The scope of this study is restricted in the HECON product family with MCC type solutions.
Temperature monitoring is also mainly restricted to points in the switchgear, which cannot easily be inspected with thermal imaging camera. However, with same methods, also visible points can be monitored, if necessary.
15 1.7 Objectives of study
The objectives of the study is to use literature to determine reasons, which can cause hazardous temperatures in switchgears. This information will be used to determine points in the switchgear, where hazardous temperatures could appear. When this information is gathered, the author will start working to find a suitable solution from the temperature monitoring system manufacturers, which could be used in HECON products. After this, the author will determine, how the chosen temperature monitoring system should be installed in the HECON switchgear by using gathered information. Final stage of this study is to test this temperature monitoring system, which will be installed in HECON, and examine, is this system operating as assumed and conclusions will be made in this thesis.
The final results of this thesis include finding a temperature monitoring system, which would be reliable in the HECON installation, and developing the system architecture and design methods. Designing is based on a research information from the critical points of the switchgear. As a result of this thesis, As Harju Elekter can provide an additional option to its customers, which includes a temperature monitoring system for HECON switchgears.
Objectives of study are presented in the thesis as follows:
- Chapter 1:
o Introduction and background of study.
o Temperatures in switchgear. The chapter presents research work for basis of design and theory of hazardous temperatures in connection points.
o Wireless sensor technology. The chapter presents selection process of manufacturer and introduction of wireless temperature monitoring system.
- Chapter 2:
o Design and development of wireless sensor system. The chapter presents principles of installation design of the system, component installation methods, part list design principles and installation design for system test.
o Monitoring system development. The chapter presents data communication topology of the system in general, system software introduction and design of preliminary system schematics.
16 - Chapter 3: Testing of system:
o Introduction of test environment, test methods, test results and conclusions.
- Chapter 4, Final conclusions:
o The chapter presents discussion and conclusions of cost of the system and benefits of the system.
o Flowcharts: The chapter presents flowcharts for design and manufacturing of the system.
1.8 Research methods
The expert interview is one of the effective methods on verifying the vision of this project.
Expert is Kalle Vuorio, who is a Chief Operating Officer of KMJ – Engineering Oy. KMJ – Engineering Oy was founded 1997 and one of the company’s service is industry thermal imaging. Kalle Vuorio has over 20 years of experience in thermal imaging and in switchgear temperature analysis in industrial applications. Interview topics are listed in the Appendix 1.
Main goal of the interview is to receive an expert opinion regarding this topic and to verify that the goals and objectives set in this project and research are reasonable and valuable. Interview is based on “free conversation” with topics listed in the Appendix 1.
When selecting temperature monitoring system, which could be used in future to improve company’s products, it must be sure that the system really works and more importantly works correctly. This will be verified with using two independent monitoring systems in addition to the chosen system. When the tests will be running, monitoring will be done with three independent monitoring systems at the same time. If the results of the temperature monitoring system that is under inspection are approximately the same as the results in two other independent systems, it can be stated that the chosen system is reliable. These systems are presented in chapter 3.2.
17 1.9 Schedule of study
Preliminary schedule of the studies is presented in Table 1. Number explanations below.
2019 company. Discussion of the topic in Harju Elekter Oy and in As Harju Elektrotehnika.
Editing the scope and the objectives of the study to correspond with the scope of the thesis and the needs and the resources of As Harju Elekter.
2. Preliminary investigation of possible temperature monitoring system manufacturers and their products. Comparison of different system. Investigation of possibility to purchase and to use those systems. Creating table of contents for the thesis.
3. Negotiations with the manufacturer of the selected system.
4. Literature study. Preliminary design.
5. Installations and testing of the system.
6. Examination of results, conclusions and discussion. Writing process.
7. Examination of the thesis. Final corrections and finishing of the thesis.
1.10 Resources for study
The required resources for this study can be divided into three different categories. The first category is the HECON switchgear for testing. As Harju Elekter has a test HECON Light switchgear in Keila, Estonia. Switchgear can be used for system testing. The second category
18 is the testing environment. As Harju Elekter has a HECON test environment, where the test HECON is located. This environment can be used for testing the system. In the test environment, there is a possibility to supply current to the test HECON switchgear and therefore analyse data from temperature monitoring system. The third category is the temperature monitoring system. Employer will purchase the chosen temperature monitoring system. In addition, supervisor of this thesis will advise in this study. He will provide assistance to operate in the As Harju Elekter test environment. He will also give advices and opinions concerning the complete study. Employer will also provide hotel in Estonia for the time being that is needed to carry out the testing of the system.
