VARIOUS TESTING TOOLS - INTRODUCTION By P. V. N. S. SARMA, Asst. Engr., NAGARJUNASAGAR SUBSTATION
VARIOUS TESTING TOOLS - INTRODUCTION WHAT IS A TEST ? It is a trial to detect and examine the parameters / properties of a specimen. It gives us an insight idea of the inherent conditions of specimen.
VARIOUS TESTING TOOLS - INTRODUCTION What is a TESTING TOOL ? A testing tool is a device which assists us in carrying out a test. It senses the parameters / properties of the specimen and presents the results in an understandable form to the operator. Ex : A Stethescope, a Thermometer, or a Multimeter, etc.
VARIOUS TESTING TOOLS - INTRODUCTION Why do we need to test ? • To ensure / confirm that the equipment has met the design parameter / specifications. • To ensure healthiness of the equipment and to take corrective action if necessary • To identify the root cause of the defect (diagnosis) and for analysis of the same. • To take corrective action in case of deviation. • To observe the trend in variation of parameters. Finally, the need to test is to ensure that the equipment lives its complete expected life without premature failure and to perform its functions as envisaged in its design.
VARIOUS TESTING TOOLS - INTRODUCTION Raw Material used in an Electrical equipment can be classified into two types 1. Conductor : Ex. Copper, Aluminum etc 2. Insulator : Ex. Porcelain, Glass, Paper etc.
VARIOUS TESTING TOOLS - INTRODUCTION CONDUCTOR :A conductor has properties of low resistance (high conductivity) & inductance A conductor is used to transmit electric current in the intended path. & INSULATOR : An insulator has properties of high resistance & capacitance. An insulator is used to restrict the flow of current in the intended path only. Failure of insulation generally leads to the failure of an equipment
VARIOUS TESTING TOOLS - INTRODUCTION • Any equipment will be healthy as long as its raw materials retain their natural properties. HENCE, The properties of the raw material used in the equipment forms the basis for the method of testing. & Finally, “ THE LIFE OF AN EQUIPMENT IS THE LIFE OF ITS RAW MATERIAL ”
The business of POWERGRID is “Transmission of bulk power at Extra High Voltage”. This is being done through a network of transmission lines and substations spread across the country.
VARIOUS TESTING TOOLS - INTRODUCTION An EHV Substation generally comprises of the following Equipment a. Transformers b. Reactors c. Circuit Breakers d. Capacitor Voltage Transformers e. Current Transformers f. Isolators & Earth Switches g. Lightning Arrestors h. Protective relays i. PLCC Equipment j. Auxiliaries like Firefighting system, AC plant, DG Set etc
VARIOUS TESTING TOOLS - INTRODUCTION TRANSFORMERS What is a Transformer ? It is a static device which transforms power from one voltage to another voltage at the same frequency.
VARIOUS TESTING TOOLS - INTRODUCTION DIFFERENT PARTS OF A TRANSFORMER A. Windings B. Core C. Transformer Oil D. Bushings E. Conservator F. Radiator Banks etc G. Protective Devices like PRD, Buchholz Relay etc
VARIOUS TESTING TOOLS - INTRODUCTION TESTS CONDUCTED ON A TRANSFORMER A. Winding Resistance : This test is done to measure the resistance of the windings in order to check any abnormalities due to loose connections, broken strands etc. This test is conducted between a. HV terminal & IV terminal b. Between IV terminal & Neutral terminal at all taps and c. Between line terminals of Tertiary winding. The temperature of the winding is to be recorded at the time of measurement of resistance. The name of the Kit used for this test is TRM 10 / TRM 25 which is of SCOPE make. The test results obtained are to be compared with the precomissioning results.
VARIOUS TESTING TOOLS - INTRODUCTION B. INSULATION RESISTANCE:This test is conducted to check the healthiness of transformer insulation. The windings are covered with paper insulation. The condition of paper insulation is revealed in this test. ie., degree of dryness, contamination, presence of foreign particles etc. This test is carried out by means of a 5 KV Megger and the results obtained are compared with precomissioning test results. The readings depend upon the temperature, applied voltage and duration of application of voltage.
