High Voltage Engineering (EX-702 (C)) - Unit 3 Short Notes

UNIT 3: HIGH VOLTAGE ENGINEERING

1.0 FUNDAMENTALS OF HIGH VOLTAGE TECHNOLOGY

1.1 Definition and Concept of High Voltage (HV)

• Definition: High Voltage (HV) refers to voltage levels significantly above standard distribution levels (typically > 1 kV AC or 1.5 kV DC), where special design considerations for insulation, safety, and measurement become critical.

• Voltage Level Classification:

  | Acronym | Classification | Typical Range (AC) | | :--- | :--- | :--- | | HV | High Voltage | 36 kV – 245 kV | | EHV | Extra High Voltage | 245 kV – 800 kV | | UHV | Ultra High Voltage | > 800 kV (e.g., 1000 kV, 1200 kV) |

• Significance in Modern Power Systems:

  • Efficient Transmission: Reduces current for same power ($$\displaystyle P = VI $$), minimizing $$\displaystyle I^2R $$ losses over long distances.

  • System Stability: Enables interconnection of large power grids.

  • Reduced Conductor Size & Tower Height: Lower current allows thinner conductors and potentially wider phase spacing.

1.2 Applications of HV Technology

• Power Applications:

  • Transmission Systems: Overhead lines and underground/underwater cables at HV/EHV/UHV.

  • Substations: Busbars, circuit breakers, instrument transformers (CTs, PTs).

  • Power Apparatus: Transformers, generators, motors (especially large hydro/turbine generators).

• Non-Power Applications:

  • Medical: X-ray generation tubes.

  • Research: Particle accelerators (Van de Graaff, cyclotrons).

  • Industrial: Electrostatic precipitators (air pollution control), electrostatic painting, food processing, ozone generation.

1.3 Need for High Voltage Generation in Laboratories

• Purposes:

  • Insulation Testing: Dielectric withstand test, impulse voltage test, partial discharge measurement.

  • Research & Development: Studying breakdown mechanisms, developing new insulating materials.

  • Calibration: Calibrating high voltage measuring instruments (dividers, sphere gaps).

• Testing Scenarios:

  • Power Frequency Withstand: Sustained AC voltage test.

  • Impulse Testing: Simulating lightning or switching surges (standard lightning impulse 1.2/50 µs, switching impulse 250/2500 µs).

  • Partial Discharge (PD) Measurement: Detecting and locating incipient faults in insulation.

2.0 BREAKDOWN MECHANISMS IN DIELECTRICS

2.1 Gaseous Dielectrics

2.1.1 Townsend Discharge Theory

• Primary Ionization: Free electrons (from natural radiation/photoemission) gain energy from electric field, collide with neutral gas molecules, causing ionization: $$\displaystyle e^- + M \rightarrow e^- + e^- + M^+ $$. The number of ions created per unit length is Townsend's first ionization coefficient (α).

• Secondary Ionization: Positive ions ($$\displaystyle M^+ $$) drift to cathode, causing:

  1. Ion Impact: Release of secondary electrons upon striking cathode.

  2. Photoemission: UV photons from de-excitation of gas molecules release electrons from cathode.

  The number of secondary electrons released per ion is Townsend's second ionization coefficient (γ).

• Breakdown Condition: When the multiplication of electrons becomes self-sustaining.

$$\gamma \left( e^{\alpha d} - 1 \right) = 1$$

where $d$ = gap distance. For $$\displaystyle \alpha d >> 1 $$, the simplified condition is:

$$\gamma e^{\alpha d} = 1$$

2.1.2 Streamer Mechanism of Breakdown

• Description: In non-uniform fields (e.g., rod-plane gap), space charge from an electron avalanche distorts the local electric field. The high field at the avalanche head ionizes new gas ahead, creating a conductive ionized channel (streamer) that propagates rapidly to the anode.

• Improvement over Townsend: Explains the fast transition from avalanche to full breakdown and the branching often seen in sparks. Townsend theory predicts a slow current rise, while streamer theory explains the rapid breakdown.

