Optimizing Partial Discharge Measurement Setups: Zm vs HFCT Explained

Understanding Partial Discharge Measurement Setups: Methods, Components, and Best Practices

What Is a Partial Discharge Measurement Setup?

Partial discharge (PD) testing is a cornerstone of high-voltage insulation diagnostics. For reliable measurements, selecting the right configuration for your test circuit is critical. This article explores the key setups and components used in PD measurements — with a focus on measuring impedance (ZmZ_m), high-frequency current transformers (HFCT), and signal handling techniques.

Branch, Series, and Bridge Configurations Explained

There are three main ways to integrate the measuring impedance Z_m into a PD test circuit:

1. Z_m in Branch (Parallel Configuration)

In this setup, Z_m is placed parallel to the test object.

•  Safer for the impedance circuit in case of breakdown

• Lower measurement sensitivity

2. Z_m In-Series with DUT

Here, Z_m is placed in series with the device under test (DUT).

• High sensitivity

• Exposes Z_m to direct high-voltage events

3. Bridge Configuration

A balanced bridge layout uses multiple Z_m elements to reject common-mode noise.

• Excellent for lab-grade sensitivity

• Requires precise balance and matching components

The Role of Measuring Impedance (Zm)

The measuring impedance Z_m converts high-frequency current pulses generated by PD events into voltage signals — which are easier to digitize and process.

Why use voltage instead of current?

• Easier to amplify

• Lower noise susceptibility

• Most acquisition systems are voltage-based

• Digital Signal Processing (DSP) works better in the voltage domain

 Using High-Frequency Current Transformers (HFCT)

High-Frequency Current Transformers (HFCTs) detect PD pulses inductively — without direct electrical contact.

Pros:

• Non-invasive

• High isolation

• Broad frequency response (100 kHz – 100 MHz)

Cons:

• Can saturate under 50/60 Hz load

• Requires integration and careful calibration

Designing the Ideal Zm Filter

A typical Z_m behaves like a high-pass filter:

Z_m = R + jωL

• High Z_m : Greater sensitivity, but may saturate

• Low Z_m : Lower sensitivity but better performance under high current

Selecting the right Z_m depends on your test object's expected discharge strength and background noise.

Why the Coupling Capacitor (C_k ) Matters

The coupling capacitor ( C_k ) provides a parallel path for high-frequency PD signals to reach the sensor.

• Higher C_k improves signal-to-noise ratio (SNR)

• Larger C_k also demands more supply current

• It directly affects pulse width: τ = RC

Managing Noise and Frequency Band Selection

Controlling noise is critical in PD measurements. Common types include:

• White Noise: Appears as a flat FFT spectrum

• Switching Noise: Comes from inverters or power supplies

• Sinusoidal Interference: From AM/FM signals or power-line carriers

• Tip: Always perform a noise spectrum analysis (FFT) before beginning PD tests. A clean background is essential for accurate detection.

Final Thoughts on Setup Optimization

Accurate partial discharge testing relies on more than just instruments — it requires a thoughtful setup. Whether you choose a simple parallel Z_m , a sensitive in-series configuration, or a lab-grade bridge network, each method has its pros and cons.

HFCTs offer non-intrusive sensing for environments where safety and isolation are key, while Z_m based setups provide more direct voltage analysis.

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