A Study of Characteristics of Transformer Switching Impulse Waves

Because the equivalent frequency of operating voltage is higher than the power frequency voltage, duration is shorter than a minute frequency. Insulation structure in the operating voltage of the discharge characteristics, the breakdown mechanism and frequency, the impact is not the same. The operation does not cause the effect of wave oscillation, a long time, for the same voltage, the coil will be more severe than the impulse test. Compared to frequency voltage,operation wave is directly caused by the oil gap breakdown. The small oil cylinder gap thin insulation structure of transformer is more necessary.

If insulation level is reduced, the power frequency and frequency test may cause partial damage. This residual weakness in the test is not easy to find. But under the long term working voltage, it will gradually develop to lead to the breakdown of insulation. Generally partial discharge occurs in operation wave , it will not caused residual damage.

Characteristics of inductance operating impulse wave

The operating waveform is close to the voltage in the operation. The authenticity is strong. The equivalent power frequency voltage is more reasonable, and the main longitudinal insulation are all evaluated, which has the advantages of portable equipment, sensitive display, no large power supply and so on.

The frequency or frequency test, ready to spend huge sums of equipment, heavy workload, the scene often give the test of safety equipment will be without clear aims. It is necessary and feasible to test the insulation of the transformer by using the operating wave test.

 Inductance operating impulse waveform

Wave front length; T_f ≥100μs

Wave tail length; T_t ≥100μs

90% amplitude duration: ≥200μs

peak reverse voltage:U2M≤0.5 UM

≥－[100×1000(0)×200(90)]μs

Circuit of inductance operating impulse wave

Operation Method of AC withstand voltage and series resonant test for Generator

With the development of power system , the capacity of the operating equipment is larger, such as thermal power generating units whose capacity is more than 1000MW, and hydraulic power unit which the largest single capacity has reached 800MW; To do AC power frequency withstand voltage test of these devices, if you use the traditional test equipment (frequency test transformer), due to the need of larger capacity , transformer and voltage regulator are very heavy. The power frequency resonance device has great advantages in the aspects of test power supply capacity, equipment weight, test waveform and investment. The device is mainly designed for the AC voltage withstand test of the generator with a capacity of 250MW or less. Specific methods of operation are as follows:

Over current protection setting, 1.2 times the rated current of high voltage side of transformer. For example: the high voltage side rated current of exciting transformer is 50A, the configuration of the transformer is 50/5, so the current relay should be adjusted to 5A.

Over voltage protection settings, according to the test voltage 1.1 times set.

For example: When the test voltage is 39kV, the display should be adjusted to 43kV.

The divider wire is connected to the high voltage current two terminal of the console.  Note: All wire must be connected, firm and reliable.

The high voltage current on the exciting transformer is connected to the high voltage current two terminal of the console.

Pressure time setting: set the test time on the time relay to the required test time.

According to the above connection that test wiring is correct, close circuit breaker of the console . At this time if the regulator is not zero, the regulator will automatically return to zero. Do not turn on the main power source, adjust the reactor core clearance, observe the lifting and clearance limit protection function to make sure whether it is normal.

Close the main power source, push the "boost" button to boost on the reactor gain hundreds of volts, by changing the air gap to tune the output voltage to reach the maximum. At this time it can be tuned, and boost to the value of the test voltage. When the voltage reaches the set value , the equipment will automatically return to zero.

Disconnect main power source.

During the test, the relevant personnel should strengthen the monitoring of the test equipment. Once the abnormal phenomenon occurs, the voltage regulator should be quickly reduced, and the power source will be cut off.

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Matters that need attention and are about the operation and off-stream of power transformer

  Transformer is outage for one month or put aside and outage for more than 6 months. 

Transformer

What test should be done before it is put into use?

1. If the transformer is outage for one month, we should use the insulation resistance tester 

DC Winding Resistance Meter

to measure insulation before resumption of power.

2. If the transformer is outage for more than 6 months, we should do insulation resistance test and insulation oil pressure test.

3. If the special transformer of drainage and irrigation which is in a dry and cold area, the outage period may be appropriately extended, but not more than eight months.

What are the requirements for transformer operation and outage?

(1) The new transformer must be tested at rated voltage for 5 times, and after the overhaul, the impulse voltage test should be carried out at least 3 times.

