Application of Intelligent Medium Voltage Switchgear in Smart Substation

Research and Analysis on the Application of Intelligent Medium Voltage Switchgear in Smart Substation

In recent years, with the rapid development of my country's economic production, the power system, as an important part of daily energy, has also tended to be large-scale, systematic and complex, and the requirements for power supply continuity, reliability and safety have also been continuously improved. Medium voltage switchgear is an important link in the power system. In industrial applications, the failure of medium voltage switchgear will affect continuous production. The most serious arc fault may also bring hidden dangers to the safety of personnel operating on site. Continuous arc faults may even spread, causing the connected switchgear in the substation to burn. Switchgear with arc faults is often unable to continue to be used due to damage caused by high temperature and high-intensity internal forces. Repairing or repurchasing new equipment requires a certain production, installation and commissioning cycle, which will cause direct and indirect economic losses to users and possible comprehensive impacts on personnel safety.

1 Advantages of intelligent medium voltage switchgear

Medium voltage switchgear is a composite device that integrates power electronic components in a cabinet packaging space. It mainly works in the distribution system and can complete data collection, transmission and control functions. It is an important link in power distribution. With the increasing development of Internet of Things technology, intelligent switchgear is gradually replacing traditional switchgear. Intelligent switchgear makes full use of the advantages of fieldbus, digital transmission, communication network and other technologies, integrating telemetry, remote adjustment, remote control, remote signaling and other functions, and is a typical representative of smart grid equipment. Once a device in the intelligent switchgear fails, the controller will quickly control the circuit breaker to disconnect the fault source, and send detailed fault information to the remote monitoring center so that technicians can be quickly arranged to go to the site for repair, which not only improves maintenance efficiency but also reduces the safety risk of the power system [1].

2 Main problems of conventional switchgear

(1) Insulation failure: cabinet discharge, CT overlap and phase, poor coordination of vehicle cabinet, insufficient insulation gap, moisture absorption through the insulation wall, insufficient creepage distance, unnecessary conversion of old circuit breakers, etc. (2) Current: poor plug contact, cable overheating, etc. (3) Mechanical failure: poor coordination of carriages can cause deformation or damage of the contact arm, incorrect opening and closing of the grounding switch, burnout of the opening and closing coils, and failure of the lock. Ordinary medium-voltage switchgear can directly monitor the external operating status. If the switch is found to be deteriorating or faulty during operation, it can be shut down to replace the faulty parts and restore its original function. It is difficult to diagnose the internal state of the switchgear (such as insulation state, ablation state of conductive and arc components, etc.) and eliminate defects during operation. The emergence and development of such problems often cause a certain degree of confusion.

3. Main technologies of intelligent medium-voltage switchgear

3.1 Temperature monitoring technology

Temperature monitoring is based on a variety of high-performance temperature sensors. At present, there are also a variety of temperature sensors on the market, with different performances and different adaptability to different occasions. Intelligent medium-voltage switchgear is generally selected according to the temperature measurement range of the temperature sensor. The temperature monitoring of the medium-voltage switchgear needs to meet the fast temperature response of 20℃～120℃, and the accuracy of temperature measurement is not the focus. In addition, the volume of the sensor needs to be considered to avoid occupying a large amount of valuable space in the cabinet, so a compact temperature sensor should be used. During the monitoring process, the thermistor converts the temperature change into a change in resistance value, and finally converts the current into a voltage signal, which is then digitized by a high-precision A/D converter to complete the temperature acquisition. The temperature digital signal is transmitted from the wireless transmission unit to the main control unit for processing. The main control module identifies the sensor according to the sensor address or code, and processes the temperature signal in real time. When the temperature exceeds the threshold, the fan is started for heat dissipation; when the temperature is lower than the threshold, the heater is started for heating, so that the temperature of the medium-voltage switch cabinet is maintained within a reasonable range [2].

3.2 Power monitoring technology

The power monitoring in the medium-voltage switch cabinet is mainly for voltage and current. Since the working voltage of the electrical equipment in the switch cabinet is high, the general main control chip cannot directly measure it, so a voltage transformer is used to reduce the high voltage. The primary side of the voltage transformer is connected to the high voltage through a large resistor to achieve milliampere current output, which is then converted into a voltage signal through a resistor.

