UNIT – 4 HARMONICS & UNIT – 5 POWER QUALITY MONITORING TWO MARKS

UNIT – 4   HARMONICS

TWO MARKS

1.      What are the important concepts to bear in mind to understand power system harmonics?

                  There are two important concepts to bear in mind with regard to power system harmonics. The first is the nature of harmonic current producing loads (non­linear loads) and the second is the way in which harmonic currents flow and how the resulting harmonic voltages develop.

2.      Draw the relationship between between P, Q, S in sinusoidal condition.

3.      Define true power factor.

                  True power factor is calculated as the ratio between the total active power used in a circuit (including harmonics) and the total apparent power (including harmonics) supplied from the source

True power factor = Total active power (P) / apparent power (S)

4.      What is the reason for existence of harmonic distortion?

·         Harmonics distortion exists due to the nonlinear characteristics of the devices and loads on the power system .

·         These devices act as current sources that inject harmonic currents into the power system.

5.      Differentiate between linear loads and non-linear loads.

                  Linear load: Any load that draws current at supply fundamental frequency only is a linear load. The current drawn does not contain any harmonics (multiples of the supply frequency). Motors, resistors, inductors and capacitors are all linear loads.

                  Non Linear load: Any load that draws harmonic currents from the supply is a nonlinear load. The current waveform of such non-linear loads, is discontinuous and non sinusoidal because of the presence of harmonics.

6.      What is voltage and current distortion?

·         Voltage distortion is any deviation from the nominal sine waveform of the AC line voltage .

·         Current distortion is any deviation from the nominal sine waveform of the AC line current.

7.      Mention the commonly used indices used for measuring harmonic component of waveform.

                  The two most commonly used indices for measuring the ham10nic content of the waveform are the total harmonic distortion (THD) and total demand distortion (TDD).

1. If a generator produces a non-ideal sinusoidal waveform, the voltage waveform will contain a certain amount of harmonics

2. In motors, decreased efficiency, excessive heating, and vibration are symptoms of harmonic voltage distortion.

8.      Mention at least two causes of harmonics made on distribution systems.

      In the distribution system, transformers are capable of producing harmonics due to magnetic core saturation. This is more prevalent at a lighter loading of the transformer

      Large load currents in the neutral wires of a 3 phase system. Theoretically the neutral current can be up to the sum of all 3 phases therefore causing overheating of the neutral wires. Since only the phase wires are protected by circuit breakers of fuses, this can result in a potential fire hazard.

9.      What is harmonic index? State its significant.

      The power quality industry has developed certain index values that help us assess the quality of service as it relates to distortion caused by the presence of harmonics. These values, or harmonic indices, serve as a useful metric of system performance. The two most commonly used indices under harmonic studies are

(a)    Total harmonic distortion (TI-ID) (b) Total demand distortion (TDD)

10.  Mention the problems created by harmonics.

            A large load current flows in the neutral WIres of a 3 phase system.

            Theoretically the neutral current can be up to the sum of all 3 phases therefore causing overheating of the neutral wires.

     

Poor power factor conditions that result in monthly utility penalty fees for major users (factories, manufacturing, and industrial) with a power factor less than 0.9.

11.  Mention the harmonic effects on devices and loads Insulation stress (voltage effect)

      Thermal stress (current effect)

      Load ruptures (abnormal operation)

12.  What is the effect on transformer due to Harmonics?

      The primary effect of power system harmonics on transformers is the additional heat generated by the losses caused by the harmonic contents generated by the load current

13.  Mention he harmonic sources from commercial loads.

      Single phase loads such as Switch mode power supplies, fluorescent lighting and UPS systems

Three phase loads such as high voltage AC drives system

14.  Mention the harmonic sources from industrial loads .

            Three phase converter with Adjustable speed drives (DC drives and AC drives)

      Arcing Devices (Arc furnaces, weiders, Discharge lamps etc)

      Saturable devices (transformer, electromagnetic devices etc with steel core)

15.  What is the advantage of three phase converter?

      Three-phase electronic power converters do not generate third-harmonic currents mainly when compared with single-phase converters. This is a great advantage because the third harmonic current is the largest component of harmonics shown in harmonics spectrum

16.  What is the disadvantage of 12 pulse drive?

            The disadvantages of the l2-pulse drive are that there is more cost in control design and an extra transformer is usually required

17.  State the different types of inverters

            Variable voltage inverter (VVI)

      Current source inverter (CSI)

      Pulse width modulated (PWM)

18.  What is Variable Voltage Inverter?

      The variable voltage inverter (VVI), or square-wave six-step voltage source inverter (VSI), receives DC power from an adjustable voltage source (either from thyristor converter or DC-DC converter fed by Diode Bridge) and adjusts the frequency and voltage.

