Comparison of four methods for detecting faulty insulators on line

According to the working principle of high voltage insulator and the development of electric power industry , we explore new methods of detecting faulty insulators in recent years. Some people who have developed and used the methods for the new insulator tester scene. Some are still at the experimental stage. There are four main types of these methods. This paper introduce the advantages and disadvantages of four kinds of test methods for convenient power industry flexible to choice.

1.Self-climbing type poor insulation detector

The self-climbing poor insulator detector used in 500kV EHV line is mainly composed of self-climbing driving mechanism and insulation resistance measuring device. The capacitor is used to bypass the AC voltage component of the insulator under test, and the insulation resistance of the insulator is measured in the live state. Judging whether the insulator is good according to the size of DC insulation resistance. When the insulation resistance insulator value is lower than the predetermined resistance value, we can determine by monitoring a poor insulator, but also can clearly see that part of the waveform reproduction device defect of insulator in automatic recording from the cassette. When the V type string and the suspension string are detected, the weight of the insulator can be moved down along the insulator without any special drive mechanism.

2.Corona pulse detector

The utility model is a detector specially used on the ground, which can be used for detecting the lines in the plain area and also for detecting the routes in the mountain areas:

(1) Light weight, small size, power supply is battery of number 1, easy to use and safe.

(2)You can detect on the ground without using a pole.

(3) The first tower unit coarse, determine if the tower has a poor insulator, then the insulator fine test one by one.

(4) the microcomputer system is used for logical analysis and processing, and the detection efficiency is higher.

In the transmission line operation, the insulator string of the insulator will produce corona, and form a corona pulse current flowing through the tower into the ground. The corona current corresponds to each phase voltage and occurs only in a certain phase range. If you separate the positive and negative polarity of current , the polarity of each phase current pulse width phase range is smaller than the phase voltage of each phase. With proper phase selection method, each phase pulse current,i_a、i_b、i_c can be observed.

Each phase corona pulse is counted respectively, and the maximum and minimum values are selected, and the ratio (maximum / minimum) of them is different index, which is used as the basis of discrimination. When the same three-phase insulator tower on poor insulator, the corona pulse is in equilibrium, the ratio is close to 1; when there is a poor insulator, each phase of corona pulses in an unbalanced state, the ratio will have a large deviation with the 1 pulse corona detector is developed according to this principle.

It consistes of four parts:

(1) the formation of the circuit of corona pulse signal;

(2) periodic signal forming circuit;

(3) each phase counting circuit of corona pulse;

(4) calculation and display circuit of different indexes of iron towers. 

3.Electron optical detector

The motion of the electron optical detector of electrons and ions in the electromagnetic field, the concept and principle and light propagation in an optical medium similarity that charged particles (electrons and ions) in electromagnetic field (electromagnetic lens) capable of focusing and imaging and deflection] manufacture.

The voltage distribution of each insulator in an insulator string of an overhead transmission line is uneven, and the voltage drop is the largest on the insulator nearest to the conductor. When the zero value insulator, redistribution along the insulator string voltage, voltage from the conductor insulator on the recent sharp rise will cause partial discharge. Partial discharge surface or increase the strength of the surface. According to the intensity of light radiation produced by partial discharge on the surface, the insulation performance of insulator string can be known.

By monitoring the insulator partial discharge, corona discharge and insulator surface light image by the lens input light brightness intensifier cathode; electronic form electronic current from the photo cathode to escape, according to the planar distribution of the electron current density can show the original image brightness distribution. The focal length adjustment system causes electrons to accelerate so that the brightness intensifier screen is lit. In this way, the original light and shadow, like midway through an electronic image, is again transformed into light and shadow. In the process of image transmission, the magnetic field system accelerates the electrons, increasing the brightness of the original image of light and shade (up to a hundred times). The luminance intensifier can measure the weak light of partial discharge on the suspended insulator string of transmission line from the ground distance (5 -50m).

