Instrument Transformers – Quick Notes for GATE Electrical & Electronic Measurements

Purpose of Instrument Transformers

  • Safety: Isolate measuring instruments from high voltage circuits

  • Convenience: Step down high currents and voltages to measurable levels

  • Standardization: Provide standard secondary values (5A for CT, 110V for PT)

  • Protection: Enable protection relay operation

  • Metering: Facilitate accurate measurement in power systems

  • Economic: Reduce size and cost of measuring instruments

Current Transformer (CT) - Basic Principle

  • Function: Steps down primary current to secondary current

  • Construction: Primary has few turns, secondary has many turns

  • Connection: Primary in series with line, secondary connects to ammeter

  • Turns Ratio: \(\dfrac{N_s}{N_p} = \dfrac{I_p}{I_s}\) (ideal condition)

  • Standard Secondary: 5A or 1A

  • Important: Secondary should never be open-circuited

  • Magnetizing Current: \(I_m = I_p - \dfrac{N_s}{N_p}I_s\)

CT - Types and Construction

  • Wound Type: Separate primary and secondary windings

  • Bar Type: Single conductor acts as primary

  • Window Type: No primary winding, cable passes through window

  • Toroidal Type: Doughnut-shaped core, uniform flux distribution

  • Core Material: High permeability silicon steel, grain-oriented

  • Insulation: Paper, oil, or SF6 for high voltage applications

CT Equivalent Circuit

  • Primary: Referred to secondary as \(I_p' = \dfrac{N_p}{N_s}I_p\)

  • Magnetizing Branch: \(I_m = I_c + I_L\) (core loss + magnetizing current)

  • Secondary: \(I_s = I_p' - I_m\)

  • Burden: \(Z_b\) connected to secondary

  • Leakage: Primary and secondary leakage reactances

  • Resistance: Primary and secondary resistances

Potential Transformer (PT) - Basic Principle

  • Function: Steps down primary voltage to secondary voltage

  • Construction: Similar to power transformer but smaller rating

  • Connection: Primary in parallel with line, secondary connects to voltmeter

  • Turns Ratio: \(\dfrac{N_p}{N_s} = \dfrac{V_p}{V_s}\) (ideal condition)

  • Standard Secondary: 110V or 100V

  • Important: Secondary should never be short-circuited

PT - Types

  • Electromagnetic Type: Conventional wound transformer

  • Capacitor Voltage Transformer (CVT): Uses capacitive voltage divider

  • Single Phase: For single phase or phase-to-ground measurements

  • Three Phase: For three phase measurements

  • Ferro-resonance Type: For very high voltage applications

CT Errors

  • Ratio Error: \(\epsilon_r = \dfrac{K_n I_s - I_p}{I_p} \times 100\%\)

  • Phase Error: \(\delta\) = phase difference between primary and secondary currents

  • Causes: Magnetizing current, core losses, leakage flux

  • Minimization: Low reluctance core, proper burden selection

  • Saturation: Occurs at high fault currents, affects accuracy

Where \(K_n\) is nominal ratio = \(\dfrac{N_s}{N_p}\)

PT Errors

  • Ratio Error: \(\epsilon_r = \dfrac{K_n V_s - V_p}{V_p} \times 100\%\)

  • Phase Error: \(\delta\) = phase difference between primary and secondary voltages

  • Causes: Magnetizing current, core losses, winding resistance

  • Minimization: High permeability core, low burden

  • Ferranti Effect: Secondary voltage may exceed primary in long lines

Where \(K_n\) is nominal ratio = \(\dfrac{N_p}{N_s}\)

Phasor Diagrams

  • CT Phasor: \(I_p = I_s' + I_m\) where \(I_s' = \dfrac{N_s}{N_p}I_s\)

  • Phase Error: \(\delta = \tan^{-1}\left(\dfrac{I_m \sin \phi}{I_p + I_m \cos \phi}\right)\)

  • PT Phasor: \(V_p = V_s' + I_s Z_{eq}\) where \(V_s' = \dfrac{N_p}{N_s}V_s\)

