Technical Proposal for Intelligent Grounding Box of Transmission Line

1、 Overview

The metal sheath of 110 kV and above single core cables generally adopts the operation mode of cross interconnection double end grounding or single end direct grounding. Under normal circumstances, the metal sheath has only tens of volts of induced voltage to the ground, and the induced current ranges from several amperes to ten amperes. Power cables are mostly solid insulated cables. There are many reasons for cable deterioration, including electrical degradation, thermal degradation, chemical degradation, mechanical degradation, theft, etc. For high-voltage cables (110kV and above), the shielding layer can only be grounded at a single point. If the cable sheath is chemically In case of damage to machinery, even rats and insects, and multi-point grounding, the circulating current of metal sheath to ground will rise to a very dangerous value. Once the cable grounding system is damaged, the voltage of the metal sheath will change from the power frequency induced voltage during normal operation to the suspension voltage. When the suspension voltage on the cable metal sheath will rise above the allowable power frequency withstand voltage value of the cable outer sheath, in this case, it will lead to the breakdown of the outer sheath or the burning of the sheath protector, and more seriously, it will lead to the breakdown of the cable main insulation and other potential safety hazards. However, cable operation management generally adopts manual periodic inspection, which is difficult to prevent such incidents. Therefore, scientific means must be used to take effective preventive measures.

At present, the circulation detection of HV cable metal sheath is an important technical detection work for the daily operation and maintenance of the main network cable line. The existing traditional mode of grounding circulating current technology detection mainly relies on the manual measurement of clamp type ammeter by operators and the quarterly detection cycle, which cannot effectively and timely reflect the grounding circulating current and is not convenient for monitoring the health level of the main network cable lines. Therefore, it is necessary to establish a set of anti-theft and real-time online monitoring system to solve the problems of cable circulation, cable sheath voltage, cable temperature and equipment safety in the current real-time operation of power cables. According to the above current needs, the intelligent high-voltage cable grounding box developed and produced by our company is equipped with a set of high-voltage cable online monitoring system to monitor the status of grounding circulation and the insulation status of cable intermediate joints, so as to ensure the safe operation of power cables.

2、 System composition

HV cable and intelligent HV cable grounding box are widely distributed. To achieve efficient management, a system must be formed to achieve real real-time monitoring and reduce management labor and operation and maintenance costs.

The system adopts a two-level architecture, which is divided into monitoring center and remote acquisition. The remote collector is responsible for information acquisition and preprocessing, converting current and voltage signals into digital signals for transmission. The monitoring center is responsible for processing the signals uploaded by the remote collector in the area under its jurisdiction, transmitting the alarm information to the relevant maintenance personnel, and processing and statistics of the relevant information in the area. The monitoring center will be set up as required, mainly responsible for the summary and processing of front-end information, and providing necessary information and decision-making basis for the unified management of power cables.

In the later system expansion, the configuration of the monitoring center does not need to be changed, but the corresponding RTU monitoring points need to be added, and the centralized monitoring and management of the grounding current and voltage data of the single core cable in the area under its jurisdiction can be completed by converging to the monitoring center according to the network configuration. In case of alarm in the system, the system can alarm by telephone, SMS, local multimedia sound and light and other means, and notify each responsible person in time to take effective measures to prevent.

3、 Main functions

²     It has the functions of monitoring the circulating current data and sheath voltage of cables, presetting the early warning value and alarming to the center;

²     It has three detector defense zones to monitor the inclination, room demolition and illegal climbing of the high-voltage grounding box;

²     It has the function of integrated monitoring and alarm for the equipment of high-voltage grounding box;

²     It has the function of monitoring the cable surface temperature in the cable trench;

²     Have the equipment image monitoring function; (Option)

²     With the equipment positioning function, the data is transmitted to the power system intranet monitoring center in real time through optical fiber and PMS/SIM/GPRS communication module/RJ45, and the specific location is displayed on the electronic map;

²     It has the function of starting and exhausting the temperature immediately when the equipment temperature is too high;

²     The equipment power supply mode includes CT induction power supply, photovoltaic power supply and 220V lighting power supply.

4、 Technical parameters

4.1 Service conditions

(1) Ambient temperature and humidity: temperature range - 45 ℃~+55 ℃, relative humidity<95%;

(2) The altitude shall not exceed 4500m;

(3) The air in the installation site shall be free of chemical corrosion gas;

4.2 Electrical performance (primary and secondary circuits)

(1) Cable voltage: ≤ 110KV

(2) Rated frequency: 50Hz;

(3) The primary circuit of the product can withstand DC 20KV for 1min without flashover or breakdown;

(4) The primary circuit of the product can withstand 10 positive and 10 negative voltage pulse tests of 40KV without flashover or breakdown;

(5) The secondary circuit of the product can withstand 3 positive and 3 negative voltage pulse tests of 5KV without flashover or breakdown;

(6) The electric shock resistance value of each connecting point of the product is not greater than 40 μ Ω；

(7) The insulation resistance between the product connecting bar and the shell shall not be less than 20M Ω.

