High voltage testing guide

To Be A World Class Maritime Academy HIGH VOLTAGE TESTS HV Course March 2007 To Be A World Class Maritime Academy Objective • Need for periodic testing to guarantee integrity and reliability of HV system • Need for diagnostic test for comparative measurement and determination of fault serenity • Need to simulate fault conditions for verification of system components stability HV Course March 2007 To Be A World Class Maritime Academy Introduction • The high voltage (e.g 6.6 kV) installation covers : the generation, main supply • cables, switchgear, transformers, electric propulsion (if fitted) and a few large motors e.g for side-thrusters and air conditioning compressors HV Course March 2007 To Be A World Class Maritime Academy Condition of HV insulation is governed by factors such as: • Temperature, humidity, surface condition and operating voltage level • Guide by the manufacturers recommendations when testing and maintaining HV insulation • Before applying an IR test to HV equipment its power supply must be switched off, isolated, confirmed dead by an approved live-line tester and then earthed for complete safety in accordance with the current EPTW regulations HV Course March 2007 To Be A World Class Maritime Academy HV Test type • 2) 3) 4) 5) 6) 7) 8) HV Test on board ship -Insulation Resistance (IR) test -Polarity Index (PI) Test -Infrared Imaging test - Circuit breaker test Partial discharge test BIL (Basic Insulation level) test Life test HV Course March 2007 To Be A World Class Maritime Academy 1.0 Insulation Resistance Test (IR) Test • For all electrical equipment the key indicator to High Voltage Equipment Testing are: its safety and general condition of its insulation resistance (IR) - The IR must be tested periodically between phases and between phases and earth • HV equipment that is well designed and maintained, operated within its power and temperature ratings should have a useful insulation life of 20 years • An IR test is applied with a high d.c voltage which applies a reasonable stress to the dielectric material (insulation) • For 6.6 kV rated equipment, a periodical 5000 V d.c insulation resistance (megger) test is recommended • The minimum IR value is usually recommended as (kV + 1) MO where V is the equipment voltage rating e.g 7.6 MO would be an acceptable IR value for a 6.6 kV machine HV Course March 2007 To Be A World Class Maritime Academy Insulation tester Permanent magnet Control coil Deflecting coil α Permanent Magnet Rotor Motor HV Course March 2007 To Be A World Class Maritime Academy Procedure of IR testing • Isolate power, test and prove the conductors are dead by a proven live-line tester • The circuit is earthed • Issue EPTW • It is to be ensured that the operator never touches an unearthed conductor • With the megger reader is connected between conductor and earth, the safety earth is disconnected [The safety earth must be reconnected before the IR tester is disconnected] • A 5000 V dc megger tester now be applied between phases and earth, and between phases, and the values are recorded • The megger test should be applied for minute • The recommended minimum value is (KV rating of the machine + 1) MΩ HV Course March 2007 To Be A World Class Maritime Academy IR Test Procedure • For machines with healthy insulation, an IR test result may indicate a value up to 100 times greater than the recommended minimum • First the reading is checked for minute and for a better test it is checked for 10 minutes • The correct procedure is to connect the IR tester to the circuit under test with the safety earth connection ON • The safety earth may be applied through a switch connection at the supply circuit breaker or by a temporary earth connection local to the test point • This is to ensure that the operator never touches a unearthed conductor • With the IR tester now connected, the safety earth is disconnected (using an insulated extension tool for the temporary earth) • Now the IR test is applied and recorded • The safety earth is now reconnected before the IR tester is disconnected • This safety routine must be applied for each separate IR test HV Course March 2007 To Be A World Class Maritime Academy IR Test Procedure • Large currents flowing through machine windings, cables, bus-bars and main circuit breaker contacts will cause a temperature rise due to I2R resistive heating • Where overheating is suspected, e.g at a bolted bus-bar joint in the main switchboard, the local continuity resistance may be measured and checked against the