VARIOUS TESTING TOOLS - INTRODUCTION The test is done in the following combinations for an auto transformer : The Minimum value of Insulation resistance should not be less than 500 MΏ at 30°C after 60 Seconds. There are two types of tests called the “DIELECTRIC ABSORPTION INDEX ” and “POLARIZATION INDEX”
VARIOUS TESTING TOOLS - INTRODUCTION • DIELECTRIC ABSORPTION INDEX It is given by the ratio of 60 Second IR Value to 15 Second IR Value. This value should be more than 1.3. • POLARIZATION INDEX It is given by the ratio of 600 Second IR Value to 60 Second IR Value. The Insulation is evaluated using Polarization Index as follows :
VARIOUS TESTING TOOLS - INTRODUCTION C. CAPACITANCE & TAN δ MEASUREMENT δθ V This test indicates the quality and soundness of the insulation.
VARIOUS TESTING TOOLS - INTRODUCTION This test is carried out using a capacitance & tan δ measurement kit. This kit employs the principle of Schering Bridge for measurement. The maximum permissible value of Tan δ is 0.007 Make of Kits commonly used in POWERGRID are Biddle, Tettex, Eltel etc.
VARIOUS TESTING TOOLS - INTRODUCTION UST MODE : Test set connected for un grounded specimen test mode. This is used when specimen is isolated from earth. Ex : Transformer bushing, CT’s with test tap etc. GST MODE : Test set connected for grounded specimen test mode. This is used when specimen do not have two specific points isolated from ground for measurement. Ex: Transformer / Reactor windings, CT’s without test tap etc.
a. For Bushings : The test is conducted between the HV terminal and test tap of the bushing in UST mode of the kit.
VARIOUS TESTING TOOLS - INTRODUCTION b. For Windings : The test is done in the following combinations for an auto transformer for winding
VARIOUS TESTING TOOLS - INTRODUCTION D. VOLTAGE RATIO TEST : This test is done to determine the turns ratio of transformers. This test is done at all taps of a transformer. This tests helps us to find out shorted winding turns, which disturbs the turns ratio of the transformer. The turns ratio tolerance should be within 0.5%
VARIOUS TESTING TOOLS - INTRODUCTION CIRCUIT BREAKERS : A Circuit Breaker is switching and current interrupting device. DIFFERENT PARTS OF A CIRCUIT BREAKER A. Interrupter B. Pre insertion Resistors C. Grading Capacitors D. Pole Column E. Drive Mechanism F. Control Cubicle
VARIOUS TESTING TOOLS - INTRODUCTION TESTS CONDUCTED ON A CIRCUIT BREAKER A. Static Contact Resistance B. Timings C. Tan δ and Capacitance of Grading Capacitors D. Insulation Resistance E. Dew Point Measurement of SF6 gas. F. Dynamic Contact resistance measurement ( DCRM )
VARIOUS TESTING TOOLS - INTRODUCTION A. STATIC CONTACT RESISTANCE : This test is conducted to measure the contact resistance of main contacts of Circuit Breaker. This test is conducted with Circuit Breaker in CLOSED position. This test indicates the wear out and misalignment of the main contacts if any. Increase in resistance causes overheating of contacts and hence presence of high contact resistance if any should be attended to immediately. The Maximum permissible limit of contact resistance for a 400 KV Circuit Breaker is 150μΏ ( for both the contacts).
VARIOUS TESTING TOOLS - INTRODUCTION B. TIMINGS : This test is carried out to find out the operating timings of circuit breaker including Pre Insertion Resistor Timings. Timings of the following operations are measured in this test. i. Open ( O ) ( for TC – I and TC – II ) ii. Close ( C ) iii. Close Trip ( C – O ) ( for TC – I and TC – II ) iv. Trip close Trip ( O – C – O ) v. Pole Discrepancy ( PD ) Kits Generally used for above test are CBT -4, HISAC 2406, HISAC ULTIMA .
VARIOUS TESTING TOOLS - INTRODUCTION From test results, it should be ensured that the following conditions are satisfied a. The PIR closes first and then the Main contacts close during closing operation. b. The PIR opens first and then the Main contacts open during opening operation. The above conditions hold good for AEG, ABB, AREVA & CGL Breakers.
VARIOUS TESTING TOOLS - INTRODUCTION DYNAMIC CONTACT RESISTANCE MEASUREMENT ( DCRM ) A DCRM is carried out on circuit breakers to monitor the condition of main and arcing contacts of a Circuit Breaker without opening the interrupter physically. Why DCRM ?
VARIOUS TESTING TOOLS - INTRODUCTION A Circuit Breaker consists of two sets of contacts per break. Viz., arcing contacts and Main contacts. During closing the arcing contacts close first and then the main contacts close. Hence an arc is struck between arcing contacts only and thereby the main contacts are protected against arcing.
