Necessity and feasibility study of applying ZigBee technology in large and medium-sized power plants

1 Introduction

With the development of science and technology, the operation and maintenance of power plants have put forward higher requirements for equipment status monitoring systems. The current monitoring system's detection information volume and test point installation problems have shown a trend of not being able to meet the safe, reliable and economical operation requirements of power plants. At present, the power plant status monitoring system basically implements the monitoring function on a wired basis. The wired method is limited by wiring, power supply, installation site and maintenance. For example, the status monitoring of the rotating parts of rotating machinery, the status (temperature, insulation) monitoring of high voltage and high current equipment, etc. cannot be completed by wired methods, and it is even more impossible to ensure the real-time, reliability and integrity of the data, which will also limit the evaluation, judgment and decision-making of the equipment operation status.

Modern monitoring systems are composed of sensor networks, which are a collection of wired sensor networks and wireless sensor networks. Wireless sensor networks are wireless data transmission networks designed based on the IEEE 802.15.4 technical standard and the ZigBee network protocol. They use a large number of sensors with multi-functional and multi-information signal acquisition capabilities, adopt self-organizing wireless access networks, and connect to sensor network controllers to form wireless sensor networks. ZigBee technology is specially developed for wireless sensor networks. Using ZigBee technology to form wireless sensor networks is an inevitable trend in the development of power plant monitoring systems.

2 ZigBee Technology

(1) Introduction

ZigBee technology is an emerging short-range, low-complexity, low-power, low-data-rate, low-cost wireless network technology that operates at 2.4 GHz and 868/915 MHz. It is a technical solution between wireless tag technology and Bluetooth. It is a two-way wireless communication standard that is mainly used for medium- and short-range wireless system connections, providing sensors or secondary instruments with wireless dual-function network access, and can meet the needs of data output and input control commands and information for various sensors to make existing systems networked and wireless. ZigBee technology uses a combination of general IEEE 802.15.4 transceiver technology and an embedded ZigBee technology protocol stack; it coordinates and realizes communication between thousands of tiny sensors based on the IEEE 802.15.4 standard. These sensors are designed to require very little energy and transmit data wirelessly from one sensor to another in a relay manner, and then transmit in sequence to form a wireless sensor network. The main features of ZigBee technology are shown in Table 1.

(2) Comparison of ZigBee technology with other wireless communication technologies

The characteristics of wireless communication technologies are compared as shown in Table 2.

3 Feasibility study of ZigBee technology applied to power plants

The power plant equipment monitoring system first provides the power plant monitoring system with various parameters, data, charts, curves, switching quantities and analog quantities of the on-site equipment. Based on this information, it analyzes the equipment status, performs open-loop and closed-loop control and adjustment, and alarms and handles equipment failures and accidents accordingly to ensure the optimal operating status of the equipment; provides analysis of long-term operating data for equipment status maintenance in order to form status maintenance decisions; and provides remote control data for telemetry, telecom, remote control, and adjustment.

The sensor network system composed of ZigBee technology has the following characteristics: real-time: real-time online monitoring; low power consumption: button battery can run for more than 2 years; advanced: advanced technology, devices, and software lay the foundation for the reliability and advancement of the system; accuracy: temperature measurement accuracy can reach ±0.1℃; flexibility: users can set parameters flexibly and conveniently according to their needs; systematization: it can be integrated with power system integrated automation system, fire protection system, etc. into a more powerful integrated system, and can be easily connected with local area network, wide area network, and system to realize data sharing and convenient management; authenticity: real-time data recording and analysis, providing real data for operation, management, maintenance, dispatching and other departments; security: ZigBee technology system is safe whether it is product, engineering and its maintenance. ZigBee technology provides data integrity check and authentication functions, and uses AES-128 encryption algorithm to ensure data security. Effectiveness: Power system failures are mostly caused by temperature rise. With this system, it can save the cost of purchasing other temperature measuring instruments and other equipment (such as infrared imagers, point stability meters, etc.); it can save inspection personnel and improve the efficiency of data acquisition; it can achieve the purpose of carrying out equipment maintenance in a targeted manner, which will reduce the workload of equipment maintenance; reduce accidents and improve power supply reliability; Practicality: ZigBee technology has been successfully applied to the measurement of high-speed rotating tire pressure and temperature in automotive electrical appliances. The ZigBee technology wireless temperature measurement system is used in the 110 kV substation of Dagang Yougang.