19.  What is current Source inverter?

      The current source inverter (CSI) receives DC power from an adjustable current source and adjusts the frequency and current.

20.  What is the need of locating harmonic sources?

      When harmonic problems are caused by excessive voltage distortion on the supply system, it is important to locate the sources of harmonics in order to develop a solution to the problem.

PART-B

1.         Explain briefly about fundamentals of harmonics generation and wave form distortion.

2.         Explain in detail about classification of linear loads and non linear loads used in harmonic studies.

3.         Explain the concept of harmonic phenomena under the presence of harmonic producing loads.

4.         Explain for the following terms (i) Harmonic distortion (ii) Current distortion (iii) Voltage distortion

5.         What are the two important harmonic indices used in power system? Explain about it briefly.

6.         Give the power definitions under non sinusoidal conditions. Explain briefly about it.

7.         Differentiate between true power factor and displacement power factor under harmonic studies.

8.         Explain briefly about various harmonic characterization on power systems.

9.         Explain briefly about the phenomena of how current distortion affects the voltage

        distortion under the presence of harmonics.

10.     What are the general causes of harmonics in power system?

11.     What are the various causes of harmonics in distribution power system?

12.     What are the effect of facility and site under the presence of harmonics?

13.     List the various effects of equipments due to harmonics. Explain briefly.

14.     Explain the harmonic effects on power system equipments briefly.

15.     What are the various classifications of harmonic sources and explain briefly about it?

UNIT – 5  POWER QUALITY MONITORING

TWO MARKS

1.      What is the importance of power quality monitoring?

Power Quality Monitoring is necessary to- detect and classify disturbance at a particular location on the power system.  PQ monitoring assists in preventive and predictive maintenance.  Problems can be detected before they cause widespread damage by sending automated alerts. PQ Monitoring can be used to determine the need for mitigation equipment.

2.      What are the monitoring objectives?

·         Continuous evaluation of the electric supply system for disturbances and power quality variations.

·         Document performance of power conditioning equipment, such as static switches, UPS systems, other ride through technologies, and backup generators.

3.      What are the purposes of power quality monitoring system?

·         Preventive maintenance

·         Load analysis

·         Equipment diagnostics

·         Long time surveys

4.      What is proactive monitoring?

The traditional approach to power quality monitoring is reactive. We need to know when a problem is going to occur before it happens.Permanent power quality monitoring systems are designed to help proactively identify conditions and events that may cause problems should be addressed. This is called proactive monitoring.

5.       

6.      nd the response of equipment that may be sensitive to microsecond variations in the voltage.

2.What are the steps involved in power quality monitoring?

·         Planning for the monitoring

·         Preparing for the monitoring

·         Inspecting the site

·         Monitoring the power

·         Analyzing, monitoring and inspecting data

·         Applying corrective solutions

3.What are the requirements of monitoring for a voltage regulation and unbalance?

·         3 phase voltages

·         RMS magnitudes

·         Continuous monitoring with periodic max/min/avg samples

4.What are the requirements of monitoring for a harmonic distortion?

·         Currents for response of equipment

·         3 phase voltages and currents

·         Waveform characteristics

·         128 samples per cycle minimum

·         Synchronized sampling of all voltages and currents

·         Configurable sampling characteristics

5.      What are the Characteristics of power quality monitoring equipment?

Harmonic Analysis

            Harmonic analyses are usually conducted by obtaining and interpreting measurements of waveforms. Equipment normally required to perform a harmonic study consists of a harmonic analyzer, an oscilloscope, and an RMS responding voltmeter and ammeter. Spectrum analysis is usually performed up to the 50th harmonic (3 kHz).

