The so-called smart grid refers to the automatic monitoring of the power grid through the use of sensors, meters, digital controls, and analytical tools, optimizing grid performance, preventing power outages, and enabling faster power restoration. Among these, the real-time and precise monitoring of the operating conditions of electrical equipment is the foundation of smart grid construction. In traditional power transmission and transformation monitoring systems, the pressure monitoring of working and insulating media such as SF6 mostly adopts node alarm methods. This cannot accurately reflect the real-time information of the equipment, which has become a weak link in the power transmission and transformation monitoring system and is one of the key areas that need improvement in smart grid construction.

  1. SF6 Density Relays and Digital Transducer Sensors

  1.1 In the power system, SF6 has become the primary insulating medium, widely used in primary equipment such as circuit breakers and current transformers, and its importance and role have become increasingly significant. For example, the insulation and arc-extinguishing performance of SF6 circuit breakers largely depend on the purity and density of SF6 gas. Therefore, the detection of SF6 gas purity and density monitoring are particularly important. Currently, SF6 density relays are commonly used for condition detection, control, and protection of SF6 equipment. Its main structure is as follows:

  1- Elastic metal bellows; 2- Gear mechanism and pointer; 3- Bimetallic strip; 4- Movement direction when pressure increases; 5- Movement direction when pressure decreases.

  The density relay collects the SF6 gas pressure value of the operating equipment through the elastic metal bellows. The bellows bend under pressure, driving the bimetallic strip and gears to rotate. Follow the WeChat account "gas-weixin" for free gas knowledge. The pointer and electrical contacts indicate the gas pressure value under the drive of the gears. The mechanism that converts mechanical signals into electrical signals is called a transducer, which is the main component for realizing the detection, control, and protection functions of SF6 equipment. Traditional density relays use electrical contacts to make the transducer, and the electrical contacts are linked with the gears. When the pressure reaches the set value, the electrical contacts trigger, outputting a 4~20mA current analog signal. This type of density relay has the characteristics of simple structure, easy replacement, and low cost. However, it can only issue specific electrical signals such as alarms and locks, and cannot reflect the real-time pressure of the operating equipment, making it impossible to perform precise data analysis and condition prediction, which no longer meets the requirements of smart grid construction.

  1.2 To improve the monitoring and prediction capabilities of operating equipment and meet the needs of smart grid construction, North China Grid Company has gradually implemented SF6 digital signal sensor upgrades for high-voltage and ultra-high-voltage equipment. Digital signal sensors add modules for SF6 gas purity, humidity, temperature, and component analysis based on the original density relay. The original electrical contact structure transducer is improved to a two-wire V/I converter. While amplifying, conditioning, and converting the weak or non-linear electrical signals output by the sensor into linear voltage output, the overall power consumption current is controlled based on the output of the signal conditioning circuit, and voltage is obtained and stabilized from the loop for use by the conditioning circuit and sensor. Its working principle is as follows:

  In the figure, OP1, Q1, R1, R2, and Rs constitute the V/I converter. Analyzing the negative feedback process: If point A is higher than 0V for some reason, the output of operational amplifier OP1 increases, the voltage across Re increases, and the current through Re increases. This is equivalent to an increase in overall power consumption, and the current through sampling resistor Rs also increases, causing point B voltage to decrease (more negative). As a result, the voltage at point A is pulled down through R2. Conversely, if point A is lower than 0V for some reason, it will also be raised back to 0V by negative feedback.

  Through the use of microelectronic components such as resistance strain gauges and microcontrollers, the sensor not only retains the traditional alarm and lock signal output functions but also gains digital signal transmission capabilities. Moreover, the gas pressure and temperature are compensated and calculated by the microcontroller, making the transmitted data more accurate. After data collection, digital remote transmission is achieved through the RS485 interface, outputting digital signals and interacting according to the digital bus communication protocol, with a data transmission speed of up to 9600bps. Its digital port is as follows:

  This enables real-time detection and recording of the operating equipment's gas pressure, composition, and other conditions, laying the data foundation for detailed analysis and prediction of primary equipment conditions. Follow the WeChat account "gas-weixin" for free gas knowledge.

  2. Digital Signal Transmission and Analysis

  With the advancement of smart grid construction, the trend towards networking and digitization of unattended substations is becoming more evident. The requirements for remote signaling, remote control, and remote measurement are increasing, making online remote monitoring of SF6 gas leakage status in SF6 electrical equipment of great practical significance.

  Based on digital sensors, North China Grid Company is gradually building an SF6 online data monitoring system in conjunction with the local office network. Through various data transmission and network connections, real-time data from the field is transmitted to the computers in the data monitoring center office. The data transmission structure is as follows:

  After the data monitoring system obtains field data, it displays the data in various visual forms, achieving the purpose of remote monitoring. The functional framework and user interface of the online data monitoring system are as follows:

  The basic functions of the system are as follows:

  a) System login: Users can only enter the system after logging in, ensuring system security and stability by restricting permissions and the number of users;

  b) Alarm history query: Users can set query conditions to perform conditional queries and print past alarm records; follow the WeChat account "gas-weixin" for free gas knowledge.

  c) Real-time data monitoring of stations: Displays the parameter values of all online equipment and detailed information of the stations (such as the actual installation location and dynamic information of the station, which can be modified and entered by the system administrator).

  d) Alarm parameter settings: Set alarm limits, including high and low alarm values;

  e) Real-time alarm display: Displays real-time alarm conditions on the main screen. When a real-time alarm occurs, the system will automatically pop up an alarm window (which displays specific alarm information);

  f) Historical data query: Users can set corresponding conditions (such as time, station, and station parameters for data filtering) according to actual conditions to perform historical data query, analysis, and printing;

  g) Historical curve drawing: Users can classify and draw historical curves by time period and station parameters;

  h) The online data monitoring system automatically performs WEB publishing during operation, and other computers within the local area network can view the data monitoring system's published screens through the local area network.