UNIT – 3: OVERVOLTAGE TRANSIENTS TWO NARKS

UNIT – 3: OVERVOLTAGE TRANSIENTS

TWO NARKS

1. Define transient over voltages.

     A transient over voltage can be defined as the response of an electrical network to a sudden change in network conditions, either intended or accidental, (e.g. a switching operation or a fault) or network stimuli (e.g. lightning strike).

2. What are the' types of transient overvoltages?

1)                     Impulsive                     2) Oscillatory

3. Define impulsive transients. Give example for impulsive transient over voltages .

     An impulsive transient is a sudden, non-power frequency change in the steady state condition of the voltage and/or current waveforms that is essentially in one direction, either positive or negative, with respect to those waveforms.

       The most common cause of this type of transient is lightning.

4.         Give examples for oscillatory transient over voltages.

Switching operations within the distribution network are a major cause of oscillatory transient over    voltages. Such operations include

(a) Switching of utility capacitor banks,

(b) Switching of circuit breakers to clear network faults, and

(c) Switching of distribution feeders to rearrange the network for maintenance or construction

5.         What is the effect of capacitor switching transients on network?

                  Transients of this magnitude and duration are usually not a problem on the utility system, but they can produce problems at a user facility.

            Severe over voltages can appear on user facility capacitors through a phenomenon known as voltage magnification

6.           What are the causes of voltage magnification on network?

            The voltage magnification will not result in capacitor damage. The problem that usually occurs is the failure or mis-operation of sensitive loads in the facility where the low voltage capacitors are installed.

7.            Define voltage magnification phenomena?

                  The highest transient voltages occur at the low voltage capacitor bank when the characteristic frequency of the switching transient is nearly equal to the resonant frequency of the low voltage system and when the switched capacitor is ten or more times the size of the low-voltage capacitor

8.            Mention the two important concerns for capacitor bank switching transients.

            Voltage transients at the capacitor bank substation and neighboring substations

Power quality impact on sensitive customer loads due to variations in voltage when energizing capacitor banks

9.            Give the various aspects of equipment specific design and protection issues for the capacitor switching transients.

·      Phase-to-ground and phase-to-phase insulation switching withstand to voltage stresses

·      Controlled closing for circuit breakers (pre-insertion resistors/reactors or

            synchronous switching)

·      Capacitor bank and substation Circuit breakers ANSVIEEE C37 requirements ./ Current      limiting reactor requirements

10.         What specify the IEEE standard for shunt power capacitors causing transient overvoltages?

            The IEEE Standard for Shunt Power Capacitors, ANSI/IEEE Std. 18-1992, specifies that capacitors "may    reasonably be expected to withstand" transient overvoltages from 205% to 354% of rated peak voltage, depending on the number of times a year the overvoltage occurs.

11.What are the various Causes of overvoltages?

                 Overvoltages, i.e. brief voltage peaks (transients, surges, spikes), can be attributed to the following main causes:

1.Atmospheric discharges, i.e. lightning (LEMP - Lightning Electro-Magnetic Pulse)

2.Switching operations in the public grid and low-voltage mains

3.Electrostatic Discharges (ESD)

4.Ferroresonance

12.Give tile basic principles of overvoltage protection of load equipments.

    Limit the voltage across sensitive insulation.

Divert the surge current away from the load.

Block the surge current entering into the load.

Bonding of equipment with ground

13.What is the need of surge arrestors?

   A surge arrester is a protective device for limiting surge voltages on equipment by discharging or bypassing surge current.

               Surge arresters allow only minimal flow of the 50Hz/60Hz power current to ground.

14.Differentiate between transient voltage surge suppressors (TVSS) and surge arrestors.

   Arresters and TVSS devices protect equipment from transient overvoltages by limiting the maximum voltage, and the terms are sometimes used interchangeably. However, TVSSs are generally associated with devices used at the load equipment.

A TVSS will sometimes have more surge-limiting elements than an arrester.

15.Mention the types of surge arrestors

   Metal-oxide varistor type

Gapped silicon - carbide type

16.What is metal-oxide surge-arrester?