500kV25KJ Impulse Voltage Generator 

(2) when the transformer is put into operation, the cooler should be put into use first, and the cooler runs for a period of time (about 15min).

(3) In the 110kV and above neutral grounding system, when the transformer is put into operation and shutdown, the neutral point must be grounded first.Transformer neutral point of the arc suppression coil should be returned and then investted. The neutral point of the two transformers shall not be connected to the neutral bus of an arc suppression coil at the same time.

(4) The power operation, stop the load side of the switch, and then stop switch power supply side (side power from low to high stop); the first pull transformer side switches, pull the bus side gate. Power supply operation is opposite.

(5) Putting into standby transformer should be based on the actual location of the device and meter instructions to determine whether it is carried out the load.  the load has been carried out in order to enable the operation of the transformer power outage. The transformer on the diagonal and the 3/2 connection, although the transformer has been cut off, but the heavy gas and differential protection of the transformer can still cause the closing of the switch. It shauled be based on the actual situation and site regulations to change the location of the gas protection signal or gas out.

(6) The station transformer is not allowed long-term coordination. It can be used to cut the low pressure side of the low voltage side of the circulation with high pressure knife gate cut off the transformer station.

How to dry transformer?

Transformer is the principle of electromagnetic induction to change the AC voltage of the device. The main components are the primary coil, secondary coil and iron core (magnetic core).

The purpose of drying transformer is to remove water from the transformer insulation material and increase the insulation resistance to improve its flashover voltage. Transformers above 3kV must be dried. The transformer body is mainly composed of an iron core , a coil and an insulating material. After assembled, transformer must through the drying process to remove the water and gas of insulating materials before joining the transformer oil. It is to ensure the transformer insulation strength and service life enough. For high voltage transformer, the insulation material should be less than 0.5%. 

1. Induction heating method

The instrument itself is placed in the tank, the use of fuel tank wall eddy current loss of heat to dry. At this time the wall temperature should not exceed 115-120 degrees, the body temperature should not exceed 90-95. In order to winding the coil of convenience, as far as possible the number of turns of the coil less or less current, the wire can be  35-50mm2 wire, the general current is 150A. The oil tank wall can be filled with a plurality of asbestos bars, and the wire is wound on the asbestos strip.

2. Hot-air drying

The transformer itself is placed in a dry room through the hot air drying. The inlet hot air temperature should be increased gradually, and the filter should be installed at the inlet of hot air to prevent the entry of Mars and dust. The maximum temperature should not exceed 95. Hot air blowing device not directly, as uniform as possible from the body of the wind in all directions, so that the moisture released by the box cover hole.

Problems should be paid attention to in transformer drying:

(1) If the drying chamber is not vacuum, the vent should be opened on the lid of the box or the throttle hole can be used to make the moisture release.

(2) When the oil is heated, the insulation layer should be installed outside the tank. The insulating layer can be made of insulating material such as asbestos cloth, glass cloth and so on.

(3) In order to improve the drying quality of windings, there are two major factors to be considered: the first is to control the drying temperature; the other is to improve the vacuum of the equipment. On the first point, the general drying equipment can meet the technical requirements, to second points, subject to a number of factors, must be taken into account, reasonable arrangements in order to achieve good drying effect.

(4) The process of vacuum drying, drying in low temperature stage, not in the vacuum or low vacuum drying conditions, otherwise it is not conducive to the elimination of core temperature and moisture. When the temperature rose to 70 to 80 to improve the vacuum degree. When drying is carried out from 1 to 2h, the water vapor in the oil tank is more, the heat radiation ability is improved, the internal temperature tends to be uniform, the water is gradually reduced, and the heat radiation ability is reduced.

      (5)A method of identification of insulation after drying and transformer technical specifications for the test.

 After the transformer is dry, it is necessary to use insulation resistance tester to make a comprehensive identification of its insulation performance to check its drying effect. Identification of the project, in addition to casing, the rest are the same as the hoisting of the heart of the test items.

Shielding Design of High Voltage Test Hall

Introduction

Electric design of high voltage test hall is mainly involved in three key points: grounding, shielding and safety. The well-designed shielding system of high voltage test gall is vital to smooth operation of high voltage test hall. As for high voltage test, roles of good shielding are as follows:

(1) Ensure the reliability and precision of measuring equipment (including display and record instrument).