2.1.3 Paschen's Law

• Statement: Breakdown voltage ($$\displaystyle V_b $$) in a uniform field gap is a unique function of the product of gas pressure ($p$) and gap distance ($d$), independent of their individual values.

$$V_b = f(pd)$$

• Derivation (Conceptual): From Townsend theory, $$\displaystyle V_b $$ depends on $\alpha/p$ (which is a function of $$\displaystyle E/p = V/(pd) $$). Solving $$\displaystyle \gamma e^{\alpha d}=1 $$ yields $$\displaystyle V_b = f(pd) $$.

• Paschen Minimum: The $$\displaystyle V_b $$ vs. $pd$ curve has a minimum. For air, $$\displaystyle V_b(min) \approx 327 $$ V at $pd \approx 0.567$ Torr·cm.

  Practical Implication: For a given $$\displaystyle V_b $$, there is a minimum required clearance $d$ at a certain pressure. Operating at very low or very high $pd$ increases $$\displaystyle V_b $$. This guides insulation coordination in gas-insulated systems (GIS) and for high-altitude equipment.

2.1.4 Time Lags in Breakdown

• Statistical Time Lag ($$\displaystyle t_s $$): Time between voltage application and the appearance of the first initiating free electron (random process, depends on natural background radiation).

• Formative Time Lag ($$\displaystyle t_f $$): Time for the discharge to develop from the first electron to full breakdown (depends on $E/p$, gap geometry).

• Total Breakdown Time Lag: $$\displaystyle t = t_s + t_f $$.

• Factors: $$\displaystyle t_s $$ depends on radiation intensity; $$\displaystyle t_f $$ decreases with increasing overvoltage ($$\displaystyle V/V_b $$).

2.2 Vacuum Breakdown

• Mechanisms:

  1. Outgassing: Desorption of gases from electrode surfaces under high field.

  2. Microprotrusion Formation: Field emission from sharp points on cathode.

  3. Vacuum Arcing: Localized heating from field emission current vaporizes electrode material, creating a plasma column.

• Conditions: Occurs at very low pressures (< 10⁻³ Torr) when mean free path >> gap distance. Breakdown is often initiated by field emission and is surface/geometry dependent, not volume-dependent like in gases.

2.3 Solid Dielectrics

2.3.1 Intrinsic Strength

• Definition: The maximum electric field a perfect, defect-free dielectric material can withstand before electronic breakdown. It is an inherent material property (typically 10-100 MV/m).

• Note: Practical breakdown strength is always lower than intrinsic strength due to impurities, voids, and defects.

2.3.2 Electronic Breakdown

• Avalanche (Impact Ionization): High-energy electrons collide with valence electrons, creating electron-hole pairs. This cascade leads to conduction.

• Thermal Breakdown: Excessive dielectric loss ($$\displaystyle I^2R $$ heating) raises temperature, increasing conductivity further (positive feedback), leading to thermal runaway and melting/charring.

2.4 Partial Discharges (PD)

• Definition: Localized electrical discharges that only partially bridge the insulation between conductors. They occur in cavities/voids within or at the surface of solid dielectrics, or in gas bubbles in liquid dielectrics.

• Cavity Breakdown Phenomenon: A void has lower dielectric strength than the surrounding solid. When local field in void exceeds gas breakdown strength ($$\displaystyle V_b $$), a small discharge occurs across the void. The discharge does not completely bridge the electrodes but erodes the insulation over time.

• PD Tests on Cables: Applied voltage is increased until PD activity is detected by sensitive electrical (pulse) or acoustic sensors. Used for quality control and condition monitoring.

• Fault Location: Using time-domain reflectometry (TDR) or pulse-echo methods on the PD pulse signals to locate the position of the void along the cable length.

3.0 HIGH VOLTAGE GENERATION

3.1 AC High Voltage Generation

3.1.1 Series Resonant Circuits

• Principle: A series RLC circuit is driven at its resonant frequency ($$\displaystyle \omega_0 $$). At resonance, the inductive and capacitive reactances cancel ($$\displaystyle X_L = X_C $$), and circuit impedance is minimum ($Z \approx R$). A large current flows, creating a high voltage across the capacitor ($$\displaystyle V_C = I \cdot X_C $$) which can be much higher than the input voltage.