                 A metal-oxide surge-arrester (MOSA) utilizing zinc-oxide block provides the best performance, as surge voltage conduction starts and stops promptly at a precise voltage level, thereby improving system protection

17.Give any two advantages of metal-oxide arresters over conventional silicon carbide distribution class arresters.

Improved Surge Duty Capability

Improved Temporary Overvoltage Capability

18.  What is the need of Transmission Line Arresters?                         '.

     Transmission Line Surge Arresters conduct lightning surges around the protected insulator so that a lightning flashover is not created.

                 They are designed to be installed functionally in parallel with the line insulator. The arrester conducts the lightning surges around the protected insulator so that a subsequent 50Hz / 60 Hz fault on the circuit is not created.

19.Mention the Benefits of Transmission Line Surge Arresters

·      Lowers initial cost of new or transmission line upgrades by making construction more compact and transmitting more energy in the same right of way.

·      Reduces the height of transmission lines by eliminating shield wire

·      Improves outage statistics by eliminating back flashover from the tower ground lead to the phase conductor

20.What is the role of surge arrestor on shielded and unshielded transmission line?

                 On shielded transmission lines or under-built distribution circuits, the arrester prevents tower to phase insulator back-flashovers during a lightning strike.

 On unshielded sub transmission or distribution circuits, the arrester prevents phase-to-ground flashover.

21.What is the need of low pass filter in transient protection?

·      This LC combination provides a low impedance path to ground for selected resonant frequencies.

·      Low-pass filters employ pi principle to achieve better protection even for high- frequency transients.

22.What is tire need of Shunt protectors or surge reduction filters?

o  An in-line filter specifically designed to reduce the rate of voltage rise (dv/dt) of the pre-clamped waveform.

o  It gives some series impedance between input and output terminals. This type of product is highly recommended for the protection of sensitive electronic equipment

23.  What is the application of Power Conditioners in transient protection?

                  Low-impedance power conditioners are used primarily to interface with the switch-mode power supplies found in electronic equipment. Low-impedance power conditioners differ from isolation transformers in that these conditioners have much lower impedance and have a

filter as part of their design

1.      When on the device to position the power conditioners to avoid voltage swells.

24.  Differentiate between TVSS, Filter and Data/signal protection devices.

       Transient: focus on limiting high-voltage spikes to an acceptable level.

 Filtering: protect against low-energy transients and high frequency noise and finally Data/signal protection devices: Products that guard sensitive instrumentation against what we refer to as 'back door' transients and noise

25. Define lightning phenomena.

                  Lightning is an electrical discharge in the air between clouds, between different charge centre within the same cloud, or between cloud and earth (or earthed object).

                  Even though more discharges occur between or within clouds, there are enough strokes that terminate on the earth to cause problems to power systems and sensitive electronic equipment

26. How Overvoltages are induced due to lightning?

·         When lightning strikes occur in or near an electricity distribution system, lightning currents are generated and conducted through the power system into connected equipment.

·         Large impulsive transient over voltages are produced as a result of this current flow.

27. What are the various causes due to lightning overvoltages?

·         In transmission systems, the insulation is generally sufficient enough not to be endangered by induced voltages. However, distribution systems in which the insulation level is low, induced voltages are hazardous.

·            When the induced voltage caused by lightning exceeds the strength of the insulation, a line flashover results, causing either temporary faults or disruption of services to customers

28.  What is the range of current induced due to lightning stroke?

                  The majority of the cloud to ground lightning strokes varies from kilo-Amperes to several tenths of kilo-amperes. Strokes above 100000 amperes are rare, and the highest reported peak value of the return stroke current is 200000 A. The shape of the current wave and the related voltage wave is rather capricious and different for every stroke.

29.  What is ferroresonance?

                  Ferro resonance is a special case of series LC resonance where the inductance involved is nonlinear and it is usually related to equipment with iron cores. It occurs when line capacitance resonates with the magnetizing reactance of a core while it goes in and out of saturation.

30.  Define ferroresonance phenomena.

                  The phenomena of ferroresonance is a name given to a situation where the nonlinear magnetic properties of iron in transformer iron core interact with capacitance existing in the electrical network to produce a nonlinear tuned circuit with an unexpected resonant frequency. This phenomenon poses a hazard to an electric power system because it generates overvoltages and over currents.