3.1 ZigBee technology communication reliability guarantee

ZigBee technology communication reliability assurance: communication reliability mechanism; strong network self-organization and self-healing capabilities; ZigBee technology has strong anti-interference performance in low signal-to-noise ratio environments; ZigBee technology has superior performance in low signal-to-noise ratio environments (Bluetooth, FSK and WiFi B).

3.2 ZigBee technology security demonstration:

3.2.1 Impact of ZigBee technology radio frequency signals on electrical primary equipment

ZigBee technology radio frequency signals, i.e. high-frequency harmonics, affect the safe operation of electrical equipment and the quality of electric energy. Therefore, high-frequency harmonics must be within the permitted range. The frequency range of the current digital cellular mobile communication network is 9 to 3.53 GHz, while the frequency bands of ZigBee technology are 868/915 MHz and 2.4 GHz, i.e. the radio frequency signals of ZigBee technology are within the frequency range of the mobile communication network. In other words, regardless of whether there are ZigBee technology devices present, the radio frequency of ZigBee technology has already invaded the power plant equipment and generated harmonics. Therefore, it is necessary to test the harmonic components of the power plant operating equipment, i.e. the high-frequency harmonic current and voltage components of the equipment, and evaluate them based on the test results. It is only necessary to design a test of the impact of the radio frequency signals of the digital cellular mobile communication network on the electrical equipment.

In theory, all power plant equipment is surrounded by the current digital cellular mobile communication network. The high-voltage side of the generator's outlet step-up transformer is divided into two levels: 500 kV and 220 kV. The test main wiring and test points are shown in Figure 1.

The measuring points in Figure 1 are: A is the voltage and current at the generator terminal; B is the voltage at the high-voltage side of the step-up transformer; and C is the voltage and current of the transmission line.

Test of generator transformer group: Test instruments: 2 DZF-Ⅱ power quality meters, 1 PP1 power meter (made in the United States); Test time: April 10, 2006 and April 21, 2006; Test conditions: generator with transformer rated voltage and no load (not connected to the grid); various loads after the generator is connected to the grid.

Data summarization principle: take the phase with the most serious harmonic content among the three phases as the representative value; the field test is 2 to 50 harmonics. In order to highlight the key points, some of the following data tables only list several times with larger harmonic content.

Machine-end harmonics (test point A) (see Table 3):

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From the test data (see Table 1), the harmonic data characteristics of each test condition are similar. For the sake of comprehensiveness and intuitiveness, we draw a bar chart of the harmonic content (2 to 50 times) at rated load (see Figure 2). In Figure 2: the horizontal axis is the harmonic number, and the vertical axis is the harmonic content rate (%).

The following points can be drawn from the above data table: the total harmonic distortion rate of the line voltage at the machine end is <5% under all conditions, and there is no exceeding the standard; after the motor is connected to the grid with load, the total harmonic voltage distortion rate is reduced compared with no-load, which is mainly reflected in the reduction of high-order harmonic content; under load, the total harmonic voltage distortion rate and the content of each harmonic at the machine end do not change much, and the main harmonic components are the 5th and 3rd; under no-load condition, the main harmonic components at the machine end are the 5th and 3rd. The harmonic current at the machine end of unit 14 is shown in Table 4.

From the data in Table 4, it can be concluded that the main harmonic currents at the machine end are the 5th and 3rd harmonics, the 5th is about 90 A, and the 3rd is about 50 A, and the 5th and 3rd harmonic currents do not change much with the increase of load.

Harmonics on the high voltage side (500 kV) of the step-up transformer (test point B) (see Table 5).

From the test data, the harmonic data characteristics of each test condition are similar. For the sake of comprehensiveness and intuition, a bar chart of the harmonic content (2 to 50 times) at rated load is drawn here (see Figure 3). In Figure 3: the horizontal axis is the harmonic number, and the vertical axis is the harmonic content rate (%).

From the data in Table 5, it can be concluded that the total harmonic voltage distortion rate of the 500 kV side of the step-up transformer under each test condition is about 2.3 to 2.5, and the main harmonic components are the 5th and 3rd order. The harmonics at this point do not change much before and after the generator is connected to the grid.