• Resonance Condition:

$$\omega_0 L = \frac{1}{\omega_0 C} \quad \Rightarrow \quad \omega_0 = \frac{1}{\sqrt{LC}}$$

• Advantages for HV Testing: High output voltage with relatively small power input from the source (since power is dissipated only in $R$). Used in tuned testing transformers and resonant test sets for cable/large apparatus testing.

3.1.2 Tesla Coil

• Construction: A two-resonant-circuit coupled transformer.

  1. Primary Circuit: Low-voltage winding, capacitor, and spark gap.

  2. Secondary Circuit: High-voltage resonant winding (many turns) with a toroidal top load (terminal).

• Working Principle:

  1. Capacitor in primary charges from supply.

  2. Spark gap fires, causing primary LC circuit to oscillate at $$\displaystyle f_1 $$.

  3. Mutual coupling excites the secondary LC circuit at its resonant frequency $$\displaystyle f_2 $$ (typically higher).

  4. Voltage builds up on the top terminal due to capacitive coupling to surroundings and the high $Q$ of the secondary.

• Why Current Flows in Nearby Objects: The high-frequency AC field induces voltages in nearby objects (electrostatic induction). The skin effect confines this induced current to the surface. This is the principle of wireless power transmission (historical) and is used in high-frequency, high-voltage applications like ozone generators.

3.2 DC High Voltage Generation

3.2.1 Cockcroft-Walton (CW) Generator

• Construction: A voltage multiplier cascade of stages. Each stage consists of two diodes and two capacitors.

  
  Input AC (V_peak)
  
     |
  
    [C1]---|>|---+
  
     |          |
  
    [C2]---|<|---+--- Output (n*V_peak)
  
     |          |
  
    [C3]---|>|---+
  
     |          |
  
    [C4]---|<|---+
  
  

• Working Principle: During positive half-cycle, capacitors C1, C3, C5... charge to $$\displaystyle V_{in} $$. During negative half-cycle, charge is transferred through diodes to stack on C2, C4, C6..., adding voltages in series. The output is approximately $$\displaystyle n \cdot V_{in} $$ (for $n$ stages) minus ripple and regulation drops.

• Advantages: No need for a high-voltage transformer; portable and compact for given voltage; output is naturally DC.

• Limitations:

  • Voltage Drop under Load: Significant due to capacitive reactance and diode drops.

  • High Ripple: Especially at high loads. Requires large filter capacitors.

  • Size/Cost for HV: Many stages needed for very high voltages, increasing size, capacitance, and losses.

3.3 Impulse Voltage Generation

3.3.1 Impulse Generator Circuits (Marx Circuit)

• Configuration: $n$ identical capacitor banks ($C$) charged in parallel to DC voltage $$\displaystyle V_c $$ through charging resistors ($$\displaystyle R_c $$). They are discharged in series through spark gaps and a wave-shaping circuit ($$\displaystyle R_s $$, $$\displaystyle L_s $$) into the load (test object).

  
  Charging Supply (+) ---[R_c]---+---[C]---[G]---+
  
                               |             |
  
                              [C]---[G]---[R_s]---[L_s]--- To Load
  
                               |             |
  
                              ...           ...
  
                              [C]---[G]---+
  
                               |
  
  Charging Supply (-) ---------+
  
  

• Output Voltage: $$\displaystyle V_{out} \approx n \cdot V_c $$ (minus losses in gaps and wave-shaping circuit).

• Waveform: Front time ($$\displaystyle T_1 $$) controlled by $$\displaystyle R_s $$ and total capacitance; tail time ($$\displaystyle T_2 $$) controlled by $$\displaystyle R_s $$ and $$\displaystyle L_s $$. Standard lightning impulse: $$\displaystyle T_1 = 1.2 \mu s \pm 30\% $$, $$\displaystyle T_2 = 50 \mu s \pm 20\% $$.

3.3.2 Triggering Mechanisms (Three-Electrode Gap)

• Arrangement: A triggering spark gap has three electrodes: two main electrodes (connected to the capacitor bank) and a trigger electrode positioned near one main electrode.

• Process:

  1. Main gap is charged to $$\displaystyle V_c $$ but remains below its self-breakdown voltage.

  2. A high-voltage pulse (from a separate pulse generator) is applied to the trigger electrode.

  3. This creates a localized spark between trigger and nearby main electrode.

  4. The ionized path reduces the effective breakdown distance of the main gap, causing it to break down synchronously.

• Why Preferred: Provides precise timing control and synchronization with other test circuits (e.g., oscilloscope, control systems). Ensures consistent, repeatable impulse generation.

