Measurement & Measuring Instruments

1. Measurement of Power (1-Phase & 3-Phase)

  • Single Phase: P = V × I × cos(φ)
  • Three Phase (Balanced Load):
    • 3-Phase Active Power = √3 × VL × IL × cos(φ)
    • Reactive Power = √3 × VL × IL × sin(φ)
  • 2-Wattmeter Method: Common for 3-phase power measurement.
Total Power = W1 + W2
tan(φ) = (√3 × (W1 - W2)) / (W1 + W2)

2. Measurement of Frequency & Phase Angle

  • Measured using electronic counters, CRO, and frequency meters.
  • Phase angle can be measured using CRO by time difference method.

3. Types of Meters

  • Moving Coil: Measures DC only, high accuracy.
  • Moving Iron: Can measure AC & DC, less accurate.
  • Extension of Range: Shunt resistors (for current), series resistors (for voltage).
  • Wattmeter: Used to measure power (P = V × I × cos(φ))

4. Multimeter, Megger & Energy Meter

  • Multimeter: Measures voltage, current, resistance.
  • Megger: Used to test insulation resistance.
  • Energy Meter: Measures energy consumption in kWh.

5. AC Bridges & Instrumentation

  • Used to measure unknown inductance, capacitance, resistance (Maxwell, Wien, Schering bridges).
  • CRO: Displays waveform, measures voltage, frequency, phase angle.
  • Signal Generator: Produces known signals for testing.

6. CT (Current Transformer) & PT (Potential Transformer)

  • CT: Steps down current for metering.
  • PT: Steps down voltage for metering.
  • Used in conjunction with relays/meters in substations.

7. Earth Fault Detection

  • Detected using Earth Leakage Circuit Breakers (ELCB), relays, and fault indicators.
  • Helps in protection from shock and equipment damage.

8. Previous Year Questions

  • Q. In 2-wattmeter method, what is the power factor if both wattmeters read equal?
    Ans: 0.5 (Lagging)
  • Q. Which instrument is used to measure insulation resistance?
    Ans: Megger
  • Q. CTs are used to measure?
    Ans: High current safely
  • Q. In which method is total power the sum of two wattmeter readings?
    Ans: 2-Wattmeter method