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Physics of Corona and Gap Discharges AC and DC Transmission Line Corona Effects UV Inspection User’s Group Meeting February 11-13, 2004 ORLANDO, Florida, USA By Dr. P. Sarma Maruvada Notas en español agregadas por Ing. Ariel Lichtig exclusivamente para curso teoría de Campos-FIUBA Introduction • Electrical Design, Operation & Maintenance of HV Transmission Lines Requires Consideration of: - Air Insulation - Corona - Insulators • All Three Aspects Require Knowledge of Electrical Discharges in Air, Which May Comprise: - Partial Breakdown (Corona) - Complete Breakdown (Gap Discharges) Corona & Gap Discharges • Corona is an electrical discharge (i.e. partial breakdown of air insulation) occurring in the high electric field region, generally in the vicinity of conducting surfaces, but sometimes also near insulating surfaces, due to ionization processes in air. Resulta de procesos de avalanchas de electrones bajo condiciones de campo no uniforme que produce que la avalancha cese antes de llegar a tierra. • Complete electrical breakdown of air insulation between two electrodes separated by a very small gap is known as a micro-gap discharge or simply as Gap Discharge. Basic Ionization Processes • Ionization and excitation by electron impact A: molécula neutra A*: excitada e inestable A + e A* + e (excitation) A* A + hfp (photo-emission) A + e A+ + e + e (ionization, si E es >, e no sólo sube de órbita sino que se separa) • Photo excitation & ionization A + hfp A* (photo-excitation) A + hfp A+ + e (photo-ionization, con más energía) Basic Ionization Processes • Electron attachment A+e A • Recombination A+ + B- A + B + hfp (radiative recombination, only with electrons) The energy of photons, or the frequency of light, depends on the difference in the orbital energies of the electron El nitrógeno no tiene afinidad con electrones Discharge in Uniform Fields • Field intensified ionization & electron avalanche + Ө: iones, más lentos - : electrones libres electric field Discharge in Uniform Fields • Discharge development & breakdown Los iones positivos golpean el cátodo y liberan más electrones. Si hay suficiente campo, el proceso se autosostiene Cathode HV A i Anode Breakdown and Corona • Excitation of molecules and photon emission occur simultaneously with ionization. • Secondary ionization processes, due to impact of ions or photons, play a crucial role in breakdown. • In non-uniform fields, such as in a conductorplane gap, only partial breakdown or corona occurs (al disminuir el campo). Modes of Corona in Air • Negative DC Corona Modes: Se crea una carga espacial que cambia la distribución de campo. - Trichel Pulse - Negative Glow - Negative streamer Depende se los constituyentes (N2, O2), generación de fotones, carga espacial. Modes of Corona in Air: Visual Appearance of Negative DC Modes Para punta de d=0,8 cm sobre esfera de D=7cm, gap=19 cm, exposición ¼ segundo Trichel pulse Glow Streamer Modes of Corona in Air • Positive DC Corona Modes: - Onset Streamer (el más importante) - Positive Glow - Breakdown Streamer Modes of Corona in Air: Visual Appearance of Positive DC Modes Para punta de d=0,8 cm sobre esfera de D=7cm, gap=19 cm, exposición ¼ segundo Onset streamer Glow Modes of Corona in Air: AC Modes Escalas: 50 microA/div - 1 ms/div Se observan varios modos de corona durante cada ciclo al cambiar el campo continuamente en amplitud y polaridad Glow Breakdown Glow Gap Discharges in Air Gap Discharges may Occur: • Between metallic hardware parts of transmission and distribution lines; • Between metallic and insulating surfaces; • On the surface of polluted insulators Gap Discharges in Air General Mechanism Z 1 Divisor U capacitivo Z2 Ug Gap Gap Discharges in Air Typical Current Pulse Produced Current I cr Time Tr ns ó μs Td Light Emission from Discharges • Excitation: A + e A* + e • Photo-emission: A* A + hfp with hfp = (E2 – E1) where E2 is the energy