【Abstract】The low-voltage system of offshore platform generally adopts IT system, and the fault current is very small when a single-phase ground fault occurs, which is not enough to stimulate the action of the protection device and make the faulty equipment automatically trip, so that the operating state of the equipment can be maintained without interruption, and the continuity of power supply is good. However, if the fault cannot be eliminated in time, the fault current and overvoltage can lead to a short circuit between phases, resulting in power failure and production stoppage, and increasing the risk of fire and personal electric shock accidents. Therefore, it is necessary to install an insulation monitoring device, immediately alarm and locate the fault point when a ground fault occurs, so as to eliminate the fault in time and ensure the normal production and personnel safety of the platform.
【Keywords】offshore platform; low-voltage systems; insulation monitoring; Fault location
0 Industry background
Offshore platform power system for island system, often a single power supply, once the fault power outage, not only will lead to the platform shutdown, resulting in economic benefit loss, serious failure may also occur electric shock accidents and even cause fire to endanger the safety of the platform, so oil and gas treatment power equipment more low-voltage system generally use IT system IT system single-phase ground fault fault failure, long enough to stimulate the protection device action, so that the fault equipment automatically tripped, so that the operating state of the equipment can be maintained without interruption, Good continuity of power supply. However, if the fault cannot be eliminated in time, the fault current and overvoltage can lead to a short circuit between phases, resulting in power failure and production stoppage, and increasing the risk of fire and personal electric shock accidents. For a long time, when a single-phase ground fault occurs in an IT system, it can only be manually disconnected to hundreds of load loops one by one to troubleshoot, which is not only time-consuming and laborious, but also destroys the continuity of equipment power supply, and frequent circuit breakers may also cause operating overvoltage. Therefore, it is necessary to install an insulation monitoring device with fault location, immediately alarm and locate the fault point when a ground fault occurs, so as to eliminate the fault in time, ensure the normal production and personnel safety of the platform, and can access the monitoring information to the integrated control platform of the whole mine through industrial Ethernet to realize the remote monitoring of the entire low-voltage power distribution system.
1 Analysis of the advantages and disadvantages of IT systems
The schematic diagram of the IT system is shown in Figure 1, that is, the neutral point on the secondary side of the power transformer is not grounded, and the housing of the electrical equipment is grounded. The low-voltage power distribution system of offshore platforms generally does not lead out N lines, nor PE lines, and the shell of electrical equipment is directly grounded through the steel structure of the platform.
Figure 1
1.1 Main advantages
When a single-phase insulation fault occurs in the system, the fault current is the leakage current of the system's capacitance to ground, and the value is very small, which will not cause the action of the relay protection device and ensure the continuity of power supply; The line voltage remains unchanged during the fault, which will not affect the normal operation of the three-phase electrical equipment, and the contact voltage formed by the small fault current is very low, which will not cause harm to the human body, taking a phase in the 230V system as an example, as shown in Figure 2.
Figure 2
1.2 Main disadvantages
When a single-phase insulation fault occurs in the system, although the fault current value is small, if the current invades the insulation of cables and equipment for a long time, it may cause insulation combustion at the fault point, resulting in phase-to-phase failure; When single-phase insulation fails, the ground voltage of the non-fault phase will increase from the phase voltage to the line voltage, and if the fault is not eliminated for a long time, the non-fault phase can also have a new fault, forming a two-phase ground short circuit. When the distribution line is too long, the line distribution capacitance is large, the IT system will be equivalent to the TT system, if an insulation fault occurs, the fault current will be significantly larger, resulting in dangerous contact voltage, resulting in the occurrence of personal electric shock accidents, to a certain phase of the 230V system as shown in Figure 3.
Figure 3
2 Product Introduction
AIM-T500L series industrial insulation monitoring and fault location system is specially developed by Accuray Electric for industrial applications such as mines, glass factories, electric furnaces and test equipment, ships, metallurgical plants, chemical plants, explosion hazard locations, computer centers and emergency power supply and other IT power distribution systems.
