The generator circuit breaker (GCB) is of great importance for the undisturbed operation of modern power plant. A new infrared sensor has been developed to measure temperature inside these circuit breakers.
The GCB typically sits between a generator and its transformer to protect this equipment from damage while handling very high currents – typically tens of kA.
With such high currents, even a small increase in resistance in the current-carrying path will lead to a large temperature increase in the breaker – and this can have very dramatic effects.
Temperature supervision is, therefore, essential. This, however, can be a very challenging task in the high-voltage environment, so ABB embarked on a development programme to produce a new temperature sensor system for GCBs.
CBs are used, for example, in fossil-fuelled, nuclear, gas turbine, combined cycle, hydro power and pumped-storage power plant. They have a tough job. During normal operation of the power plant, the GCB must carry the full nominal current of the generator, which can easily reach 23 kA without bus cooling or over 30 kA with active cooling – all at potentials of up to 32 kV (see Fig. 1).
At such high currents, even a slight increase in electrical resistance in the current-carrying path leads to a large temperature increase. Increased resistance can arise from connection misalignment, dust inside the GCB or damaged contact surfaces. The consequent heating can lead to damage to internal silver-plated contact areas such as the bus duct connection zones, the line disconnector and the contact system of the interrupting chamber. Heat removal from the main conductor is partly done by radiation, so paint with high emissivity is usually applied to the conductor – but this cannot cope with significantly elevated temperatures (normal operating temperatures are in the range of 70 to 90°C).
Excess temperature can lead to loss of interrupting capability or even provoke a flashover if components start to melt.
GMS600
ABB’s GMS600 is a GCB monitoring system which indicates the need for maintenance and provides early warning to avoid unexpected downtime
(see Fig. 2). It calculates remaining time-to-overhaul based on cumulative current interruptions, total number of mechanical operations, time from last overhaul, circuit breaker main drive supervision, SF6 density and so on.
One aspect missing from its repertoire was temperature supervision. This was because no commercially available temperature sensing system fulfilled all the technical, commercial and functional requirements for an accurate and reliable temperature monitoring of GCBs during operation.
This lack of a commercial system is not surprising as the temperature supervision of HV components can be challenging. For example, the temperature sensor has to survive severe electromagnetic conditions and can also be exposed to steep temperature gradients caused by, for instance, desert climate. A new temperature sensor system had to be developed.
Fig. 1: Pumped storage power plant application with HECPS-3S.
Sensor development and design
A detailed technical analysis determined that a temperature measurement scheme based on the detection of infrared radiation (IR) was the best approach. The goal then was to take a commercially available IR sensor element and package it to operate reliably in the demanding GCB environment.
The central component of the IR temperature sensor is the IR detector element itself. Non-cooled Si-based thermopile detectors were chosen due to their good cost/performance ratio. The only way to ensure proper performance of the sensor under the severe conditions encountered in a GCB – spatial and temporal temperature gradients and high electromagnetic fields with fast transients – was to package the IR detector and the electronics appropriately.
In addition to the detector ASIC, other electronics were included to convert the digital SMBus output signal to Modbus. These electronics, too, must withstand the severe electromagnetic interference (EMI) environment of a GCB.
The sensor package, therefore, must fulfill three major objectives:
Suppression of large spatial temperature gradients at the IR sensor element.
Suppression of large temporal temperature gradients at the IR sensor element.
Suppression of EMI.
Fig. 2: Central condition monitoring device GMS600.
To fulfill the first objective, the housing of the IR sensor element was surrounded by a material with high thermal conductivity (see Fig. 3). This ensures that temperature gradients immediately equilibrate and the thermal field around the sensor remains homogeneous. The second objective can be satisfied by choosing a design that leads to a large thermal time constant (in the range of several minutes). The time constant of conductivity around the sensor to delay heat ingress to the sensor.
Fig. 3: Schematic of the cross section of the sensor package showing the main functional components.
The package is, therefore, a two-part housing concept where the inner and outer parts are weakly coupled thermally (see Fig. 3). This approach inherently satisfies the dielectric and EMI requirements, too: the outer housing acts as a Faraday cage and the thermal insulation acts as an electrical insulator as well as a heat barrier. As an additional EMI countermeasure, the outer housing is grounded through the GCB enclosure and the inner housing is connected to a local ground potential.
Package dimensioning was defined by transient thermal finite element method (FEM) simulations of a simplified thermal model (see Fig. 4)
The design goal was to achieve a thermal time constant greater than ten minutes. This duration was predicted by the simulation and verified by experimental tests later on.
