Efficient HV testing, research work or students training requires well-designed HV laboratories. This chapter is related to the planning of HV laboratories. The basis is a clear analysis of the requirements to the laboratory and corresponding selection of the HV test systems. This includes a general principle of the control and measuring systems, the internal data evaluation and communication structures. The planning of test buildings or test rooms depends strongly on the objective of the laboratory and of the available funds. The general principles for the grounding and shielding, for power supply, transportation and auxiliary equipment are explained. An important part of the planning is a safety system which guarantees both safety for the operators and reliable, quick testing. Some specialities to outdoor test laboratories and updating of existing test fields are submitted.
9.1 Requirements and Selection of HV Test Systems
9.1.1 Objective of a Test Field
The term “HV test laboratory” may range from a small single room with test equipment of few kilovolts rated voltage up to huge UHV laboratory complexes with several test fields of different test areas. An optimum planning of a HV laboratory well adapted to the objective of a company or institution is the basis for its later smooth operation. The users of HV test fields can be subdivided into the following groups:
manufacturer and repair shops of equipment for power systems (“equipment provider”),
companies of electric power generation, transmission and distribution (“utilities”),
research institutes and HV test service provider (“service provider”),
measurement and calibration service institutions, national labs (“calibration provider”),
universities and technical schools, education and training (“institutions”).
The performed HV tests can be subdivided as follows:
routine tests on new or repaired HV equipment (“routine tests”),
type tests on newly developed equipment (“type tests”);
tests for research and development (R&D) of new HV equipment (“development tests”),
tests for development of HV measurements and calibrations (“calibration”),
tests for practical training and demonstrations (“educational tests”).
The different kinds of HV tests are related to the different users of HV tests in Table 9.1. The darkness of a field shall indicate its importance for the user, dark blue means most important, light blue means useful, but not necessary, and white usually not necessary. It should also be mentioned that some users should be able to perform combinations of tests. Many universities with well-equipped HV laboratories perform HV research or even type testing. Last but not least, HV laboratories are very attractive and may support the image of an institution remarkably.
Table 9.1
Objectives of HV tests (explanations in the test)
The combination of education and research fits well together, whereas routine testing and research work would disturb each other. A routine test is a part of the production and has to follow the technological flux in the company. Short and smooth transportation from the previous workshop to the routine test field and from there to the next station is as important as a simple and quick test process. For an equipment provider, it might be useful to have different routine test fields related to its significantly different products, but in minimum separated between routine tests and type/development tests.
Utilities have to investigate very different, service-aged equipment and only in special cases new equipment. Therefore, a laboratory for multi-purpose application might be optimum. Service provider must also be very flexible and specialized in type tests and research/development tests.
9.1.2 Selection of Test Equipment
For the different kinds of tests, different test systems and different special accessories are necessary. Tables 9.2 and 9.3 give an overview on the required test systems and accessories.
Table 9.2
Test systems for the different types of HV tests
Table 9.3
Special accessories for the different types of HV tests
As in Table 9.1, the dark blue colour indicates necessary equipment, the light blue useful equipment. The mentioned voltage values in Table 9.2 indicate the highest rated voltages of test equipment, selected according to the procedure described in the example of Sect. 1.2 (Figs. 1.5 and 1.6). They are only necessary for UHV power equipment. In most other cases, the rated voltages of test equipment depend on the test objects of highest rated voltage (Tables 1.2 and 1.3) or the targets of the HV research.
For routine tests (Table 9.2), the HV test systems must be sufficient for the products to be tested now and within the coming 10 years. It is recommended to avoid any over-dimensioning. AC voltage of power frequency is by far the most important voltage for routine tests. It is more and more connected with PD measurement. Therefore, the test rooms should be shielded or a shielded test chamber should be applied (Table 9.3; see also Fig. 9.32). Also a metal-enclosed test system (Fig. 3.47) is useful for metal-enclosed power equipment (GIS). If the AC voltage test system is metal-enclosed, the whole circuit is well shielded and does not require a shielded test room. It can be arranged in the workshop itself. Impulse voltage testing is required for routine tests of only few equipment, e.g. for power transformers. Then, related LI/SI test systems belong to the scope of supply. Routine tests with DC voltages are very seldom at the moment, but will become more and more necessary with the broader HVDC application.
For type and development tests and for HV research work, all types of test voltages and most of the special accessories are required (Tables 9.1, 9.2 and 9.3). Their rated values must be carefully selected considering the possible development within the next 2 decades.
Voltage calibrations (Table 9.2) can be performed at reduced voltages of >20% of the rated voltage of the measuring systems to be calibrated. Therefore, the rated voltages for calibration are about 20% of the rated voltages for type testing (Table 9.2) and small test systems are sufficient (Fig. 9.1, courtesy of TÜBITAK, Gebze, Turkey). Calibration requires always low electromagnetic noise; therefore, test rooms for calibration should be shielded.
Fig. 9.1
Small HV test systems in a calibration laboratory
Educational tests (Table 9.2) can be performed at high voltages up to some hundred Kilovolts (Fig. 9.2, courtesy FH Mittweida, Germany). Separate small rooms or areas for each HV test are recommended to enable several training groups to work in parallel without disturbing each other (Prinz 1965; Mosch et al. 1974; Hauschild and Fahd 1978; Kind and Feser 1999; Schwarz et al. 1999).
Fig. 9.2
Test areas for students training
Note For the selection of the rated data of test systems, special accessories and test areas, it is recommended to establish a list of power equipment to be tested with their rated electrical parameters, their test voltage values, test conditions, dimensions and weight. Also the standards which shall be applied should be summarized for the design of the laboratory as described in the following.
9.1.3 Clearances and Test Area
The
size
of a test room can be defined after the necessary HV test systems have been selected and the size of the largest test objects has been estimated. The necessary clearances between test objects and any grounded or energized structure have been discussed in Sect. 2.1.2 and are given in Fig. 2.1. This clearance is necessary to avoid any influence on the voltage distribution at the test object. In opposite, the distance in air between a HV test system and its surroundings must be selected in such a way that it withstands not less than about 120% of the rated voltage Vr of the test system.
In the test voltage range up to 600 kV (peak), the simple calculation of the air distance d can be performed based on the voltage demand 5 kV/cm of the positive streamer discharge in air (Fig. 9.3, dotted, blue curve):
Fig. 9.3
Withstand voltage of a positive rod–plane gap in air
Example A 400 kV AC test circuit shall be arranged. Which distance d is required to guarantee withstand between its components and the grounded metallic fences and walls? The rated voltage is a rms value; therefore, the stressing peak value is given by
and the distance follows to
It is decided that the distance to the metallic fence shall be 140 cm.