  • Error Reduction: Minimize magnetizing current and burden

Accuracy Classes

  • CT Classes: 0.1, 0.2, 0.5, 1.0, 3.0, 5.0

  • PT Classes: 0.1, 0.2, 0.5, 1.0, 3.0

  • Meaning: Maximum permissible error at rated burden

  • Metering: Classes 0.2, 0.5 for revenue metering

  • Protection: Classes 5P, 10P for protection applications

  • Special Classes: Class X for differential protection

Burden and VA Rating

  • Burden: Total impedance of secondary circuit

  • CT Burden: \(Z_b = \dfrac{V_s}{I_s}\) where \(V_s\) is secondary voltage

  • PT Burden: \(Z_b = \dfrac{V_s^2}{VA}\) where VA is apparent power

  • Standard Burdens: 2.5, 5, 10, 15, 30 VA

  • Effect: Higher burden increases errors

  • Power Factor: Burden power factor affects accuracy

Testing of Instrument Transformers

  • Ratio Test: Verify turns ratio using bridge methods

  • Polarity Test: Check instantaneous polarity markings

  • Burden Test: Measure secondary circuit impedance

  • Accuracy Test: Determine ratio and phase errors

  • Insulation Test: High voltage and impulse tests

  • Saturation Test: For CTs to find knee point voltage

CT Saturation and Knee Point

  • Knee Point: Voltage at which 10% increase causes 50% current increase

  • Saturation: Occurs when core flux density reaches maximum

  • Effect: Severe ratio and phase errors during saturation

  • Testing: Apply voltage to secondary, measure exciting current

  • Applications: Critical for protection CT selection

  • ALF: Accuracy Limit Factor = (Knee Point Voltage)/(Rated Burden Voltage)

Safety Considerations

  • CT Secondary: Must never be open-circuited (causes dangerous voltages)

  • PT Secondary: Must never be short-circuited (causes excessive current)

  • Earthing: One point of secondary must be earthed

  • Polarity: Proper polarity marking essential for measurements

  • Short-circuiting: CT secondary terminals during maintenance

  • Insulation: Adequate insulation levels for operating voltage

Applications

  • Metering: Energy meters, power factor meters, demand meters

  • Protection: Overcurrent relays, distance relays, differential relays

  • Control: Automatic voltage regulators, load tap changers

  • Monitoring: SCADA systems, power quality analyzers

  • Measurement: Ammeters, voltmeters, wattmeters, varmeters

  • Synchronization: Synchroscopes, synchronizing relays

Special Applications

  • Multi-ratio CTs: Multiple secondary taps for different ratios

  • Summation CTs: Combine currents from multiple circuits

  • Bushing CTs: Built into equipment bushings

  • Rogowski Coils: Air-core current sensors for high frequencies

  • Optical CTs: Using Faraday effect for high voltage applications

Key Formulas for GATE

  • CT Ratio: \(K_n = \dfrac{N_s}{N_p} = \dfrac{I_p}{I_s}\) (ideal)

  • PT Ratio: \(K_n = \dfrac{N_p}{N_s} = \dfrac{V_p}{V_s}\) (ideal)

  • Ratio Error: \(\epsilon_r = \dfrac{K_n \times \text{Secondary} - \text{Primary}}{\text{Primary}} \times 100\%\)

  • Burden: \(Z_b = \dfrac{V_s}{I_s}\) (CT), \(Z_b = \dfrac{V_s^2}{VA}\) (PT)

  • Phase Error: \(\delta = \tan^{-1}\left(\dfrac{I_m \sin \phi}{I_p + I_m \cos \phi}\right)\)

  • Magnetizing Current: \(I_m = I_p - \dfrac{N_s}{N_p}I_s\)

Important Points for GATE

  • CT secondary should never be open-circuited

  • PT secondary should never be short-circuited

  • Accuracy class indicates maximum permissible error

  • Higher burden increases transformer errors

  • Polarity marking is crucial for proper operation

  • Standard secondary values: 5A (CT), 110V (PT)

  • Phase error occurs due to magnetizing current

  • Knee point voltage is critical for protection CTs

Numerical Problem Types

  • Ratio Error Calculation: Given primary, secondary currents/voltages

  • Phase Error Calculation: Using magnetizing current and burden

  • Burden Calculation: From connected instrument specifications

  • Accuracy Class Problems: Determining suitable class for application

  • Saturation Analysis: Finding knee point and ALF

  • Error Correction: Calculating true values from measured values