Study Notes on High Voltage and Current Measurement Techniques

Measurement of High Voltages and Currents

• Importance of High Voltage and Current Measurement
    - Essential in industrial testing and research laboratories.
    - Accuracy in measurement ensures safety for personnel and equipment.
    - Protection against overvoltages and induced voltages due to stray coupling is necessary.
    - Device location and layout are crucial for effective measurement.

Classification of High Voltage Measurement Devices

• High voltage measuring devices differ significantly from low voltage devices due to:
    - Calibration needs for high voltage devices (device ranges must not be extrapolated linearly).
    - Need to minimize electromagnetic interference in impulse voltage and current measurements.

Types of High Voltage Measurement Devices

• Table 7.1 - High Voltage Measurement Techniques
    - (a) d.c. voltages
      - (i) Series resistance microammeter.
      - (ii) Resistance potential divider.
      - (iii) Generating voltmeters.
      - (iv) Sphere and other spark gaps.
    - (b) a.c. voltages (power frequency)
      - (i) Series impedance ammeters.
      - (ii) Potential dividers (resistance or capacitance type).
      - (iii) Potential transformers (electromagnetic or capacitance voltage transformer).
      - (iv) Electrostatic voltmeters.
      - (v) Sphere gaps.
    - (c) High frequency a.c., impulse voltages, and rapidly changing voltages
      - (i) Potential dividers with a cathode ray oscillograph.
      - (ii) Peak voltmeters.
      - (iii) Sphere gaps.

Measurement of High Direct Current (d.c.) Voltages

Techniques Used

• General method involves extending meter range with a large series resistance.

• Typical net current through the meter is limited to one to ten microamperes for full-scale deflection.

Challenges with High d.c. Voltages

• At very high voltages (1000 kV or more), challenges arise due to:
    - Large power dissipation.
    - Leakage currents.
    - Limitations on voltage stress per unit length.
    - Resistance changes due to temperature variations.

Solution Techniques

• Resistance Potential Divider with Electrostatic Voltmeter
    - Provides better performance for high precision needs, but still suffers drawbacks of power dissipation and current drain.
    - Spark gaps, such as sphere gaps, can be used for peak voltage measurement, affected by atmospheric conditions.

Measurement Technique Details

7.1.1 High Ohmic Series Resistance with Microammeter

• High voltage measured by connecting a high resistance (hundreds of megaohms) in series with a microammeter.

• Voltage calculated via $V = IR$, where:
    - V is the voltage of the source.
    - I is the current through the calibrated resistance R.

• Protective device used across meter (e.g., paper gap, neon glow tube) ensures protection against high voltages.

Protective Resistance Setup

• Value of series resistance R designed to allow a current of 1-10 microamperes for full-scale deflection.

• Constructed from multiple wire wound resistors in series to avoid ground flashovers.

• Corona-free terminations prevent discharge issues, using materials with low temperature coefficients.

Limitations of Series Resistance Design

• Significant challenges include:
    - Power dissipation and source loading.
    - Temperature effects and long-term stability.
    - Voltage dependence of resistive elements.
    - Sensitivity to mechanical stresses.

7.1.2 Resistance Potential Dividers for d.c. Voltages

• High accuracy with designs eliminating temperature and voltage influences.

• Voltage measured using the formula:
    $V = rac{(R_i + R_2)}{R_2} imes V_2$
    where V2 is the voltage across the lower arm R2.

Protecting Against Transient Voltages

• Voltage controlling capacitors connected across elements mitigate transient issues that may arise during voltage fluctuations.

Additional Measurement Techniques

7.1.3 Generating Voltmeters

• Operate under principles that isolate source loading, especially with generators.

Principle of Operation

• Charge stored in capacitor: $q = CV$, where C is capacitance, V is voltage.

• Varying capacitance results in current: $I = rac{dq}{dt}$.

• The current for varying capacitance described as sinusoidal yields RMS value:
    $I_{rms} = rac{C_m imes rac{dV}{dt}}{ ext{angular frequency}}$.

Practical Implementations

• Design of generating voltmeters incorporates rotating mechanisms driven by synchronous motors; can measure a.c. voltages under specific speed conditions.

Measurement of High a.c. and Impulse Voltages

Techniques Used

• Commonly utilize potential dividers, transformers, and electrostatic voltmeters, with designs tailored for insulation and source loading.

Key Instruments in High a.c. Measurement

• Peak Voltmeters and Sphere Gaps
    - Specifically utilized for capturing peak values and reference during calibration.

Series Impedance Voltmeters

• Employ resistive or reactive impedance for power frequency a.c. measurements, with capacitance preferred to minimize losses.

Series Capacitance Voltmeters

• Designed to substitute for resistors to negate thermal variations and boost accuracy in high a.c. measurements.

Capacitance Potential Dividers

• High accuracy employed in measurement while addressing harmonics through additional capacitors for linearization.

Capacitive Voltage Transformers (CVTs)

• Used in power systems; configured to measure various voltages while providing isolation between high and low voltage terminals.
  

Potentials and Capacitive Devices

• Potential Transformers (PT)
    - Old devices offering simplified construction, but can experience ratio and phase angle errors, suffering from harmonic and transient limitations.

Electrostatic Voltmeters

• Operate within electrostatic fields, leveraging the attractive forces between electrodes, useful for accurate measurement of high voltage parameters.

Spark Gaps for Measurement of High Voltages

Construction and Functionality

• Uniform field spark gaps provide reliable measurements of peak voltage based upon known sparkover voltage parameters.

Sphere Gaps

• Spheres are generally made from conductive materials (copper, aluminum) with engineered dimensions.

• Peak values recorded are dependent on spacing and surrounding conditions.
  

Operational Considerations

• Precise arrangement of spheres can affect results; configurations can either be vertical or horizontal to enhance measurement reliability.
  

Sparkover Voltage Calibration

• Measurements involve finding average sparkover values across multiple trials, recommending preliminary measurements before critical ones.

Capacitance and Environment Effects

• The sphere gap measurements involve accounting for nearby earthed objects, temperature, humidity, and the effects of irradiation on voltage readings.

Worked Examples

Example 7.1: Generating Voltmeter Design

• Capacitance calculated based on designed range from 20 kV to 200 kV while ensuring operational stability under specified current readings.

Conclusion of Calculations

• Generated capacitance for the voltmeter deduced to meet operational parameters effectively, suggesting an ideal capacitor configuration.

Example 7.2: Peak Reading Voltmeter Configuration

• Design criteria established based on specific voltage requirements, employing potential divider ratios to ascertain meter specifics for usability up to specified peak voltages.