For higher voltages, the increase of the breakdown voltage (V50) distance characteristic becomes remarkably lower due to the leader discharge (Fig. 9.3; Carrara and Zafanella 1968). According to the withstand voltage (V01, green curve) of the positive rod-to-plane arrangement a clearance of about 16 m to other objects would be required for a 2MV SI or AC (peak) test voltage. Because of the high cost of the space in a HV laboratory, a much shorter distance must be reached by the application of suited shielding and control electrodes. Therefore, the impression of a UHV laboratory is determined by huge electrodes (Fig. 9.4).
Fig. 9.4
Toroid electrodes in the UHV test laboratory of HSP Cologne (Germany)
There are many publications related to the design of HV electrodes, e.g. Moeller et al. (1972), Feser (1975), Mosch et al. (1979), Lemke et al. (1983) and Hauschild (1995), and electric fields can precisely be calculated now. The big problem remains to select the acceptable critical field strength. For PD-free electrodes, the concept of the streamer inception (Bürger 1976; Engelmann 1981; Dietrich 1982) can be used; otherwise, also the streamer–leader transition is applicable. For large electrodes, the streamer inception leads via a heavy streamer discharge which is immediately transferred into a leader discharge. The size of the electrode influences—in addition to the field strength distribution—also the probability of surface defects. This causes a distribution function of the inception field strength with a low increase for low probabilities and a steep one for high probabilities (Fig. 9.5). The dimensioning of electrodes is based on the low probabilities under consideration of the enlargement rule (See Subclause 2.4.7, Hauschild and Mosch 1992; Hauschild 1995). Usually, the dimensions of the electrodes are determined in an iterative process of field calculation and application of the streamer or leader inception criteria as described in the literature. Considering the enlargement of the electrode area and the prolongation of stressing time, the acceptable maximum field strength for a large electrode can be assumed to be in a range of 10–15 kV/cm for AC and DC peak voltage and of 15–20 kV/cm for SI voltage.
Fig. 9.5
Performance function of the streamer inception field strength in air (area element of 100 cm2)
For test voltages below 2000 kV (peak), smooth electrodes like spheres and especially toroids are available. The double toroid (Fig. 9.4) is an ideal shielding element, because the necessary connections can be performed in the field shadow of the two rings.
For higher test voltage, composite electrodes are applied. Huge toroids can be realized by cylinder segment electrodes (Fig. 3.15). These are cylinder elements welded together to a toroid (Fig. 9.6). If the electrode is correctly designed, the higher field strength at the welded joint is related to a small area and the danger of a discharge would be not higher than at the larger area of the cylinders of lower field strength.
Fig. 9.6
Cylinder segment electrode for a 2.25-MV AC voltage cascade transformer
So-called polycon electrodes are sometimes huge spheres consisting of many metal plates fixed on a spherical scaffold (Fig. 9.7). The design of the polycon electrodes is well developed (Singer 1972; Hauschild et al. 1987; Schufft 1991).
Fig. 9.7
Electrodes for UHV AC voltage components. a Different polycon electrodes and full-sheet sphere, respectively, toroid electrodes (EHV Laboratory of Dresden Technical University). b Framework of a polycon electrode (diameter 3 m)
9.1.4 Control, Measurement and Communication
The controls in a HV laboratory should not only be related each to a single HV test system (Fig. 2.8), but also to their interaction among each other and with auxiliary equipment. It is recommended that all control and measuring devices are equipped with industrial personal computers (IPC) which are connected to a common bus system. This can even be made in such a way that a certain HV circuit can be controlled from any of the IPCs in the control room (Fig. 9.8). Also external test data, e.g. the atmospheric conditions, data of wet or pollution tests, etc., and the correct function of the safety system shall be recorded. The test engineer should use the computer-aided evaluation of the test and the preparation of the test record.
Fig. 9.8
Control room of a transformer test field (Courtesy Siemens AG, TBD Dresden). a Principle circuit diagram. b Control desk and rack
The IPC control and measuring system enables automatic operation of the HV test system, which guarantees a better reproducibility of the test parameters, the direct recording and the evaluation of the measured data. Therefore, it may shorten the test duration and safe manpower. This is especially important for the testing of mass products (possibly in combination with other non-high-voltage tests), for lifetime testing and for large-scale tests for research and development. A fully automatic test procedure cannot be recommended for HV tests on valuable, single equipment as power transformers or GIS bays. In this case, a computer-aided procedure with decisions from the operator is optimum.
The computer system of the laboratory can be connected with the local area network (LAN) of the company or institution. Most important is the connection between the HV test field and test fields for other tests and measurements. This enables a common test record for the product. Furthermore, the status of the actual test can also be observed from any other place, e.g. a customer room during an acceptance test or a student’s gallery in a university during HV demonstrations. A connection to the Internet enables remote service from the supplier (see Sect. 2.2).
9.2 HV Test Building Design
There are many publications on the design of HV test laboratories, and their study is always useful for HV laboratory planning. It is impossible to mention here all of them, but the following section considers—in addition to the experience of the authors—especially the books of Prinz et al. (1965), Hylten-Cavallius (1988) and Schwarz et al. (1999) as well as publications by Läpple (1966), Mosch et al. (1974), Hauschild and Fahd (1978), Krump and Haumann (2011) and Hopke and Schmidt (2011). Each HV laboratory must be designed according to the special demand of the later user. This section will give suggestion for details to be considered during the planning of new or the refurbishment of existing HV laboratories.
9.2.1 Required Rooms and Principle Design
Minor details in a HV laboratory make the work of the test engineers efficient and easy. Good planning is therefore the necessary precondition for later smooth operation of the laboratory. Therefore, the application determines the basic design of the HV laboratory.
For routine testing, the HV test field may be a single test room or only a test area at the end of the production area. Then, the selected HV test systems and the dimensions of the test objects determine the size of the room. As an example, Fig. 9.9 shows such a quite compact, well-shielded test field for routine and type tests on power transformers up to 245 kV. It includes the facilities for induced and applied AC voltage tests and for LI/LIC/SI voltage tests in a room of only L × W × H = 18.3 m × 13.3 m × 12.3 m (Hopke and Schmidt 2011). Such a test field must be well adapted not only to the requirements of the HV tests, but also to the demand of the production and the flow of products. This includes the kind of transportation of the test objects (e.g. air cushion, rails, crane), the necessary space and clearances for the HV tests as well as the necessary conditions of measurement (e.g. of PD) in an industrial environment. The test room must be completed by a shielded control room and a power supply room.
