High Voltage Engineering (EX-702 (C)) - Unit 1 Short Notes

1.0 Introduction to High Voltage Technology

Definition and Significance

High Voltage (HV) technology deals with generation, measurement, transmission, and application of voltages significantly above standard levels (typically > 1000 V AC or 1500 V DC). Its significance in modern power systems is paramount:

• Efficient Power Transmission: Enables long-distance transmission with reduced current ($$\displaystyle I = P/V $$), minimizing $$\displaystyle I^2R $$ losses and allowing thinner, cheaper conductors.

• System Integration: Facilitates interconnection of large, remote power generation sources (hydro, thermal) with load centers.

• Evolution: Progression from High Voltage (HV: up to 245 kV), to Extra High Voltage (EHV: 345 kV - 800 kV), and Ultra High Voltage (UHV: > 800 kV AC / ±500 kV DC) for ultra-long distance, high-capacity corridors.

Applications

Need for HV Generation in Laboratories

Essential for:

1. Insulation Coordination Studies: Determining withstand levels of equipment.

2. Type & Routine Testing: Verifying design integrity (e.g., impulse, power-frequency withstand tests).

3. Diagnostic Measurements: Partial discharge (PD) testing, dielectric response measurements.

4. Research & Calibration: Developing new materials, calibrating measuring instruments.

[!TIP] Exam Focus: Be prepared to differentiate between HV, EHV, UHV levels and cite specific examples for both power and non-power applications.

2.0 Breakdown Mechanisms in Dielectrics

2.1 Gaseous Dielectrics

Ionization Processes

• Primary Ionization: Free electrons (from natural radiation/photoemission) gain energy from the electric field, collide with neutral gas molecules, and ionize them, releasing more electrons. This creates an electron avalanche.

• Secondary Ionization: Processes at the cathode that replenish electrons lost to the anode, sustaining the discharge:

  • Ion Impact: Positive ions strike cathode, emitting secondary electrons.

  • Photoionization: UV photons from the avalanche excite/ionize gas molecules or cathode material.

  • Thermionic Emission: Localized heating at cathode.

Townsend's Theory of Breakdown

• Townsend's First Ionization Coefficient (α): Number of ionizing collisions produced by one electron per unit path length (function of E/p, where E=field, p=pressure).

• Townsend's Second Ionization Coefficient (γ): Number of secondary electrons emitted from cathode per incident positive ion.

• Breakdown Condition: When the number of electrons multiplies sufficiently to cause a self-sustaining discharge.

$$n_d = n_0 e^{\alpha d}$$

where $$\displaystyle n_d $$ = electrons at anode, $$\displaystyle n_0 $$ = initial electrons, $d$ = gap distance.

For breakdown, secondary emission must compensate electron loss to anode:

$$n_0 e^{\alpha d} (1 + \frac{1}{\gamma}) = 1$$

**Final Townsend Breakdown Criterion:**

$$\boxed{\alpha d = \ln\left(1 + \frac{1}{\gamma}\right)}$$

> This theory fails for large gaps as it doesn't account for space charge distortion of the field.

Streamer Mechanism

• Necessity: Explains breakdown in non-uniform fields and larger gaps where space charge significantly distorts the electric field.

• Process:

  1. A strong avalanche develops near the electrode with high field.

  2. Space charge of the avalanche (positive ions, electrons) locally distorts and enhances the electric field ahead of the avalanche head.

  3. This enhanced field initiates a new, independent electron avalanche from the gas ahead.

  4. The process repeats, forming a conducting plasma channel (streamer) that propagates across the gap at high speed (~10^6 m/s).

• Key Difference: Streamer theory includes space charge effects, making it valid for practical gap breakdown.

Time Lags in Breakdown

• Statistical Time Lag ($$\displaystyle t_s $$): Time required for a free electron to appear and initiate the avalanche. Inherently random, depends on background radiation and gas purity.

• Formative Time Lag ($$\displaystyle t_f $$): Time from first electron initiation to final breakdown. Depends on gap geometry, voltage magnitude, and gas properties.

• Total Breakdown Time ($$\displaystyle t_b $$): $$\displaystyle t_b = t_s + t_f $$.

2.2 Vacuum Breakdown

• Mechanism: Different from gases due to absence of molecules for ionization.

• Process: Initiated by field emission of electrons from microprotrusions on electrode surfaces (due to high local field). These electrons strike the anode, causing:

  • Localized heating → vaporization of electrode material → plasma formation.

  • Ion bombardment of cathode → more material release.

• Sustenance: A vacuum arc is maintained by a self-sustained plasma of vaporized electrode metal.

• Key Factor: Electrode surface finish and material are critical. Smooth, clean electrodes have higher breakdown strength.