of the excited state and E1 is the energy of the ground state to which the molecule returns. • Light spectrum emitted in air is mainly that of molecular nitrogen. • Excitation potentials of N2 = 6.3 eV and of O2 = 7.9 eV La mayoría de los fotones está producida por N2 Diagram of the Electronic and Vibrational Energy Levels of the Nitrogen Molecule Distintos tipos de fotón según el salto de energía Light Emission from Discharges • The frequency band of light emitted is in the UV range, with the stronger emissions having wavelengths in the range of 300 nm to 500 nm and the weaker emissions in the range of 80 nm to 200 nm. • The excitation coefficient (i.e. number of molecules excited by an electron drifting 1 cm in the field direction) depends on the composition of air and is a function of E/p (cociente campo eléctrico/presión) Light Emission from Discharges • Presence of any trace gases such as argon, carbon dioxide etc., can change the light spectrum emitted by discharges in air. • Spectroscopic data in air suggest that sparks (breakdown) produce more intense light than streamers (corona). Photoabsorption • Photons emitted during the avalanche development in air are absorbed: a) partly by other gas molecules; b) partly by the negative oxygen molecules in the gas, leading to photo-detachment; O2- + hfp O2 + e • Other mechanisms leading to the loss of photons are: photoionization, step ionization, dissociation and dissociative ionization Photoabsorption • Overall photoabsorption may be characterized by I (intensidad de fotones): I I0 e x where μ is the absorption coefficient. Typical values of μ at atmospheric pressure are: For N2, μ = 0.3 cm-1 , O2, μ = 30 cm-1 , -1 Air, μ = 5 cm A menor μ, se propaga mayor distancia The presence of moisture in air reduces μ by about 25%. Radiation from a Corona Discharge Radiation from Sun Corona Onset Gradient (en kV pico/cm) Ec K m E0 1 rc • E0 and K are empirical constants (for positive dc, E0=33.7 & K=0.24, for negative dc & ac, E0 =30.0 kV/cm & K = 0.30) • = (273+t0).p/(273+t)p0 is the relative air density; t is the temperature and p the pressure of ambient air and t0 and p0 are reference values; (t0 = 25 C and p0 = 760 mm) • rc is the conductor radius in cm • m conductor surface irregularity factor, depende de la rugosidad superficial del conductor Corona Effects on AC and DC Transmission Lines For both ac and dc lines: • Corona (power) Loss (CL) • Electromagnetic Interference (EMI) (Includes RI, TVI, etc.,) • Audible Noise (AN) • Ozone, NOx etc. For dc lines: • Space Charge Effects AC Space Charges and Corona Loss - - - - - - - +++ - - - - - - - - ++ + + - + ++ + + ++ - - - - - - (c) + + + + --- + + - -- - - + + --- + + + + (e) Icor (b) (a) + + + ++ ++ + + ++ ++ ++ + + + + ++ + ++ + ++ + +++ + U I + + a + + b c e c f d + -- + + + + + + + (d) - -- - - - -- - - - - - - - - - --- - -- -- - - - - - - - - --- - -- - - - ----- (f) Icorona es capacitiva, por desplazamiento de la carga espacial. g t Main Types of DC Transmission Lines • Unipolar Lines AC System AC System Metallic return ( Optional) AC System • Bipolar Lines AC System Physical Description of Unipolar Corona • Unipolar ions created near the conductor drift towards the ground, filling the entire space Physical Description of Bipolar Corona • Ions of both polarities fill the space, creating two unipolar regions and a bipolar region Bipolar Region Negative Region Positive Region Generation of RI Corona current pulse trains are injected into conductors DC Positive time Pulsos aleatoriamente DC Negative Pulsos menores time T c- AC T c+ Ambos tipos T c+ time The high-frequency current components propagate along the conductors and produce RI near the transmission line. Corona & Gap Discharge Current Pulse Characteristics I cr 0.9 I cr Current • Both positive and negative corona, as well as gap discharge, current pulses have a fastrising front (1 a 50 ns) and a slowly decaying tail (50 a 200 ns) as shown 0.1 I