EE 3183-Laboratory Practice V

BREAKDOWN CHARACTERISTICS OF AIR GAPS Semester 5 Instructed by: Mr. S.B. Wijesundara Group Members: Name : H. N. T. Wijesekara Index No. : 120716 UGroup: G-22 Field : Electrical Engineering Date of Perform : 29/07/2015 Date of Submission: 11/08/2015 H.N.T.Wijesekara120716U L.S.Wijesinghe120720C W.M.K.M.Wijesooriya120724R D.T.P.Wijesooriya120726B V.Y.Wong120737J CALCULATIONS Gas density correction factor is given by, ( )( ) Here P- Atmospheric pressure in mmHg T- Room temperature in 0C Hence Breakdown voltage (

) at standard pressure and temperature is given by,

Here Vis the Breakdown voltage at pressure P and temperature T. Specimen calculation for a rod type electrode when the gap distance of 0.5 mm ( )( )

Since the transformer ratio is 220V/100 kV, breakdown voltage at S.T.P is given by

= 10.811 kV Similarly breakdown voltage at S.T.P can be calculated for other instances Gap Distance (mm) Breakdown Voltage at S.T.P (kV) Rod typePlate typeSphere type 0.510.81111.75112.221 1.013.63215.98222.093 1.515.98225.85331.964 2.018.80235.72439.955 2.522.56340.42551.706 3.026.32346.06658.757 3.530.08456.40763.928 4.032.43461.10769.568 4.533.84462.04876.149 Breakdown Voltages of Air Gaps For Different Electrode Types Gap Distance (mm) Breakdown Voltage at S.T.P (kV) Rod typePlate typeSphere type 0.510.81111.75112.221 1.013.63215.98222.093 1.515.98225.85331.964 2.018.80235.72439.955 2.522.56340.42551.706 3.026.32346.06658.757 3.530.08456.40763.928 4.032.43461.10769.568 4.533.84462.04876.149 01020304050607080900 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5Breakdown Voltage (kV) Gap distance (mm) Breakdown Characteristics of Air Gaps For Diffenrent Electrode Types Rod Plate Sphere Power (Sphere) Power (Plate) Power (Rod )DISCUSSION 1.HV lab arrangement with diagrams The laboratoryhas of two sections,namely thecontrolling section (located in upstairs) and the testing area (located in downstairs). All the controllingpanels are located in controlling section. This is to ensure the safety of us.The testing area is covered using a perfectly earthed metal mesh for the protection purposes and to avoid the unexpected entrances during the testing periods. There is an earthing rod to earth the testing equipment before touching. 2.Necessity & usefulness of safety procedures taken in conducting the experiment High voltage experiments always deal with very high voltages and the hazards can occur if not carefullyconducted.Occurrenceofflashoversthroughairispossiblewhenconductingthese experiments.Eventhoughthesupplyisdisconnectedtherecanbestaticchargesinthe equipment and might cause electric if touched even if the person is wearing shoes. Hence it is essential to obey necessary safety precautions. Indication bulb There is a bulb in front of the High Voltage laboratory which indicates whether there is an ongoing practical or not. Iron Mesh and Warning sign TheHighVoltagetestareaisseparatedfromthelaboratoryarea,byusing2mhighmesh fence. Also there is a warning sign in the gate to mentioning that is a high voltage test area and do not enter. Entrance Impulse Generator & other related equipment Other equipment HV DC Capacitor Earthing rod Iron Mesh Inter lock Equipment used for breakdown of air gaps practical Ground rod It is necessary to earth all the equipment in order to ensure the safety first. Therefore, there isagroundingsystem(aspecialgroundrod)toeliminategettinganelectricshock,while workingwiththehighvoltageequipment.Thatisnormallylocatedattheentranceofthe test area. During the practical we have to make some adjustment in the high voltage equipment, such as changing the gap distance etc. But there might be some remaining static electric charge, duetotheprevioustestingandpeoplemightgetshockwhentouchingthatequipment.So we have to discharge all these remaining charges by using the special ground rod. Interlocking system Thereisaspecialinterlockingsystem(interlockingswitch)inlaboratory,toensurethe safety.If someoneaccidentlyenters to the highvoltage test area during the test, he or she hastoplugouttheinterlockingswitchfirst.Whenopeningthatswitch,thepowersupply for high voltage equipment will be automatically disconnected. Separate floors High voltage lab designed with two floors where the high voltage supply transformers and equipment are placed in ground floor and the control and supervision is done in upper floor. Andalsogroundfloorisvisibletotheupperfloor.Thisseparationisprovidedprotection for people. Safety relays AlltheHVgeneratorsareprotectedwithsafetyrelaysinordertogivethemaximum protection. These relays automatically disconnect immediately after a HV discharge during an experiment. 3.Methods of HVDC and HVAC generation HVAC generation Cascade arrangement Cascadearrangementcanbecontinuedfurtherifahighvoltageisrequired.Thismethod used to reduce the insulation cost and to make transportation easier. Following figure shows the principle cascade arrangement. But the actual arrangement may be differ from that due to practical reasons.

