Digital Partial Discharge Tester: Application Guide for High-Voltage Bushings, Instrument Transformers, and Surge Arresters

While transformers, GIS, and cables often dominate PD testing discussions, several smaller yet equally critical components deserve dedicated attention: high-voltage bushings, current transformers (CTs), voltage transformers (VTs), and surge arresters. These components are numerous, relatively inexpensive individually, but their failure can be just as catastrophic—and often more sudden—than that of a large transformer. A digital partial discharge tester provides the diagnostic sensitivity needed to detect insulation defects in these components before failure. This article provides a focused application guide for testing bushings, instrument transformers, and surge arresters.

Why These Components Deserve Dedicated PD Testing

Bushings, CTs, VTs, and surge arresters share several characteristics that make PD testing both challenging and valuable:

• High population: A single substation may contain dozens or hundreds of these components, making individual monitoring costly but fleet-wide testing highly beneficial.
• Sudden failure mode: Unlike transformers, which often show gradual degradation, these components can fail explosively with little warning due to their compact insulation systems.
• Critical function: A failed CT or VT disables protection and metering. A failed surge arrester leaves equipment unprotected. A failed bushing can destroy the transformer it connects.
• Accessibility for testing: Many of these components have test taps or grounding connections that facilitate PD measurement without disassembly.
• Well-defined failure mechanisms: PD in these components typically originates from a limited number of known defect types, making pattern recognition highly reliable.

High-Voltage Bushings

Bushings are the interface between transformer windings and the external power system. They are among the most failure-prone transformer components, accounting for approximately 25% of transformer failures.

Bushing Types and PD Characteristics

PD Testing Procedure for Bushings

1. Connect the digital partial discharge tester to the bushing tap using a calibrated coupling capacitor or direct tap adapter.
2. Ensure the tap is clean and dry; contaminated taps introduce noise.
3. If the bushing is installed on a transformer, perform testing offline with the transformer de-energized and energized from a separate test source, or online using the bushing tap as a PD sensor.
4. Apply voltage in steps: 50%, 70%, 90%, 100%, and 110% of rated line-to-ground voltage. Hold each step for 2–5 minutes.
5. Record PDIV, PDEV, and PD magnitude at each step.
6. Compare PD magnitude across all three phases; significant asymmetry indicates a defect.

Bushing Acceptance Criteria

• New bushings (factory test): PD <10 pC at 1.1× rated voltage per IEC 60137.
• In-service bushings (online or offline maintenance test): PD <100 pC at rated voltage; no sudden increase from baseline.
• PD >500 pC: Investigate immediately; schedule replacement during next available outage.
• PD >1,000 pC: Consider emergency action, especially if PD is increasing rapidly.
• PDIV below 0.8× rated voltage: Indicates significant defect; replace.

Current Transformers (CTs) and Voltage Transformers (VTs)

Instrument transformers provide measurement and protection signals. Their insulation systems are compact and often operate at high stress.

CT and VT Insulation Types

PD Testing Procedure for Instrument Transformers

1. For oil-filled or cast resin CT/VT, clamp HFCT around the grounding conductor.
2. For SF6 units, deploy UHF sensor at inspection port or acoustic sensor on enclosure.
3. Energize from the primary side if possible, or use an external test source for offline testing.
4. Measure PD at 0.8×, 1.0×, and 1.2× rated voltage.
5. For CTs, also test with secondary open (worst-case voltage stress across insulation).
6. Compare PD among similar units in the same switchyard.

Instrument Transformer Acceptance Criteria

• New units: PD <10 pC at 1.2× rated voltage per IEC 61869 (CTs) and IEC 61869 (VTs).
• In-service: PD <50 pC at rated voltage for oil-filled; PD <100 pC for cast resin.
• PD >200 pC: Investigate; compare with historical data.
• PD >500 pC: Plan replacement during next outage.
• For CVTs, any sudden increase in PD or change in capacitance (>5%) requires immediate attention.

Surge Arresters (Metal-Oxide Varistors)

Surge arresters protect equipment from overvoltages. Their metal-oxide varistor (MOV) blocks degrade with repeated energy absorption, and PD activity is a key indicator of impending failure.

PD Mechanisms in Surge Arresters

• Internal partial discharge: Occurs in voids within the MOV blocks or along the block-epoxy interface. Indicates manufacturing defects or thermal degradation.
• Surface discharge: Occurs on contaminated or moist external surfaces. More common in polluted environments.
• Thermal runaway precursor: Increased resistive leakage current generates heat, which accelerates degradation, leading to PD and eventual failure.