When detecting at night, in order to distinguish the partial discharge of the insulation porcelain surface and the interference of other external light sources (moonlight and illumination), and improve the signal-to-noise ratio, the pulse power supply can be used to power the brightness intensifier. Because the surface partial discharge occurs near the maximum value of AC voltage applied by insulators, the frequency is 100Hz, while the external luminous intensity is independent of the frequency of the grid. When the insulating porcelain in the instrument surface partial discharge (1-6ms), press close to 100Hz frequency brightness intensifier input, will make the background light and weak interference light weakened. On the screen of the electron optical detector, the bright region pulsation of the surface partial discharge in line with the grid frequency and brightness intensifier will be observed. The pulse can distinguish the light intensity of the partial discharge of the surface with the attenuation of the non pulsation and the external interference. In actual test, the radiation of partial discharge on the faulty insulator string exceeds the average intensity of radiation.

To evaluate the difference of light radiation intensity from the partial discharge of conductor surface recently on the first insulator and the average radiation intensity is the use of electronic optical detectors, analyze the relationship between the sensitivity threshold of the optical detector Ø₀ electronic and optical input system parameters and its relationship to

Ø₀＝τ（D/F）²A／L²

Τ - Transmission coefficient of input system;

D/F - measure the light intensity (relative aperture, aperture) of the input eyepiece;

A - constant;

L -- the distance from the emitter.

Reduce Ø₀ (turn off the small input aperture). When the D is reduced to a certain value, the average light intensity no longer occurs on the screen of the electron optical detector, and the screen will only show partial discharge on the surface of the defective insulator. Then, further light radiation intensity on the first insulator surface discharge near the wire and the average radiation are compared, if the light radiation intensity than light radiation intensity in the presence of faulty insulators, according to voltage relationship curve of light radiation intensity and insulator partial discharge on the surface of the first insulator to find the voltage distribution on the wire. According to the difference between the distribution voltage value and the normal distribution voltage value of the first insulator of the good insulator string, the bad insulator can be judged. This method of detection is efficient.

However, the electronic optical detector can only determine whether there is a zero value insulator in the insulator string, and it is not sure how many pieces of zero value insulators and their positions are in the end.

4.Detecting poor insulators with infrared thermal imager

There is a difference in the surface temperature between the poor insulators and the good insulators, but this difference is very small, so it is difficult to distinguish them with the general method of temperature measurement. In recent years, infrared thermal imager has been widely used to convert the temperature distribution of the insulator surface into an image, which is displayed by an intuitive and vivid thermal image, and then the poor insulators are detected according to the thermal image.

To detect faulty insulators by infrared imaging method, simple and convenient, fast speed, high efficiency, and even census every string insulators can also be combined with detection of transmission line,which is the direction of detection of poor insulator HV and EHV and UHV transmission line. However, at present, two problems widely exist: one is the thermal infrared imager is expensive, each about hundreds of thousands of RMB; second is to be the instrument used for mountain or both line, should be equipped with aerial aircraft, these are general and not the unit force. Of course, it is beneficial to organize the research in this field at present, sum up experience and grope for the law.

Analysis of Oil Immersed Transformer Fault

In today's industrialized society, power supply is closely related to the production of industrial facilities, the operation of national defense and military facilities, the development of scientific and technological research, and the normal conduct of people's lives. With the improvement of people's living standard, the demand for energy is increasing, so the transformer load increases with the increase of accidents. Transformer accident is harmful to the safe operation of power grid. so it is necessary to find fault and eliminate the fault in time, so that the power grid can be unblocked.

The operation condition of oil immersed transformer has direct influence on the power supply quality of the whole power system. In the actual operation, due to various factors, the transformer will inevitably appear in the course of a variety of faults. These failures lead to power outages, heavy equipment, equipment outages, and even major power accidents. Therefore, it is necessary to make a scientific analysis of the oil immersed transformer fault.

1.Fault analysis of abnormal sound of transformer

When the transformer is in normal operation, its sound mainly comes from two aspects: the transformer body and the cooling system. The vibration of iron core caused by magnetic and magnetic leakage of silicon steel sheet is the main cause of vibration of transformer body. When the following faults occur, the transformer will produce abnormal sound:

(1) The core bolt of the iron core is not firmly fixed, which will lead to loose core and cause irregular vibration between the silicon steel sheets, thus causing strong and uneven noise.