Fig. 9.9
Test field for routine and type tests on power transformers.
Courtesy Siemens AG, TBD Dresden
A universal HV laboratory for research, development and training requires many test and auxiliary rooms. The size of the largest laboratory room is determined by the highest required test voltages. It is useful to have a second universal test room for equipment of clearly lower rated voltage. For training, smaller test rooms are recommended with one or two test areas each. Furthermore, special laboratories for cable testing, pollution testing, calibrations as well as for oil and solid insulation are often required. The selection of rooms shall be explained based on the following example:
Example (Fig. 9.10) A universal EHV laboratory shall be erected as the National HV Laboratory of a country. It has been decided to erect the laboratory near the campus of an university and to use it also for student’s training. The largest equipment to be tested is for a rated voltage of 550 kV. Therefore, the highest test voltages are 1550 kV LI, 1705 kV LIC, 1175 kV SI (Table 1.2) and 680 kV AC (IEC 60076-3:2012). According to the principles described in Sect. 1.3, the following rated values of the largest HV test systems are selected: 3000 kV impulse voltage (LI; LIC: SI) with a highest SI output voltage of 1800 and 1000 kV (AC). Additionally, it is decided to have a DC voltage test system of 1000 kV (extendable up to 2000 kV) to meet future requirements on DC testing. After consideration of the necessary clearances and the necessary space for the test objects, an EHV laboratory of a length of 40 m, a width of 30 m and a height of 25 m was selected (Fig. 9.10). The test systems are arranged in three corners of the EHV hall. The control room (yellow) is in the fourth corner.
Fig. 9.10
Planning of the arrangement of rooms of an universal HV laboratory
The HV laboratory shall be used for testing power equipment of rated voltage up to 145 kV. It is therefore equipped with an 800-kV impulse test system and a 400-kV AC test system. The related control room (Fig. 9.10) is in a corner. The three MV labs are planned for students training. Each of them shall be equipped with two HV test bays for voltages in the range up to 100 kV.
The laboratory is completed by a number of special test rooms. There is a special HV laboratory for cable testing with a big door to outside for the cable drums. As the other two laboratories, this laboratory including its control room is well shielded. The cable test laboratory is completed by an open-air field for pre-qualification tests of cables. The related control room with a window to outside is in the mezzanine. The area in front of the EHV hall is kept empty for a later outdoor EHV or UHV test field. Also a related control room in the mezzanine is planned for the future. A pollution test laboratory can be connected via a bushing to the AC test system of the big hall (Alternatively, the room is big enough for its own powerful test transformer. Also an air-conditioned calibration laboratory is part of the planning (left).
There are many auxiliary rooms required, such as the powersupply room(s)
, stores, workshops, seminar and meeting rooms, offices for the director and his staff. Additional functional rooms as IT server room, kitchens, washrooms, etc., should be planned. The arrangement of these rooms shall be explained with the continuation of the above example:
Usually, a HV laboratory cannot have too many store rooms. Possibly, the one between the EHV and the HV laboratory is not sufficient. In opposite to that, there is a mechanical workshop in the ground floor and an electronic workshop in the mezzanine. The area under the calibration laboratory can be used as a meeting room. In the upper floor (Fig. 9.10), a seminar room enables the teaching of up to 60 students. The teaching room is connected with a visitors’ gallery which enables the observation of experiments in the EHV hall (similar to Fig. 9.11). Additionally, the upper floor is used for all offices, for functional rooms and for small laboratories, for example for mechanical or chemical investigation of solid insulating materials and insulating oil.
Fig. 9.11
Building and students gallery at the HV laboratory of Damascus University
Rectangular test rooms can be considered as optimum (Figs. 9.11, 9.12 and 9.13). They can be well subdivided into two separate test areas by safety fences for parallel testing. The relation between width W and length L depends on the necessary test voltages. If only AC and impulse voltages are required, the relation W/L ≈ 0.5 … 0.6 seems to be optimum. In case of the three voltage sources (AC; DC and impulse), the relation W/L ≈ 0.7 … 0.8 is better adapted. Large test objects are arranged in between the test systems. Today, the HV generators can be placed on air cushions (see Sect. 9.2.5.3) and moved to the optimum place for testing or to a corner when not used. Smaller components of the HV test circuit can be equipped with wheels and also be placed at optimum positions for testing or stored on an empty area.
Fig. 9.12
EHV laboratory of Dresden Technical University (building erected in 1930)
Fig. 9.13
EHV laboratory of Cottbus Technical University (established in 1998)
Usually, the largest clearances to walls and ceiling are required at the top of the test systems. Therefore, a limitation of the clearance which is sometimes applied by a parabolic cross section (Fig. 9.14a) cannot be recommended, but a rectangular one (Fig. 9.14b). The lower demand of clearance near its floor can be used for additional small built-in test areas of lower test voltages or for stores within a rectangular structure.
Fig. 9.14
Cross sections of HV laboratories
HV laboratories are impressive technical rooms. Therefore, an advanced planning should not only consider the technical aspects, but also a certain aesthetic planning. The three-dimensional design offers best opportunities for the aesthetic planning (Fig. 9.15, which is related to Fig. 9.4). The balanced planning of the colours between HV test systems, wall, ceiling and floor is highly recommended. The control room is often the place where also visitors appear who conclude from their impression to the quality of the HV testing. Therefore, the control desk (or table or rack) should be clearly arranged, of uniform design (even for test systems of different suppliers) and well placed in the room. The height of a desk should correspond to the lower frame of the window (Fig. 9.8). There should be additional space in the desk for later extensions by different instruments, to avoid the poor impression of additional stand-alone device on a desk with built-in instruments.
Fig. 9.15
Three-dimensional planning of a HV laboratory
9.2.2 Grounding and Shielding
Building grounding
, test field grounding, electromagnetic shielding and earth return of a test circuit are often mixed up, not only theoretically, even worse in real test field operation. First, the different terms shall be defined by their descriptions:
Building grounding: Each HV laboratory building must be grounded, mainly for its lightning protection. The building grounding consists of the building’s steel reinforcements, the foundation earth electrodes and possibly additional earthing rods. In a complex of buildings, the different building groundings are connected. The building grounding takes no care for external noise reduction.