2.3 Solid Dielectrics

• Intrinsic Dielectric Strength: Maximum electric field a perfect, defect-free material can withstand without breakdown (typically 10-100 MV/m). Determined by electronic structure.

• Electronic Breakdown Mechanisms:

  • Avalanche (Zener) Breakdown: High field accelerates electrons to energies sufficient to ionize valence electrons via collision, causing a chain reaction.

  • Thermal Breakdown: High field causes excessive Joule heating ($$\displaystyle I^2R $$), leading to thermal runaway and material degradation.

  • Intrinsic Breakdown: Direct tunneling or excitation of electrons across the band gap under extremely high fields.

2.4 Paschen's Law

• Statement: The breakdown voltage ($$\displaystyle V_b $$) of a gaseous gap is a unique function of the product of gas pressure ($p$) and gap distance ($d$), for a given gas and electrode material.

$$V_b = f(pd)$$

• Derivation (Simplified): Based on Townsend's criterion, assuming α ∝ exp(-Bp/E) and γ constant. Solving αd = constant leads to $$\displaystyle V_b $$ being a function of pd.

• Paschen Minimum: The curve $$\displaystyle V_b $$ vs. $pd$ has a minimum. At this point, the probability of an electron gaining enough energy to ionize and having a collision within the gap is optimal.

  • Significance: For a given gap $d$, there is a critical pressure $p$ where breakdown voltage is lowest. For very low or very high $p$, $$\displaystyle V_b $$ increases.

• Practical Implications:

  • Insulation Design: Equipment must operate at pd values away from the Paschen minimum to avoid unexpected breakdowns.

  • Gas-Filled Equipment: SF₆ circuit breakers operate at pressures where $$\displaystyle V_b $$ is high.

  • High-Altitude Equipment: Reduced atmospheric pressure shifts the Paschen curve; clearances must be increased.

[!TIP] Common Pitfall: Paschen's Law applies to uniform field gaps and static voltages. It does not hold directly for non-uniform fields or very fast impulses.

3.0 High Voltage Generation Methods

3.1 AC High Voltage Generation - Series Resonant Circuit

• Principle: Uses a tank circuit (L and C in series) connected across a low-voltage AC source. At resonance, the circuit impedance is minimum ($$\displaystyle Z = R $$), and current is maximum. This high current flows through the series combination of the high-voltage transformer winding and the capacitor, generating high voltage across the capacitor.

• Resonance Condition:

$$\omega L = \frac{1}{\omega C} \quad \text{or} \quad f = \frac{1}{2\pi\sqrt{LC}}$$

where $$\displaystyle \omega = 2\pi f $$.

• Voltage Magnification: $$\displaystyle V_C = Q \cdot V_{source} $$, where $$\displaystyle Q = \frac{\omega L}{R} $$ is the quality factor.

• Advantage for HV Testing: Requires a much smaller power rating from the source compared to a conventional test transformer, as the high voltage is generated by resonant magnification of current.

3.2 DC High Voltage Generation

A. Cockcroft-Walton (CW) Voltage Multiplier

• Construction: $n$ stages of capacitor-diode pairs. Charged from a transformer secondary (typically high-frequency AC).

• Working:

  1. Positive Half-Cycle: Capacitors $$\displaystyle C_1, C_3, ... $$ charge through diodes $$\displaystyle D_1, D_3, ... $$.

  2. Negative Half-Cycle: Capacitors $$\displaystyle C_2, C_4, ... $$ charge. The voltage across each stage adds.

  3. Output Voltage (No Load): $$\displaystyle V_{out} \approx 2n V_{peak} $$ (for ideal components).

• Advantages: No need for large HV transformer; portable; output is DC.

• Limitations: Significant ripple and voltage drop under load due to charging/discharging currents; poor regulation; capacitance limits current output.

B. Tesla Coil

• Construction:

  • Primary Circuit: Low-turn coil ($$\displaystyle L_p $$) in series with a capacitor ($$\displaystyle C_p $$) and a spark gap ($SG$).

  • Secondary Circuit: High-turn coil ($$\displaystyle L_s $$) with one end grounded and the other connected to a toroidal terminal (for corona control). $$\displaystyle L_s $$ is magnetically coupled to $$\displaystyle L_p $$.

  • Power Supply: Transformer charging $$\displaystyle C_p $$.

• Operation:

  1. $$\displaystyle C_p $$ charges until spark gap fires.

  2. Oscillatory discharge in primary circuit excites secondary circuit at its resonant frequency.

  3. Due to high Q-factor and tight coupling, very high voltages (MV) are induced in $$\displaystyle L_s $$ and appear on the toroid.