(2) When the winding flashover discharge or the grounding wire of the iron core is broken, it will cause the iron core to discharge the high voltage outer shell, and cause the discharge sound inside the transformer.

(3) A few parts of transformer is not firmly fixed, within the transformer there is iron tapping sound. 
(4) In the case of light load or empty load, some of the end of the laminated silicon steel sheet will vibrate, and there will be a mosquito flapping sound inside the transformer.

(5) If there is a breakdown in the transformer. There will be a big and small discharge sound.

2. Analysis of Oil immersed transformer oil level and oil temperature abnormal fault

(1) The oil level of transformer is too high or too low

The oil level of the transformer is affected by the following factors: a. internal oil content  b.oil temperature  c.transformer load  d.ambient temperature  e.transformer sealing and so on. Oil pillow volume is generally about 10% of the transformer volume. If the oil level is too high, easy to cause oil spill. If the oil level is too low, the transformer leads to the wiring part to be exposed to the air, which reduces the insulation effect and may cause an internal short circuit. At the same time, as the oil and air contact area becomes larger, the insulation strength of the oil decreases rapidly. In case of power failure or low temperature in winter, the oil level will drop rapidly, triggering the action of light gas relay. From the above analysis, we can see that the transformer must keep the normal oil level in operation. Operators must always check the level of oil level gauge, and take corresponding measures to maintain normal oil level, to ensure the safe operation of the transformer.

(2) Oil leakage fault of transformer

Transformer oil has a strong penetration force, usually after the infiltration of oil seal for poor sealing or welding point welding two categories.

To achieve the prevention of transformer oil, first, need to carefully check the record of the original data, and experience of all seal welds and check the transformer oil, according to find all leaks; second, the leakage point welding in time; third, material, try to choose the installation process of high quality seals and improved transformer.

(3) oil temperature continues to rise

Transformer in the daily operation of the oil temperature rising phenomenon, indicating that the transformer internal problems. The main reason is the iron core overheating or winding turn to turn short-circuit. The bolt clamping core insulation or eddy current caused by the destruction of the core overheating. The eddy current will keep the core temperature high for a long time, resulting in iron loss and rising oil temperature. Through that bolt insulation was destroyed will cause overheating of the screw. When the temperature rises to the extent that the iron core is fused together, the transformer should be cut off immediately so as to avoid a major accident such as fire or explosion.

3. Analysis of oil immersed transformer circuit fault

(1) Insulation material damage caused by failure

First, when the transformer is installed, the cover is checked and exposed to air, causing moisture. The vacuum treatment is not complete before oiling, so that the moisture is concentrated around the insulating parts. Second, maybe due to equipment manufacturing cycle is short, if the insulation factory is not fully dry, insulating paper and board out after running for a period of time, water absorption water, resulting in low voltage winding of the transformer insulation performance is reduced, resulting in DC leakage current increases. Third, poor sealing, easy to cause winding insulation part of the damp. Fourth, if the volume of hydrogen and carbon monoxide in the transformer oil is relatively high, the adsorption of the gas produced by the solid insulating material or stainless steel in the manufacturing process should be taken into account.

Insulation damp is one of the main causes of insulation failure, so it is necessary to strengthen the prevention and supervision work, do a good job of checking the insulation effect of the important components of the transformer, and find out and eliminate the fault as soon as possible. When installing and overhauling transformers, minimize the time that the transformer is exposed to the atmosphere. In the production of transformers, the standard of drying process should be strictly carried out, and the moisture content of insulating paper (sheet) shall be controlled within 0.3. The leakage points of the transformer body and accessories should be treated in time to avoid infiltration of moisture into the transformer. Improve design and manufacturing processes, and monitor material selection.

(2)Influence of discharge fault on transformer insulation

The influence of the discharge on the transformer insulation is divided into two kinds of damage: a. the discharge particle directly impacts the insulation and makes the insulation breakdown. b. ozone and nitrogen oxides generated by the discharge, chemical corrosion, insulation components, and finally lead to thermal breakdown.