Test field grounding: The upper layers of the ground are influenced by the earth return of power-electronic-driven machines and equipment in a factory. In brief, they are not free from noise signals. Therefore, a conductive contact of the test field grounding with the upper layers of soil and with the building grounding should be avoided. The test field grounding shall be realized by earthing rods which dip some metres into the ground water and which are electrically insulated along their first metres. The effective ground resistance of the parallel connection of all earthing rods should not exceed 2Ω.
Electromagnetic shielding: An electromagnetic field caused by noise signals may disturb measurements in a HV circuit (Chap. 4). According to the famous observation of Faraday, no electric field exists inside a closed metallic container. Therefore, HV laboratories are shielded against the penetration of electromagnetic fields by a closed metallic structure which is separated from both the soil and the building grounding and only at one point connected to the test field grounding.
Earth return: Each HV test circuit must be connected to the test field grounding at one point near to the voltage divider or the test voltage generator. It should have its own earth return of lowest possible inductance (see Sect. 7.1.2.3). Only in special cases, it can be recommended to use the test field grounding as the earth return.
Building grounding, test field grounding, electromagnetic shielding and the earth return(s) of the single test systems must be connected only at one point to guarantee a common stationary ground potential and to avoid any grounding loop which may act as an antenna for noise signals. For power cables from the regulators to the test room, a cable trench along the walls with metallic frames and covers is necessary. Frames and covers must be connected to the shielding of the floor. The connection of the floor shielding to that of the walls has to be made via the cable trench.
When a HV laboratory is erected, the mechanical design of the floor is related to the required maximum load by test generators and test objects. Also the kind of transportation, e.g. by air cushions, must be taken into consideration. The steel reinforcement of the concrete forms one part of the shielding of the floor. It must be isolated from the soil by a suited plastic foil, the single steel rods must be welded together, and a wide mesh of metal band, also welded to the reinforcement steel, completes the shielding of the floor (Fig. 9.16a). The basic concrete is usually finished up by a layer of final concrete which might contain a special fibre-glass reinforcement. The final concrete requires a very fine surface or an epoxy coating for air cushion transportation (see Sect. 9.2.5.3).
Fig. 9.16
Shielding of the floor of a HV laboratory. a Preparation of the shielding of the floor. b Earthing and connection boxes
The surface of the floor is interrupted by earthing boxes and connection boxes (Fig. 9.16b). The metallic earthing boxes are connected to the floor shielding. The internal part is isolated from that and connected to the earthing rods and/or their connections by isolated copper bars. Furthermore, inside the steel reinforcement of the floor, metallic tubes between the HV test systems and the control room are provided for control and measuring cables. They are accessible via the connection boxes. Even measuring instruments can be arranged in a connection box, for example the PD measuring impedance and the PD measuring instrument in a box near to the coupling capacitor (Fig. 9.17).
Fig. 9.17
Connection box with a measuring instrument.
Courtesy of HSP Cologne
The shielding of the walls shall enable multi-functions: In addition to the electromagnetic shielding, it shall be a heat isolation and a sound absorber. A perfect shielding of >100 dB up to 100 MHz—as usual for EMC shielding of computer centres and EMC test areas—is not necessary for a HV test room. The PD measurement according to IEC 60270 is usually performed at frequencies up to 1 MHz, and therefore, a damping ≤100 dB up to 5 MHz is sufficient for most laboratories. This can be reached with standard steel panels of two layers of galvanized steel (Fig. 9.18). The lower layer (Fig. 9.18a) carries the heat isolation (black, e.g. rock wool) which is at the same time the sound absorber. The upper layer is made of perforated steel panels and fixed to the lower panels (Fig. 9.18b, c). The panels of each layer among one another as well as of the two layers with each other should overlap and carefully screwed together for a reliable electric contact. Instead of screwing, welding of points every 50–100 cm is even more reliable.
Fig. 9.18
Shielding of the walls (Courtesy of HSP Cologne). a Lower layer carrying the heat isolation (black). b Upper layer of perforated steel panels. c Check of electrical contact
The shielding of the ceiling (Fig. 9.19) may follow the same principle as that of the walls. But the ceiling usually contains panels of the heating system, the lighting system and the air conditioning (ventilation and aeration). The heating panels are also of steel and can be used as a part of the shielding. The shielding panels may be screwed or welded to the heating panels. For the lamps and the air conditioning, openings in the ceiling are necessary which must be covered by a wire netting of steel wires with welded crossing points (width of meshes <30 mm). Such a wire netting is also recommended for windows, for example of the control room or a visitor’s gallery.
Fig. 9.19
Shielding of the ceiling including heating panels, lighting and air conditioning.
Courtesy of Siemens AG, TBD Dresden
It can be recommended to design a shielding also for the control room (Fig. 9.8b). In that case, the shielding can be made of expanded steel under the plaster of walls and ceiling. The single sheets of expanded steel must overlap and be welded together. For windows and doors, the sheets should be welded to their metallic frames. The openings of the windows should be covered with glass and the mentioned wire netting which has to be welded to its frame. It is useful when also the power supply room is shielded (for details see 9.2.3).
The most sensitive and expensive parts of a shielding are the shielded doors. For very sensitive PD measurements, for example of extruded cables down to very few picocoulombs, special doors with metallic feeder contacts which are pressed to the frame of the door are available. In many cases, such doors are not required. Then, sliding doors of steel or roller shutter doors with metallic lamellae—both moving in overlapping frames—are economic alternatives.
It should not be forgotten that all supplies (water, compressed air, oil, etc.) entering into the test field and carrying the building ground must be isolated and inside connected to the shielding of the test room. Otherwise, they may carry noise signals into the shielded test area. Electric supplies must be filtered.
9.2.3 Power Supply and High-Frequency Filtering
The power for a complete HV laboratory is usually supplied from a medium-voltage network (Fig. 9.20). One or several three-phase distribution transformers in a nearby substation should be used for that purpose. Their ratings depend on the equipment to be supplied. One has to take into consideration the maximum required test power (active and reactive components), the supply power for control and measuring systems, the power for all auxiliary equipment (see Sect. 9.2.4) and an extra charge for later extensions.