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Because the output current is extremely small, the voltage transformer almost runs in a no-load state, has a good linear relationship, and achieves a high measurement accuracy. The current measurement mostly uses a shunt solution to complete the current collection. The shunt has the characteristics of no magnetic saturation, low price and high precision. It can meet the measurement of large AC current and is very suitable for current monitoring of medium-voltage switchgear.

3.3 Switch quantity monitoring technology

The core function of the switchgear is to connect and disconnect the power grid line, and this state is achieved through the mechanical action of relays, circuit breakers, contactors and other equipment. Therefore, monitoring the opening and closing state of these devices is an effective way to grasp the current working state of the switchgear and one of the important bases for analyzing power grid failures. During a fault or regular power outage maintenance, it is necessary to remotely open and close the remote switchgear, and this function can be achieved by using relays. At present, the relays used for switch quantity monitoring and control mainly use photoelectric couplers because this device has the advantage of photoelectric isolation. General sensors are usually powered by low voltage and collect low-voltage signals and small currents, while switchgear is a high-voltage device and is prone to generate huge induced currents or voltages during operation, which puts ordinary sensors at risk of breakdown or burnout. Through photoelectric coupling, the voltage signal can be converted into an optical signal, and then the light can be converted into a weak current signal, so that the use of switch quantity can be realized. By connecting the relay to the IP control device, remote opening and closing control can be achieved [3].

3.4 Wireless transmission technology

After the status data is collected, it must be transmitted to the remote monitoring center and analyzed accordingly in order to realize the intelligent management of the medium-voltage switchgear. Therefore, the communication system is an important step in realizing intelligence. The development of communication technology can be roughly divided into two types: wired and wireless. The former has different physical carriers such as cables and optical fibers. No matter what communication carrier is used in wired communication, the trouble of wiring cannot be avoided. Therefore, wireless communication is the main application in the intelligent application of medium-voltage switchgear. From the perspective of technological development, modern wireless transmission technologies such as GPRS, Bluetooth, ZigBee, Wi-Fi, etc. have been formed in different fields. Among them, ZigBee has become one of the mainstream technologies due to its self-organizing network. In the application of intelligent medium-voltage switchgear, ZigBee can complete the data interaction between the acquisition end, router and coordinator, making the layout of communication nodes more flexible, and realizing centralized monitoring and control of large-scale decentralized switchgear.

4 Application of intelligent medium-voltage switchgear in smart substations

4.1 Checking the characteristics of mechanical performance of circuit breakers

The indirect measurement method measures the contact distance, first directly receives the change of the angular displacement of the rotating shaft of the switching mechanism, and then indirectly receives the contact displacement data through calculation. This method is suitable for mechanisms with precise assembly, high reliability and high stability. If the components of the mechanism are deformed, large deviations may occur. Since the switching speed of the circuit breaker has a greater impact on the arc extinguishing performance, the speed curve is more accurate than the average speed and can more accurately describe the mechanical performance of the circuit breaker. In order to obtain a complete and effective speed curve, it is necessary to measure the opening (closing) displacement more accurately. However, it is not easy to obtain accurate and precise opening (closing) speed data from widely used sensors and single-chip systems at present [4].

4.2 Image data processing

Image sensors are installed at the main monitoring position to monitor the switch status and the position of the trolley in the control cabinet in real time during operation. The space in the control cabinet is narrow, the busbar components are entangled, and there are blind spots in the lighting area. It is difficult to identify the observation image picked up by the image sensor. Standard identification points (such as closed position and position) can be added to compare the relative positions of key points and image sensors. Image enhancement technology can also be used to determine whether the lock and position are in place so that the display in the cabinet is clearly visible [5].

Conclusion

In recent years, with the development of medium-voltage intelligent switchgear technology, this type of switchgear technology has become more and more mature. However, there are the following problems: the technical standards of different manufacturers are inconsistent, different systems are relatively scattered, and the compatibility between different systems is poor. The components of the smart substation add some complexity. This system solution integrates the decentralized system into a system that reflects a high level of intelligence, and can provide stable and reliable medium-voltage switchgear for smart substations.