DEVELOPMENT OF A LOW-COST ZIGBEE AND GSM SMS-BASED CONDUCTOR TEMPERATURE AND SAG MONITORING SYSTEM

DEVELOPMENT OF A LOW-COST ZIGBEE AND GSM SMS-BASED CONDUCTOR TEMPERATUREAND SAG MONITORING SYSTEM

ABSTRACT

This paper deals with the design, construction, instrumentation and testing of a GSM and ZIGBEE based monitoring system for the measurement of Overhead High Voltage (HV) Conductor Temperature and Sag. The main advantage of this concept is the real time direct measurement of the parameters (i.e., conductor sag and temperature) needed for the operation of the transmission system without intermediate measurement of conductor tension and ambient weather conditions, by which the temperature controlling of transmission lines conductors is realized the stoppage caused by raised temperature can be avoided and some accidents caused by the increased temperature can be avoided. The principle and the feature of GSM SMS and ZIGBEE communication are analyzed. The construction of this system is outlined, and the force modal of calculating the variety of the sag due to the increased temperature of conductors is built. Finally, the software and hardware design of the online temperature monitoring unit of conductors and fittings are outlined. In this paper, a self-designed industrial GSM module is selected to finish the transmission and the decoding of the monitoring data through AT command and coding of short message PDU (Protocol Data Unit).

Key words: GSM SMS, Zigbee, Temperature, Conductor, DTLR

INTRODUCTION

With fast development of economy in India, the demand of electricity is higher and higher, and the problem between lag of construction of network and inadequacy of transmission capacity becomes increasingly prominent, which exacerbates the unharmonious contradictions of development between power grids and power generation structure. Some provinces and cities have begun to take power limited policies to alleviate contradiction of the current electricity supply-demand, how to resolve this problem has become imperative responsibility for many power workers. Recently, in order to prevent overloading of transmission lines, domestic power system usually adopts the static, conservative transmission capacity value in design, which is a conservative static value based on the severest weather conditions. However, such severe weather conditions rarely occurred, and it has resulted in the inefficient use of potential transmission capacities in most time. Now, according to the traditional technology, the transmission capacity can be increased only by adding transmission lines. However, it is becoming more and more difficult to build new transmission lines with the transmission lines increased. From the perspective of sustainable development and environmental protection, we should pay more attention from power grids expansion to increase the potential transmission capacity of available transmission lines, and enhance the transmission capacity of power grids, so as to resolve the problems between high requirement of electricity and difficulty of new transmission line. At present, some areas adopt the allowable temperature value of 70º to 80ºor even 90º. Properly increasing the allowable temperature of existing conductors can increase stable carrying capacity of transmission lines; thereby the normal transmission capacity is improved. The method is a breakthrough of current technical regulations, the impact caused by improving conductor temperature on conductors, the mechanical strength and the lifespan of matched fittings, the increase in sag and so on should be studied. In addition, if the conductor temperature and the sag can be real-timely monitored, the dynamic regulation of the transmission capacity, such as day and night, cloudy and sunny, summer and winter under the different environmental conditions can be realized to improve the transmission capacity.

In order to meet these demands, the monitoring system of temperature of conductors and fittings conductor sag based on GSM SMS and ZIGBEE is studied and developed in this paper. In any interconnected HV transmission system, there is the need to define in quantitative terms the maximum amount of power that may be transferred without violating the system safety, reliability and security criteria that are in place. Hence, real time ratings of circuits are critical to system capacity utilization. The current carrying capability of many transmission circuits is limited by the conductor temperature (thermal limits) and sag. For this reason, real time conductor temperature and sag measurements and real time current rating hold promise for the improvement of system transfer capability. Traditionally, overhead conductor sag has been considered for line rating by using indirect measurements. Recent commercialized techniques include the physical measurement of conductor surface temperature using an instrument mounted directly on the line, and the measurement of conductor tension at the insulator supports. These measured parameters can be used to estimate conductor sag. The pertinence of conductor sag to circuit operation relates to the calculation of Dynamic Thermal Line Rating (DTLR).

