Direct Answer

Periodic outage testing per the preventive-test code is costly, and there is concern that online monitoring cannot replace it. How should the two relate? The answer: DL/T 596 specifies the items and intervals of periodic preventive tests for electrical equipment — a periodic check in the de-energized state; online monitoring is continuous observation in the energized state. They are complementary: preventive tests find existing defects, online monitoring tracks developing trends; qualified online-monitoring data can serve as a reference for extending test intervals, but cannot replace mandatory preventive-test items.

At one plant, a 10 kV transformer followed the DL/T 596 interval for preventive tests and also had online monitoring. A preventive test found winding DC-resistance imbalance over limit (2.1%, standard 2%) and it was handled per the code; the online curve over the same period was smooth (no step change). Cross-analysis: the preventive test found an "existing defect" (a joint loosening that already existed); online monitoring did not trigger because the defect had not yet produced a measurable change — each tool covers its own segment. Conclusion: preventive tests + online monitoring in parallel gives more complete defect discovery. DL/T 596 and online monitoring are complementary: preventive tests find existing defects during outages, online monitoring catches trend step-changes during operation — together they form complete equipment health management.

Because:

  • Preventive tests are periodic checkups: systematic items and fixed intervals in the de-energized state;
  • Online monitoring is real-time supervision: continuous trend observation in the energized state;
  • Defect forms differ: existing defects (joint loosening) are found by tests, developing defects (PD growth) are caught by online;
  • Decision split: tests decide "whether to repair," online decides "when to repair."

Key insight: DL/T 596 and online monitoring are not substitutes but complements. Outages find existing defects, operation catches trends — both in parallel give the full health picture.

At one power plant's GIS: preventive tests per the DL/T 596 interval, plus online PD monitoring. Test data were normal, but online PD rose slowly for three consecutive months (from 30 pC to 120 pC), triggering an alert; at the next maintenance window the equipment was de-energized and metal particles were found on a spacer surface — the test could not catch it (outage test items are insensitive to this kind of developing defect), while online monitoring detected it early. The two tools each cover one segment: tests find problems that "already exist," online monitoring finds problems that are "developing" — together nothing is missed.

1. Items and Intervals of the DL/T 596 Preventive-Test Code

Step one: what the code governs.

Positioning of the code:

  • DL/T 596, Preventive Test Code for Electric Power Equipment;
  • Specifies items, intervals, and criteria for periodic preventive tests;
  • Covers transformers, breakers, instrument transformers, cables, GIS, and other major equipment;
  • A periodic check in the de-energized state (scheduled inspection).

Typical intervals:

  • Transformer: winding DC resistance, insulation resistance, dielectric loss (1–3 years);
  • Breaker: circuit resistance, operating characteristics (1–3 years);
  • Cable: insulation resistance, DC withstand (1–3 years);
  • GIS: SF6 gas, circuit resistance, withstand (3–6 years).

Nature of test items:

  • Outage tests: the equipment must be taken out of service;
  • Test conditions: standard conditions (voltage, temperature, humidity controlled);
  • Clear criteria: each item has an explicit pass standard;
  • Result handling: over-limit items are scheduled for treatment (repair/replacement).

Limitations of tests:

  • Long intervals: the gap between two tests is a blind zone;
  • Outage cost: critical equipment is hard to de-energize (continuous production);
  • Instantaneousness: a snapshot at the test moment (trends unseen);
  • Item limits: insensitive to developing defects (PD growth).

Key insight: DL/T 596 is the outage checkup code. It sets items, intervals, and criteria — finds existing defects, but the gap between tests is a blind zone.

2. Division of Labor Between Online Monitoring and Tests

Step two: how the two tools divide work.

Positioning of online monitoring:

  • Continuous observation in the energized state (real-time data);
  • Measured quantities: temperature, PD, insulation, current, etc.;
  • Trend analysis: catching change trends (developing defects);
  • Early-warning capability: early anomaly detection.

Division logic:

  • Tests: find existing defects (present, not yet developing);
  • Online: find developing defects (currently worsening);
  • Tests: define items and criteria (pass standards);
  • Online: define trend thresholds (rate-of-change alerts).

Complementary scenarios:

  • Test pass + online normal: healthy;
  • Test pass + online abnormal: developing defect (online detects first);
  • Test abnormal + online normal: existing defect (test detects first);
  • Both abnormal: defect already developed (handle immediately).

Data relationship:

  • Online data cannot directly replace test items;
  • Online trend anomaly → schedule an outage test/repair for verification;
  • Test results calibrate online thresholds (validation);
  • Analyze both data sets side by side (cross-validation).

Key insight: the division is "existing vs developing." Tests find existing, online catches developing — cross-validation leaves no defect missed.

3. Can Online Data Extend Test Intervals

Step three: can outages be reduced.