3.3.3 Control Tripping

• Method: Using a pulse transformer or a triggered spark gap (as above) to initiate the discharge of the main Marx circuit.

• Necessity of Triggering:

  • Synchronization: To fire the impulse generator at a precise point on the AC supply cycle (for AC withstand tests on wave) or in sync with measurement instruments.

  • Control: To generate a single, controlled impulse on demand, rather than relying on random self-breakdown of the main gaps.

  • Safety & Repeatability: Prevents accidental firing and ensures test reproducibility.

4.0 HIGH VOLTAGE MEASUREMENT TECHNIQUES

4.1 Potential Dividers

4.1.1 Types and Comparison

4.1.2 Conditions for Impulse Work

1. Low Impedance: Divider impedance must be much lower than the test object's impedance to avoid waveform distortion.

2. Damping: Sufficient damping (often inherent in RC design) to suppress ringing oscillations caused by inductance of leads and components.

3. Frequency Response: Capacitive components must dominate to ensure linear voltage division over the wide frequency spectrum of an impulse (kHz to MHz).

4. Shielding: Proper shielding to prevent electromagnetic interference (EMI) pickup.

4.2 Sphere Gap Arrangement

4.2.1 Standard Sphere Gap

• Construction: Two identical, polished metal spheres of defined diameter (e.g., 6.25 cm, 12.5 cm, 25 cm, 50 cm, 100 cm, 200 cm). Gap distance is variable. Mounted in a well-ventilated, earthed enclosure.

• Operation:

  • AC/DC: Measures peak value of voltage. Breakdown occurs at a statistically determined voltage for a given gap. Voltage is read from standard tables (IEC 60052) based on sphere diameter and gap.

  • Impulse: Measures peak value of impulse voltage. Breakdown is more statistical; requires multiple shots for average value.

• Advantages: Primary standard, independent of waveform (for impulse, measures peak), simple, no need for calibration.

• Disadvantages: Statistical scatter in breakdown voltage. Requires careful alignment. Slow for repeated measurements. Affected by humidity, temperature, air density.

4.2.2 Factors Influencing Measurement

1. Sphere Diameter & Gap Distance: Primary variables in calibration tables.

2. Atmospheric Conditions: Air density (pressure, temperature) and humidity affect breakdown voltage. Corrections (air density factor, humidity factor) are applied per standards.

3. Voltage Polarity: For DC and impulse, breakdown voltage differs for positive vs. negative polarity (especially for non-uniform field spheres). Tables specify polarity.

4. Surrounding Objects: Must be at least 2.5 x sphere diameter away from spheres to avoid distortion.

4.3 Other HV Measuring Instruments

4.3.1 Electrostatic Voltmeter

• Principle: Based on attraction force between fixed and moving plates caused by electrostatic charge. The deflection of a pointer (via a system of levers) is proportional to the applied voltage ($$\displaystyle F \propto V^2 $$).

• Advantages: Very high input impedance (negligible loading), independent of frequency (works for AC, DC, impulse peak), no need for external power.

• Disadvantages: Low sensitivity (requires high voltage for full scale), fragile, parallax error, typically limited to < 100 kV.

4.3.2 Generating Voltmeter

• Principle: A variable capacitor (rotating vane or disk) is driven at constant speed by a small motor. The AC current generated ($$\displaystyle I = V \cdot dC/dt $$) is proportional to the DC voltage $V$ being measured. This current is measured by a sensitive DC ammeter.

$$V \propto \frac{I_{avg}}{(dC/dt)_{avg}}$$

• Construction: Rotating electrode system inside a shielded housing, with a commutator or rectifier to produce a DC reading.

• Use: High DC voltage measurement (up to MV range), where a direct connection would draw significant current. Used in Van de Graaff generators, HV DC labs.

4.3.3 Surge Current Measurement

• Methods:

  1. Rogowski Coil: A flexible, toroidal coil around the conductor. Output voltage $$\displaystyle V_o = M \cdot di/dt $$, where $M$ is mutual inductance. Requires integration to get $i(t)$. Advantages: non-invasive, no saturation, high bandwidth.