A new direct method for the measurement of overhead conductor temperature and sag factors based on GSM SMS and ZIGBEE has been proposed in this dissertation work for the purpose of DTLR. This temperature and sag monitoring device responds to the weather conditions. The main advantages of the method include the accurate measurement of conductor sag and temperature values without recourse to simplified assumptions that could otherwise affect its accuracy. With this method, errors caused by insulator swings could be eliminated. To be able to directly monitor and display the conductor temperature and sag values in real time will enable prospective engineers to physically capture the conductor behavior, and to take judicious steps towards a reliable system loading.

The primary objectives consist of the following:

Development, design, construction and performance of selected tests on an instrument based on the GSM and Zigbee technology to measure, in real time, overhead HV transmission conductor temperature and sag values. This new approach is expected to provide a competitive alternate tool for real time monitoring of overhead conductor temperature and sag values.

 

Figure 1: Cross Sectional Diagram SOP

Figure 2: output versus absolute pressure

This Figure.2 shows the sensor output signal relative to pressure input. Typical minimum and maximum output curves are shown for operation over 0 to 85°C temperature range. The output will saturate outside of the rated pressure range. A gel die coat isolates the die surface and wire bonds from the environment, while allowing the pressure signal to be transmitted to the sensor diaphragm. The gel die coat and durable polymer package provide a media resistant barrier that allows the sensor to operate reliably in high humidity conditions as well as environments containing common automotive media.

Transfer Function:

Volt = VS x (0.009 x P - 0.095) ± (Pressure Error x Temp. Factor x 0.009 x VS)

VS = 5.0 ± 0.25 Vdc

Temp.Factor =1

Pressure Error = ± 1.5 kPa

STUDY ON RELATIVE MODELS

Preambles of Conductor Capacity Ratings

Transmission lines across the country are recently being operated at higher temperatures. Two key factors driving the changes in the way utilities operate their transmission systems can be attributed to the increased population growth, and the necessity to maximize equitable return on investment in the electricity deregulation era. The population growth has not only increased power demand, but also reduced the available rights-of-way for new transmission lines. For the purpose of curtailing investments, a probable option for increasing power transfer capability is to operate lines at significantly higher loading levels than ever before. It is very important for electric power utility companies to know the power level that can be transmitted over their power transmission lines at any given time. This enables them to serve load reliably and to secure adequate and equitable financial gains without compromising system-wide reliability during normal operating conditions. For this reason, both the conductor thermal and mandated sag limits must be evaluated. The conductor thermal limit relates to conductor temperature and sag, and it is often a main concern especially for circuits that are heavily loaded. The thermal capacity of overhead conductors depends on conductor temperature due to ambient air temperature, Ohmic heating, incident solar radiation, local wind speed and wind direction, limiting physical conductor characteristics, conductor configuration and geometry. For purposes of DTLR (Dynamic

Thermal Line Ratings), these parameters must be accurately determined since operating conductors at higher temperatures for longer duration of time could cause irreversible aging phenomena, referred to as annealing and creep. This could lead to a total loss of conductor life.

In order to better utilize existing transmission circuits therefore, utility companies must also strive to match closely the conductor thermal ratings by taking into consideration actual weather conditions. The conventional steady state thermal ratings of certain overhead conductors have been based on the 1971 standard worst case conditions such as wind speed of 2 ft/s, summer ambient temperature of 40oC and maximum allowable conductor temperature of 95oC. The conservative nature of these assumptions is due to the lack of actual knowledge of the conductor operating conditions. The utilization of the extra capacity of the system by operating conductors at higher load levels in real time could serve as an option for an improvement in power wheeling. This is a potential source of reduction in capital and operating costs.