(2) Prevent shock wave from disturbing low-voltage control circuit or other test equipments in the impulse test.

(3) Stop shock wave from affecting outside the high voltage test room and disturbing the power supply.

(4) Restrain external electromagnetic field from disturbing PD measurement test, which leads to measurement error.

(5) Protect the health of human body within the range of electromagnetic radiation and interference.

Combined with the construction example of a high voltage test hall, its shielding design and measures are presented in this paper.

1. Project Overview

The high voltage test hall covers 700m². The single-floor high voltage test hall is divided into east area and west area. With 1620m² building area, the east test hall is 27m × 60m × 20m while west test hall is 37m × 24m × 32m covering 888m². North of east test hall is used as capacitive room and equipment shed; south of east and west test halls is three-layer control room, debugging room and office. The high voltage test hall mainly conducts the AC and DC withstand voltage, PD test, impulse voltage, insulation test and over-current test for UHVAC power transmission and distribution equipment . Most tests fall into the category of high voltage and low current (600 ~ 880kV) and a minority of tests belong to the category of high current and low voltage (2000 ~ 3000A). The frequency range that high voltage test hall needs to shield is 500kHz~100MHz. The  shielding effectiveness SE ≥ 55 dB. 

2. Issues Needed to Be Pay Attention to When Designing High Voltage Test Hall   

(1) Correctly analyze properties of electromagnetic shielding field of high voltage test to determine shielding material, thickness as well as the method to deal with pores.

(2) Pay attention to the integrity of shielding body. The overall shielding effectiveness depends on the weakest link on the shielding body. In order to make shielding effectiveness reach some value, all parts and components on the shielding body need to reach this value. Therefore, shielding effectiveness of each component should be greater than the one the design required. The match of shielding effectiveness level of each component in the shielding system is very important.    

(3) The shielding design is involved in many majors, such as architecture, water heating and electrical engineering etc. One major among these majors had better be responsible for organizing and coordinating. Generally speaking, the electrical engineering major bears the responsibility.

(4) Owning to big volume of high voltage test hall, adopting shielding material with excellent quality and reasonable price should be taken into account when pursuing good shielding effect. Besides design factor, factors which can determine shielding effectiveness also include construction factor and its constructability.  

(5) The pores on shielding body have a significant effect on the shielding effectiveness, which needs to be carefully dealt with.   

3. Analysis and Measures of Shielding Field of High Voltage Test Hall

3.1 Shielding Situation of Part of High Voltage Test Hall at Home and Abroad

High voltage test halls in overseas advanced countries mostly adopt multi-layer steel plates to shield while part of halls use aluminum plate, copper plate or expanded metal to shield. The shielding effectiveness is above 78 dB. One-layer expanded metal is mostly utilized in China and the shielding effectiveness is 40~50 dB.

3.2 Shielding Material and Structure of High Voltage Test Room

JBJ7-1996 Code for Design of Machinery Factory Building stipulates shielding material and structure of electromagnetic shielding room, which is shown in Table 1.

      

  In the project, high voltage and low current interference source is focused on electric field while high current and low voltage interference source is focused on electromagnetic field, belonging to variable electromagnetic field shielding. Hence, electric and electromagnetic field need to be taken into account at the same time. The electromagnetic wave has electric field component and magnetic field component. The high magnetic permeability is as important as high conductivity.     

The effectiveness of shielding body is measured by shielding effectiveness (SE). The definition of SE is as follows:

SE = 20 lg(E₁/E₂)dB        (1)

In the formula:

E₁ — field strength without shielding

E₂ — field strength with shielding 

Generally, the shielding effect can be classified into following categories:

0~10 dB  almost no shielding effect

10~30 dB small shielding effect

30~60 dB middle shielding effect, which can be applied in the general industry or commercial electronic equipment

60~90 dB high shielding effect, which can be used for shielding the aerospace and military equipments

＞90 dB best shielding effect, which is suitable for products with high precision and sensitivity. This project belongs to the middle shielding requirement.

