Methods for Identifying the Frequency Response of Voltage Measurement Systems

Why Wideband Voltage Measurement Matters in Modern Power Systems

The global energy transition is fundamentally changing how transmission and distribution systems behave. High penetration of inverter-based resources (IBRs), power electronics, HVDC links, FACTS devices, and increasingly dynamic operating conditions have introduced voltage phenomena far beyond the traditional 50/60 Hz sinusoidal waveform. Harmonics, interharmonics, sub-harmonics, fast transients, and very fast transient overvoltages (VFTOs) are now part of normal grid operation.



At the center of this transformation lies the voltage measurement system the critical interface between the primary power system and secondary applications such as protection, control, metering, power quality monitoring, PMUs, and digital substations. If the frequency response of the voltage measurement system is not properly understood, verified, and documented, even the most advanced protection or monitoring scheme can produce misleading or unsafe results.

What Is a Voltage Measurement System?

A voltage measurement system is not just a voltage transformer. It is the entire measurement chain, which may include:

• Primary voltage sensor (IVT, CVT, LPVT, optical sensor)
• Intermediate converters or signal conditioning
• Cabling and termination
• Burden and connected secondary equipment
• Digital interfaces and merging units (IEC 61869-9)

The overall frequency response is determined by the combined behavior of all these components not by the primary sensor alone.

Drivers for Wideband Frequency Response Requirements

1. Changes in the Primary Power System

Modern grids experience:

• Increased harmonic distortion due to power electronics
• Bidirectional power flows
• Frequent switching events in GIS and AIS substations
• Ferro-resonance and resonant overvoltages
• Lightning and switching transients with frequency components up to hundreds of kHz

Voltage measurement systems must therefore reproduce signals far beyond rated frequency without unacceptable ratio error or phase displacement.

2. Expansion of Secondary Applications

Voltage signals are now used for:

• Power quality compliance (IEC 61000-4-7, -4-30)
• PMU-based wide-area monitoring
• Traveling-wave fault location
• High-speed protection and control
• Asset condition monitoring

Each application imposes different bandwidth and accuracy requirements, making frequency response identification essential.

Overview of Voltage Measurement Technologies

1. Inductive Voltage Transformers (IVTs)

Strengths:

• High accuracy at rated frequency
• Excellent long-term stability
• High burden capability



Limitations:

• Limited bandwidth due to core magnetics
• Susceptibility to ferro-resonance
• Resonance behavior at higher frequencies

IVTs are well suited for protection and metering at power frequency but require careful evaluation when used for harmonic or transient measurements.

2. Capacitor Voltage Transformers (CVTs / CCVTs)

Strengths:

• Lower weight and cost at EHV levels
• Integrated PLC capability
• Established technology

Limitations:

• Frequency-dependent accuracy
• Significant resonance effects
• Strong sensitivity to burden and damping systems

CVTs must be treated with caution for power quality and transient applications unless their frequency response is explicitly characterized.

3. Low-Power Instrument Transformers (LPIT / LPVT)

LPITs include resistive dividers, capacitive dividers, and optical voltage sensors.

Key advantages:

• Wide bandwidth (from DC to tens or hundreds of kHz)
• Linear response over wide voltage range
• Ideal for digital substations and power quality

Challenges:

• Sensitivity to termination and cabling
• Dependence on secondary electronics
• System-level testing is essential
  

Understanding Frequency Response Characteristics

Frequency-Domain Perspective

Frequency response describes how the magnitude and phase of the measured voltage change as a function of frequency. Key parameters include:

• Ratio error versus frequency
• Phase displacement versus frequency
• First resonance frequency
• Application bandwidth (e.g., 1%, 5%, or 10% error limits)

Time-Domain Perspective

Transient performance is equally important. Relevant parameters include:

• Step response
• Settling time
• Peak overshoot
• Reproduction of fast-front and very-fast-front transients



Time-domain and frequency-domain responses are mathematically equivalent and must be assessed together.

Identification Methods for Frequency Response

1. Mathematical (Model-Based) Identification

This approach uses equivalent circuit models:

• White-box models based on physical design
• Black-box models based on measured input-output behavior

Applications include:

• Design optimization
• Sensitivity analysis
• Simulation of resonance phenomena

Keentel Engineering routinely applies such models to evaluate transformer suitability for power quality and EMT studies.

2. Measurement-Based Identification

Practical testing methods include:

• Single-frequency sweeps
• Multi-tone excitation
• White-noise excitation
• Impulse and step-response testing

These tests may be performed:

• In high-voltage laboratories
• Using low-voltage injection methods
• On-site with mobile test systems

Measurement-based identification is essential for compliance verification and forensic investigations.

Influence Factors Engineers Must Consider

• Burden magnitude and power factor
• Secondary cable length and impedance
• Temperature variation
• Stray electric fields and grounding
• Merging unit sampling and filtering
• Ferro-resonance damping devices



Ignoring these factors can invalidate otherwise compliant laboratory results.

Technology-to-Application Mapping

Different applications require different measurement technologies:

• Revenue metering → IVT or LPVT (rated-frequency accuracy)
• Protection → IVT, LPVT with verified transient response
• Power quality → LPVT with wideband class
• PMU / WAMS → LPVT + digital interface
• Transient studies → Resistive or optical LPVT

There is no universal solution engineering judgment is required.

How Keentel Engineering Supports Clients

Keentel Engineering provides end-to-end support for voltage measurement system performance, including:

• Technology selection studies
• Frequency response testing and interpretation
• IEC 61869 wideband compliance support
• Power quality and PMU readiness assessments
• Forensic analysis of measurement errors
• Digital substation integration reviews

Our experience spans HV, EHV, renewable interconnections, HVDC, and complex grid environments.

Frequently Asked Questions (FAQ)