What is partial discharge and why has it become a priority in hydrogenerator monitoring?
- AQTech Power Prognostics
- Jun 26
- 4 min read
Hydropower has a new job. For decades, the machines ran at base load, with stable output over long periods. Today, with the growth of solar and wind, the hydrogenerator has become the flexibility element of the grid. It starts when a cloud covers the panels, ramps fast when the wind shifts, runs at part load and even takes on synchronous condenser duty. On some days, a single machine can reach up to 20 start and stop cycles.
The machine is the same. The duty it performs is not. And that is where a phenomenon many operators still keep at arm's length comes in: partial discharge.
Designed for S1, operating in S4
IEC 60034-1, the standard that classifies the duty types of rotating electrical machines, makes this shift clear. Many hydrogenerators were designed for the S1 regime, continuous duty under constant load. In today's reality, inside hybrid grids, many machines work in S3 or S4 regimes, with daily start and stop cycles and, in some cases, more than 300 starts per year.
It is the same machine subjected to a thermal and mechanical effort it was never sized for. Monitoring has to keep up with this change and, above all, measure the damage it generates.
What is a partial discharge?
Partial discharge, or PD, is a microscopic spark inside the insulating material. It happens in small voids and defects of the insulation when the electric field becomes too strong at that point, generating extremely fast electrical pulses. It is called "partial" because it does not cause a full short circuit. Only a fraction of the insulation is involved.
The problem is not the isolated event. Each discharge is microscopic, but the effect is cumulative. Pulse after pulse, the insulation degrades, the asset's lifetime shortens and the risk of a forced outage grows. Discharges are measured in picocoulombs (pC), and tracking that level over time turns an invisible curve into a clear indicator of stator winding health.
Why this matters more now
A CIGRE survey of 1199 hydrogenerators found that roughly 56% of failures originate in the insulation. A word of caution about that number: the survey reflects a fleet operating at base load. The market has changed. Under the start and stop regime, the insulation goes through a thermal cycle on every start, and the real failure rates are likely to be considerably higher than the ones observed in that survey.
The reason is simple. Every start subjects the winding to thermomechanical stresses that favor delamination, micro-void formation and bar looseness, exactly the conditions that trigger and accelerate partial discharge. Damage no longer scales with hours of operation, it scales with the number of cycles. And the events that damage the machine the most happen outside the stable operating range, precisely where monitoring designed for constant load tends to give a false sense of security.
Monitoring PD online, with the machine in service, is no longer a differentiator. It has become a necessity.
Megger ICMobserver: state of the art in online PD
The heart of this solution is the ICMobserver, from Megger. It is a compact unit, in extruded aluminium housing, that delivers the most advanced online partial discharge monitoring available:
Parallel measurement on 4 channels, with independent synchronization per channel
IEC 61850 communication and integrated web server
No software licenses required
Also available in a portable configuration
Behind it is Power Diagnostix, now part of Megger, the pioneer in making PRPD analysis accessible to the market. Phase-resolved partial discharge analysis links each pulse to the voltage angle and reveals the type and location of the defect, going well beyond a simple amplitude value. Megger brings more than 130 years of insulation engineering, with over a thousand monitoring systems and 30,000 sensors in operation. When the subject is partial discharge, it is one of the best references there is.
Why the coupler makes the difference
Partial discharge is captured by capacitive couplers installed at the generator terminals, typically one per phase. The detail that separates a reliable measurement from a misleading number lies in the frequency range that coupler works in.
Megger couplers measure at lower frequencies than competing solutions. This is possible thanks to an advanced system of analog and digital filters that performs very efficient noise filtering. Working at a lower frequency, with noise under control, brings two direct advantages: more sensitivity and far broader coverage of the generator coils. The result is a measurement compliant with IEC 60270, traceable and trendable over the years, instead of a loose value that is hard to compare.
The combined AQTech + Megger solution
Partial discharge tells the electrical story of the machine. But the cyclic regime attacks the whole assembly: bearings, rotor, turbine, air gap and magnetic flux. That is why AQTech, a specialist in mechanical condition monitoring for hydrogenerators, joined its VibraOne platform with Megger's PD solution.
The integration between AQTech and Megger happens in hardware and in software. These are not two systems sitting side by side, but a single fully integrated system, able to monitor mechanical failures and electrical failures of the machine at the same time. Vibration, air gap, magnetic flux and partial discharge in one architecture, with local workstation and server and remote access. It is the mechanical and electrical reading of the asset on the same screen, correlating what happens during the start with what happens in the insulation. This combination is already in the field, supplied in plants of different countries and sizes, and increasingly demanded by the market.
The message for those operating hydro today
The flexibility the grid needs comes at a price, and that price is paid in asset wear. The good news is that this wear is measurable. Monitoring partial discharge online, with the right coupler at the right frequency, is the way to see insulation degradation before it turns into a forced outage.
Maintenance in hydro today needs to be condition-based, not calendar-based. And condition monitoring has never been more important. Want to understand how the combined AQTech + Megger solution applies to your plant? Let's talk. Get in touch with me and schedule a demo: https://linktr.ee/thiago.kleis
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