1.11 Temperatures in switchgear
1.11.1 Increase of resistance by loose connections, corrosion and impurity
Busbars are usually connected with bolts, which are installed through one or several holes predrilled in the busbar. A bolt is tightened with a nut to press two busbars together, thus two separated solid objects are connected together. It is important to accomplish connections correctly. Bolts, nuts and pressure plates must be suitable for connection and torque for the tightening must be correct. The target is to accomplish a connection, where the resistance is matching the resistance of solid objects as closely as possible. If for some reason the connection is not done correctly, it may have a higher resistance than the solid busbars. In the connection two surfaces are connected together. When looking very closely at the busbar surface, it can be noticed that it is not purely flat (Figure 3). The surface is slightly rough. When two surfaces are tightened together with the applicable torque value, the two surfaces will have connections with each other at nearly all areas. If the torque value is not correct, the two flat rough surfaces will not have a full area connection with each other (Yovanovich, M., Culham, J. and Teertstra, J.
2004).
19
Figure 3. Rough surfaces (Yovanovich, M., Culham, J. and Teertstra, J. 2004)
When a current is flowing through the connection, the actual cross section of the connection is smaller than the solid busbar cross section. The connections may include areas, which are not tight enough to form a connection due to the properties of rough surfaces. Similar problem may evolve over a longer period of time. If the nut starts to slowly loosen up over time, the pressure between two surfaces also decreases. This could lead to a situation, where the connection of the surfaces is finally lost. (Zhou, X. and Schoepf, T. 2011). In addition, contamination and corrosion are remarkable phenomena, which can affect the connection. Impurities and corrosion can penetrate in between the two connection surfaces over a longer period of time and start to degrade the connection even though the connection is not loose. Loosening connections, contamination and corrosion are problems, which may occur in any kind of connection. Usually these will evolve slowly during time, but at some point, this phenomenon can lead to a dangerously high temperatures by increase in resistance (Alhainen, J. 2015).
For an example, let us investigate an imaginary situation, where an electric heater is supplied from a switchgear. For example, there is a 800 W/ 230 V heater supplied through MCB (miniature circuit breaker), which is connected to a terminal block (Figure 4).
20
Figure 4. 100 W heater supply via MCB and terminal block
We can calculate the resistance of the heater (1) by active power and voltage in the electric circuit.
U=230 V, P=800 W, R=U2 P =
230 V × 230 V
800 W = 66.1 Ω (1)
Figure 5. Circuit diagram of heater supply
We can calculate the electric current in the electric circuit (2) (Figure 5).
𝑈 = 230 𝑉, 𝑅 = 66.1 Ω, 𝐼 =𝑈
𝑅 = 230 V
66.1 Ω= 3.5 A (2)
We can see that the 800 W/230 V heater has an internal resistance of 66.1 ohms. At full load, the electric current in the electric circuit is 3.5 A.
21 Let us assume that that over time, the auxiliary block and the wire connection starts to weaken.
Reasons could be loose connection, corrosion or impurity. Let us assume that five ohm additional resistance is applied on the terminal, which could be a typical resistance for small wire cross section (Master Samurai Tech. 2015) (Figure 6).
We can now calculate the new electric current by voltage and resistance in the electric circuit (3) (Figure 7). In the Figure 7 there is a serial connection of two resistances.
𝑈 = 230 V, 𝑅 = 66.1 Ω + 5 Ω = 71.1 Ω , 𝐼 =𝑈
𝑅 = 230 V
71.1 Ω= 3.2 A (3) It can be seen that the electric current decreases in the new situation, when an increase of a resistance occurs. Let us calculate the active power for heater in this situation (4).
𝐼 = 3.2 A, 𝑅 = 66.1 Ω , 𝑃 = 𝐼 × 𝑅 = 3.2 A × 3.2 A × 66.1 Ω = 691.5 W (4)
It can be seen that the heater power decreases in the new situation, when an increase of a resistance occurs. Now we can calculate the power loss in the auxiliary terminal as a result of the increase in the resistance (5).
𝐼 = 3.2 A, 𝑅 = 5 Ω , 𝑃 = 𝐼 × 𝑅 = 3.2 A × 3.2 A × 5 Ω = 52.3 W (5)
Figure 6. Increase of resistance in heater supply Figure 7. Increase of resistance in circuit diagram
22 We can see that 52 W power loss occurs in the terminal block because of the increase in the resistance. This power loss is basically released as a thermal energy by conducting and radiating. The temperature of an area depends on the amount of power loss and cross section of the area, where the thermal energy is released. If large amount of power is released through a very small area, the occurring heat in the area is high. Same principle concerns all kind of connections of electrical conductors (Master Samurai Tech. 2015).
