MLX microcomputer harmonic elimination small curre

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MLX microcomputer harmonic elimination small current grounding line selection device and substation operation safety

I Overview

a safe and reliable power supply system is the most basic requirement for enterprise development. The safety and reliability of power supply system must be supported by reliable power supply equipment. However, power supply equipment will have different degrees of faults under the influence of the environment. Troubleshooting is the key to ensure safe power supply, and the search of fault points is the top priority

our company is a petrochemical enterprise with an annual processing capacity of 700000 tons of crude oil. The power supply system is 66kV with three incoming lines. During normal operation, one transformer is in operation and the other is standby. It is a single bus section connection mode. During normal operation, the bus tie switch is in the closed state. There are four sub substations with 6.3kV voltage level, and the four sub substations supply power dispersedly. For petrochemical enterprises with strong flammability, explosion and continuity, any electrical fault will seriously threaten the safety production of the enterprise

at 18:56 on July 13, 2002, the optical signs of "section I and II grounding" and "low cycle AC voltage disconnection" on the central signal screen of the main transformer lit up and gave an alarm, which could not be reset. Immediately send personnel on duty to each substation to find the grounding point. Because the whole 6.3kV system is connected by the bus coupler of the main transformer, it shows that the system of section I and II is grounded, and it is impossible to judge. At 19:11 2 \a, 10 minutes have passed, and the PT of section II of substation a has grounding breakdown. PT insulation breakdown caused by single-phase grounding overvoltage. Considering the continuity and stability of the production system, report to the dispatcher in time to reduce the load, and the focus is to ensure that the production device operates under low load, so as to avoid sudden shutdown, explosion and other serious accidents. At the same time, the director decisively decided to stop non important lines and equipment for further investigation. After investigation, it was found that the line 1# of the outer railway line was grounded, and the railway 1# power supply was immediately cut off to restore the normal operation of other equipment. This treatment process took 40 minutes

in order to prevent the recurrence of the above faults, monitor the line and operation status, and timely record the faults and the data at the time of faults, we have timely installed a microcomputer harmonic elimination small current grounding system

at 8:58:51 on June 20, 2005, due to the external line grounding, the system resonance was caused. The resonance bus was section I and II of the main transformer, with a frequency of 70Hz and a resonance voltage of 100.1v. Due to the strong system resonance, the personnel on duty quickly reduced the reactive power of the generator to 0, but the resonance was not eliminated. 1 # - A, 2 # - A, 3 # - a capacitors were still in operation, so they went to the unattended substation and cut off the container, making the resonance disappear immediately, However, the PT of section I of the main transformer was burnt out, which took only about 8 minutes, greatly shortening the time to find the fault point and eliminate the fault. In addition, the record of equipment failure is also an important basis to judge the aging degree of the equipment

since the device was put into operation in October 2002, single-phase grounding has occurred for more than 20 times, and resonance has occurred for 7 times. The time of each grounding and the fault line have been clearly recorded. The frequency and voltage of each resonance can be quickly identified at the time of occurrence, which has achieved the expected purpose and played its due role in substation operation

II Working principle of microcomputer small current grounding line selection device:

1. Single phase grounding line selection principle: the principle of zero sequence harmonic current direction is adopted, that is, the direction of zero sequence harmonic current of fault line and non fault line is different

2. harmonic elimination principle: digital harmonic elimination principle. That is, a bidirectional thyristor is connected in parallel at the open triangle output end of the transformer, and the microcomputer automatically recognizes and outputs pulses to control the conduction and shutdown of the thyristor, automatically absorb the resonance energy and dynamically eliminate the ferromagnetic resonance of the system

3. Small current grounding system refers to neutral ungrounded system or arc suppression coil grounded system

4. Characteristics of grounding operation of small current grounding system

(1) in case of single-phase grounding fault, the size and phase of line voltage remain unchanged and symmetrical, and the phase to phase insulation of the system can meet the requirements of line voltage operation, so it is allowed to maintain operation in case of single-phase grounding, and the general regulations stipulate that it shall not exceed 2H

(2) influence and harm of single-phase grounding on equipment

○ 1 in case of single-phase grounding fault, the non fault phase to ground voltage increases, and the weak insulation links in the system may break down and cause short-circuit fault

○ 2 intermittent arc is generated at the fault point, which is easy to cause resonance, and resonance overvoltage is generated, which is harmful to the system equipment. At the same time, intermittent arc may burn out the equipment and expand the fault into interphase fault

III. main features of mlx198g device:

mlx198g upgraded products. Compared with mlx198, mlx198g has superior performance and more perfect functions. Its main features are as follows:

1. The hardware design adopts the latest bus and does not introduce CPU chips. It has high integration, strong anti-interference ability and fast running speed (the average execution time of each instruction is about 100ns):