  4. Output is high-frequency, high-voltage AC.

• Applications: HV laboratory for impulse testing, dielectric testing of insulation, demonstration of corona and streamers.

3.3 Impulse Voltage Generation - Marx Circuit

• Basic Configuration: $n$ identical capacitor banks ($C$) charged in parallel to a DC voltage $$\displaystyle V_c $$ through charging resistors ($$\displaystyle R_ch $$). They are discharged in series through a set of spark gaps and a wave-shaping circuit (R, L, C) to produce a standard impulse wave (1.2/50 µs).

• Triggering Mechanisms - Three-Electrode Gap:

  • Arrangement: Each main spark gap has a third trigger electrode placed near its cathode.

  • Process:

    1. A high-voltage trigger pulse (from a separate pulser) is applied to all trigger electrodes simultaneously.

    2. This initiates a small discharge between trigger electrode and main cathode, providing initial electrons/ions.

    3. These initiate the main spark gap breakdown in a controlled, synchronized manner.

  • Advantages: Precise timing, independent of gap conditioning, reliable triggering at desired instant.

  • Method of Control Tripping: The trigger pulse is the "control tripping" signal that initiates the main discharge sequence.

[!TIP] Exam Focus: Be ready to sketch the CW multiplier and Marx generator. Clearly state the triggering advantage of the three-electrode gap (synchronization) vs. uncontrolled self-breakdown.

4.0 High Voltage Measurement Techniques

4.1 Sphere Gap

• Standard Sphere Gap: Two identical metal spheres of defined diameter (e.g., 25 cm, 50 cm, 100 cm, 150 cm). Gap distance ($d$) is variable.

• Operating Principle: Measures the peak value of AC, DC, or impulse voltage by determining the 50% breakdown voltage for a given gap setting. Breakdown voltage is looked up in standard tables (IEC 60052).

• Factors Influencing Measurement:

  • Sphere Diameter: Determines the calibration curve.

  • Gap Distance ($d$): Must be within specified limits relative to sphere diameter ($$\displaystyle d/D < 0.5 $$ typically) to ensure a well-defined uniform field.

  • Atmospheric Conditions: Pressure and temperature corrections are applied (using air density factor).

  • Polarity Effects: For DC and impulse, breakdown voltage differs for positive vs. negative polarity due to differences in electron avalanche initiation (from cathode vs. anode). Standard tables specify polarity.

4.2 Potential Dividers

• Resistance Potential Divider:

  • Principle: Simple voltage division: $$\displaystyle V_{out} = V_{in} \cdot (R_2 / (R_1+R_2)) $$.

  • Limitations for Impulse: Stray capacitance ($$\displaystyle C_s $$) across $$\displaystyle R_1 $$ causes frequency-dependent error (division ratio changes with wavefront time). Not suitable for fast impulses.

• Capacitance Potential Divider:

  • Principle: Uses two capacitors: $$\displaystyle V_{out} = V_{in} \cdot (C_1 / (C_1 + C_2)) $$.

  • Limitations: Output impedance is high; sensitive to loading. Division ratio depends on frequency if stray capacitance is present.

• Mixed RC (Resistive-Capacitive) Potential Divider:

  • Principle: Combines a low-inductance resistor ($R$) in series with a capacitor ($C$) as the high-voltage arm. The low-voltage arm is a pure capacitor ($$\displaystyle C_2 $$).

  • Advantage for Impulse: The RC arm compensates for its own inductance and stray capacitance, providing a frequency-independent division ratio over a wide range (up to MHz).

• Conditions for Impulse Work:

  1. Linear Response: Output voltage waveform must be an exact scaled replica of input.

  2. Minimal Distortion: No ringing or overshoot.

  3. Proper Damping: To suppress oscillations from circuit inductance/capacitance.

  4. Low Time Constant: $RC$ time constant of the divider must be much smaller than the impulse wavefront time.

4.3 Other HV Measuring Instruments

• Electrostatic Voltmeter:

  • Principle: Based on electrostatic attraction between fixed and moving charged plates. Deflection is proportional to $$\displaystyle V^2 $$.

  • Use: Direct reading for DC and power-frequency AC. Not suitable for impulses. Very high input impedance.

• Generating Voltmeter:

  • Principle: A rotating vane (or disc) passes between fixed electrodes. The changing capacitance generates an AC current proportional to the DC voltage applied. This current is measured by a sensitive AC ammeter.

  • Suitability: Excellent for very high DC voltages (MV range), as it has no direct electrical connection to the high-voltage terminal.

[!TIP] Comparison: For impulse voltage, Mixed RC divider is standard. For peak AC/DC, sphere gap is the primary standard. Generating voltmeter is for high DC, electrostatic voltmeter for lower AC/DC.