At present, in accordance with different natural environment, different countries adopt different boundary conditions to calculate the transmission capacity of conductors such as wind speed, sunlight, temperature and conductor temperature, which has a large impact on the calculation results. Different countries have different allowable temperature value about the ACSR, Japan and the United States 90°C, France 85°C, Germany, 80°C, India 75 °C, the Soviet Union 70°C, Britain 50°C. When the allowable temperature of conductor increases from 70° to 80°C in short time, its cumulative loss of mechanical strength for 30 years fall in the permitted scope of 7% to 10%. If the allowable temperature of conductor exceeds current operating temperature of +70°C, it will bring the following questions:

(1) It does not comply with current design standards (in the current standards the maximum temperature of conductor is +70°C), but increasing the maximum allowable temperature to +80°C or +90°C, it does not affect its safety operation of conductor itself;

(2) It brings some impacts on conductors, mechanical strength and lifespan of fittings. When the temperature of linear linking tube of conductor and the combination fittings of tension resistible clinch is below the temperature of the conductor, the grasp strength after the thermal cycling tests is also in compliance with the international standard;

Dynamic Thermal Line Ratings (DTLR)

Deregulation has opened the doors of power industries to a more competitive electricity market. This raises the level of interest on the thermal capability of overhead conductors for the maximum power transfer capacity from one point of a transmission circuit to another. The recognition of the limitations of the conservative steady state ratings and the potential benefits of a DTLR system has been an interesting issue in recent years. Real time thermal rating methods have been given various names including DTLR. DTLR is a method described by the process of favorably adjusting the thermal ratings of power equipment for actual weather conditions and load patterns. This is the case, particularly if an overload which causes a small conductor loss of life or strength but never violates the code mandated clearance is to be applied for an acceptable period of time. There appears to be no firm industry standard for DTLR methods.

Traditional Methods for DTLR

In recent years, many authors including and EPRI have intensified research and proposal of various DTLR methods as a strategic option for transmission system operators. Most of the proposed methods measure some related parameters, which are then used to indirectly compute the overhead conductor sag. Although the existing DTLR systems have not been thoroughly assessed, there seems to exist a potential source of weakness in terms of measurement precision and cost since they do not measure the overhead conductor sag directly. The GSM and ZIGBEE based sag instrument is likely to require installation of fewer units for a given transmission network compared to existing systems. The overhead conductor temperature and sag information can be used to:

(1) Determine the load carrying capabilities of overhead conductors,

(2) ensure that conductors do not violate their code mandated clearances,

(3) For estimating the conductor loss of strength caused by annealing, and

(4) To limit the elevated temperature creep of conductors.

Three traditional methods can be identified in industry practices for DTLR based on the measured parameters. These are:

(1) weather-based models,

(2) Conductor temperature-based model, and

(3) The conductor tension-based model.

All the three models have some advantages and some disadvantages, for example, the monitoring method based on weather-based is simple and less calculation, but at a certain interval of time the anemometer should be corrected. In the model based on conductor temperature and meteorological environment, a method of directly detecting the temperature of conductor avoids the influence of anemometer. But because the temperature detection equipments directly contact with the high-voltage conductors, the high voltage magnetic field has a certain impact on measurement accuracy, some methods should be adopted to reduce or avoid the impact of the magnetic field. The model based on pulling force need monitor some effective factors of meteorological environment, but pulling force sensors can only be installed when conductors is unloading, that is to say, they can only be installed in maintenance period. In addition, the reliability of pulling sensor also should be higher. In the present industry DTLR methods, the sag information is a calculated output, whereas in this new proposed approach (i.e., GSM and ZIGBE based instrument); the sag information is a measured input.

THE STRUCTURE OF THE MONITORING SYSTEM

The monitoring system of temperature of conductors and fittings and conductor sag based on GSM SMS and ZIGBEE is mainly composed of the provincial monitoring center, the municipal monitoring center, the communication unit, the temperature and pressure monitoring unit and the expert software, the topology of system is shown in Figure.3. The communication unit is installed on the tower with both GSM and ZIGBEE communication modules, and the temperature and pressure monitoring unit on the corresponding conductors with the same potential.