cr Tr Time T d Corona Current Pulse Characteristics Type of Pulse Amplitude (mA) Rise-time (ns) Duration (ns) Repetition Rate (pulses/s) Positive Corona 10 – 50 50 250 103 – 5.103 Negative Corona 1 – 10 10 100 104 - 105 Gap Discharge 500 - 2000 1 5 102 – 5.103 Corona Current Pulse Characteristics • Frequency Spectra of Corona and Gap Discharge Pulses Positive Gap 110 100 F ( ) , dB Negative 90 80 70 60 50 0.01 0.1 1.0 10.0 Frequency, MHz. 100.0 1000.0 RI Characteristics of AC Lines • RI from transmission lines is generally defined in terms of three characteristics: 1. Frequency Spectrum 70 RI, dB ( µ V/ m ) QP 60 50 40 30 20 10 0 0.1 0.2 0.3 0.5 1 2 3 Frequency, MHz 5 10 20 30 RI Characteristics of AC Lines 2. Lateral Profile (proporcional a 1/D) 70 RI, dB ( µ V/ m ) QP 60 50 40 30 20 10 0 - 100 - 75 - 50 - 25 0 Distance, m 25 50 75 100 RI Characteristics of AC Lines 3. Statistical Distribution 0.5 1.0 2.0 Percentage Time Above Abscissa 5.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0 95.0 98.0 99.0 99.8 99.9 20 25 30 35 40 45 RI, dB ( µ V /m ) QP 50 55 60 65 RI Characteristics of DC Lines Lateral Profile 70 El positivo contribuye mucho más a la radiointerferencia. Producida por las descargas tipo streamer. RI, dB ( µ V/ m ) QP 60 50 40 30 20 10 0 - 100 - 75 - 50 - 25 0 Distance, m 25 50 75 100 RI Characteristics of DC Lines Statistical Distribution 0.5 1.0 2.0 Percentage Time Above Abscissa 5.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0 95.0 98.0 99.0 99.8 99.9 20 25 30 35 40 45 RI, dB ( µ V /m ) QP 50 55 60 65 Audible Noise Generation and Propagation 2 Generated Corona Acoustic Pulse Pressure, Pa. 1 0 -1 -2 0 10 20 30 40 50 60 70 80 Time, µs. x x - 1 2 O R r P • AN Propagation AN Characteristics of AC Lines Sound Pressure Level, dB above 20 µ PA • Audible noise from AC lines is described, similar to RI, in terms of frequency spectrum (figure below), lateral profile and statistical distribution 60 50 40 30 31 63 125 250 500 1000 Frequency, Hz 2000 4000 8000 16000 Corona-generated Hum Noise Sound Pressure Level, dB above 20 µ PA • Oscillatory movement of the ionic space charge creates hum noise at twice power frequency; Figure shows lateral profile of hum noise 70 60 50 40 0 10 20 30 40 Lateral Distance From Center Phase, m. 50 AN Characteristics of DC Lines Sound Pressure Level, dB above 20 µ PA • Lateral profile & Statistical distribution are similar to those for RI; Frequency spectrum is given below 60 50 40 30 31 63 125 250 500 1000 Frequency, Hz 2000 4000 8000 16000 DC Electric Field & Space Charge Profiles Negative Positive Pole Pole + 25 Computed Electric Field 100 20 90 E g (kV / m) 2 j g (nA /m ) Measured Electric Field 80 15 70 60 Computed Current Density 50 10 40 30 Measured Current Density 5 20 10 0 -10 0 10 20 30 Distance from Centre of Line, m 40 50 Corona Effects Design Criteria • Corona Loss - Economic Choice of Conductor Bundle Total Cost B A dc1 por pérdidas dm dc2 por radiointerferencia Conductor Diameter, d Corona Effects Design Criteria (at 1 MHz) • Radio Interference USA RI from power systems is governed by the FCC Rules Canada Design Limits Nominal Interference Phase-toField Strength Phase Voltage (dB above (kV) 1 μV/m) Below 70 70 – 200 200 – 300 300 – 400 400 – 600 Above 600 43 49 53 56 60 63 Corona Effects Design Criteria • Audible Noise USA The Environmental Protection Agency (EPA) published guidelines for AN in general. However, each state is responsible to legislate noise regulations and these regulations may vary widely from state to state. The EPA document recommends that the daynight average sound level, Ldn, be limited to 55 dB(A) outdoors and 45 dB(A) indoors. DC Fields & Ions Design Criteria • Design criteria for electric fields and ion currents under DC lines are established on the basis of human perception studies • Based on such studies, the following design limits have been proposed: E = 25 kV/m (en ca 10 kV/m) j = 100 nA/m2 (corriente iónica)