The bigger the attenuation value is, the better the shielding effect is. According to the principle of electromagnetic shielding, shielding effect of the material can be expressed as:

SE = SE_R + SE_A + SE_B      (2)

In the formula:  

SE_R — single return loss of electromagnetic shielding body material (dB)

SE_R — absorption loss of electromagnetic shielding body material (dB)

SE_B — multiple return loss correction terms of electromagnetic shielding body material (dB)  

A＞10 dB, SE_B can be neglected. The formula (2) can be expressed as:

SE = SE_R + SE_A           (3)

SE_R = 168 + 10 lg (ơ_r/μ_rf)  (4)

 SE_A = 1.31 t (f ơ_rμ_r )^1/2      (5)

The plate thickness adopts the following formula:

t = SE - 168 - 10 lg (ơ_r/μ_rf)/1.31 (f μ_rơ_r )^1/2

In the formula:

t — material thickness (cm)

SE — shielding effectiveness (dB)

ơ_r — material to copper conductivity steel-0.17

μ_r — material to copper magnetic permeability  steel-200   

f — frequency  

The steel is a good conductor and the permeability magnitude is satisfactory as well. It is relatively cheap and can provide the material with strong mechanical strength. Hence, the satisfactory shielding effectiveness can be obtained via using cheap steel. The main interference source of high voltage test hall is low-frequency electromagnetic wave, which has higher magnetic-filed component than high-frequency electromagnetic wave. Thereby, as for low interference frequency, magnetic permeability of shielding material is far more important than high frequency. In the strong electromagnetic environment, the material is required to shield electric field and electromagnetic field so the ferromagnetic material with perfect structure is necessary. The shielding effectiveness is directly affected by the thickness of material and the method to lap and ground.  

The frequency that high voltage test hall needs to shield ranges from 500kHz to 100MHz. Based on the relation between wave length and frequency, corresponding wave length can be obtained. 500kHz corresponds to 600m wave length and 100MHz corresponds to 3m wave length.  

In order to prevent shock wave in the impulse test from disturbing the low voltage control circuit or other test equipments, high voltage test hall can be viewed as interference field source. Meanwhile, control room can be viewed as receiving point. The project belongs to low-frequency alternating electromagnetic field. Because electric field and magnetic field exist at the same time, shielding of electric field and magnetic field need to be taken into consideration. When the frequency is low, the electromagnetic interference mainly displays in the near-field area. In the near field, because properties of interference source are different, the size of electric field is greatly different from that of magnetic field.

When the interference generated by interference source appears in the manner of high voltage and low current and is focused on the electric field, electric field shielding method can be taken into account. The electric shielding can view electric field induction as coupling of distributed capacitance. Key points of the design are as follows: the shielding shell had better be totally-enclosed metal box and be grounded well. Bad grounding of metal shielding body will reduce shielding effectiveness. The material of shielding plate might as well use good conductor but there is no requirement for thickness. When the interference generated by interference source emerges in the manner of low voltage and high current and is focused on magnetic field, magnetic field shielding method can be taken into account. If the interference source belongs to low frequency field, shielding plate should select high magnetic conductive material and increase the thickness of shielding body. Pay attention to the structure design of shielding body and deal with pores which may increase the magnetic resistance of shielding body, thus reducing the shielding effect.

The baseboard adopts double-layer expanded metal and mesh size is 9mm × 25mm. The lower layer adopts 1.5mm expanded metal while the upper adopts 3mm expanded metal. The thickness of liner shielding plate is 0.65m and lapping＞120m. The interior adopts metal ceiling and all metal ceilings and part of metal walls adopt the micro plate.           

In order to prevent the shock wave in the impulse test from imposing an impact on outside the high voltage test room and disturbing the power source, high voltage test hall can be viewed as internal electric field shielding of interference source. However, the equipment of high voltage test hall cannot be completely independent, such as water and electricity. It is inevitable that the equipment is related to surrounding while the shock wave affects outside the high voltage test room and power source via this connection. When designing, the shielding shell of high voltage test hall had better adopt the totally-enclosed metal box and the grounding should be good. At the same time, 10kV cable and water pipe enter the lab 20m and should be grounded. Due to single pointing grounding of high voltage test hall, this connection grounding should be separated from shielding grounding.        