Policy basis:

  • Condition-based maintenance: good equipment state allows adjusted intervals;
  • Online-monitoring data is an important input to condition assessment;
  • Qualified online monitoring can serve as a reference for extending intervals;
  • But it cannot replace mandatory test items (per the code).

Conditions for extending:

  • Online monitoring covers key quantities (temperature/PD/insulation, etc.);
  • Long-term data stable (no abnormal trends);
  • Historical tests consistently pass;
  • Stable operating environment (no overload/harsh conditions).

Notes:

  • Extension requires approval (condition assessment + expert review);
  • Conservative for critical equipment (do not extend lightly);
  • Restore the original interval when monitoring fails/data is missing;
  • Extension ≠ cancellation (still perform minimum items per interval).

Key insight: extension is a conditional adjustment. Good state, full monitoring, stable data — extension may be referenced; mandatory items remain non-replaceable.

4. Mandatory Test Items That Cannot Be Replaced

Step four: which items must be done during outages.

Non-replaceable items:

  • Winding DC resistance (transformer): contact-state judgment;
  • Insulation resistance/dielectric loss (accurate measurement requires outage);
  • DC withstand/leakage current (cable insulation strength);
  • Breaker operating characteristics (mechanical-state verification);
  • Circuit resistance (contact state);
  • SF6 gas analysis (GIS insulation-medium state).

Why non-replaceable:

  • Measurement requires outage conditions (accuracy requirements);
  • Test energization (withstand) is possible only in the de-energized state;
  • Criteria are based on standard test conditions;
  • Online measurements ≠ test items (different quantities).

What online monitoring supplements:

  • Operating temperature trends (rise deterioration);
  • PD development trends (early insulation degradation);
  • Load-health correlation (operating-condition effects);
  • Alert + localization (narrow the repair scope).

Key insight: mandatory items are non-replaceable. Tests requiring outage conditions can only be done by preventive testing; online monitoring supplements operational trends — the two do not swap.

5. Combined Maintenance Strategy

Step five: what combination is most reasonable.

Combination strategy:

  • Tests as the base: mandatory items per the code's intervals;
  • Online coverage: online monitoring on critical equipment;
  • Anomaly linkage: online anomaly → schedule outage verification/repair;
  • Data accumulation: both data sets accumulated side by side (historical comparison).

Critical equipment first:

  • Important-load circuits (high outage cost) get online monitoring first;
  • Aging equipment (higher defect probability) first;
  • Harsh environments (outdoor/high-humidity/high-temperature) first;
  • General equipment: tests primarily + point online monitoring.

Maintenance decisions:

  • Online trend normal + test pass: maintain per interval;
  • Online alert: advance the schedule (do not wait for the interval);
  • Test over-limit: handle per criteria;
  • Both abnormal: immediate outage repair.

Key insight: the combination is "tests as the base + online as reinforcement." Mandatory items not skipped, critical equipment online, anomaly-linked repairs — a closed-loop health-management loop.

6. Product Landing: Predictive-Maintenance Scheme

FEXLINK products bring DL/T 596 into daily operation:

  • Predictive-Maintenance Scheme: online monitoring of critical equipment (temperature/PD/insulation);
  • Trend analysis: early detection of developing defects;
  • Side-by-side with test reports: cross-validation (existing + developing);
  • Maintenance recommendations: online alert → schedule outage verification/repair.

Design principle in one sentence: "tests find existing, online catches developing" — dual-track. Code checkups + real-time supervision — full equipment health visibility.

7. Common Misconceptions in Interpretation

  1. Treating online monitoring as a substitute: no tests once online is installed. Consequence: mandatory items missing (existing defects missed). Correct: tests as base + online reinforcement.
  2. Tests only, no trends: at ease once tests pass. Consequence: developing defects missed between tests. Correct: online monitoring fills the blind zone.
  3. Extending without basis: extending with incomplete data. Consequence: equipment state out of control. Correct: condition assessment + approval + conservatism.
  4. Isolating the two data sets: no cross-validation. Consequence: information silos (incomplete defect judgment). Correct: side-by-side analysis (tests calibrate online).
  5. Flat outage-cost view: all equipment hard to de-energize. Consequence: critical equipment unmonitored. Correct: grade by importance (critical circuits get online first).

Scope and Limitations

This article provides a framework for interpreting the complementary relationship between the DL/T 596 preventive-test code and online monitoring (test items and intervals, division of labor, conditions for extending intervals, non-replaceable mandatory items, combined maintenance strategy) for planning preventive testing and online monitoring, budget decisions, and maintenance-strategy design. Specific test intervals and criteria follow the current DL/T 596 version and the clauses for the equipment type; extending test intervals must be approved per condition-based-maintenance regulations; outage tests must be performed by qualified test personnel per procedures. All cases are anonymized (a certain plant / a certain power plant) and do not refer to any specific customer.