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 = SER + SEA + SEB (2)
In the formula:
SER — single return loss of electromagnetic shielding body material (dB)
SER — absorption loss of electromagnetic shielding body material (dB)
SEB — multiple return loss correction terms of electromagnetic shielding body material (dB)
A＞10 dB, SEB can be neglected. The formula (2) can be expressed as:
SE = SER + SEA (3)
SER = 168 + 10 lg (ơr/μrf) (4)
SEA = 1.31 t (f ơrμr )1/2 (5)
The plate thickness adopts the following formula:
t = SE - 168 - 10 lg (ơr/μrf)/1.31 (f μrơr )1/2
In the formula:
t — material thickness (cm)
SE — shielding effectiveness (dB)
ơr — material to copper conductivity steel-0.17
μr — 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 ≥ Sd/32 for round waveguide pipe while L ≥ Sd/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).
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