31.  What are the problems associated with ferroresonance?

Transformer overheating

Audible noise

High overvoltages and surge arrester failure

32. What are the various capacitance produced by power system elements?

a)                  The circuit-to-circuit capacitance

b)                  Parallel lines capacitance

c)                  Conductor to earth capacitance

d)                 Circuit breaker grading capacitance

e)                  Bus bar capacitance

f)                   Bushing capacitance

33. List some common circumstances leading to ferroresonance problems.

                  Some common circumstances leading to ferroresonance include transformer fuse blowing, line or switch fuse blowing, energizing a new transformer by manual cable switching up-line from transformers, cable connector or splice opening, manual cable switching to reconfigure a cable circuit during emergency conditions, and open conductor fault in overhead line feeding cable.

34. Mention the common methods used for utility for protecting distribution transformer.

                  Generally utility to provide two common ways for protecting distribution transformer:

1.      Use transformers with interlaced secondary windings.

2.      Use surge arresters at low voltage terminals

35. Which are the most widely used protection devices in protection of transformer? Also  list the types     mostly used.

                  Usually, in distribution transformers, MOV type surge arresters are used for overvoltage protection.

                        External Live Front arresters

Under Oil arresters

External Elbow arresters

36. Give the cable life equation as a function of impulses.

                  The cable life is an exponential function of the number of impulses of a certain magnitude that it receives, according 10 Hopkinton. The damage to the cable is related by

Dc = P . Ve
P =       Number of impulses

                               V =       Magnitude of impulses

                                e =       empirical constant ranging from 10 to 15

37. List the important types of arrestor used in protection of cable.

       (i) Under oil arresters

(ii) Elbow arresters

 (iii) Lower discharge arresters

38. What is the need of Computer analysis tools for transient studies?

                  Computer analysis simulation tool can simulate the time response of the transient phenomena in the power system with a very high degree of accuracy.

39. List the advantages of computer analysis tools for transient studies.

                  The application of commercial time-domain simulation packages (PSCAD/EMTDC) has many advantages over the conventional mathematical analytic methods, in terms of:

1.      There being no need to develop a complex nonlinear differential equation for the system;

2.      The complex power system phenomena such as ferroresonance, harmonics, etc being easily modeled with modules of lines, transformers, power converters, loads and protection equipment from the library.

40. What is the need of PSCAD/EMTDC?

·         EMTDC (Electromagnetic Transients including DC) represents and solves differential equations for both electromagnetic and electromechanical systems in the time domain.

·         Solutions are calculated based on a fixed time step, and its program structure allows for the representation of control systems, either with or without electromagnetic or electromechanical systems present

41. Give any two analysis examples available in PSCAD/EMTDC?

      Transient Studies

·      Transient over voltage studies (TOV)

·      Line energizing (charging and discharging transients)

·      Capacitor bank back to back switching, selection of inrush and out-rush reactors

Power Quality

·      Voltage dips, swells and interruptions

·      Induction motor starting

·      System faults

·      Voltage fluctuation

42. How to model a surge arrestor in PSCAD? (Anna University April/may-2008)

     The frequency dependent model which was recommended by IEEE WG 3.4.11 (1992) is the most accurate representation based on single phase line model.

PART – B

1.What are transient overvoltages? Explain the different types of transient overvoltages.

2.Draw the CBEMA curve for transient overvoltages and explain

3.What are the different sources of transient overvoltages? Discuss the Capacitor switching transient.

4.What are the important concerns for capacitor bank switching?

5.Define lightning? Discuss in detail about the overvoltages due to lightning and the problems associated with it.

6.Explain in detail the mechanism of lightning.

7.Draw the standardized waveform of the lightning induced voltage. Discuss about

      the wave shape of the lightning current.

8.Explain the phenomena of ferroresonance.

9.Explain the problems associated with ferroresonance.

10.What is the need for protection against overvoltages? What are the basic

       principles of overvoltages protection of load equipments?

11.Explain in detail about various methods to mitigate voltage swells

12.Explain in detail about the surge arrestors and surge suppressors.

13.What are the advantages of surge arrestors? Discuss about the application module.

14.Explain the following:

(a)   Low pass filters (b) Power conditioners (c) Surge filters