Continuous partial discharge monitoring for medium and high voltage rotating machines
Tracking the stator insulation condition of a medium or high voltage motor or generator without stopping it calls for a continuous partial discharge monitor that also correlates every reading with the temperature, load and humidity at that moment. The RMM does this with 15 partial discharge channels per module acquired concurrently, a 70 dB dynamic range (3 mV to 10 V) with no gain adjustment and a 1 MHz to 20 MHz bandwidth, plus inputs for six winding temperatures, three load currents, three voltages and six 4-20 mA signals. It runs without a connected PC, stores up to two years of data and also works on machines fed from variable frequency drives.
RMM key data
- Acquisition: 15 partial discharge channels per module with concurrent data acquisition, with one or two optional 15-channel modules; the RMM accepts any brand of PD sensor, including those already installed on the machine.
- Measurement: 70 dB dynamic range, 3 mV to 10 V, with no gain adjustment; 1 MHz to 20 MHz bandwidth; 7.5° power frequency phase resolution and 32 magnitude windows.
- Operating dynamics: inputs for 6 RTD winding temperatures, 3 load currents, 3 voltages and 6 user-defined 4-20 mA signals; every base system includes an auxiliary CT and ambient humidity and temperature sensors.
- Detection zone: coupling capacitors near the terminals and the RTD-PD module, which picks up through the RTDs the discharge produced deep inside the winding.
- Versions and analysis: base systems for hydro generators, turbo generators or motors; motor current signature analysis to detect broken rotor bars in induction machines; suitable for VFD-fed machines.
- Data and communications: up to 2 years in FIFO memory (8 MB internal) and standalone operation without a PC; Ethernet, USB and RS-485 with ModBus as standard and DNP-3 on request.
- Power and mounting: 90-264 VAC (47-63 Hz) or 100-300 VDC; -40 to 70°C; standalone, panel, NEMA 4X enclosure or 316 stainless steel hazardous location enclosure (Class I Div. 1 B, C, D; Class II Div. 1 E, F, G; Class I Zone 1 IIB T4), the latter rated -20 to 40°C.
Advantages
Trend instead of snapshot
Periodic measurement shows the condition of the insulation on the day of the test. Continuous monitoring shows how it evolves, which is what makes it possible to plan an intervention before the fault appears. The unit stores up to two years of data on a first-in, first-out basis.
Discharge and operating dynamics together
Inputs are available for six winding temperatures, three load currents, three voltages and six user-defined 4-20 mA signals, so the discharge activity can be correlated with what the machine was doing.
Standalone operation
The monitor runs on its own, with no computer attached, and still reports through its built-in interfaces, so it can be left permanently installed in a plant that has no dedicated maintenance network.
Applications
- Turbogenerators and hydrogenerators in power plants.
- Large medium and high voltage motors driving critical processes.
- Machines fed from variable frequency drives.
- Detection of broken rotor bars in induction machines through current signature analysis.
- Installations in hazardous areas, with the explosion-proof enclosure version.
Detection across the whole winding
Coupling capacitors pick up the discharge activity near the machine terminals. The RTD-PD module reaches the discharge produced deep inside the winding, where the temperature detectors already respond to its high-frequency content. Used together, both techniques widen the detection area, and four parameters are recommended for correlation with partial discharge on any machine: winding temperature, ambient temperature, relative humidity and load current. Existing sensors can be reused where the machine already has coupling capacitors fitted, which shortens the commissioning work and keeps the outage to a minimum.
Results and software
Results are presented in every format used in the industry: two-dimensional and three-dimensional phase-resolved patterns, the polar view over the phase circle, the pulse height distribution and the trend of each quantity over time, with several channels compared on the same screen. The dashboard software configures the instrument, downloads and stores the data and provides the analysis tools.
Specifications and mounting
- 15 partial discharge channels per module, acquired simultaneously.
- 70 dB dynamic range, from 3 mV to 10 V, with no gain adjustment.
- Measuring bandwidth from 1 MHz to 20 MHz and phase resolution of 7.5°.
- Ethernet, USB and RS-485 interfaces, with ModBus as standard and DNP-3 on request.
- Supply from 90 to 264 V AC at (47-63) Hz, or from 100 to 300 V DC.
- Standalone, panel, enclosure or hazardous area mounting, the latter in 316 stainless steel.
Frequently asked questions
Why monitor partial discharge continuously on a high voltage motor?
Studies by IEEE indicate that up to 40% of electrical-origin failures on HV motors are attributable to the stator insulation, mainly through partial discharge in windings and cables. A periodic measurement only shows the condition on the test day; the RMM records the trend continuously and stores up to two years of data, so the intervention can be planned before the failure.
How many sets of coupling capacitors does the machine need?
Turbo generators, small hydro generators and motors commonly use two sets: one at the line terminals and one in the isolated-phase bus duct, at least 2 m apart in a time-of-arrival configuration. Large hydro generators often need two to four sets, one per parallel phase group, with coaxial cable lengths adjusted. The RMM works with sensors that are already installed.
The Dynamic Ratings RMM is manufactured by Dynamic Ratings and distributed by Amperis.