VARIOUS TESTING TOOLS - INTRODUCTION During opening of CB, the main contact open first followed by the arcing contacts and hence an arc is struck between arcing contacts. The function of main contacts is to carry current in closed condition of circuit breaker.
VARIOUS TESTING TOOLS - INTRODUCTION In Static contact resistance measurement the resistance of breaker contact is measured with breaker in CLOSED condition. The resistance obtained is of Main Contacts. Hence the resistance of arcing contacts and therefore its condition is not known.
VARIOUS TESTING TOOLS - INTRODUCTION DCRM is a very sensitive measurement in which a continuous measurement of resistance is made in CLOSE – OPEN Operation of a circuit breaker. The test is made at a sampling frequency of 10 kHz whereby the variation of the resistance of the circuit breaker contacts in motion is obtained in a graphical manner.
VARIOUS TESTING TOOLS - INTRODUCTION Using a DCRM we can find out the inherent defects in moving mechanism, linkages and contacts without dismantling the breaker and physical inspection.
VARIOUS TESTING TOOLS - INTRODUCTION D.Dew Point Measurement of SF6 gas : SF6 gas has excellent arc quenching properties and hence the same is being used in EHV breakers. Dew Point of SF6 gas is a measure of its purity and it indicates the change in the value of dielectric properties. The unit of Dew Point is °C and it is the temperature at which moisture content in SF6 gas starts condensing.
VARIOUS TESTING TOOLS - INTRODUCTION CAPACITOR VOLTAGE TRANSFORMER(CVT ) A Capacitor Voltage Transformer transforms High voltage to a low voltage for the purpose of measurement. It is also used for Coupling of High Frequency Protection and Speech signals for the purpose of PLCC ( Power Line Carrier Communication) Ratio of CVTs used in POWERGRID are 400 kV / 110V. It consists of a potential divider circuit employing two capacitances ( C1) and (C2). The voltage across C2 is fed to an intermediate transformer which steps down the voltage to the order of 110V.
VARIOUS TESTING TOOLS - INTRODUCTION CURRENT TRANSFORMER ( CT ) : A Current Transformer transforms high current to a low current for the purpose of measurement & Protection. Difference between an ordinary transformer and a current transformer : In ordinary transformer, the primary current is dependent on the Secondary current ( Load ) Whereas, In a Current Transformer, the secondary current is a function of primary current. It is independent of the loading of the secondary winding. Rating of CT s is of the order of 1A or 5A ( Secondary current )
VARIOUS TESTING TOOLS - INTRODUCTION TESTS CONDUCTED ON A CT Tan δ and Capacitance Tan δ test for CTs with a test tap is carried out by connecting HV lead to the Primary (HV terminal ) and LV lead to the test tap. The test is carried out in UST mode For CTs without Test tap, the jumpers are to be disconnected, HV lead connected to the Primary ( HV terminal and LV lead to the CT tank or base of the CT. The test is to be carried out in GST mode. B. CT Ratio test C. CT Secondary resistance test
VARIOUS TESTING TOOLS - INTRODUCTION • ISOLATORS & EARTH SWITCHES : An Isolator is a disconnecting device. It is used to isolate an equipment and is operated only on NO LOAD. TEST ON ISOLATORS : CONTACT RESISTANCE This test is carried out to measure the contact resistance of the contacts of an isolator. The contact resistance of isolators should be less than 300μΏ. Presence of high resistance indicates improper contact caused by loose connection, improper alignment or pitting of contacts. Corrective action should be taken accordingly, otherwise it may lead to a HOT SPOT.
VARIOUS TESTING TOOLS - INTRODUCTION EARTH SWITCH : An Earth switch is used to earth the equipment and to discharge the charge. CONTACT RESISTANCE TEST is the test carried out on this equipment. DMO 100 of Eltel make and CRM 100 of SCOPE make kits are used for this test.
VARIOUS TESTING TOOLS - INTRODUCTION LIGHTNING ARRESTOR (OR) SURGE ARRESTOR A SURGE ARRESTOR diverts Over voltages to earth and protect substation equipment from over voltages. A surge arrestor is provided near every important equipment like transformer, Bay entry etc.
VARIOUS TESTING TOOLS - INTRODUCTION TESTS ON LA Measurement of Leakage Current ( III Harmonic Resistive current ) PURPOSE : A Surge Arrestor continuously conducts a small leakage current. The resistive component of this leakage current may increase with time due to different stresses caused due to ageing and may lead to failure. Hence to monitor the health of Surge Arrestor, III Harmonic Resistive current from the leakage current is monitored.