Fig. 9.20
Schematic power supply of a HV test laboratory
Power cables connect this external power supply with the power supply room. This room should be arranged close to the HV test hall (e.g. Figure 9.10) and should also be shielded in connection with the shielding of the related test room (Figs. 9.20 and 9.21). The connection of the external power cables to the equipment for distribution and regulation in the power supply room is realized via high-frequency (HF) power filters attached to the shielding of this room (Fig. 9.22). The power control room contains the equipment explained in Sect. 2.2 (Fig. 2.5) for the “power supply”:
Fig. 9.21
Shielded power supply room.
Courtesy HSP Cologne
Fig. 9.22
HF filter for the power supply. a Principle circuit of low- and medium-voltage filters. b Arrangement of HF filters at the wall of an inside shielded power supply room.
Courtesy HSP Cologne
switching cubicles with main and operation switches, measuring equipment for primary voltages and currents, protection devices and control components (Fig. 2.8) like programmable logic controllers (PLC);
regulators, one for each HV test system and
compensation reactors and/or capacitor banks, to reduce the demand of external supply power.
The size of this power supply room depends on the mentioned single components and their arrangement. The HF-filter consists mainly of a L-C network (Fig. 9.22a). Conducted noise signals of frequencies remarkably higher than the power frequency 50/60 Hz are blocked by the inductances and conducted to ground via the capacitances. The frequency characteristic of the HF filters shall be adapted to that of the shielding.
Note If the power supply room is not shielded, the HF filters must be attached between the regulators and the test generator at the shielding of the test room. Then, a separate filter is required for each test system and each of the auxiliary equipment. For the AC test system, even a larger filter is necessary because the compensation reactors are in the—unshielded—power supply room. The shielding of the power supply room simplifies the filtering and the wiring.
Communication lines—as long as they are not realized by fibre optic links—must also be filtered. As HF filters for power connection, also such for communication lines are available on the market.
9.2.4 Auxiliary Equipment for HV Testing
If accessories—as mentioned in Table 9.3—are used, consequences for the building design must be drawn. It might be necessary to have supply tubes for water, compressed air, etc. into the shielded area [test room(s), control room(s), power supply room(s)] from a machinery room or the unshielded outer areas; these connections must be isolated. This means that all metallic tubes must be interrupted by suited insulating tubes where they enter the shielding.
The size of the artificial rain equipment (see Sect. 2.1.3) must be adapted to the size of the largest HV equipment to be tested. This determines also the demand of water of the required conductivity, the water processing tank and the water connection from there to the rain equipment. Furthermore, the floor in the area of wet tests must be equipped with water drainage. This means it should have a certain slope which may come in conflict with the air cushion transportation system. At a too high slope, the air cushions with their load may run away by themselves. Therefore, no air cushion transportation in that area should be allowed or the slope must be very low. A slope below 1 cm/m might be necessary, but it must follow the instructions of the supplier of the air cushions.
Thepollution chamber must be equipped with a spray system, connections to water and compressed air, as well as drainage in the floor. The bushing for the pollution chamber must withstand the worst pollution conditions inside the chamber and therefore be well selected. Because the conductivity of the pollutant is controlled by salt and water, all materials used in the pollution chamber must be resistant to corrosion.
Using of a trench in a HV laboratory: Tanks for insulating liquids, usually for mineral oils, are necessary for testing transformer bushings and for R&D experiments on liquid-impregnated insulation structures. They are connected to oil (or liquid) processing units with related reservoirs outside the shielded area. It can be recommended to arrange the test tanks under the floor in a trench. Also other HV components can be arranged in a trench, especially when they include grounded tanks. This may even include test transformers or standard capacitors (Fig. 9.23). Figure 9.23a shows such an arrangement of a multifunctional capacitor which may even act as a central electrode for the HV laboratory. The trench must be carefully planned together with the floor shielding and the grounding system (see Sect. 9.2.2). All walls and the floor of the trench should be shielded. The tank should be isolated from the shielding of the floor. During a test, the tank is part of the test circuit and must be included into the ground return of the test circuit. If it is not used, it should be directly connected to the earthing system, but never to the shielding system. The trench must be equipped with well-fitted covers which close the trench not only mechanically, but also close the shielding.
Fig. 9.23
Use of a trench in a HV laboratory. a Arrangement of a multifunctional capacitor. b Tank for testing bushings.
Courtesy HSP Cologne
Tanks for compressed gases are used for testing bushings from air to SF6 and for R&D experiments with gas-insulated structures. They are connected to gas processing units for conditioning the used gases. The arrangement of gas-filled test tanks is identical to those of liquids (Fig. 9.23).
Climatic chambers should be adapted to the principles described here for HV test circuits, grounding and shielding. The design of a commercially available chamber determines the necessary measures.
9.2.5 Auxiliary Equipment and Transportation Facilities
9.2.5.1 Lighting
Usually, HV test laboratories are designed without windows for natural light. Natural light might be useful in very special cases, e.g. when the room is also used for production or larger assembling work. Then, the windows must be supplied with a metallic frame connected to the shielding and covered by the above mentioned wire nettings with welded crossing points, also welded to the frames. Darkening of the windows should be possible.
Artificial lighting—the standard case—should be realized with incandescent lamps in all shielded rooms—and not with fluorescent lamps which may cause noise signals during sensitive PD measurement. For many observations and photographs, it would be helpful if the light can be dimmed. Because this is realized with a power electronic adjustment, another source of noise signals, the lamps should be outside the shielding behind wire nettings (Fig. 9.19). Preferably, the lamps can be arranged at the ceiling. A separate emergency lighting is necessary. The local requirements and safety rules have to be considered.
9.2.5.2 Heating, Ventilation and Air Conditioning
HV test equipment should be specified for a temperature range not smaller than between 10 and 35 °C; for the control and measuring systems, a minimum range between 15 and 30 °C is required. The installation of a comfortable heating system in a large HV test hall seems to be only necessary when the minimum temperature of 10 °C cannot be guaranteed for a longer period than few days per year. In opposite to that, it is recommended to have an air-conditioned control room with a temperature in the range of 20 °C. The power supply room requires no heating as long as the temperature remains above 5 °C. Small HV test rooms where HV test equipment and the control and measuring system are in one common room should be heated for an acceptable room temperature.
The experience of the last years has shown that an optimum heating system for a big HV test room is a radiant ceiling heating system which is used as a part of the shielding as described above (Sect. 9.2.2 and Fig. 9.19). Such radiant heating systems of quite large panels are usual for industrial buildings today.
