Important Short Questions and Answers: Introduction to Power Quality

Introduction to Power Quality

1.  Definition of Power Quality

Power quality means supply of the power within the permitted variation of the voltage and frequency and without any deviation of sinusoidal waveform in balanced condition.

Power quality is any deviation of the voltage or current waveform from its normal sinusoidal wave shape. These disturbances include, but are not limited to sag, overvoltage, interruption, swell and any other distortions to the sinusoidal waveform.

2.  Define voltage swell

A swell is defined as an increase to between 1.1 pu and 1.8 pu in rms voltage or current at the power frequency durations from 0.5 to 1 minute

3.  Define momentary interruption?

Momentary interruption is said to occur when the RMS voltage decreases less than 0.1 per unit for time duration of 0.008333 second to 3 second.

4.  Define voltage sag

Sag (dip) a decrease to between 0.1 and 0.9 pu in rms voltage or current at the power frequency for durations of 0.5 cycles to 1 minute. Voltage sag is defined as a decrease in RMS voltage magnitude lasting from 0.5 to 30 cycles.

5.  Define over voltage

Over voltage is an increase in the rms ac voltage greater than 110 percent at the power frequency for duration longer than 1 min. Over voltages is usually the result of load switching (e.g., switching off a large load or energizing a capacitor bank). The over voltages result because either the system is too weak for the desired voltage regulation or voltage controls are inadequate. Incorrect tap settings on transformers can also result in system over voltages.

6.  Define Interruption

An interruption occurs when the supply voltage or load current decreases to less than 0.1 pu

for a period of time not exceeding 1 min.

7.     Why is power quality so important?

Power quality is an increasingly important issue for all businesses. Problems with powering

and grounding can cause data and processing errors that affect production and service quality.

1.     Lost production: Each time production is interrupted, your business loses the margin on the product that is not manufactured and sold.

2.     Damaged product: Interruptions can damage a partially complete product, cause the items to be rerun or scrapped.

3.     Maintenance: Reacting to a voltage disruption can involve restoring production, diagnosing and correcting the problem, clean up and repair, disposing of damaged products and, in some cases, environment costs.

4.     Hidden costs: If the impact of voltage sag is a control error, a product defect may be discovered after customer delivery. The costs of losing repeat sales, product recalls and negative public relations can be significant and hard to quantify.

8.  What causes power quality problems?

Most causes can be divided into two categories:

1.     Internal causes: Approximately 80 percent of electrical disturbances originate within a business facility. Potential culprits may include large equipment start-up or shutdown, improper wiring and grounding, overloaded circuits or harmonics.

2.     External causes: About 20 percent of power quality problems originate with the utility transmission and distribution system. The most common cause is a lightning strike; other possibilities include equipment failure, vehicle accidents, weather conditions, neighboring business, and even normal operation of utility equipment.

9.       Define total harmonic distortion.

Total harmonic distortion is the term used to describe the net deviation of a non linear waveform from ideal sine wave characteristics. THD is the ratio between the RMS value of the harmonics and the RMS value of the fundamental.

10. Define total demand distortion.

The total demand distortion is defined as the square root of the sum of the squares of the RMS value of the currents from 2^nd to the highest harmonic divided by peak demand load current and is expressed a s a percent.

11.  What are the various power quality issues?

1.     Power frequency disturbance

2.     Power factor

3.     Power system transients

4.     Grounding and bonding

5.     Electro magnetic interference

6.     Power system harmonics

12. Define power frequency variations?

Power frequency variations are a deviation from the nominal supply frequency. The supply frequency is a function of the rotational speed of the generators used to produce the electrical energy.

13.            List any four standards available in power quality.

IEEE power quality standards: Institute Of Electrical and Electronics Engineer. IEEE power quality standards: International Electro Technical Commission.

IEEE power quality standards: Semiconductor Equipment and Material International. IEEE power quality standards: The International Union for Electricity Applications.

14.            Name any four IEC standards that define power quality.

1.       Definitions and methodology  61000-1-X

2.       Environment        61000-2-X

3.       Limits         61000-3-X

4.       Tests and measurements         61000-4-X

5.       Installation and mitigation      61000-5-X

6.       Generic immunity and emissions 61000-6-X

15. Define voltage fluctuation.

Voltage fluctuation is rapid changes in voltage with the allowable limits of voltage magnitude of 0.95 to 1.05 of nominal voltage.

16. Define voltage imbalance.

Voltage imbalance or Voltage unbalance is the deviation of each phase from the average voltage of all three phases.

17.            What are the effects of power quality problems?

1.    The effects of power quality problems can be understood by looking at the various types of loads that are affected by power quality problems, including computers, consumer products, lighting, meters, aeromagnetic equipment, telephones, manufacturing processes and capacitors.

2.  Computers and computer controlled equipment are most subject to power quality problems. They freeze up and lose data. Most power quality problems on computers are caused by voltage variations.

18. List out the IEEE and IEC standards.

IEEE standards:

1.     IEEE Std 519-1992, IEEE Recommended practices and requirements for Harmonic control in Electric power systems.

2.     IEEE Std 1159-1995, IEEE Recommended practices for monitoring electrical power quality.

IEC standards:

1.       Definitions and methodology  61000-1-X

2.       Environment        61000-2-X

3.       Limits         61000-3-X

4.       Tests and measurements         61000-4-X

5.       Installation and mitigation      61000-5-X

6.       Generic immunity and emissions 61000-6-X

19.            Differentiate between sag and swell.

Voltage sag is an event in which the RMS voltage decreases between 0.1 and 0.9 pu at the

power frequency. It lasts for duration of cycles to 1 min

Swell is an event in which the RMS voltage increases between 1.1 and 1.8 pu at the power frequency. It lasts for duration of 0.5 cycles to 1 min

20.            What are the causes of sags?

1.     Voltage sags are usually associated with system faults but can also be caused by the switching of heavy loads.

2.     Voltage sags are caused by motor starting.