In a presentation at the Emerson Exchange EMEA 2024, Federico Martincigh, instrumentation and electrical maintenance specialist at Edison, described the implementation of an online monitoring solution for SF6 420 kV circuit breakers at the Torviscosa combined cycle gas turbine (CCGT) power plant in Italy.
Circuit breakers are used to switch electrical equipment and networks under normal and fault conditions. The primary function is to interrupt the flow of current by opening its contacts, thereby isolating the switched parts of the system. SF6 circuit breakers utilize sulfur hexafluoride (SF6) gas as the surrounding insulating medium for extinguishing the arc established between the moving breaker contacts.
Every SF6 circuit breaker is composed of two breaking chambers filled with SF6 gas. The high pressure (HP) compartment is composed of two breaking chambers and one column. This compartment is filled with SF6 at design pressure. The low pressure (LP) compartment is filled with SF6 at a pressure lower than atmospheric pressure and is needed to operate the switch. Both the HP and LP compartments have double static seals.
The energy for actuating the main contacts is provided by SF6 pressure gas differential between two chambers. The interrupting chamber is under rated SF6 gas pressure for insulating the live parts and extinguishing the electric arc. On receiving a control signal, the valve group directs the SF6 in front of the piston to open the sliding contact. The pressure drop, which can be observed every time the switchgear is operated, is restored by a sealed compressor installed in the expansion enclosure.
The SF6 gas volumes inside the device are designed to perform a complete cycle without compressor operation. SF6 gas is essential for the safe and reliable operation of the breaker because if the device starts losing gas, it will not be able to extinguish the electric arc, increasing the risk of faults, damage and explosions. It is therefore important to monitor the SF6 pressure and sealed compressor efficiency through predictive maintenance and online diagnostics.
Edison, part of the EDF group, is one of the leading energy operators in Italy. At its Torviscosa plant, the existing breaker design only includes a visual pressure gauge, but this provided limited monitoring, no automatic alarm and no recorded data in the SCADA system. With readings needing to be performed manually, this increased risk, requiring personnel to walk near high voltage devices in every weather condition.
Edison decided to install an automated solution from Emerson, which included a Rosemount™ wireless pressure transmitter for each HP chamber, a wireless discrete transmitter to check the compressor duty cycle, and a wireless temperature transmitter to linearize the pressure value. The linearization of the pressure is used to compare actual pressure and rated pressure.
The monitoring system consists in 25 wireless instruments from Emerson.
SF6 monitoring through traditional wired devices can lead to electromagnetic field disturbances. A WirelessHART network was therefore deployed to transmit measurement data to the control room. The complete monitoring system consists of 25 wireless instruments, a WirelessHART gateway and one remote antenna. The gateway was situated in a centrally located control room, with the antenna installed externally to connect to the field instrumentation. The wireless network is very stable and unaffected by the electric field of the high voltage (380 kV) substation.
Configuration, set-up and calibration of instrumentation was performed using an existing HART communicator device. These activities can also be performed using Emerson’s AMS software, which can be used to configure and start the wireless gateway.
The OPC standard protocol has been utilized to seamlessly integrate the data from the wireless monitoring system into the power plant distributed control system (DCS). The AMS suite includes an OPC Server. The OPC client (the DCS) uses the OPC server to get the data. The DCS linearizes the SF6 pressures based on ambient temperature and can monitor the compressor state and the number of start-ups per day. Historical trends are now available, and it is possible to set alarms based on the rated pressure.
A lot of hours have been saved because of the wireless solution. Operator rounds have been reduced, increasing safety. Having a WirelessHART network in place has also allowed the identification of new applications and the seamless integration of additional measurement points.
Federico Martincigh, instrumentation and electrical maintenance specialist at Edison.
