Introduction
Dynamic Ratings was asked to test a 300 MW, 20 kV generator equipped with Stator Slot Couplers (SSC). This application also included Resistance Temperature Detectors (RTD) installed in the same slot directly under the SSC, with the RTD located between the top and bottom bar. This case study documents the effectiveness of using RTDs as a reliable partial discharge (PD) sensor, while informing the benefits of using RTDs and Coupling Capacitors for making more thorough informed decisions. Dynamic Ratings is not recommending the use of RTDs in place of an SSC. We are providing documentation about the effectiveness when using RTDs.
The Challenge
Some large turbine generators that utilize SSCs are commonly between the top coil and the slot wedge. SSCs have a limited “zone of sensitivity” and typically only provide PD detection for the slot where they are installed. Some partial discharge sensors and monitor manufacturers attempt to convince clients that the use of RTDs for PD is ineffective.
The Solution
Measurements were simultaneously made using both the RTD and SCC devices. In the graph above, the upper trace shows data from the RTD and the lower trace shows data from the SCC.
The partial discharge plots documented that the PD patterns look almost identical. The measurements showed that both types of sensors have nearly identical sensitivity. Minor differences are explained by the symmetrical position of RTDs between stator bars and the asymmetrical position of SSC. Note the PD from the bottom bar will be shielded by the top bar, therefore one can expect lower sensitivity of SSC to PD pulses in the bottom bar.
When a system is installed, there is no way of knowing which sensor will be detecting the most PD. Therefore, it is recommended that both RTD’s and Coupling Capacitor sensors be used to capture PD activity.
Due to signal attenuation issues, coupling capacitors installed at the line terminals of machines will only detect PD signals for around 10 – 15% of the winding. Defects that might be occurring deeper in the winding will not be detected by coupling capacitors.
Using the RTDs is like using a pair of binoculars which allow you to see more of the winding than just using coupling capacitors alone. The Dynamic Ratings PD monitoring systems do not require the use of RTDs, however it is strongly recommended.
The Result
With so many global installations and millions of existing data records, we have documented the value of using existing RTDs in motors and generators as a PD sensor. Further, our rich data records show that the use of RTD’s and coupling capacitors provide customers thorough, uncompromised views of the PD activity occurring then SSL’s or RTD’s alone. Dynamic Ratings reminds the industry to use care in the application of RTDs on generators. Here are two examples:
- A major global turbine generator manufacturer, historically locates its’ RTDs not in the windings (preferred) but in the gas space, consequently the use of the RTDs in this application is not recommended for partial discharge activity measurement.
- A second turbine generator manufacturer, historically locates its’ RTDs in every slot, making partial discharge measurement possible and reliable.
Dynamic Ratings recommends and promotes the use of RTDs and traditional coupling capacitors for maximum system PD monitoring coverage. Use of both devices is complementary and not mutually exclusive. The complementary data provides a more thorough and deeper view of PD activity, necessary for making better equipment decisions.
Continuous monitoring of partial discharge activity can provide early indication of potential failure. By providing this early warning, asset management and maintenance personnel can schedule condition-based maintenance via a planned outage. Taking a planned outage is far more cost effective than having an unexpected outage. Contact Dynamic Ratings for an application specific audit to better understand the optimal methodology of PD sensing in your application.