For countries with tropic conditions or very high summer temperatures, a ventilation system for the test hall which uses low night temperatures for cooling the HV test hall may be sufficient. Smaller test rooms should be air-conditioned. When during a test, a larger amount of heat is generated, for example during heat run tests of transformers, a well-designed ventilation system has to transfer the heat to the environment (Fig. 9.19). This avoids an unacceptable temperature rise in the test room. The necessary ventilators are arranged behind openings (covered with wire nettings) of the shielding of the ceiling.
9.2.5.3 Transportation Facilities
One or two portal cranes covering the whole area of a large HV test hall guarantee its efficient use (Fig. 9.24a). This may come into conflict with HV components which could be fixed at the ceiling, for example voltage dividers, central electrodes or rectifiers. In some laboratories, this had been done; consequently a single-rail crane had to be applied (Fig. 9.24b; Prinz et al. 1965, courtesy of TU Munich). The comparison of the two solutions shows clearly that in minimum for industrial HV test laboratories, the portal crane is by far superior to the one-rail crane. With two or three portal cranes in one HV test hall, a compromise between the two solutions is reached. Figure 9.25 (Krump and Haumann 2011) shows a test room with three cranes; the crane in the middle carries the large, PD-free connection electrode (1200 kV rms.), between the central connection point and the test object, whereas the cranes of both sides are available for transportation and fixing of test objects. In addition to the portal crane, lifts with a single rope from a hole in the ceiling can be taken into consideration. They are for fixing a test object only during a test. Such lifts and hand-operated cranes are also useful in small test rooms.
Fig. 9.24
Cranes in HV laboratories. a Portal crane (Courtesy HSP Cologne). b Single-rail crane
Fig. 9.25
UHV laboratory with three cranes.
Courtesy of HSP Cologne
The maximum load of a crane depends on the maximum load of a test object, in some cases also of the weight of the heaviest HV test component, whatever is heavier. For the small hand-operated cranes and lifts, a load up to 1000 kg should be sufficient. It should be mentioned that available transportation by air cushions can reduce the maximum necessary load of a crane.
Air cushions are a very helpful tool for the ground transportation in a HV laboratory. On the one hand, they can be used for the generators and HV components (see Sect. 9.2.1), on the other hand for the transportation of heavy test objects as, e.g. power transformers or GIS. The air cushions replace the transportation on rails as it has been applied for generators or test objects in the past quite often. The rails interrupt the smooth floor and may even influence the optimum erection of test circuits. The ground transportation of smaller HV components can be made on wheels. Also transportation by the crane is often applied.
Well-selected air cushions, usually arranged under the base frame of the generator, can carry up to the highest loads required for HV test systems. Number and size of the single-air-cushion modules depend on the load and the load distribution. But they require a well-levelled, smooth and stable floor. The surface can be a fine concrete, in most cases covered with a special epoxy resin. Air cushions can damage the floor if it shows gaps and cracks or is not smooth enough. Usually, the air pressure is 0.2 MPa, in special cases up to 0.4 MPa. It is recommended to order HV test equipment with frames and with the necessary stability suitable for air cushion transportation (Fig. 9.26). When the movement of air cushions suddenly stops due to an obstacle, the mechanic stress to the insulating supports, for example of an impulse voltage generator, is considerable.
Fig. 9.26
Frame of a test generator with air cushions
9.2.5.4 Technical Media
The electric power supply is described in Sect. 9.2.3. In the single test and control rooms, a sufficient number of LV sockets should be arranged. This includes the sockets in the control desk or rack for instruments. Everywhere, sockets are necessary for electric tools, spot lights or additional instruments. Again, all electric power lines which enter shielded rooms must be filtered.
Data communication and telephone lines should be as far as possible realized by fibre optic links. They do not require filtering. All wire lines for communication must be filtered.
Compressed air is necessary for the air cushions (see Sect. 9.2.5.3), but also certain tools as well as pollution tests require compressed air. The necessary pressure depends on the local demand. The metallic tubes for compressed air must be isolated when they enter the shielded rooms.
A water supply is necessary for artificial rain and pollution equipment (see Sect. 7.2.4), for other special tests and for cleaning. Therefore, few water taps might be arranged in a big and only one in smaller test rooms.
Other technical media as oil and other liquids, SF6 and other gases should be supplied via installed facilities if there is a permanent demand (see Sect. 9.2.4).
9.2.5.5 Fire and Environmental Protection
The fire and environmental protection in a HV laboratory has to follow both, the national laws and the technical knowledge on HV testing. Special consideration requires insulating oil and other inflammable liquids. This includes also oil-filled HV test components and equipment of power supply, as e.g. the mayority of test transformers, reactors, regulators, etc. One should consider that a leak of a related tank should not pollute the nature, the soil or the water. But a leak in the tank of a modern HV test component is very seldom and if it occurs it would remain very small. Consequently a volume which may overtake a certain part of the oil after a small leak should be taken into consideration. In some cases even cable ducts are used for that purpose. The application of the hard rules for a collector for all the oil of the related power transformer cannot be recommended. A power transformer in a power system—which may explode and dramatically burn due to heavy internal failures caused by over voltages. In opposite to that, test transformers and other test equipment are usually stressed remarkably below their rated voltage. The HV tests are connected with sensitive PD measurements, which show not only the defects of the test object, but also of all components of the test circuit. This means, a fault inside oil-filled equipment is very early indicated, the related component has to be taken out of operation. There is practically no danger of a powerful fault in an oil-filled test transformer or reactor, with dramatic tearing up the tank followed by fire. Nevertheless, a realistic concept for fire and environmental protection must be established during the planning of a HV laboratory and approved on the basis of the technical knowledge and the relevant local rules.
For tests of insulations with SF6 gas, one has to consider that SF6 is a green-house gas and a totally proof gas handling system must be planned. The GIS-enclosure shall fulfill the technical requirements of high-pressure containers.
9.2.6 Safety Measures
High electric fields, generated by high voltages between energized electrodes and all earthed objects in the HV test rooms, are very dangerous: When the electric field exceeds the dielectric strength of the surrounding air, electrical discharges appear with currents up to kiloamperes. But the rate of accidents in HV laboratories is low, because the operators are conscious of the high risk when the “safety concept” is not carefully considered. The safety concept includes all technical matters related to the test laboratory as a whole and related to the single HV test systems as well as the instructions for the personnel. The latter includes the general behaviour in HV testing and the instructions for the operation of test equipment.