4.3 Performance of cable sheath protector

(1) The performance of this product meets the requirements of GB11032-2010 AC gapless metal oxide arrester;

(2) Protector pass 8/20 μ s. Residual voltage of 10KA impulse current is not greater than 10KV;

(3) The protector can withstand 5s without damage under 6 (or 12) KV power frequency voltage;

5、 Main online monitoring functions

1. Online monitoring function of cable circulating current: 0~100A single mode current transformer and grounding circulating current acquisition module are installed in the intelligent grounding box to monitor the current parameters of each high-voltage cable metal sheath grounding point of 110kV and above high-voltage cables in real time, which can realize the measurement of 0~100A grounding circulating current and regular patrol measurement, replacing the traditional manual regular grounding circulating current patrol measurement, greatly reducing the labor intensity of on-site measurement, and improving the system maintenance efficiency.

The grounding circulating current monitoring is used to monitor the gradual change value of the sheath grounding circulating current, reflecting the good grounding degree of the sheath, the aging degree of the cable, the change of the core load, etc; At the same time, in the case of normal operation of high-voltage cable lines, when the ground circulating current value suddenly decreases or is zero, the analysis and diagnosis model of electric regulation and cable operation state shall be connected to effectively judge whether the grounding box is stolen or the grounding wire is stolen.

2. Cable sheath voltage monitoring and protection function: each phase cable sheath is installed with a voltage monitoring sensor, which can monitor the voltage value on each phase cable sheath in real time. If the preset value is exceeded, alarm information can be sent. At the same time, a cable sheath voltage protector is installed between each phase cable sheath and the ground. When the voltage value of the cable sheath reaches the protection rating, it will conduct and discharge to the ground, so as to protect the cable sheath.

3. Cable surface temperature monitoring function: mainly to cooperate with the circulating current detection, fix the temperature measuring triode on the outer sleeve of the grounding cable and seal it, transfer the temperature information of the relevant grounding cable to the temperature measuring module of the monitor through the temperature measuring probe, or place the monitoring point on the cable connector surface to detect the cable connector temperature. It can realize 24-hour continuous online monitoring of the operating temperature of high-voltage cable joint surface, and the temperature acquisition range is - 55 ℃~+125 ℃. Through the statistical analysis of the temperature of the protective layer of the high-voltage cable body, the operating personnel can fully understand its working conditions, timely understand the aging of the cable protective layer, and send an alarm signal to remind the relevant personnel of emergency treatment when the joint temperature rises sharply to the limit temperature.

4. Anti theft alarm function: the intelligent grounding box has the door opening alarm function, as well as the out of limit grounding circulation alarm and out of limit voltage alarm functions. If the door of the box is opened or pried open, the grounding box will send out a harsh alarm (about 120 dB), and send out an alarm message (after the grounding box sends out an alarm). The monitoring software will immediately display the alarm message, which is displayed in red font. It will also automatically dial the alarm reception telephone set in advance (2 SMS alarm reception numbers and 3 voice alarm reception numbers can be set), monitor the surrounding activities, call out on the scene, and determine whether to report the case according to the situation.

5. With the equipment positioning image monitoring function: the monitoring center displays the specific location of the intelligent high-voltage cable grounding box on the electronic map through the longitude and latitude data set in the module or the base station information of the GPRS module. When the door of the grounding box is opened for no reason and gives an alarm, the host equipment in the grounding box will automatically control the video to capture the current scene, and the captured images will be saved in the monitoring camera, The SD card in the camera can store 200 photos, and can also take remote photos via software at any time and send them back to the monitoring software.

6、 Box and internal connection parameters

1. Box material: the grounding box box is made of SMC unsaturated resin and other materials, which are fully insulated, corrosion resistant, high strength, no recycling value, environmental protection and safety;

2. The box structure is fixed with special bolts, fully sealed, with anti-theft effect;

There are ventilation holes on the upper part of the back of the box, and warning words are posted on both sides: Do not open the high pressure danger;

3. The grounding box with a lockable door (smart lock) has anti-theft effect;

4. Cabinet protection grade: IP54

5. Internal connection: T1 red copper bar with tin plated surface, size: 4 * 40 and 3 * 30

6. Box size: H X W X D: 1265X650X405 (mm)

7、 Product structure and composition (taking cross interconnection as an example):

The structure and composition of the product are shown in the figure below:

① Case ② Grounding bus bar (4mm * 40mm) ③ Grounding copper bar (3mm * 30mm) ④ Current transformer ⑤ Phase copper bar (3mm * 30mm) ⑥ Cable sheath protector ⑦ Controller installation box (including battery, control panel, signal acquisition device, etc.) ⑧ Anti theft detection device ⑨ Solar power panel (one on the left and one on the right)

8、 Field case