manufacturers recommendations or compared with similar equipment that is known to be satisfactory • A normal ohmmeter is not suitable- as it will only drive a few mA through the test circuit • A special low resistance tester or micro-ohmmeter must be used which drives a calibrated current (usually I=10 A) through the circuit while measuring the volt-drop (V) across the circuit • The meter calculates R from V/I and displays the test result For a healthy bus-bar joint a continuity of a few megerOhm) would be expected HV Course March 2007 10 PARTIAL DISCHARGES To Be A World Class Maritime Academy • Partial discharges are small electrical discharges that takes place in a gas filled void or on the dielectric of a solid or liquid insulation system • The discharges are basically small arcs that only partially bridge the gap between phase to ground and phase to phase insulation • Partial discharge serves to provide an early warning of an imminent equipment failure • The ultimate failure is the result of the heating effect caused by the discharges • This leads to deeper pits and finally puncture the insulator • Oil impregnated paper deteriorates very rapidly • Some epoxy resin insulators are moderately resistant • Porcelain, ceramics and glasses are practically immune to partial discharges HV Course March 2007 17 To Be A World Class Maritime Academy ACCEPTABLE LIMIT OF PARTIAL DISCHARGES • To except zero discharges is not practical and it is generally acceptable to accept a maximum limit for partial discharges • Experience has shown that a discharge of less than 10 pico coulombs at 0.75% of line to ground voltage is acceptable • [Ref: Page 252, High Voltage Circuit Breakers by Ruben D Garzon] HV Course March 2007 18 To Be A World Class Maritime Academy Partial Discharge and Dielectric Strength • The term ‘Dielectric Strength’ is used to describe the capacity of an insulating material to withstand electrical stresses It is not a constant value • The dielectric strength of a material is considerably influenced by numerous parameters- temperature, form and frequency of voltage, field distribution, size of the stressed volume, duration of stress, etc • If the dielectric strength of a cable insulation specified under definite conditions is exceeded, discharge processes always occur, and these can be divided into two categories; partial discharges and complete break down • Dielectric Strength: The Potential gradient necessary to cause breakdown of an insulating medium is termed its dielectric strength and is usually expressed in MVs/meter of thickness HV Course March 2007 19 To Be A World Class Maritime Academy Dielectric Strength of different insulation materials If thickness of the insulation material is mm Air- 4.46 MV/m Mica- 61 MV/m Glass (density)- 28.5 MV/m Ebonite- 50 MV/m Paraffin-waxed paper- 40-60 MV/m Transformer oil- 200 MV/m Ceramics- 50 MV/m Ref: Page 119, Hughes Electrical Techonology SF6 = About twice of Air HV Course March 2007 20 To Be A World Class Maritime Academy PARTIAL DISCHARGE IN INSULATION OF THE HIGH VOLTAGE CABLES • The occurrence of partial discharge (PD) within a dielectric -means that either the electric field or the dielectric strength or both are distributed in a highly inhomogeneous manner Referring figure 1: • Gases usually have a considerably lower dielectric strength than solid, liquid or impregnated dielectrics • Field strength inside the void (εr=1) exceeds the stress in surrounding dielectric by a factor of nearly εri, the permittivity of the insulation material • As a result, discharges occur in the void above a definite voltage that can be measured externally and can lead to a gradual erosion of the surrounding material HV Course March 2007 21 To Be A World Class Maritime Academy CONTINUED • The figure below shows an example: a gas-filled cavity in the dielectric that disturbs both the field pattern and the distribution of the dielectric strength Fig: Field Strength in void increased (doubled) HV Course March 2007 22 To Be A World Class Maritime Academy Simplified relationship between Electric stress and relative permittivity or dielectric constant Fig: In such an arrangement of this type, the electrical field strength E of neighboring individual components behaves as inversely proportional to the relative dielectric constants εr That is if relative permittivity is less Electrical Stress goes high (V/m) Ref: Page 40, Cable Systems for High and Extra-High Voltage by E Peschke, R von Olshausen HV Course March 2007 23 To Be A World Class Maritime Academy