5.0 High Voltage Testing of Electrical Equipment

5.1 Circuit Breaker Testing

• Short-Circuit (Making & Breaking) Test:

  • Objective: Verify ability to close onto and interrupt a short-circuit current. Tests arc extinction capability, contact speed, and mechanical endurance under electromagnetic forces.

• Dielectric Test:

  • Objective: Verify insulation strength of main and auxiliary circuits.

  • Types: Power-frequency withstand test, impulse voltage withstand test (lightning & switching surges).

5.2 Insulator Testing

• Mechanical Strength Test:

  • Purpose: Verify ability to withstand specified tensile, compressive, bending, and torsional loads (from conductor weight, wind, ice).

• Puncture Voltage Test:

  • Purpose: Determine the internal dielectric strength of the insulator material (porcelain, polymer). Voltage is applied between the pin and the conductor clamp.

• Flash-Over Voltage Test:

  • Purpose: Determine the surface insulation performance. Voltage is applied between the pin and the conductor clamp with the insulator surface wet (to simulate rain). The voltage at which a flashover occurs along the surface is recorded.

5.3 Transformer Testing

• High Voltage (Withstand) Test:

  • Purpose: Verify integrity of main insulation (winding-to-ground, winding-to-winding).

  • Types:

    1. Power Frequency Withstand: Applied between windings and ground/other windings.

    2. Induced Voltage Test: Applies overvoltage to one winding to test turn-to-turn insulation of other windings (at higher frequency).

    3. Impulse Voltage Test: Simulates lightning surges. Standard 1.2/50 µs wave applied to terminals.

5.4 Cable Testing

• Partial Discharge (PD) Tests:

  • Principle: Detects localized electrical discharges (PD) within insulation voids or at defects. PD pulses are measured in pico-coulombs (pC) or millivolts (mV) using a coupling capacitor and a PD detector.

  • Fault Location:

    • Time Domain Reflectometry (TDR): Sends a low-voltage pulse; reflections from impedance discontinuities (faults) are timed to calculate distance.

    • PD Pulse Propagation: PD pulses travel along the cable at known velocity ($$\displaystyle v = 1/\sqrt{LC} $$). Measuring time difference between PD pulse arrival at both cable ends locates the fault.

[!TIP] Key Distinction: Puncture is internal material failure. Flashover is surface discharge. Withstand test is a pass/fail voltage application; PD test is a diagnostic sensitivity measurement.

6.0 Specialized Topics and Measurement Challenges

Surge Current Measurement

• Methods:

  1. Rogowski Coil: Air-cored toroidal coil around conductor. Output voltage $$\displaystyle v = M \cdot di/dt $$, where $M$ is mutual inductance. Requires integration to get $i(t)$.

  2. Shunt Resistor: Low-value, non-inductive resistor (e.g., manganin). Voltage drop $$\displaystyle v = i \cdot R $$ directly gives current.

  3. Magnetic Field Sensors (Hall Effect, Fiber Optic): Measure magnetic field around conductor, proportional to current.

• Challenges:

  • High Magnitude & Fast Rise Time: Requires very low inductance and high bandwidth (>10 MHz).

  • Electromagnetic Interference (EMI): Need proper shielding and grounding.

  • Insertion Impedance: Sensor must not disturb the circuit.

  • Safety: High voltage isolation required.

Cavity Breakdown

• Explanation: Breakdown occurring in voids or cavities within solid insulation (e.g., in transformer windings, cable joints) or between solid-gas interfaces.

• Mechanism: The cavity (often filled with gas at lower pressure) has a lower breakdown strength than the surrounding solid. Under high stress, the gas in the cavity breaks down first, causing partial discharges (PD).

• Relevance to PD: Cavity breakdown is the primary physical mechanism behind internal partial discharges. Repeated PD erodes insulation, leading to eventual failure.

Motor Coil Insulating Paint (Impregnation)

• Pouring Methods (Impregnation Techniques):

  1. Dip & Drain: Coil is dipped into resin vat, then drained. Simple but may have voids.

  2. Vacuum-Pressure Impregnation (VPI): Coil placed in pressure vessel, vacuum applied to remove air/moisture, then resin introduced under pressure. Most effective for complete penetration and void elimination.

  3. Trickle Impregnation: Resin is slowly "trickled" onto a rotating coil under vacuum. Good for large machines.

  4. Autoclave: Similar to VPI but uses steam pressure for better flow.

[!TIP] Link Topics: Cavity breakdown is the cause, Partial Discharge is the measurable effect. VPI is the premium method to eliminate cavities that cause PD.