1.11.2 Measurement point
The temperatures inside the switchgear varies. Temperatures inside the switchgear is a combination of the ambient temperature and also radiating and convective heat from different sources. Sources in this content are equipment or other parts of the switchgear, where electric current can flow. Power losses in the electric circuit mean that a certain amount of power is dissipated in a form of heat. Heat will be released from the circuit mainly by free convection via conductor and to a small extent by radiating from the conductor to the environment. It can be assumed, that ambient temperature is approximately the same at any point of the switchgear, when the switchgear is not operating. When electric loads and electric supply are connected to the switchgear, electric current starts to flow in the switchgear and convective heat will occur due to power losses. In this situation, there are different temperatures inside the switchgear depending on where the temperatures are being measured at. Distances from the heat sources affect all the temperature measurements. Another factor is the timing of measurements. In the switchgear, electric loads can vary in time and therefore also electric currents vary. This leads to varying power losses and therefore varying heat convection at different times. Due to varying convections and different distances from sources inside the switchgear the temperatures should be measured closely at the conductor.
1.11.3 Invisible switchgear points for wireless temperature monitoring
In this thesis the main target is to focus on invisible connection points in the switchgear.
Switchgear may consist of hundreds of different electrical connection points. It is not reasonable to try to monitor all of these points. This kind of monitoring would be almost impossible to accomplish, or it would be at least very expensive. Thermal imaging is the best way for fast
23 monitoring of all the visible connection points of the switchgear. In contrast, it is very difficult to monitor invisible points in the switchgear with a thermal camera. One solution for this could be to install sensors to the invisible connection points.
Invisible connection points of the highest priority can be split into two categories. The first category is busbar connection points. Switchgears are split into transport units, which could be 2 – 4 meters wide parts of the switchgear. On site, the switchgear’s transport units will be
assembled. The busbar connections parts of the transport units maybe located later in non-accessible locations for thermal imaging (e.g., behind covers or cables installed in cable
canals). The second category is withdrawable ACBs and MCCBs (air circuit breakers and molded case circuit breakers), where the connection points are either behind a fixed part of device or behind touch protection plate and therefore not accessible for thermal imaging (Vuorio, 2021).
1.12 Wireless sensor technology
1.12.1 Introduction
There are many wireless temperature monitoring systems in the markets. Usually systems include sensors, antennas and the main unit. Sensors are measuring temperature and wireless communication happens between sensors and antennas. Antennas are connected to the main unit, which will process the temperature data from antennas. Usually, antennas are connected to the main unit via cables. There are some limitations for wireless systems in general. It is usual that antennas have to be installed in the same space with sensors. This is because most systems are communicating by radio frequencies, which cannot go through metal walls (Britannica, 2021). Switchgears are built from metal and all the fields and cells are surrounded by metal walls. Sensors and antennas, which are separated with a metal wall, do not work.
Sensors may also be battery powered. In that case, limitations in the battery life may be a problem. However self-powered sensors are pretty common in today’s systems. Self-powering is based on electromagnetic field occurring around cable, where the current is running (Micropelt, 2015), or technology, where an antenna sends a signal, which is then reflected back to the antenna (Benes, E., Groschi, M. and Seifert, F, 1997).
24 After search of suitable wireless temperature monitoring system for this project, manufacturer IntelliSAW system was selected. This system is described in chapter 1.13.
1.13 Topology
Figure 8. Intellisaw temperature monitoring topology (Intellisaw. 2020. IntelliSAW Monitoring Solutions Overview 2020)
Sensors will be mounted to the points of the switchgear, where temperature shall be measured.
Sensors are transmitting data to “antennas” (or more officially air interfaces) with radio signal.