  2. Shunt Resistors: A low-inductance, high-power resistor (e.g., manganin) placed in series. Voltage drop $$\displaystyle V = i(t) \cdot R $$ is measured. Must be carefully designed to minimize inductance and handle high currents/energy.

• Challenges:

  • High Bandwidth: Need to capture fast rise-times (nanoseconds to microseconds).

  • Safety & Isolation: Measurement circuit must be isolated from high potential.

  • Electromagnetic Interference (EMI): Large $di/dt$ creates strong magnetic fields that can induce errors.

  • Bandwidth Limitation of Cables & Oscilloscopes: Entire measurement chain must have sufficient bandwidth.

5.0 HIGH VOLTAGE TESTING OF POWER EQUIPMENT

5.1 Circuit Breaker Testing

• Short-Circuit Test (Making & Breaking Test):

  • Objective: Verify the breaker's ability to interrupt a short-circuit current and withstand the associated thermal and mechanical stresses.

  • Procedure: Simulate a fault using a special test circuit (e.g., parallel current source with a fast switch). Measures arcing time, re-strike voltage, current chopping, and ability to interrupt at various current levels and power factors.

• Dielectric Test (Withstand Test):

  • Objective: Verify the insulation strength of the breaker (main contacts, bushings, support insulators) between live parts and ground, and across open contacts.

  • Procedure: Apply a power-frequency voltage (usually 1 minute) or impulse voltage (standard lightning/switching) at specified levels. No breakdown should occur.

5.2 Insulator Testing

• Flash-Over Voltage Test:

  • Procedure: Apply increasing AC or impulse voltage across the insulator (mounted as in service) until a flashover (surface discharge) occurs over the insulator. The voltage at which 50% of applications cause flashover is recorded.

• Mechanical Strength Test:

  • Purpose: Ensure the insulator can withstand tensile, compressive, bending, and torsional loads as per service conditions (e.g., conductor weight, wind, ice). Tested to specified minimum destructive load (MDL).

• Puncture Voltage Test:

  • Purpose: Determine the internal dielectric strength of the insulator material (porcelain, glass, polymer). A special electrode arrangement is used to apply voltage across the insulator body (not across the surface). The voltage at which internal breakdown occurs is the puncture voltage.

• Necessity of Tests: Ensure reliability, safety, and long service life under combined electrical, mechanical, and environmental stresses. Mandatory for type testing and routine testing per standards (IEC, IS).

5.3 Transformer Testing

• High Voltage Test (Withstand Test):

  • Purpose: Verify the integrity of the main insulation system (winding-to-winding, winding-to-core, winding-to-tank).

  • Procedure: Apply a power-frequency voltage (usually 1 minute) at a specified level (e.g., 1.5 to 2 times rated voltage) between each winding and ground, and between windings. Induced Overvoltage Test (AC) or Impulse Test (LI) is also performed to check insulation between turns.

5.4 Insulation Tests Comparison

6.0 SPECIALIZED TOPICS AND DEVICES (SHORT NOTE CATEGORY)

6.1 Power Transformer

• Construction: Core (laminated silicon steel), windings (primary/secondary on separate limbs or concentric), tank, bushings, tap changer, conservator, breather.

• Operation: Based on mutual induction. AC in primary creates alternating flux in core, inducing EMF in secondary. Voltage ratio = turns ratio.

• HV Relevance: Critical HV equipment. Requires rigorous dielectric testing (IR, HV withstand, impulse). Bushings are critical insulation interfaces. Tap changers (OLTC) are major maintenance and failure points. Cooling systems (oil, fans) are vital for insulation life.

6.2 Electrostatic Voltmeter

• Detailed Principle: A fixed stator plate and a movable vane/plate form a capacitor. Applied voltage $V$ creates an attractive force $$\displaystyle F \propto V^2 $$. This force moves the vane against a spring. A mirror on the vane deflects a light beam onto a scale for reading. No current flows from the source.

• Advantages for HV Measurement:

  • True RMS for AC (force $$\displaystyle \propto V_{rms}^2 $$).

  • Measures peak of impulse directly.

  • Infinite DC resistance (no loading).

  • Frequency independent up to MHz.

  • Primary standard capability.