Harmonics on the high voltage side (220 kV) of the step-up transformer (test point B) (see Table 6):

From the data in the above table, it can be concluded that the total harmonic voltage distortion rate and the harmonic voltage content rate of the 220 kV side of the step-up transformer under various test conditions are within the standard, the main harmonic component is the third harmonic, and the harmonic changes before and after the generator is connected to the grid are not significant.

From the above test results (see Tables 3 to 6), there are very few harmonic components above the 13th order, and the harmonic content tends to zero as the order increases, indicating that the harmonic voltage and harmonic current of the generator transformer group surrounded by mobile communication signals are within the standard range under various test conditions, and the total distortion rate of harmonic voltage is not exceeded. At the same time, the generator transformer group operates normally during the test. In other words, the mobile communication signal, that is, the ZigBee technology radio frequency signal, did not cause the deterioration of the power quality of the power grid and did not threaten the safe operation of the generator transformer group. It proves that the application of ZigBee technology in the monitoring system of the generator transformer group is feasible.

Transmission line (220 kV) harmonics (test point C): 220 kV GIS station Gelu I loop voltage and current harmonics test results; measurement time: January 15, 2007; measurement tool: Zhongyuan Huadian ZH-2 fault recording device; CT ratio: 1000/1 PT ratio: 220 kV/100 V;

In Table 7:

(1) Harmonic components above the 14th order have tended to zero; the measured value of the load current of the Gelu I loop is 600 A, and the maximum value of the 3rd order harmonic current is 3 A; the measured value of the voltage of the Gelu I loop is 220 kV, and the maximum value of the 5th order harmonic phase voltage is 677 V; By comparing Tables 5, 6, and 7, it can be seen that the voltage and current harmonics of the 500 kV and 220 kV systems of the switch station are at the same level as the Gelu I loop. From the above data analysis, it can be seen that the ZigBee technology radio frequency signal has no impact on the transmission line, indicating that the application of ZigBee technology in the power plant transmission system is also feasible.

Test data analysis:

When the generator is at rated voltage without load, the total harmonic distortion rate of the line voltage is 2.01% < 5%. When the generator is loaded, the total harmonic distortion rate of the line voltage at the machine end decreases (see data table 3). The total harmonic distortion rate of the line voltage at the machine end does not change much under each load, and the main harmonic components are the 5th and 3rd. The harmonic current at the machine end is mainly the 5th and 3rd (see data table 4), and there is little change in each test condition.

(2) Before and after the generator is connected to the grid and under various loads after the connection, the total harmonic voltage distortion rate on the high-voltage side of the step-up transformer does not change much (see data tables 5 and 6), with a maximum value of 2.57%, and the main harmonic components are the 5th and 3rd harmonics;

(3) The main harmonic components of the transmission line are the 5th, 3rd, 14th and above harmonic currents and voltages tending to zero. The voltage and current harmonics of the 500 kV and 220 kV systems at the switch station are at the same level as the Gelu I loop (see data tables 5-7).

In summary: the intrusion of mobile communication signals, namely ZigBee technology radio frequency signals into the primary equipment of the power plant will not deteriorate the power quality. The content of ZigBee technology radio frequency in the equipment is very low. The maximum output power of ZigBee technology radio frequency signal is: ≤1 mW. If a single network has 240 sensors, its maximum transmission power is: ≤240×1 mW. It has almost no effect on the temperature rise of the stator and rotor of large and medium-sized generators of hundreds of thousands of kilowatts or hundreds of thousands of kilowatts. Moreover, the radio frequency signals of 240 points do not work at the same time, and the sampling time can be optimized. In addition, the shielding technology can be used to adjust the radio frequency distance and direction as needed by using the motor rotor hub (the hub itself has a shielding effect) to ensure that the radio frequency signal intrusion into the motor is minimized. The above test shows that the main components of the harmonics inside the motor are the 1st, 2nd, 3rd and 5th harmonics, and the total harmonic distortion rate of the line voltage at the machine end decreases after the motor is loaded. These components do not and will not overlap with the radio frequency signals of ZigBee technology. [page]

In short, the ZigBee technology network will not deteriorate the power quality of the power grid, nor will it cause the temperature rise of rotating motor equipment to exceed the limit. It is safe for the primary equipment and power system operation of the power plant.