PD Testing Procedure for Surge Arresters

1. Measure reference voltage (U_ref) and leakage current (I_res) first—these are the primary diagnostic parameters.
2. For PD measurement, use HFCT around the arrester ground lead or a capacitive coupler at the base.
3. Apply voltage up to 1.0× continuous operating voltage (U_c). Do not exceed 1.1× U_c to avoid damaging the arrester.
4. Record PD magnitude. For new arresters, PD should be <10 pC at U_c.
5. Compare with previous measurements. A doubling of PD over 12 months indicates degradation.
6. If PD exceeds 100 pC or I_res increases by >20%, plan replacement.

Surge Arrester Acceptance Criteria

• New arresters: PD <10 pC at U_c per IEC 60099-4.
• In-service: PD <50 pC at U_c; stable over time.
• PD >100 pC: Investigate; check leakage current.
• PD >500 pC: Replace immediately; risk of explosion.
• Any arrester with visible damage, oil leakage, or cracked housing should be replaced regardless of PD measurement.

Common Challenges and Solutions

Case Study: Bushing PD Detection Prevents Transformer Failure

A 230 kV transformer underwent routine online PD monitoring using HFCT on the bushing ground leads. One phase showed PD of 180 pC, while the other two phases measured <20 pC. The PRPD pattern indicated internal void discharge. Offline testing with a bushing tap coupler confirmed 220 pC at rated voltage, with PDIV at 0.7× rated voltage. The bushing was replaced during a scheduled outage 6 weeks later. Inspection revealed significant paper degradation and carbon tracking within the OIP bushing. The transformer was saved from a catastrophic failure that could have cost $3 million and months of outage time.

Case Study: Surge Arrester PD Indicates Impending Failure

A 132 kV surge arrester showed increasing PD from 15 pC to 85 pC over 18 months, with a corresponding 25% increase in resistive leakage current. The PRPD pattern shifted from symmetrical (normal) to asymmetrical with clusters near voltage peaks—indicating internal MOV degradation. The arrester was replaced proactively. Post-mortem analysis revealed partial block failure and early thermal damage. Replacement cost $8,000; avoided failure cost: $250,000 in equipment damage and outage.

Selecting a Digital Partial Discharge Tester for These Components

For bushings, instrument transformers, and surge arresters, prioritize:

• HFCT input with pC calibration for quantitative measurement on ground leads.
• Bushing tap adapter for direct connection to capacitive taps.
• High sensitivity (1–10 pC) for detecting early-stage defects.
• Multi-channel capability for simultaneous testing of three phases.
• Portable, lightweight design for testing multiple components in a switchyard.
• Software with component-specific pattern libraries and acceptance criteria.
• Battery life sufficient for a full day of testing (8+ hours).

Fleet Management for High-Population Components

With hundreds of bushings, CTs, VTs, and arresters in a typical substation, systematic fleet management is essential:

• Asset register: Unique ID for each component, with manufacturer, type, installation date, and test history.
• Test schedule: Annual or biennial PD testing for all critical components; more frequent for units showing elevated PD.
• Baseline establishment: First measurement establishes baseline; subsequent measurements compared to baseline.
• Trend analysis: Plot PD vs. time for each component; flag any increase >20% per year.
• Risk ranking: Combine PD level, trend, criticality, and age to prioritize replacement.
• Spare parts planning: Maintain spares for components with rising PD trends.

Standards and Acceptance Criteria Summary

Best Practices Summary

• Test all three phases and compare—asymmetry is a powerful diagnostic indicator.
• Establish baseline PD for every critical component when new or after repair.
• Perform PD testing at multiple voltage levels to determine PDIV and PDEV.
• Combine PD with other diagnostics (leakage current for arresters, capacitance and tan delta for bushings and CVTs).
• Record environmental conditions (temperature, humidity) with each measurement.
• Train technicians on component-specific defect patterns and acceptance criteria.
• Integrate PD data with asset management systems for fleet-wide visibility.
• Act on trends, not just absolute values—a bushing at 80 pC that was 20 pC last year is more concerning than a stable 80 pC for five years.

Bushings, instrument transformers, and surge arresters may be small compared to power transformers, but their failure consequences are equally severe. A digital partial discharge tester provides the sensitivity and diagnostic capability to detect insulation defects in these components early, enabling planned replacement instead of emergency response. For asset managers responsible for substation reliability, including these components in a routine PD testing program is one of the most cost-effective reliability investments available.