2. It adopts large screen LCD display and full Chinese operation prompts, which is intuitive, clear and convenient to operate

3. Different systems, different operation modes and different grounding resistances make the grounding current very different, which can reach more than 1000 times in extreme cases. How to adapt to system changes is an important link for the correct operation of the device

4. This device automatically tracks the change of the zero sequence current of the system without adjusting and setting the amplification factor. It can be applied to the system with the field zero sequence current of 0.2a~100a (single line): that is, when the primary grounding current is small, the device has no dead zone, and when the primary grounding current is large, it is set to unsaturated

5. After single-phase grounding, the transition process of capacitor charging and discharging is long. This product adopts optimization algorithm, which can avoid the influence of system transition process

6. It has three output modes of tripping, alarm and coding, which can meet various needs of the site. The action delay time (range is 0 ~ 6553 seconds) can be set by software, and it has a perfect tripping locking software and hardware structure

7. Bus grounding alarm (alarm time can be set by software), device fault alarm, power failure alarm, and two groups of normally open contacts are led out from the main alarm contact of this product, which can meet the needs of different sites

this product can select the communication interface through software without changing the chip. It is equipped with 6 communication protocols, and the corresponding parameters of small baud rate can be set by software

8, that is, after the system resonates on the basis of different fixtures, PT secondary will also have zero sequence voltage. This device can correctly distinguish whether it is resonance or single-phase grounding. Using the method of combining software and hardware, the resonant frequency of the system can be measured, and it has perfect harmonic elimination function

9. The working power supply of the device is AC and DC

10. 16 times of grounding fault and resonance fault can be memorized respectively, and the information will not be lost after the device is powered off

11. Four sets of fixed values can be set, and one of them can be easily selected during operation. The on-site CT transformation ratio, starting voltage and tripping voltage can be set by software, and there is no need to adjust on site

12. Perfect self checking and self debugging functions

13. Working process

(1) single phase grounding. When a single-phase grounding fault occurs, the PT open triangle appears zero sequence voltage, and the device starts. At the same time, the device exits normal monitoring, enters fault data processing, and automatically selects and determines which circuit of outgoing line grounding and bus grounding is. And display and print the grounding time and grounding line number

(2) ferromagnetic resonance. When ferromagnetic resonance occurs in the system, zero sequence voltage with different frequency components appears in the open triangle of the voltage transformer. According to different frequency components and different voltage values, the computer automatically outputs pulses to control the conduction and shutdown of the thyristor, dynamically eliminates the ferromagnetic resonance of the system, and displays the resonance frequency, but does not alarm

from the characteristics of this device, it has its outstanding advantages. Small size and light weight, its weight is less than 15kg, and the opening size of the equipment panel is only 425 × 170mm。 Due to the principle of zero sequence harmonic current direction, the requirements for current transformer characteristics are low, and there is no polarity requirement for zero sequence voltage. And the device basically does not need maintenance

and put forward three key development directions. Fourth, deepen the discussion of this technology.

this device also has its shortcomings. It can automatically distinguish and eliminate ferromagnetic resonance of five frequencies in the system, and has 16 recall functions. For the resonant frequency range, there are also its limitations, and only 16 times of memory. For the system with more overhead lines and frequent grounding, less memory storage is also its deficiency. Discussion on the above problems:

1. Limitations of scope of application. In the era of increasing power transmission voltage, the proportion of small current grounding system is becoming smaller and smaller. In China, electricity below 35KV is generally small current grounding system. Therefore, the limitations of this device are quite prominent. The global 110kV and above are large current grounding systems. For this device, the range of 35kV ~ 110kV is its limited range. In order to improve the voltage range of small current grounding system, it is suggested that the content of 11% and 0.6% increase range and voltage range of small current grounding system should be added when formulating power industry standards, so as to improve the applicability of various safe and reliable devices and improve the applicability of this device

2. This device can eliminate 5 kinds of ferromagnetic resonance of system power frequency: 1/3 fundamental frequency (17hz), 1/2 fundamental frequency (25Hz), fundamental frequency (50Hz), double frequency (100Hz), triple frequency (150Hz). From the perspective of eliminating resonance range, the bandwidth is not enough. Judging from the resonance I have experienced several times, it does not completely eliminate the resonance. In my personal opinion, we should start with the transformation of the device itself, improve the performance of the device, so as to improve the frequency range, and achieve the purpose of basically eliminating the vibration to promote the intensive and differentiated development of the petrochemical industry

3. It has only 16 recall functions. For systems with many overhead lines and frequent grounding and resonance faults, if not checked in time, it is likely to cause the loss of original data. In this era of explosive development of microcomputer function and capacity, it is also necessary to adopt large capacity storage. Therefore, installing a large capacity hard disk and memory in the storage unit of this device is also a good way to solve the problem of data expiration and loss

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