Every utility and industrial asset owner eventually faces the same question: should we start with handheld partial discharge (PD) detectors for periodic inspections, or invest directly in online PD monitoring systems? Both approaches detect the same underlying phenomenon, but they answer different operational questions, require different budgets, and fit different stages of a condition-based maintenance programme.
This guide compares the two approaches across the criteria that actually drive purchasing decisions — coverage, cost, manpower, data quality and deployment — and offers a practical framework for choosing the right starting point.
What Portable PD Testing Delivers Today
Portable instruments such as handheld PD detectors and live-line testers give a technician a snapshot of asset condition at a specific point in time. A patrol can cover switchgear, GIS, transformers and cables in a day, applying TEV, ultrasonic (AE), UHF and HFCT methods as the situation demands.
- Broad coverage per visit — one operator can screen many bays and panels in a single shift.
- Low entry cost — a single instrument replaces multiple dedicated monitors for survey work.
- Flexible method selection — the same unit can switch between TEV, AE, UHF and HFCT depending on the asset and access.
- Fast triage — results tell you immediately whether a defect is present, and roughly how severe it is.
The limitation is obvious: a periodic inspection only sees the hours when the operator was present. Intermittent discharge activity — the kind that appears under certain loads, temperatures or weather conditions — can be missed entirely. For a deeper look at portable instrument selection, see our guide to live-line partial discharge testing.
What Online PD Monitoring Delivers Today
Online monitoring installs sensors on the asset and continuously streams PD data to a local concentrator or cloud platform. Instead of a snapshot, the utility gets a time series: trend curves, phase-resolved patterns and alarm events that show exactly when activity started and how it evolves.
- Continuous visibility — activity is captured at 2 a.m. as well as at peak load, under every weather condition.
- Trending and early warning — small increases in PD magnitude become visible weeks before a failure.
- Lower long-term labour cost — remote access reduces repeated site visits for routine screening.
- Data for planning — maintenance intervals and outage plans can be based on measured condition rather than fixed schedules.
The trade-offs are a higher initial investment and the need to select the right sensor type and installation point. A transformer, for example, typically needs AE or HFCT sensors plus careful installation, as discussed in our transformer online monitoring guide.
Comparing the Two Approaches
The table below summarises how the two approaches differ across the dimensions that matter most in practice.
| Dimension | Portable PD Testing | Online PD Monitoring |
|---|---|---|
| Coverage | Many assets per visit, point-in-time | Fixed assets, continuous |
| Initial cost | Low (one instrument) | High (sensors + gateway + platform) |
| Manpower | Skilled operator per inspection | Mostly automated; review on alarm |
| Data value | Snapshot severity and location | Trends, patterns, early warnings |
| Installation | None (non-intrusive) | Sensor installation and wiring |
| Best suited for | Fleet screening, small sites, pre-outage checks | Critical assets, remote sites, load-driven defects |
A Practical Decision Framework
There is no single correct answer, but three starting paths cover most situations:
- Start with portable testing when you have many assets, a limited budget, or no clear failure history. One survey campaign identifies the 5–10% of bays that need attention, which then justifies targeted monitoring. This is the most common first step for utilities building a PD programme.
- Start with online monitoring when specific assets are already known to be critical or at risk — a main transformer, a long cable circuit, or a GIS bay where access for inspection is difficult. Protecting one high-value asset can justify the system cost immediately.
- Run both in parallel once the programme matures: portable testing screens the fleet, while online systems watch the critical core. The inspection results also validate and calibrate the monitoring installation.
Whichever path you choose, instrument quality matters more than the label on the product. A poorly calibrated portable detector produces misleading severity rankings, and a monitoring system with the wrong sensor type can miss the very defects it was installed to catch. For buyers evaluating suppliers, our PD detector FAQ and the handheld detector product page cover the specification points worth checking.
Frequently Asked Questions
Can a portable PD detector be used for continuous monitoring?
Not in practice. A portable instrument is designed for periodic, operator-led inspection. For continuous coverage you need permanently installed sensors feeding an online monitoring system. Some handheld units support data export and basic trend comparison between visits, but they cannot replace a fixed installation.
Is online PD monitoring worth the cost for a small substation?
It depends on what the substation serves. If a single transformer or feeder supplies a critical load — a hospital, data centre, industrial process — the cost of one unplanned outage usually exceeds the monitoring investment. For low-criticality sites, periodic portable testing is often the more economical first step.
Do I need both portable testing and online monitoring?
Many mature programmes use both: portable screening across the fleet, online monitoring on the critical subset. The two are complementary rather than competing, and inspection results help verify that monitoring sensors are correctly installed and sensitive enough.
How many sensors does an online PD monitoring system need?
There is no fixed number. A switchgear panel may need one to three sensors (TEV, AE or UHF depending on construction), a transformer typically two or more (AE on the tank plus HFCT on bushings or neutral), and a cable circuit uses HFCT or directional sensors at terminations and joints. A supplier should size the system from the asset layout, not sell a fixed package.
Can online monitoring replace routine inspection?
For the monitored assets, it reduces but does not fully replace inspection. Visual checks, thermal imaging and portable re-verification still catch issues that fixed PD sensors are not designed to see. The monitoring system’s main job is to tell you when to send a technician, not to remove the technician from the process.
Talk to a Manufacturer That Builds Both
Zhuhai Huawang Technology Co., Ltd. (HUWOR) manufactures both portable PD detectors and online monitoring systems, including TEV, AE, UHF, HFCT and wireless sensors with IoT gateways and cloud platforms. R&D centres in Zhuhai and Xi’an, ISO 9001 certified manufacturing and factory calibration to IEC 60270 support everything from a single instrument to a full fleet monitoring programme. Contact us to discuss which starting point fits your assets.
All-in-One PD Testing System: UHF, TEV, AE & HFCT Methods Explained – the technical foundation behind both online and portable approaches.