6.      What are the Characteristics of power line monitors?

·         Portable, rugged, lightweight

·         Simple to use, with proper training

·         Designed for long-term unattended recording

·         Definition of line disturbance parameters varies between manufacturers

7.         What is the Types of power quality measurement equipment?

·         Hand-held single-phase power quality monitors

·         Portable three-phase power quality monitors

·         Harmonic analyzers

·         Distortion analyzers

·         Multimeters

8.        Mention the factors that should be considered  for selecting the instrument.

·         Number of channels (voltage and/or current)

·         Temperature specifications of the instrument

·         Input voltage range (e.g., a to 1000 V)

·         Ability to measure three-phase voltages

9.       What is the use of oscilloscope?

         Oscilloscopes with fast sampling rates and automatic triggering function can be very useful for trace of transients.

10.  What is the use of spectrum analyzer?

            A spectrum analyzer can be used for trace of high frequency harmonics.

11.  What is the use of simple single phase hand-held power quality monitor?

         Power quality problems like measuring the occurrence of harmonics or checking the voltage level or the power frequency can easily be made by using a simple single phase hand-held power quality monitor.

12.  Mention the Instruments used for the analysis of non-sinusoidal voltage and currents?

·         Oscilloscope

·         Spectrum analyzer

·         Harmonic analyzer

13.  Mention the basic categories of instruments for harmonic analysis?

·         Simple meters

·         General-purpose spectrum analyzers

·         Special-purpose power system harmonic analyzers

·         Digital Harmonics Measuring Equipment

·         Distortion Analyzers

·         Data Logger

14.  What is Spectrum analyzer?

            An instrument used for the analysis and measurement of signals throughout the electromagnetic spectrum.

Spectrum analyzers are available for sub audio, audio, and radio-frequency measurements, as well as for microwave and optical signal measurements.

15.  What is the operation of spectrum analyzer?

            A spectrum analyzer separates the signal into two components: amplitude (displayed vertically) and frequency (displayed horizontally). In some low frequency analyzers, phase information can also be displayed.

·         Low-frequency analyzers are sometimes called as "Harmonic analyzers"

·         Vertical scale displays the amplitude and horizontal scale displays the frequency.

16.  What is Swept heterodyne technique?

         Any signal at the input, at a frequency such that the difference between its frequency and the local oscillator is within the bandwidth of an intermediate- frequency filter, will be detected and will vertically deflect the spot on the display by an amount proportional to the amplitude of the input signal being analyzed.

17.  What is FFT (or) digital technique used for harmonic analysis?

         The signal to be analyzed is converted to a digital signal by using an analog to digital converter, and the digital signal is processed by using the FFT algorithm.  The algorithm analyzes the time domain waveform, computes the frequency components present, and displays the results.

18.  What are the advantages of FFT?

·         FFT technique is much faster.

·         Measurement is virtually real time.

19.  What are the disadvantages of FFT?

·         Restricted to lower frequencies.

·         Complex due to need of A/D converter.

20.   What is the use of digital storage?

         Digital storage gives the effect of a constant display, even though a very slow sweep may have been used to acquire the displayed data.

21.  What is tracking generator?

         The tracking generator enhances the applications of spectrum analyzers. Its output delivers a swept signal whose instantaneous frequency is always equal to the input tuned frequency of the analyzer.

22.  What is harmonic analyzer?

            Spectrum analyzers covering up to typically 100 kHz can also be called harmonic analyzers.

Part-B

1.            Bring out the significance of power quality monitoring. What are the important

         power quality monitoring objectives?

2.            Explain proactive monitoring.

3.            Discuss in detail about the selection of power quality monitoring sites.

4.            Explain the steps involved in power quality monitoring. What are the information from monitoring site surveys?

5.            Bring out the important characteristics of power quality variation~.

6.            Write short notes on power quality measurement system. What are the characteristic of power quality measurement equipments?

7.            Write notes on power line disturbance analyzer.

8.      What are the various instruments used for power quality measurements? What are the factors to be considered when selecting the instruments?

9.            Discuss in detail about the instruments used for analyzing non sinusoidal voltage and currents.

10.        Explain Harmonic/Spectrum analyzer.

11.        Define voltage flicker. Discuss some of the flicker sources. Write notes on common methods for mitigation of flicker.

12.      Discuss in detail about the flicker meter.

13.        Explain the modem power quality monitors.

14.        Bring out the applications of expert system for power quality monitoring.

15.        Draw and explain the functional structure of expert systems.

16.        Draw the block diagram of advanced power quality monitoring systems. Explain it.