THEORY EXAMINATION (SEM–VIII) 2016-17 POWER QUALITY

Power Quality – Section Wise Explanation

Section A – Basic Concepts of Power Quality

Section A contains short questions designed to test the basic understanding of power quality disturbances and electrical system performance. Students must attempt all questions in this section. 

Power quality refers to the ability of an electrical power system to deliver stable voltage, current, and frequency without disturbances. Good power quality ensures that electrical equipment operates efficiently and safely.

Poor power quality can lead to equipment malfunction, increased energy losses, and system failures. Therefore, monitoring and maintaining power quality is very important in modern electrical systems.

One important disturbance in power systems is voltage swell, which occurs when the voltage level rises above the normal value for a short duration. Voltage swells can damage electrical devices and insulation systems.

Another common disturbance is electrical transients, which are sudden spikes in voltage caused by lightning strikes, switching operations, or faults in power systems.

Power systems may also experience harmonics and notching. Harmonics are distortions in voltage or current waveforms caused by nonlinear loads such as power electronics devices. Notching refers to periodic disturbances in voltage waveforms caused by switching operations.

A flicker meter is an instrument used to measure voltage fluctuations that cause visible flickering in lighting systems.

Another important concept is custom power, which refers to the use of advanced power electronic devices to improve power quality in distribution systems.

Overvoltage protection devices such as surge arresters and voltage regulators are used to protect equipment from high voltage conditions.

Distributed generation systems, such as solar and wind power plants, can also affect voltage regulation in power networks because they introduce variable power sources into the grid.

Understanding these basic concepts helps engineers identify and solve power quality problems.

Questions for Section A

What is power quality?

What is voltage swell?

What are the sources of electrical transients in power systems?

What are notching and harmonics?

What is a flicker meter?

What are the impacts of poor power quality on system efficiency and reliability?

What is custom power?

Name two devices used for overvoltage protection.

What causes voltage and current harmonics?

How does distributed generation affect voltage regulation?

Section B – Power Quality Disturbances and Control

Section B focuses on detailed explanations of power quality disturbances and the technologies used to control them. Students must attempt any five questions from this section. 

One device used for improving power quality is the Active Series Compensator. This device is connected in series with the distribution line and injects voltage to compensate for voltage disturbances such as sag or swell.

Power systems also experience waveform distortion, which includes harmonics, interharmonics, notching, and noise. These distortions change the shape of voltage or current waveforms and affect system performance.

Another important disturbance is voltage sag, which is a short-term reduction in voltage level. Voltage sag often occurs when large motors start or when faults occur in the power system.

Electrical transients are also common disturbances. These transients can be classified into impulsive transients and oscillatory transients, depending on their waveform characteristics.

Harmonics have harmful effects on electrical equipment. They can cause overheating in transformers, additional losses in motors, and malfunction of protective devices.

To reduce harmonics, several harmonic mitigation techniques are used, such as passive filters, active filters, and hybrid filters.

Power quality can also be improved using network reconfiguration devices, which help modify the network structure to maintain stable voltage levels.

Engineers also use various equipment such as voltage regulators, surge arresters, and dynamic voltage restorers to minimize voltage disturbances.

Understanding these techniques helps maintain reliable and efficient power systems.

Questions for Section B

Explain the working of an active series compensator.

What are the different types of waveform distortion?

What is voltage sag and what are its causes?

What are the different types of electrical transients in power systems?

Explain the effects of harmonics on transformers.

Explain the effects of harmonics on AC motors.

What are the harmonic mitigation techniques?

Explain the network reconfiguration devices used in power systems.

What equipment is used to minimize voltage disturbances?

Section C – Advanced Power Quality Control Systems

Section C contains advanced questions related to harmonic analysis, power backup systems, and voltage regulation techniques. Students must attempt any two questions from this section. 

One important topic discussed in this section is harmonic measurement techniques. Engineers use instruments such as harmonic analyzers and spectrum analyzers to measure harmonic distortion in power systems.

Understanding harmonic sources and measurement techniques helps engineers design effective solutions to reduce waveform distortion.

Another important system discussed in this section is the Uninterruptible Power Supply (UPS).

UPS systems provide backup power when the main electrical supply fails. They are commonly used in data centers, hospitals, and industrial facilities.

There are two main types of UPS systems:

Online UPS – provides continuous power through an inverter and offers the highest level of protection.

Offline UPS – supplies power directly from the mains and switches to battery backup when power fails.

Another advanced power quality device is DSTATCOM (Distribution Static Synchronous Compensator). DSTATCOM is used to regulate voltage and compensate reactive power in distribution networks.

It helps improve load compensation and maintain voltage stability in electrical systems.

Questions for Section C

What are the causes of voltage and current harmonics?

Explain harmonic measurement techniques.

Explain the working difference between online UPS and offline UPS.

How does DSTATCOM provide load compensation?

How does DSTATCOM help in voltage regulation?

Conclusion

The Power Quality exam paper evaluates knowledge at three levels. Section A focuses on basic definitions and disturbances in power systems. Section B examines detailed analysis of power quality problems and mitigation techniques. Section C explores advanced technologies such as harmonic measurement, UPS systems, and DSTATCOM for improving power quality. 

Maintaining good power quality is essential for reliable operation of modern electrical systems and sensitive electronic equipment.