There is no special IEC Document on safety in HV test laboratories; the IEC Publication 62061:2005 is partly applicable to the control- and power-feeding equipment. IEEE Guide 510 recommends a practice for safety in high-voltage and high-power testing. There is also the European Standard EN 50191 (2000) on the erection and operation of electrical test systems. The hints in this subsection cannot release the users from applying these internationally accepted documents, standards and special national rules. Also the instructions and hints of the suppliers of HV test systems shall be considered.
9.2.6.1 Safety in HV Test Fields and Areas
A HV test field is a room with one or several fenced-in HV test areas and related control areas, completed by a power supply area. A HV test laboratory may contain several HV test fields (Fig. 9.10). The basis for the safety of a whole test field is its correct grounding and shielding as described in Sect. 9.2.2. The recommended current return of a test circuit is only connected at one point to the grounding system. In special cases, the grounding system may be used as the current return. The shielding shall never be used as a current return.
In a HV test area, the safe clearances (see Sect. 9.1.3) of HV components to the walls, the fences or the ceiling as well as to earthed or energized objects in neighbouring fenced-in test areas shall be selected according to the maximum test voltages which can be generated there. If several fenced-in test areas are in one test field, metallic safety fences with a minimum height of 2 m and a maximum mesh size of 4 cm shall be applied. The single elements of the fence shall be electrically connected to each other and connected to the earthing system only at one point. A complete loop of fences shall be avoided. If the HV components in a test area are rearranged, interactions to neighbouring fenced-in areas must be considered (Fig. 9.27).
Fig. 9.27
Safety fences with warning signs and lamps, safety loop and emergency-off switch
The HV test area must be marked from outside by a warning sign and by warning lamps indicating the condition within the area: The “red” light indicates the HV test circuit is degrounded and possibly energized. The “green” light means the HV test circuit is grounded, the area is safe for entering (see Sect. 7.2.6.3). Warning signs must be arranged at all doors into the test area. In addition to the signs and lamps, the test area must be equipped with a safety loop and emergency-off switches.
A safety loop is a ring main integrated into the walls, fences and doors of a test area. The loop must be closed, before the test system is degrounded and energized. Contacts at the door shall ensure that the loop is closed. As soon as the loop is opened anywhere, the emergency-off switch has to operate and the test system shall be grounded (see Sect. 9.2.6.2). Emergency-off switches shall be large, red pushbuttons mounted preferably on a yellow background at the related control desk or rack, near the doors of the test areas and in observation areas. The actuation of an emergency-off switch results in the operation of the power (main) switch and operation switch as well as of the earthing equipment of the relevant HV test system. It has to be checked whether also all HV sources of the whole test field shall be switched off. It should not deenergize the lighting system (in minimum an emergency lighting must remain).
Control and observation areas are outside the test area and its safety loop. Both shall be equipped with warning signs and lamps and emergency-off switches. For the power supply room, the relevant national regulations for the erection of electrical substations shall be applied.
9.2.6.2 Safety of HV Test Systems
There is a close connection between the safety system of a HV test area and that of the related HV test system. Both work together, and if the first fails, the second cannot operate. Therefore, a safety check of the related safety equipment should precede each test.
A HV test generator is energized in two steps: first, the power (main) switch supplying the energy from the grid up to the power supply unit, and then, the operation switch connects the generator (see Sect. 2.2). The two switches are part of the safety concept of a HV test system (Fig. 3.1).
The earthing of the HV test circuit shall guarantee a safe stationary operation, whereas the earth return has to minimize transient phenomena, e.g. after a breakdown of the test object (see Sect. 9.2.2). If not in use, the HV components must be discharged and permanently grounded. For that, they are equipped with discharging switches and earthing switches. The switches (Fig. 9.28a) can be realized by earthed rods with an electric or hydraulic drive. They open the grounding when the main switch is switched on. They close the earthing, e.g. by gravity or by a spring, when the power switch opens and the voltage is off after a test or an emergency off.
Fig. 9.28
Earthing equipment. a Earthing switch (earthing by gravity). b Earthing rope with motor drive
Earthing ropes (Fig. 9.28b) consist half of a flexible metallic cord and half of an insulating rope. In case of grounding the HV component, the metal rope connects the HV electrode and all intermediate electrodes with the earthing system. In case of HV potential at the component, the metallic rope is replaced by an insulting rope. Both ropes are moving by a motor drive. One should look for reliable earthing ropes; otherwise, they might become the weak point of the equipment.
Earthing bars for manual operation (Fig. 6.14) are often applied for smaller HV test systems. They are from insulating material with a handle at one side and a hook at the opposite site. The hook is connected to the earthing system by a flexible cord. It is recommended to arrange earthing and discharging bars at the door to the test area not to forget their application after a HV test.
Capacitors are charged during operation, keep the charge—at least partially—after switching off the test system and must be discharged, shorted and earthed, also when they are not in service. The discharging is performed by resistors in series with earthing bars, earthing switches and earthing ropes.
The control of a HV test system realizes not only the HV test procedures, but activates also the safety functions. This includes the interactions between position of the power switch and the operation switch with the status of the safety loop, the signal lamps and the earthing equipment. Furthermore, the control system shall react in case of breakdown of the test object, opening of the safety loop or emergency-off application. All these functions must be periodically checked according to the relevant instructions of the supplier of the test systems. Table 9.4 shows a timetable for such instructions as a brief example.
A HV laboratory should have a “safety concept” which describes the mentioned safety measures for test fields, test areas and test systems. Furthermore, it should include the following safety instructions for operating HV test systems and also the list of necessary inspections of safety-relevant equipment (Table 9.4). On the basis of the safety concept, the personnel should be instructed about the danger at HV tests according to national rules, but at least annually.
Before a HV test starts, the test engineer should take the following actions:
1.
Final check of earth connections and clearances of the test set-up!
2.
Check that no other personnel is in the test area, take off all manually operated earthing devices!
3.
Close the safety loop!
4.
Switch on the control power for the control and measuring system (green lamps “on”)!
5.
Switch on the power (main) switch (red lamps “on”)!
6.
Warn by horn and loudspeaker “Attention, high voltage is switched “on” (automatic earthing “off”)!
7.
Switch on the operation switch and start the test. When a test is terminated the following should be done:
1.
Reduce the voltage to a level <50% or to zero!
2.
Switch off the operation switch (automatic discharging and earthing moves to “on”, also red lamp still “on”.)!
3.