One air interface can receive data from up to three sensors. Air interfaces are connected to a reader via cables and the reader is processing the incoming data. Up to four air interfaces can be connected to one reader. Data from the reader can be transferred to a PC via USB cable and analysed by the PC software IntelliSAW Configuration Tool. With the same tool the configuration of the system can be done. The reader also has a Modbus RTU communication option, where data can be transferred forward from the system. Basically, maximum topology for a one reader system consists of a one reader, which is connected with four air interfaces, each air interface is reading data from three sensors. This is also a system that was used in this thesis. For a larger operation, IntelliSAW also provides CAM-5 touch panel HMI with monitoring. Up to nine readers can be connected to one CAM-5 monitoring unit. CAM-5 also supports Modbus TCP, DNP3 and IEC-61850 communications. In this thesis, the focus was just to test the operation of system and monitoring accuracy, so a larger than one reader system
25 was not necessary. However, it has been taken into account that in the future it could be possible to provide also larger systems with using CAM-5. IntelliSAW also provides humidity monitoring and partial discharge monitoring for the same system. Additional humidity sensors and partial discharge air interferes can be connected to the reader. IRM-48 readers have different types of measurement supports. These options are temperature and/or humidity and/or partial discharge. In this project, IRM-48 type, which supports only temperature monitoring, was used. Scope of this project was just temperature monitoring, so the other optional measurement possibilities are not handled in this thesis. However, it is reasonable to address, that this kind of option exists (Intellisaw. 2020. IntelliSAW Monitoring Solutions Overview 2020) (Figure 8).
1.13.1 Temperature sensors
The system includes temperature sensors, which are wireless and will be mounted to the points of the switchgear, where temperature shall be measured. IntelliSAW provides two different types of sensors. IntelliSAW (IS) Sensor (Figure 11) has bolt mounting possibility for maximum of 13 mm or ANSI ½ inch, so it can be mounted for example between busbar and busbar connection bolt or between cable shoe and cable shoe connection bolt. Sensors can also be mounted by cable tie or bonding tape. Another model is Low Profile (LP) Sensor (Figure 12). Operation of this model is same as in IS model, but the physical dimensions are smaller and LP sensor does not have a bolt connection possibility, so it has to be mounted by using cable tie or bonding tape (Intellisaw. 2020. IntelliSAW Monitoring Solutions Overview 2020).
Air interference is transmitting 425 – 442 MHz radio frequency for sensor. Signal is transduced into acoustic wave by an interdigital transducer. Then the surface acoustic wave propagates along the surface of the substrate and the acoustic impulse response wave is transduced back (Moussa. H. 1997) (Figure 9). Two resonators provide a differential measurement for better signal reliability. Temperature is known from resonator signal frequency difference (Intellisaw.
2018. IntelliSAW Overview Customer Presentation 2018) (Figure 10).
26
Figure 9. SAW IDT technic principle (Intellisaw. 2018. IntelliSAW Overview Customer Presentation 2018)
Figure 10. SAW IDT frequency-temperature charasteristics (Intellisaw. 2018. IntelliSAW Overview Customer Presentation 2018)
These sensors can measure temperatures in range of -25 celsius degrees to 125 celsius degrees with resolution of ±0.2 celsius degrees. Accurary of the sensors is ± 2 celsius degrees in area of 0-80 celsius degrees and ± 4 celsius degrees in full range. Base plate material is 260 Brass, Tin Plated and cover material is polycarbonate, UL94-HB. Cover Dielectric strength of the sensor is 15 kV. Bolt mounting maximum torque is 102 Nm. (Intellisaw. 2015. IntelliSAW Critical asset monitoring temperature sensors)
27 Sensors are grouped with unique colours. Sensors are all ordered by certain bandwidth and bandwidths are identificated from band one to band twelve. Different bands are grouped by different colours. When selecting three sensors for the air interface, same colour group sensors must be selected to automatically have bands that are not adjacent. For example, orange colour group has bands 1,3 and 5 and so on. This how the possibility of reading wrong sensor by air interface is eliminated (Intellisaw. 2016. IntelliSAW Sensor Installation Manual).
1.13.2 Air interfaces
Air interface (Figure 13) must be mounted in the same space, where the sensors are, to be able to communicate with the sensors. Air interface can be mounted by magnets or bolts. Ingress protection rating of the air interface is IP40 and the operation condition is in range of -20 to +79 celsius degrees. Air interfaces are connected to a reader with double shielded coaxial cables. Air interfaces can be ordered with 3, 5, 7 or 10 meter cable. In this project, air interfaces are with 7 meter cable length (Intellisaw. 2016. IntelliSAW Sensor Installation Manual).
Air interface (Figure 13) must be mounted in the same space, where the sensors are, to be able to communicate with the sensors. Air interface can be mounted by magnets or bolts. Ingress protection rating of the air interface is IP40 and the operation condition is in range of -20 to +79 celsius degrees. Air interfaces are connected to a reader with double shielded coaxial cables. Air interfaces can be ordered with 3, 5, 7 or 10 meter cable. In this project, air interfaces are with 7 meter cable length (Intellisaw. 2016. IntelliSAW Sensor Installation Manual).