3.1.2 Analysis of the impact on secondary equipment in power plants

The frequency range of ZigBee technology is 868 MHz, 915 MHz and 2.4～2.4835 GHz, while the general motor microcomputer protection collects fundamental wave, second harmonic and third harmonic. Moreover, both the current transformer and the voltage transformer are inductors, which have the property of suppressing high-frequency signals. In addition, the microcomputer protection input also adopts photoelectric isolation technology, and the primary equipment is also equipped with a switch capacitor (see Figure 1), which has the function of filtering and conducting high-frequency harmonics. Therefore, the RF signal of ZigBee technology has no effect on the protection and will not cause the protection to malfunction. It is safe for the operation of the protection device.

3.2 Demonstration results

Based on the reliability guarantee of ZigBee technology communication, ZigBee technology is safe and feasible whether applied in primary or secondary equipment of power plants, and will not affect the safe operation of the power grid and the quality of power.

4 Research on Application Objects and Data Characteristics of ZigBee Technology

4.1 Generally, applications that meet one of the following conditions can consider using ZigBee technology

(1) The equipment cost is very low and the amount of data transmitted is very small;

(2) The device is very small and it is not convenient to place a large rechargeable battery or power module;

(3) There is no sufficient power support and only disposable batteries can be used;

(4) Frequent battery replacement or repeated charging is impossible or difficult;

(5) It needs to support a large number of network nodes and a large range of communication coverage. There are many devices in the network, but they are only used for monitoring or control.

(6) Low requirements for communication service quality (QoS) (or even no QoS);

(7) Requires selectable security levels (using AES-128): encryption, transmission authentication, and message integrity;

(8) Requires multi-faceted and complex network topology applications;

(9) Requires high network self-organization and self-recovery capabilities.

4.2 Power plant equipment monitoring ZigBee technology meets one of the above conditions

(1) Online monitoring of the temperature rise and insulation of the internal windings of the motor, such as the temperature rise and insulation of the rotor, stator, and transformer, especially the temperature rise and insulation monitoring of the joints at the motor end and busbar;

(2) Online monitoring of the status and temperature rise of switch contacts, busbar joints, wire joints, and cable joints;

(3) Online monitoring of the insulation and overvoltage protection equipment of power generation, distribution, and transmission equipment in power plants;

(4) Online monitoring and alarm of the temperature of oil depots, cable corridors, etc., i.e. fire alarm;

(5) Online monitoring of various switch positions, secondary equipment connection (pressure plate) status and other switch quantities;

(6) Hydrological and meteorological monitoring of the water basin of power plants;

(7) Dam safety monitoring;

(8) Environmental monitoring.

4.3 Data transmitted in ZigBee technology network can be divided into three categories

(1) Transmission of periodic analog data: data on the oil, water, and wind systems of power plants, data on various electrical quantities (current, voltage, active and reactive power, etc.), data on temperature and insulation of various electromechanical equipment, hydrological and meteorological data, etc.;

(2) Transmission of intermittent switch data: a large amount of data on various switch quantities (contacts, switches, pressure plates/connections and valve status, etc.), the action counts of electrical equipment and the accumulation of operating time, etc.;

(3) There is also repetitive data transmission with low response time.

5 Conclusion

With the development of science and technology, the power plant equipment monitoring system will be incomplete without the participation of wireless sensor networks. ZigBee technology is specially developed for wireless sensor networks. ZigBee technology communication has reliability guarantee. Using ZigBee technology to form a wireless sensor network is an inevitable trend in the development of power plant equipment monitoring systems and is very necessary. ZigBee technology radio frequency signals will invade power plant equipment to generate high-frequency harmonics. Excessive harmonics will affect the safe operation of the power plant. This article gives the harmonic test method and test results, and combines the test results with the technical characteristics of ZigBee equipment for analysis and summary. It further gives the basis and technical conditions for the application of ZigBee technology in power plant equipment monitoring systems, the applicable objects of ZigBee technology and the characteristics of the detected data, proving that the application of ZigBee technology in power plants is safe, feasible and economical. In short, the application of ZigBee technology in power plant equipment monitoring is necessary and feasible.