Partial discharge test on polymer-insulated cable C1 • • The best way to explain the processes taking place here is by using the simplified equivalent circuit diagram comprising three capacitances representing the void itself, the dielectric connected in series with it and the intact dielectric connected in parallel with them both C2 Parallel to the void the equivalent circuit diagram provides a spark gap which breaks down when a specific voltage Uz, assumed to be constant, is exceeded, and thus has the effect of discharging the capacitor This results in repeated voltage collapses at capacitor C1 as indicated in the oscillogram C3 Display unit C1 C3 C2 HV Course March 2007 24 To Be A World Class Maritime Academy Test on Cables for partial discharge Transfor mer Insulation under test C1 C3 C2 Display unit Test voltage Voltage at void without discharge Voltage at void with discharges HV Course March 2007 25 To Be A World Class Maritime Academy On-site partial discharge monitoring • It is available for the accessories, particularly joints, and only where they are equipped with sensors • Inductive Coupler: Pulses from the joint that is being monitored pass through Rogowiski coils in opposite directions and, in summation, produces a signal with almost twice the amplitude because the winding in the coil in in opposite directions • For the same reason, pulses that originate from the right or left of the joint and are consequently passing through the coils in the same direction are largely cancelled out during summation Rogwisky Coil Joint + - Signal addition HV Course March 2007 26 To Be A World Class Maritime Academy 5.0 Basic Impulse Insulation Level (BIL) TEST • Insulation can withstand very high voltage, if it is applied for a very brief period • If a 60 Hz sinusoidal voltage between the insulation and ground is applied and if the voltage is slowly increased, a point will be reached where break down occurs • On the other hand if we apply a dc impulse voltage for a extremely short period, it takes much higher voltage before insulation breaks down • Same happens with other insulators, bushing, etc • In the interest of standardization, and to enable a comparison between the impulse withstand capability of insulators, the insulators are tested by a defined impulse wave as follows HV Course March 2007 27 To Be A World Class Maritime Academy CONTINUED 100% Peak voltage 50% 0% 1.2μs Time 50μs 1.2 X 50 μs BIL Puse HV Course March 2007 28 To Be A World Class Maritime Academy CONTINUED • The BIL of a device is usually several times higher than its normal ac operating voltage • For example, the standards require that 69 kV distribution transformer must have a BIL of 350 kV • The peak voltage at which a safety begins to conduct must always be lower than the BIL of the apparatus it is intended to protect • A phase, 69 kV transmission line having a BIL of 300 kV is supported on steel towers and protected by a circuit breaker The ground resistance at each tower is 20Ω whereas the neutral of the transmission line is solidly grounded at the transformer just ahead of the circuit breaker During an electric storm, one of the towers is hit by a lightning stroke of 20 kA • Calculate the voltage across each insulator string under normal conditions • Describe the sequence of events during and after the lightening stroke HV Course March 2007 29 To Be A World Class Maritime Academy CONTINUED • Under normal condition: • Line to neutral voltage= 69/√3 = 40kV • The insulator is therefore at the same potential to the ground • When lightening strikes: • Voltage across the insulator and the ground resistance suddenly jumps to 20kA X 20 = 400kV • Therefore insulator burns immediately causing short circuit in all three phases HV Course March 2007 30 To Be A World Class Maritime Academy 6.0 Life Test • A factory test to determine expected life Impregnated paper Field strength, E Polymer Time to break down HV Course March 2007 31 ... That is if relative permittivity is less Electrical Stress goes high (V/m) Ref: Page 40, Cable Systems for High and Extra -High Voltage by E Peschke, R von Olshausen HV Course March 2007 23 To... Insulation can withstand very high voltage, if it is applied for a very brief period • If a 60 Hz sinusoidal voltage between the insulation and ground is applied and if the voltage is slowly increased,... temperature measurement with an recording infra-red camera • Electric Propulsion and High Voltage Practice Infrared image testing distance • The camera is used to scan an area and the recorded infra-red