Fig. 4: FEM simulation of the sensor package heating. This gives a first estimate of the thermal time constant of the whole sensor package and was used to define the package dimensions.
Fig. 5: Climate chamber simulation of GCB overheating:
IR temperature sensor response.
Prototyping and testing
Temperature shock experiments were performed to check the thermal design and to verify good thermal coupling of the IR sensor element to its surroundings.
The IR temperature sensor was exposed to an ambient temperature change of 25 to 70°C. The rise time of 5°C per minute was limited by the heating power of the climate chamber used. For the duration of the experiment, the IR temperature sensor stared at a black-body radiator held at a constant 80°C. Very good sensor performance (error less than 2°C) was found if the thermal coupling to the inner housing was guaranteed by a thermal grease or adhesive.
To verify the performance of the IR temperature sensor, 21 sensor prototypes were built and subject to different environmental scenarios simulated by the climate chamber. Sensor response to a black-body radiator temperature range of 30 to 120°C at a constant ambient (sensor) temperature of 25°C was tested. The sensor response displayed a linear behavior. The linearity error remained below 3°C over the entire object temperature range. The variations (standard deviation) between the individual prototypes were found to be 0,8°C and 1,2°C at an object temperature of 75°C and 120°C respectively.
A very important task for the IR temperature monitoring system is the detection of GCB overloading, when the temperature of the main conductor can approach 120°C. This scenario was simulated by changing the object temperature range from 80 to 120°C (see Fig. 5a).
Fig. 5a: IR temperature sensor signal at an object temperature from 80 to 120°C over several hours.
The IR sensor accurately captured this temperature change and the measurement deviation stayed well within the required accuracy interval of ±3°C. To assess the influence of changes in ambient temperature, the IR temperature sensors were exposed to three consecutive temperature cycles from –5 to 60°C at a rate of
0,1°C/min. This rate of temperature change was chosen to simulate a typical day/night scenario. Again, typical sensor measurement error remained below 3°C. Furthermore, in humidity tests, the sensor measurement error was less than 2,5°C up to 90% relative humidity, at an ambient temperature of 60°C. The IR temperature sensors were also tested for other disruptive factors encountered in a GCB environment. This included extensive vibration testing to simulate mechanical shock experienced during GCB switching operations. Electromagnetic immunity was tested according to IEC 61000-4 and
IEC 61000-6, addressing immunity to RF electromagnetic fields and electrostatic discharges, as well as electrical fast transient tests (severity level 3 required). All tests were passed successfully and the sensor system qualified for operation in a GCB.
Productisation phase
The involvement of a potential manufacturer early on in the project resulted in a very mature technology demonstrator. Only a few changes were necessary for full productisation (modification).
Productisation was done in parallel to the adaptation work on the sensor itself. This covered the sensor assembly, cable harness design, mechanical integration of the sensors into the GCB enclosure, routing of cables, and a GMS600 monitoring software update to log, store and present the temperature data to the customer for nine sensors (three per phase).
The supply chain was put in place in co-operation with the manufacturer, who also preassembles sensors and cables on a mounting rack to speed installation in the GCB.
Fig. 5b: Measurement deviation stays well within the required accuracy interval of ±3°C.
Extended service
Cost-efficiency can be improved significantly by intelligent service approaches such as predictive maintenance. However, efficient, adaptive and sustainable predictive maintenance and equipment life strategies are highly dependent on meaningful sensor signals from the field.
The robust and cost-effective temperature sensor system described here enables reliable temperature monitoring of GCBs during operation. In combination with other sensor information (e.g. vibration or contact ablation), a clear picture of a device’s health condition can be derived and predictive maintenance strategies formulated.
This is especially important for GCBs where overheating of the main conductor can lead to the power plant shutting down, which can result in high cost and potentially disastrous equipment damage. Access to this kind of condition data also enables new service concepts and business models to be created, and provides valuable feedback for the design of new devices.
Finally, statistical analysis of data from an entire fleet of devices can reveal information unobtainable from a single device. This opens up new opportunities and value propositions for ABB to offer to end customers through its service portfolio.
Acknowledgement
This article was originally published in ABB Review 2/14, and is republished here with permission.
Contact Shivani Chetram, ABB, Tel 010 202-5000, shivani.chetram@za.abb.com
The post Temperature measurement in generator circuit breakers appeared first on EE Publishers.