruptions, which are terminated through self-restoration.

Typical Waveforms

Interruptions are generally a result of the opening of an upstream protective device (usually following a fault).

Harmonics

Definition

The presence of frequencies other than the power frequency component in the voltage or current waveforms.

Typical Waveform

Harmonics result from nonlinear loads that convert AC line voltage to DC. They flow into the electrical system due to nonlinear electronic switching devices, such as variable frequency drives (VFDs), arc furnaces, UPS systems, computer power supplies, energy-efficient lighting, electric vehicle chargers, renewable energy systems (like solar inverters and wind turbines) and welding machines. Additionally, household appliances with electronic controls, such as microwave ovens, washing machines, and inverter fans can contribute to harmonic distortion.

Voltage Unbalance

Voltage unbalance is sometimes defined as the maximum deviation from the average of the three-phase voltage or current, divided by the average of the three phase voltages, or currents, expressed in percent.

Unbalance can also be defined using symmetrical components (ratio of negative sequence to positive sequence)

The primary source of voltage unbalance, having magnitudes less than two percent, is single-phase loads on a three-phase circuit.

Severe voltage unbalance is usually the result of blown fuses in one phase of a three phase capacitor bank.

Power Frequency Variations

Power frequency variations are defined as the deviation of the power system fundamental frequency from its specified nominal value (50 or 60 Hz).

The power system frequency is directly related to the rotational speed of the generators supplying the system.

The size of the frequency shift and its duration depend on the load characteristics and the response of the generation control system to load changes.

On modern interconnected power systems, significant frequency variations are rare.

Frequency variations of consequence are much more likely to occur for loads that are supplied by a generator isolated from the utility system.

Voltage notching can sometimes be mistaken for frequency deviation. The notches may come sufficiently close to zero to cause errors in instruments and control systems that rely on zero crossings to derive frequency or time.

Power Quality Terms to Avoid

These terms should be purposely avoided because no single or adequate technical definition exists for them:

Modified power quality disturbances

Blackout

Brownout

Clean Ground

Clean Power

Computer Grade Ground

Conducting Barriers

Counterpoise Ground

Dedicated Ground

Dirty Ground

Dirty Power

Equipment Safety Grounding Conductor

Frame Ground

Frequency Shift

Glitch

Natural Electrodes

Power Surge

Raw Power

Raw Utility Power

Shared Circuits

Shared Ground

Spike

Sub cycle Outages

Sucks

Type I, II, III, Power Disturbance

Modified power quality disturbances

Impacts of Poor Power Quality

The effects of poor power quality have serious implications for utilities, industries, and other customers. These implications include:

• Higher losses in transformers and cables
• Energy meters will give faulty readings
• Solid state protective relays, electronic devices and computers may damage
• Variable speed drives may shut down
• Motors will increase core and copper losses
• Non-sinusoidal waveforms will reduce the efficiency of motors
• Loss of synchronization in processing equipment
• Malfunctioning of process devices
• Computer lock-ups and data loss
• Nuisance tripping of protective devices
• Domestic TV and other appliances are affected by the poor power quality

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