Fort Collins Utilities, Light and Power Department has initiated an automation project to monitor co-generation facilities and control and monitor distribution load centres (switches and fuse locations). Various data is gathered for transmission back to the control room SCADA system using 900MHz wireless technology. All substation feeders are currently monitored by SCADA at the substation, but this only provides a total value for the entire feeder. By applying current monitoring at each 15KV distribution switch using, the RCTrms ac current transducer, exact real time currents may be obtained for long range planning studies and for switching operations during outage restoration. There are about eighty locations in total that require monitoring.
Current tap points are not available on the main feeders so current measurement devices are installed around 600A bolted T bodies and 200A elbows. This application would be very difficult for traditional iron-cored current transformers (CTs) because of the connection points large size and ‘inconvenient shape’. Additionally, conventional CTs have the potential for an open circuit secondary which could develop extremely large secondary voltages, the RCTms is intrinsically safe.
The flexible Rogowski coil is very easy to install saving many hundreds of dollars at each of the eighty locations. The RCTms also offer several other advantages in this application:
- There is no saturation because unlike conventional CTs, Rogowski transducers do not have an iron core.
- Clip-on coils eliminate the need for extensive switch modifications (which would be cost prohibitive) or disconnection of the 600A T-bodies to install the sensors.
- The output of RCTrms transducer is fed directly into the SCADA node (Micro Control Systems SCADA Pack). No additional expensive signal conditioners are required.
The RCTrms is ideal for arc furnace applications where accurate measurement of large load currents, in the 10kA to 100kA range, are required. Conductors are often large and the RCTrms can be supplied with coil lengths up to 5.0m if necessary. In one such application the RCTrms is used to monitor three individual secondary currents of a 16MVA submerged arc furnace transformer. The secondary current is 37.5kA at 60Hz on 3 secondary windings per transformer. The output of the RCTrms 4-20mA signal is connected to a PLC for continuous monitoring.
The secondary windings of the transformer are connected via multiple contact clamps to the electrodes. Continuous monitoring of the secondary currents using the RCTrms is important to ensure that the secondary currents are all of the same magnitude. An imbalance in the transformer secondary currents can be used to immediately identify a failure in the electrode contacts. Without current monitoring, the contact failure may not be identified and can lead to reduced transformer lifetime due to localized overheating of the transformer windings.
The paired bus tube configuration of the secondary conductors made installation of standard CT’s impractical. The RCTrms flexible sensing coil was easily installed around the bus tube pairs and provided a simple / cost effective solution for this application.
Other applications include
- VSD motor drives and generators
- Large pumps and compressors
- Building services
- Custom versions
- Rogowski coils immersed in transformer oil
- Long coils and cable runs
- Versions with a faster response time units up to 50ms have been supplied
- Versions with high frequency performance > 100kHz