According to the sampling interval time set up remotely by the monitoring center, the communication unit can regularly or real-timely call the temperature and pressure monitoring units controlled by the communication unit in turn by ZIGBEE communication. The monitoring unit, installed on the conductor can measures the actual operating temperature of conductor and pressure values under local weather conditions, sends the pressure and temperature of conductor to the communication unit by ZIGBEE communication whose frequency is 2.4 GHz. All the temperatures of conductors and pressure values coming from various monitoring points will be packed as GSM SMS to send to the municipal monitoring center by GSM communication module. All the information of the temperature and altitudes of various points can be managed by the expert software, and the current capacities can be real-timely stored into the database. Then the expected temperature of conductors, the expected current capacities, the expected time, the real-time sag, the expected sag of conductors and so on can be calculated according to the computing model. When the measured or calculated temperature or the safe distance exceeds the allowable value, an alarm message can be send by GSM SMS to some managers. The operating parameters of the communication unit, such as time interval, system time of unit and requests of real-time data etc., can be remotely modified by GSM communication. The municipal monitoring centers are connected to the provincial monitoring centers by LAN, and the provincial monitoring center can directly browse the monitoring data of all measured conductors and fittings. By comparing with allowable temperature and analyzing, the transmission capacity will be enhanced with no break of the available technical regulations. Of course, the operating temperature of conductors can also be monitored by this system when the transmission capacity is increased.

 

Figure 3: The Topology of System

STRUCTURE AND DESIGN OF MONITORING UNIT

In order to improve the transmission capacity of conductors with no break of the available technology, the monitoring of the conductor temperature is very important. However, the traditional wireless temperature measurement methods cannot meet requirements, for example, using infrared to measure temperature should keep the distance close (within 5m) and the accuracy of measurement is low. Using fiber temperature measurement will not be able to meet the requirements of insulation for the high-voltage and long-distance transmission lines. Using radio to transmit data directly will be difficult to organize an effective star network with multi-points to one. The temperature of conductor is the most direct and important parameter during the operation of transmission lines, how to real-timely and accurately monitor the temperature of conductor is the key technique to solve this problem. The temperature of environment, conductor, and the crossing, Altitude values can be measured by the monitoring unit, which is composed of power module, MCU (Micro Control Unit), ZIGBEE communication module, temperature sensors, Barometric pressure sensors and so on, as shown in Figure.7. Here, LM35 is selected as the temperature sensor which is a single-bus digital sensor and MPXAZ6115A selected as pressures sensor which is Integrated Silicon Pressure Sensor Altimeter/Barometer Pressure Sensor On-Chip Signal Conditioned, Temperature Compensated and Calibrated. Using single-bus (1- wire) technology, LM35 and MPXAZ6115A are blends with address bus, data bus and control bus for a bidirectional serial signal wire, which provides a simple structure, the convenient bus expansion and maintenance. Zigbee modules are developed independently by authors to achieve the short-distance communication. The specific structure is shown as Figure.4.

Figure 4: The structure of temperature monitoring device

Figure 5: The structure of temperature monitoring unit

 

Figure 6: The structure of the communication unit

The block diagram of monitoring system consists of power supply unit, microcontroller, temperature and pressure sensors, ZIGBEE module, and GSM module.

 

Figure 7: the Block diagram of Monitoring System

DESIGN OF SOFTWARE

The main function of the monitoring unit is to monitor temperature of conductors and the pressure station value. On one hand, monitoring unit works in interrupted mode, which starts to convert the temperature and pressure values when the sampling time is coming, then sends data to the communication unit after conversion by Zigbee module.

The interrupted program flowchart is shown as Figure.8. On the other hand, monitoring unit works in a cyclic mode for an alarm (the upper limit temperature of LM35 can be set to 70°C or other values), which will ignore the limitation of sampling interval time and send signals to communication unit by ZIGBEE module when the temperature beyond the limit, and the communication unit sends the messages to workers to take measures timely.

Functions of Expert Software:

All the information of the monitoring systems of various points can be managed by the expert software. Then the expected temperature of conductors, the expected current capacities, the expected time, the real-time sag, the expected sag of conductors and so on can be calculated according to the computing model, and shown in graphic.