The system has rich functions, including insulation resistance monitoring, insulation fault early warning, insulation fault alarm, event recording, parameter setting, communication networking, etc., when the system has a ground fault, it can alarm in time, and accurately locate the specific circuit where the fault occurs, in order to remind relevant personnel to troubleshoot the fault in time.
Insulation monitoring is to monitor the insulation impedance of the system, based on Ohm's law, using the low-frequency current injection method, the system consists of an insulation monitor, a test signal generator, a fault locator and a sensor, that is, a current transformer, as shown in Figure 4.
Insulation monitors inject low-frequency AC signals into low-voltage systems, and insulation monitors contain very low impedance to power frequency currents and very low frequency currents. When no insulation fault occurs in the system, the host can measure the insulation resistance to ground and leakage capacitance of the system and each circuit, and the insulation fault can be prevented by setting the insulation resistance preset; When the insulation fault occurs in the system, the low-frequency current signal returns to the insulation monitor through the earth through the fault point, and the insulation monitor issues a fault alarm after calculation and analysis, and the fault location device locates the fault loop through the current transformer.
Figure 4
3 Function introduction
AIM-T500L insulation monitor has the following functions:
Real-time monitoring of the insulation resistance of the IT system to the ground, the more the resistance starts the fault warning or alarm function; Relay alarm output, LED alarm output and other fault indication methods; The event recording function is convenient for operators to view and analyze the fault type and occurrence time, and judge the system operating status; Self-test function, which can realize fault self-test of the instrument hardware circuit with one button; Wire break monitoring, real-time monitoring of PE/KE function grounding wire connection status; One RS485 interface, standard Modbus-RTU protocol; One CAN communication, custom protocol, used to interact with signal generator, fault locator information; Wide range of applications for AC, DC and AC/DC hybrid IT systems.
The ASG200 test signal generator has the following features:
Indicates the phase line where the fault is located; Support manual start positioning for testing communication lines; Using CAN bus technology, it is convenient to interact with other devices in the system.
AIL200-12 insulation fault locator has the following functions:
locate and indicate the branch where the fault is located; A single AIL200-12* can locate up to 12 circuits; Each IT system* can connect up to 90 locators, with a total of 1080 circuits; Using CAN bus technology, it is convenient to interact with other devices in the system. Use AKH-0.66L series current transformer as monitoring transformer, change ratio 1000:1, and select the appropriate aperture according to the rated current of the loop;
4 Technical indicators
AIM-T500L insulation monitor technical indicators
5 Platform low-voltage system system configuration method
The low-voltage system of offshore oil platform includes 400V system and 230V system, the power side of the 400V system is generally powered by two transformers, double busbar with normally open bus link, operating separately under normal working conditions, and the power supply circuit of emergency diesel generator under emergency working conditions is mainly for oil and gas treatment power equipment, and the power supply circuit can reach hundreds. The power supply side of the 230V system is a single transformer power supply, mainly for lighting, low power and electric heating system power supply, each 230V lighting or heating system circuit number is generally about one hundred.
The 400V system insulation monitoring device can be equipped with an insulation monitoring host at each power inlet to meet the insulation monitoring under different working conditions. It should be noted that under normal working conditions and black start conditions, when there are two or more insulation monitoring hosts in a system, it is necessary to ensure that only one host can inject low-frequency current into the system, otherwise the insulation monitoring results will be inaccurate. Therefore, it is necessary to connect the status of the power inlet switch and the bus tie breaker to each monitoring host and set the interlock.
Considering that there are many load circuits in the 400V system, if an insulation fault locator is installed for each circuit, the investment is large, and the fault locator can be installed only for the important circuits required for process flow or safety, and other circuits can be manually located after the fault alarm using a portable fault locator without disconnecting each circuit. A 230V system can be configured in the same way as a single 400V system.
6 Conclusion
After equipping the low-voltage IT system of offshore oil platforms, the insulation resistance and leakage capacitance of the system can not only be monitored online, but also the insulation resistance of some important circuits can be monitored to prevent insulation failures. After the insulation fault occurs, the insulation monitoring device can issue an alarm and automatically or manually locate the fault circuit, which can cut off the fault circuit in time, eliminate the occurrence of secondary faults, ensure the continuity of power supply, and improve system safety.