The main form of aging of insulating materials is  a. the damage of chemical structure of insulating materials is caused by partial discharge. b.the thermal effect of the discharge point causes the thermal cracking of the insulation and accelerates the aging process. c.the ozone and nitrogen oxides generated by the discharge process corrode the insulator by chemical reaction and weaken the insulation performance. (d.during the discharge process, the transformer oil decomposes to produce bubbles, which make it difficult to heat the solid insulation and cause damage to the solid insulation.

(3)Partial discharge fault

Within the insulation structure, the phenomenon of non - penetrating discharge at the edge of the hole is known as partial discharge under the action of voltage. The reason is: a hole or cavity in transformer oil gas, gas of low dielectric constant reduces the compressive strength of insulating material, causing the discharge gap between the transformer oil; processing net, will make the bubble in the transformer oil caused by discharge; discharge between metal parts or conductive contact will cause the bad. Although the energy intensity of partial discharge is not large, but the potential harm can not be overlooked, if long-term partial discharge will cause breakdown or damage to equipment, is a serious safety hazard.

(4)analysis of automatic trip fault

In case of standby transformer, if the main transformer is automatic tripping, the working transformer should be deactivated immediately, and the standby transformer can be used at the same time. To check the cause of automatic tripping of the transformer. If internal trouble is found during inspection, internal inspection should be carried out immediately. If it is found that the tripping is caused by differential protection, the electrical equipment in the differential protection range shall be checked and power transmission shall be carried out after troubleshooting.

4.Epilogue

According to the above fault analysis, the staff should monitor the sound, the oil level and the circuit condition of the transformer. According to the characteristics of the relevant fault, make timely and ready analysis, and quickly formulate the corresponding repair plan, to repair the fault at the fastest speed. In short, when the oil immersed transformer after the failure, the staff should do the analysis according to the real-time fault situation, and formulate the corresponding countermeasures, eliminating the trouble in the bud, to ensure the safe and stable operation of transformer.

Characteristics of Using Series Resonant System to Conduct Power Frequency Dielectric Test

In order to meet the requirement of power frequency voltage withstand test for large electrostatic capacitance test , some departments have installed AC  series resonant test equipment.

The samples which have the large electrostatic capacitance are usually refers to cable, six sulfur fluoride pipeline, the capacitor and the large capacity generator of more than 300MW.

The characteristics of power frequency withstand voltage test using series resonant test equipment.

(1) The capacity of power supply transformer T and voltage regulator AT is small. This is because the voltage Uc=QUs.  Us is a high supplied voltage. Since the current flowing in the high voltage circuit is the same, the capacity of the power supply transformer and the voltage regulator is Q times smaller than the required capacity in theory.

(2)The output voltage waveform of the series resonant device is better. This is because only the power frequency resonance, and for other power supply by the high harmonic component, the total impedance of the circuit is very large, so the harmonic component is very weak, the test waveform is better.

(3) If the flashover occurs during the test, the high voltage will disappear immediately because of the absence of resonance condition, so the arc will be extinguished immediately.

(4) The recovery of voltage is longer, and it is easy to control the power trip before the flashover voltage is reached again, so as to avoid repeated breakdown.

(5)The recovery voltage does not appear any overshoot caused by over-voltage.

Because of the characteristics of the above (3) and (4), the burn point formed after the breakdown of the sample is not large, which is beneficial to the study of the breakdown of the test sample. Due to the above characteristics, the device is safe to use, and can not generate large short-circuit current and will not restore over-voltage.

Below we will analyze and explain the above two characteristics about (4) and (5) .