VARIOUS TESTING TOOLS - INTRODUCTION Kit used for measurement of III Harmonic Resistive current is called Leakage Current Monitor. Make of kit being used in POWERGRID is Trans i nor. The maximum permissible current for LAs in service is 150 micro Amps.
VARIOUS TESTING TOOLS - INTRODUCTION GENERAL TESTS : • DISSOLVED GAS ANALYSIS ( DGA) This test is performed on equipment filled with insulating oil like Transformers and Reactors. The device used for performing DGA is called Gas Chromatograph.
VARIOUS TESTING TOOLS - INTRODUCTION SWEEP FREQUENCY RESPONSE ANALYSIS(SFRA) This test is made to assess the mechanical integrity of the transformer. Transformers experience severe mechanical forces during short circuit which may cause deformation of windings insulation, core etc. These changes cannot be detected through conventional condition monitoring techniques such a Dissolved Gas Analysis. Winding resistance measurement, Capacitance and Tan Delta measurement etc. Sometime even during transportation, the transformer may experiences severe shocks which may cause internal mechanical displacements / damages.
VARIOUS TESTING TOOLS - INTRODUCTION In SFRA, a Variable frequency ( of the order of 5 Hz to 2 Mhz ) voltage is fed to the equipment and the response is plotted in a graph of Frequency Vs dB. The Signature is compared with previous signature and deviations if any are analysed. The Kit used is of DOBLE make.
VARIOUS TESTING TOOLS - INTRODUCTION GENERAL PRECAUTIONS OF TESTING • Ensure that the test environment, kits and accessories are clean and free from dust. • Read Manufacturer’s instruction before operation of kit. • Ensure compliance of safety precautions of the instrument manufacturer. • The Ground cable should be connected first and removed last. • Clean the porcelain parts of test specimen with clean cloth to remove dust, moisture etc before testing. Otherwise the same may lead to erroneous results.
VARIOUS TESTING TOOLS - INTRODUCTION GENERAL PRECAUTIONS OF TESTING • Always try to carry out testing in a fair weather. Avoid testing during atmospheric extremities. • Always record the ambient temperature / temperature of the specimen wherever applicable. • Be extremely careful while working with sensitive parts of the equipment like CT Secondary terminals, test taps, aircell of conservator, PRD, Pressure switches, Density monitor, gaskets etc.
VARIOUS TESTING TOOLS - INTRODUCTION GENERAL PRECAUTIONS OF TESTING • Connecting leads from specimen to kit should be as short as possible, without joints and ( even insulated lead ) shall not touch tank or earth. • IR measurement should be performed with windings and leads completely immersed in oil. Never perform the tests with equipment under vacuum • Ensure that the test taps of CTs and Earth connection of CVTs are reconnected to ground after completion of test. • Ensure that the test leads make a firm contact with the test specimen. If required, clean the surface of the equipment where leads are to be connected with a fine emery paper. Loose connections and improper contact may lead to sparking and erraneous results.
VARIOUS TESTING TOOLS - INTRODUCTION GENERAL TERMS RELATED TO TESTING a. PTW : Permit to Work b. SFT : Sanction for Test c. PSI : POWERGRID Safety Instruction d. EHV : Extra High Voltage e. OTI : Oil Temperature Indicator f. WTI : Winding Temperature Indicator g. PRD : Pressure Relief Device h. DGA : Dissolved Gas Analysis i. OLTC : On Load Tap Changer j. NGR : Neutral Grounding Reactor k. REF : Restricted Earth fault l. HF Terminal : High Frequency Terminal
VARIOUS TESTING TOOLS - INTRODUCTION SAFETY IN TESTING • Ensure that the equipment under test is totally isolated and earthed. • Obtain a Permit to Work ( PTW ) before commencement of work • Use Personal Protective Equipments ( PPE ) • Prefer to have an additional local earthing at the point of work. • Ensure that the equipment is totally discharged before commencement of work • Never Bypass any Protection / Safety devices of the equipment under test or the testing kit.
VARIOUS TESTING TOOLS - INTRODUCTION SAFETY IN TESTING • Ensure that the testing kit is earthed wherever a provision for the same exists. • Read thoroughly the Manual of the Kit for its operation before testing. • Be thorough of the Procedure, modes, and precautions of the test to be performed. • Discharge the kit and test terminals after completion of test and before being touched by any personnel.