To prevent external electromagnetic field from disturbing PD measurement test and resulting in measurement error, interference field source can be viewed as electromagnetic wave of far-field high frequency. The key point of design is higher requirement for dealing with pores on control room shielding body.  

Take GB 7195-1988 Hygienic Standard for Environmental Electromagnetic Waves and GB 8702-1988 Regulations for Electromagnetic Radiation Protection for the design standard in order to protect the health of human body within the range of electromagnetic radiation and interference. The design point is to strengthen shielding measures at the office area, which is adjacent to high voltage test hall, and adopt metal plate to shield and ground.

4. Methods to Deal With Pores

Methods such as welding, spring leaf contact, manganese metal mesh and cut-off waveguide etc can be used to handle pores on the shielding body.

4.1 Cut-off Waveguide

The heating, ventilating, water pipe, power cable and control measurement cable on the shielding body are connected externally and the connection cannot cause electromagnetic leakage, pores have to be handled. Methods such as welding, spring leaf contact, manganese metal mesh etc can be applied to handle pores on the shielding body. Or the cut off waveguide can also be used. When applying cut-off waveguide to shielding body, here are some tips:  

(1) The waveguide must be cut off. The waveguide pipe does not play any role in attenuating the electromagnetic wave, which is above cut off frequency. The requirement for applying the above formula is to make waveguide cut-off frequency five times of highest shielding frequency. This project pick the diameter d of waveguide pipe based on f = 500MHz and λ = 60mm. As for round waveguide pipe, d (cm) ≤ λ/1.71 while d (cm) ≤ λ/2 for rectangular waveguide pipe. The attenuation S is proportional to the length of waveguide pipe L. L ≥ S_d/32 for round waveguide pipe while L ≥ S_d/27.3 for rectangular waveguide pipe. A section of honeycomb plate can be added on the waveguide pipe if the diameter of metal pipe exceeds the maximum diameter corresponding to cut off frequency.        

(2) Prohibit metal material from passing through the cut-off waveguide pipe. Otherwise, it will cause serious electromagnetic leakage.

(3) Continuous welding should be adopted between the waveguide pipe and shielding body; adopt Flange to fix the waveguide pipe on the shielding body or take gap shielding measures.  

4.2 Shielding Measures of the Gap

(1) Shielding of Construction Joints. In order to ensure the safety, lapping length between plates and internal corner as well as external corner sheet should be lengthened. The interval adopts 40 copper meshes as filling zone. Welding effect of shielding hall is the best but only riveting or screw is used to fix owing to restrictions of construction condition. When using riveting or screw to lap, starts with middle gap, and then extend to both ends in order to avoid bending of mental surface. The riveting distance should be less than 1% of highest working frequency and at least no more than 1/20λ. The result in this project is 150mm.      

(2) Shielding of Pipelines. Connect a section of non-mental insulating pipeline before ventilation and drainage pipelines lead to the shielding area. Its length is 1.5-2 times of pipeline diameter. Set copper mesh or waveguide filter in the non-mental insulation pipeline which goes through the shielding layer.   

(3) Shielding of Doors and Windows. The shielding layer of observation window must be tightly connected with shielding body. The shielding door adopts steel plate door, which should be equipped with compress equipment. The upward and both sides of the door is set with shielding bucket. The seal adopts comb grid.    

(4) Shielding of Cable and Cable Channel. Measuring cable and primary cable are applied respectively in the metal cable tank, which multi-connects to test hall shielding body. The cable channel gap in the junction between shielding area and non-shielding area uses the aluminum foil to shield.   

(5) Shielding of Luminaries. All the luminaries adopt 2mm × 2mm copper mesh to shield.  

5. Conclusions

After the project is completed, the minimum of shielding effectiveness measured on site is 58dB, which meets the design requirement. That proves that shielding design and construction of this high voltage test hall are successful.

By reviewing the whole design process, conclusions are made about this kind of design. First, correctly analyze properties of electromagnetic shielding field of high voltage test hall and then select reasonable shielding material; pay attention to the integrity of shielding body and its good grounding; shielding effectiveness of each shielding component selected should be greater than required effectiveness; pay great attention to ways to deal with pores, and strengthen the coordination of each major; take implementation feasibility into consideration (cost and construction cycle).