When the measured or calculated temperature or the safe distance exceeds the allowable value, an alarm message can be send by GSM SMS to some managers. The operating parameters of the communication unit, such as time interval, system time of unit and requests of real-time data etc., can be remotely modified by GSM communication.

 

Figure 8: Flow Chart for Monitoring Unit:

 

Figure 9: Flow Chart for Communication Unit

The ZIGBEE transceiver is used for the communication between the monitoring and communication units, in the present work. The AVR Microcontroller takes data and decides where it should be sent. This involves looking at the data type and the destination to determine whether the data should be sent over the serial port. The ZIGBEE module is responsible for encapsulating the data in the required packet format for sending it to another ZIGBEE, or to the serial port. ZIGBEE’s SPI protocol performs tasks, such as timing and parity checking, that are needed for data communications. The data enters the DO buffer and is sent out the serial port to a host device. It has been seen that the data transmitted over the communication link is uncorrupted.

Laboratory Bench-Testing

A selected number of experiments were performed on the GSM and ZIGBEE based overhead conductor temperature and sag measuring instrument at different environmental conditions. The main objectives of the bench-testing experiments were to evaluate the proper functioning of the radio communication links. In this case the experiments were performed at the Laboratory. Note that in these experiments, the proposed GSM and ZIGBEE based conductor sag instrument was not directly mounted on an energized overhead HV conductor due to lack of logistics and high cost in terms of the availability of necessary facility. To be able to perform such an experiment in a real life application is beyond the capability of the university research resource at this time.

Example Calculations:

The received Monitoring System values from GSM Module are shown in the below figure, Figure. 10.

 

Figure 10: Received Monitoring System Values from GSM Module as SMS

Pressure Altitude Calculator:

This calculator is designed to give a value for a calculated pressure altitude, based on data entered. The term station is the designation for the vertical point that you take your measurements; vertical meaning above (or below) sea level. The absolute air pressure is the calculated air pressure, but not corrected for altitude. In our calculator, enter the station pressure (absolute); be sure to click on the proper designation if using measurements. Click on Calculate and the calculated pressure altitude will be returned in both feet and meters. Based on this Pressure Altitude Calculation, we can calculate the conductor’s sag value.

 

Figure 11: Pressure Altitude Calculation

CONCLUSIONS

The main contribution of this paper is the design, construction, field testing and analysis of a GSM SMS and ZIGBEE based instrument for the real time direct measurement of overhead HV conductor temperature and sag. The resulting conductor sag information can be used to enhance the operation of electric power systems, particularly the DTLR. The proposed GSM and ZIBBEE based measurement of overhead HV conductor sag is a more direct technique in some ways as compared to similar alternative methods. This is concluded because the direct measurement of overhead conductor position involves no intermediate calculations and measurements of conductor tension, ambient weather conditions, or makes any assumptions to that effect. It also presents a potential source for cost reduction and better accuracy in the conductor sag measurement, since there is no need to directly measure conductor tension, and weather conditions. The real time direct measurement of overhead conductor temperature and sag is a clear advantage. With the power grids gradually increasing and the new lines building difficult, improving allowable temperature of conductors can fully exploit massive transmission capacity of existing transmission lines, and the number of new lines or the cost of investment in new lines can be deduced, the economic and social benefits brought by it is very large.

The measuring unit measures the temperature of the sensor and pressure station values. The conductor temperature is the result of the flowing to and effluent thermal power. The ambient conditions, which affect the conductor temperature, are the current, the wind velocity, the angle between the conductor and the wind direction, the global radiation, the solar radiation and fluent cooling mechanism. These components have different influences on the conductor and the sensors, but both are influenced by the same factors. In general the measured sensor temperature is not equal to the conductor temperature because the sensor is a heat sink in this complex thermal system. So the conductor temperature must be computed by using the measured temperatures of the sensor. For this purpose a calibration of the sensor and conductor must be made in the laboratory.

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