Resonant transformers ResonanceprincipleofaseriestunedL-Ccircuitcanbemadeuseoftoobtainahigher voltage with a given transformer. Following figure shows the resonance principle at power frequency. Air cored coils are used to simplify the construction and the insulation. HVDC Generation Economical voltage transformation can only be done using AC power transformers. Hence, to generate HVDC first high voltage AC is generated and then this HVAC is converted to HVDC byusingrectificationmethodsavailable.Mostcommonlyusedmethodofrectificationused here is thyristor rectification. One of the methods of producing high direct voltages for testing is to use either a half wave or full wave rectifier circuit, with a high AC voltage source. This produced a peak inverse voltage of at least twice the peak value of the AC supply. A low pass filter may be used to smooth the output, but when the test device is highly capacitive then no smoothing is needed. Figures are shows the half-wave and the full wave arrangements. For Power transmission, direct current is used for transmission of electric power in bulk, rather thanthemorecommonlyusedalternatingcurrentsystems.Forremotetransmission,HVDC systemscanbelessexpensivesincetheysufferfewerelectricallosses.PowercablesHVDC submarinetoavoidheavycurrentsrequiredbythecablecapacity.HVDCallowspower transmission between the alternative distribution systemswhich are not synchronized, and can increase system stability by preventing the spread of a cascading failure in a network of power transmission to the next set. 4.Need of 2 M Resistor Breakdownvoltageofgaseousinsulationsuchasairisveryhighandisatabout30kV/cm stress.Breakdownofgassesoccursduetoavalancheeffectandmuchhighercurrentflowsin breakdown.Thishighercurrentcandamagetotheequipmentusedintheexperimentand nonlinear 2M resistor is used to limit this current. 5.Factorsthat affect the breakdown voltage of air for a given pair of electrodes Electrode separation For vacuum gaps less than about 1 mm the breakdown voltage is approximately proportional to the length, all other parameters remain constant gives constant breakdown strength. For gaps greaterthanabout1mmthebreakdownvoltagedoesnotincreaseatanequalrateandthe apparent breakdown strength for larger gaps is much reduced. Temperature The variation of breakdown voltage with temperature is very small but cooling the electrodes to liquid Nitrogen temperature increases the breakdown voltage Frequency of applied voltage It is known that a given gap stands a higher impulse voltage than an alternating voltage, and a higheralternatingvoltagethanadirectvoltage.However,ithasbeenshownthatforasmall gap (2 mm) there is no dependence of the breakdown voltage on the frequency in the range 50 Hz to 50 kHz. Vacuum Pressure For smallgaps, decreasing in vacuum pressure increases the breakdownvoltage until belowa certainpressurethereisnochange.Thevacuumbreakdownregionistheregioninwhichthe breakdownvoltagebecomesindependentofthenatureofthepressureofthegapbetweenthe electrodes.However,forlargegaps(about200mmspacing)itisfoundthatbelowacertain pressurethebreakdownvoltagestartsdecreasingagain,sothatthebreakdownstressatthis stage could in fact be improved by actually worsening the vacuum. Conditioning of electrode Thebreakdownvoltageofagapincreasesonsuccessiveflashovers,untilaconstantvalueis reached. The electrodes are then said to be conditioned. Material and surface finish The breakdown characteristics of a given gap space is highly depend on the material of that electrodes. And also smoothness of the surface of the electrode is greater the breakdown voltage. Surface contamination Presence of the contamination on the electrode surface causes the breakdown voltage to reduce by nearly 50% of the clean electrode value. Area and configuration of electrodes A breakdown voltage significantly decreases with increase in surface area. So therefore, it is very difficult to maintain the given breakdown voltage.