6.3 Tesla Coil

• Detailed Construction:

  1. High-Frequency Transformer: Primary (few turns, heavy wire) and secondary (many turns, fine wire, often solenoid or helical).

  2. Capacitor: Connected across primary to form LC circuit.

  3. Spark Gap: Between primary capacitor and primary winding. Often a rotary gap for better control.

  4. Top Load (Toroid): Large conductive ring at top of secondary. Increases capacitance, reduces electric field stress, and improves Q-factor.

• Applications: Historical (radio transmission), educational demonstrations, high-frequency, high-voltage sources for vacuum tubes, X-ray tubes, ozone generators, plasma globes.

6.4 Statistical and Formative Time Lags

• Statistical Time Lag ($$\displaystyle t_s $$): The random waiting time for the first initiating free electron to appear from cathode (via natural radiation, photoemission). Follows an exponential probability distribution. Decreases with increasing overvoltage ($$\displaystyle V/V_b $$).

• Formative Time Lag ($$\displaystyle t_f $$): The deterministic time for the electron avalanche to develop into a full breakdown streamer. Depends on:

  • Overvoltage ($$\displaystyle V/V_b $$): Higher overvoltage $$\displaystyle \rightarrow $$ faster avalanche $$\displaystyle \rightarrow $$ shorter $$\displaystyle t_f $$.

  • Gap geometry: Uniform field $$\displaystyle \rightarrow $$ longer $$\displaystyle t_f $$ (Townsend); non-uniform $$\displaystyle \rightarrow $$ shorter $$\displaystyle t_f $$ (streamer).

  • Gas type and pressure.

• Total Lag: $$\displaystyle t = t_s + t_f $$. In impulse testing, $$\displaystyle t_f $$ is typically a few nanoseconds to microseconds, while $$\displaystyle t_s $$ can vary significantly.

6.5 Insulation Application Techniques (Motor Coil)

• Pouring Methods:

  1. Dip & Bake: Coil preheated, dipped into liquid varnish, drained, and baked. Simple, common.

  2. Brush/Spray Painting: Manual or automated application. Good for complex shapes, but thickness control is difficult.

  3. Vacuum Pressure Impregnation (VPI): Most effective. Coil placed in pressure vessel, vacuum applied to remove air/moisture from windings and voids, then liquid resin is forced in under pressure. Results in void-free, homogeneous insulation with excellent mechanical strength and dielectric properties.

  4. Powder Coating: Electrostatic application of dry powder, then cured. Used for some stators.

6.6 Generating Voltmeter

• Detailed Diagram & Operation:

  
        High Voltage DC (V)
  
              |
  
             [ ] Fixed Electrode (Stator)
  
              |
  
        [====] Rotating Vane (Rotor) <--- Shaft from Motor
  
              |
  
             [ ] Fixed Electrode (Stator)
  
              |
  
             GND
  
  

  • The rotating vane changes the capacitance between the high-voltage terminal and ground cyclically: $$\displaystyle C = C_{max} \cos(\omega t) $$.

  • The displacement current is: $$\displaystyle i = V \frac{dC}{dt} = -V \omega C_{max} \sin(\omega t) $$.

  • This AC current is rectified (via a commutator or electronic rectifier) and measured by a DC ammeter calibrated in volts.

  • Key: The average current $$\displaystyle I_{avg} \propto V $$. The motor speed must be stable and known.

6.7 Cavity Breakdown

• Relation to Partial Discharges (PD): A cavity/void within a solid dielectric (e.g., in resin, between paper layers in a transformer) is filled with gas (air, or gas evolved from decomposition).

• Process:

  1. The electric field in the cavity is higher than in the solid because the cavity has lower permittivity ($$\displaystyle E_{cavity} = \frac{\varepsilon_{solid}}{\varepsilon_{gas}} E_{applied} $$).

  2. When the field in the cavity exceeds the breakdown strength of the gas (Paschen's law applies to the small cavity), a small PD occurs across the void.

  3. The PD is localized (does not bridge electrodes) but causes erosion of the cavity walls and chemical degradation of the insulation over time.

• Significance: PD is a key indicator of insulation health. Continuous PD leads to treeing and eventual complete breakdown. PD measurement is a critical diagnostic test for cables, transformers, and motors.