When an AC high voltage series resonant device is used for testing, the recovery voltage of the sample after the first flashover has been eliminated:

U_0-1＝cos(ωt)－[cos(βt)＋αsin(βt)／ β]exp(－at)

β＝√1／(LC)-R2／(4L )2, α＝R／(2L)

Because Q＝ωL／R=40~80，1／（2Q）≈10^-2

β＝√ω²－[ω²/(4Q²)] ＝ω√1－1/4(Q²)≈ω

α＝R／(2L) ＝ω√(2Q)

So,

U_0-1＝cosωt－[cosωt＋sinωt／(2Q)]exp[－ωt／(2Q)]

Because  1/(2Q) is small

U_0-1≈{1-exp[－ωt／(2Q)]}cosωt

ωt=ωnT

n:The number of cycles counted from 0

T:Period of a Sine Wave

f:Sinusoidal frequency

Then

ωnT=2πfn/f=2πn

So, ωT/(2Q)＝πn/Q

If the second times (the flashover voltage in voltage than reignition) first appeared slightly lower values, assuming exponential decay of e to 5%, nπ/Q≈3，n≈Q.

Since the Q value in the resonant device is always larger, the n value is greater. So, before another flashover occurs, it takes dozens of cycles, and it's easy to cut the power off for a long time.

Figure 1: Recovery voltage waveform of test sample after flashover

Figure 2: Full waveform of recovery voltage after flashover test

Figure 1 and Figure 2 draw the recovery voltage waveform of an actual series resonance test device after the test case flashover, the quality factor Q=40. It can be seen from the diagram that the negative overshoot is not occurred after the test case has been flashover. The time interval for recovering the voltage to reach the secondary breakdown value is close to 1s.

The use of series resonant devices to do withstand voltage test is limited, for example, it can not be used for external insulation of wet flashover and pollution flashover test.

Test Report of 900KV Lightning Impulse Voltage Generator

900KV lightning impulse voltage generator acceptance test

1.Physical inventory and technical indicators  

Product name	Model	Technical indicators to be achieved	Number	Remarks
Equipment A	900KV/27KJimpulse voltage generator	1.Lightning full wave (1.2/50us) 2.steep wave (1000 - 1500/ S) meet the national technical requirements。	A set	√
Equipment B	900KV/1000PF Weak Damping Capacitor Voltage Divider	The voltage divider has a precision of 1% Lightning wave measurement Steep wave measurement	Two sections	√
Equipment C	DC charging Apparatus	100kV	A set	√
Equipment D	Wave modulation resistance	Two waveforms	Two set	√
Equipment E	Automatic shock control measurement and analysis system	It contains computer, measurement and control software, oscilloscope.	A set	√

2.Acceptance record

After the installation and testing, the detailed acceptance test of the equipment and the experiment are as follows:

1). Operation steps and methods: carry out wave full wave and steep wave experiments, and the experimental results are recorded.

2). Lightning full wave: the use of wave head resistance value of 50 Ω (no sense resistance), wave tail resistance value is 180 Ω (no sense resistance). The waveforms generated by the attached experiments are as follows:

a. Full wave positive polarity of lightning:

b.Full wave negative polarity of lightning:

c.Steep wave

The steep wave head has a resistance value of 20 Ω(no sense resistance) and a resistance of 180 Ω with a tail resistance value. The experimental waveforms are as follows:

3). Operation result: equipment running with test sample is running normally.

a.Operation procedure and method: to increase the lightning withstand voltage required by the equipment or the sample, and the steep wave withstand voltage. And recorded its experimental waveform.

b.Operation result: the equipment is running normally and the waveform produced by the experiment conforms to the national standard.

3. Acceptance conclusion

Through the tests and experiments, it is proved that the full wave and steep wave waveforms produced by the 900 KV/27KJ full automatic impulse voltage generator of this equipment can meet the requirements of the national standard. Technical parameters and quantity of each device, including weak damping capacitance divider, high response resistor divider and sharpening device are consistent with the contract, after installation and commissioning and good performance, normal operation, and technical parameters of the waveform are in line with the requirements of the relevant experiments, acceptance.

(Subsidiary description: The equipment shall be dustproof, damp proof and regularly energized to prevent the aging of the circuit boards and components. The equipment should be powered by a single power supply in the course of experiment, and the ground wire should be grounded separately. It should not be carried out simultaneously with other large capacity high-voltage experimental equipment, such as power frequency test)