Switch off the power (main) switch (red lamp “off”, green lamp “on”)!
4.
Open the safety loop and perform the manual discharging and earthing, if necessary!
5.
Switch off the control power, the test system is out of operation (green lamp “off”)!
The described steps of operation shall also be part of the instructions for the personnel working in the HV laboratory. The instructions shall include all international and national rules on safety in electrical testing, the information of the supplier of the test systems and the own experience. The instructions may include demonstrations in the laboratory. They should be performed in minimum once a year and shall be recorded.
9.3 Outdoor HV Test Fields
When an outdoor test field completes a traditional “indoor” laboratory (compare Fig. 9.10), the test voltages might be transferred to outside via bushings or big doors. Even complete HV test systems can move on air cushions to the open-air test area. Outdoor test fields are usually equipped for research and development of EHV and UHV overhead transmission lines including the related open-air substations. In most cases, they are connected to test lines of some hundred metres length to test the design of towers, the arrangement of the conductors or to measure corona losses.
The HV test systems are placed on a concrete area which gives also space for tests on substation equipment. This area may be applicable for air cushions (see Sect. 9.2.5.3) and should be well connected to roads for the transportation of test objects. Transportation and assembling work must be done by mobile cranes. The HV test systems placed outdoor have a special design for the local climatic situation. Sometimes, the local conditions are the reason to have an outdoor test field, e.g. on high altitude to perform tests under low air density. Pollution of the test systems is a challenge which needs both good design and permanent maintenance.
For LI and SI voltage testing, impulse generators of more or less indoor design are arranged in an insulating tower for weather protection (Fig. 9.29). The air inside such a tower is conditioned to avoid dew on inner surfaces due to too low outer temperatures. Also too high inner temperatures shall be avoided. The rated voltage of outdoor impulse generators exceeds usually 3MV.
Fig. 9.29
Outdoor impulse voltage generator 4000 kV/400 kJ.
Courtesy KEPRI Korea
For AC voltage testing, metal-tank transformers (see Sect. 3.1.1) are well suited for both single-transformer application and transformer cascades (Fig. 9.30). They have proven high reliability for that application, because the design uses the experience with power transformers. The rated voltages of AC test systems are usually above 1MV.
Fig. 9.30
Outdoor transformer cascade 1800 kV/1.25 A.
Courtesy of Siemens AG, Berlin
For DC voltage testing on test lines, quite powerful generators (of rated voltages up to 2MV and currents of several 100 mA) are required (Fig. 9.31). It is very difficult to design the outdoor DC insulation of such HV test components.
Fig. 9.31
Outdoor DC generator 1300 kV/1 A
The safety measures in outdoor laboratories may follow the principles described in 9.2.6, but have to consider the broad range of atmospheric influences as temperature, humidity, rain, snow, ice, sandstorm, etc. This requires larger clearances. The earthing system of an outdoor laboratory can be made as for indoor laboratories (see 9.2.2), but the steel reinforcement of the concrete should be welded and used—in addition to earthing rods—as an area earthing, no isolation from the soil should be made. It has to be connected to the earthing system of the indoor laboratory only at one point.
9.4 Updating of Existing HV Test Fields
The erection of a new HV laboratory is a big investment. The updating (sometimes also called upgrading or refurbishment) of an existing test field is often an economically and technically acceptable alternative to a new test laboratory. In the simplest case, it is related to the replacement of complete HV test systems, the addition of new or the exchange of old components. The updating of existing HV test rooms is often more difficult. Viewpoints for updating existing HV test fields are given in the following. Generally, the updating of existing HV test fields shall follow the principles described in this chapter for new test fields as far as possible (see Sects. 9.1, 9.2, 9.3).
9.4.1 Updating of HV Test Systems
The lifetimes of HV and power components are usually higher than those of the components of control and measuring systems. Sometimes, also regulating transformers or thyristor controllers have a shorter lifetime than generators for high AC, DC or impulse voltages. Therefore, updating is often related to the control and measuring units. It cannot be recommended to replace only some of these components and to operate with a mixture of new and old control and measuring instruments. The control and measuring system should enable easy and reliable control, safe data recording and evaluation, communication within local data networks and remote service as shown in Fig. 2.8 and described in Sect. 9.1.4. The necessary interfaces between digital control and measurement systems and the components of the power circuit should be individually established depending on the design and age of the equipment to be refurbished.
If a new HV test system—possibly of higher rated voltage—shall be arranged in an existing laboratory, necessary clearances must be taken into consideration (see Sect. 9.1.3). In case of limited clearance between HV components (generator, divider, connections, etc.) and neighbouring grounded or energized objects, the application of larger control electrodes can be taken into consideration. This requires the calculation of the conditions of the electric field between the HV components and their surroundings. Additionally, the necessary clearance for test objects must be guaranteed (Fig. 2.1). To save space in the test area, tank-type test transformers and reactors can be arranged outside with their HV bushing into the test room (Fig. 9.21). For the safety requirements, see Sect. 9.2.6.2.
9.4.2 Improvement of HV Test Rooms
The safety system has to follow the principles explained in Sect. 9.2.6.1 completely. Any reduction in the safety requirements is not acceptable. When auxiliary equipment (Sect. 9.2.5) is modified or improved, the reliable operation of the test systems including the necessary sensitivity of PD and dielectric measurement must not be influenced: Clearances should sufficiently be maintained, and shielding effects should not be reduced.
Most expenditure is necessary to improve the grounding and shielding of a HV test field. This is often necessary because withstand tests are more and more completed by sensitive PD and/or dielectric measurement. Often, an unshielded test room is not longer sufficient for these monitored withstand tests. Then, consequences for filtering the supply power, for shielding and improved grounding become unavoidable.
Usually, old grounding rods are corroded and must be replaced to reach an effective ground resistance of the order of 1Ω. The grounding should be independent from the shielding (Sect. 9.2.2), but in older test fields, the grounding is realized by a combination of earthing rods and an area grounder covering the whole test field area. This means grounding and shielding are combined in the floor. It should be investigated whether it is necessary to separate grounding and shielding in the floor (Sect. 9.2.2) or not. The shielding of the floor is not as important as that of the walls and the ceiling, which shall be performed as described above (Figs. 9.18 and 9.18). If the floor grounding is considered to be necessary, one had to put an insulation foil over the area grounder, followed by a suited metal mesh or metal panels forming the floor shielding and a protection layer, usually of concrete with an upper layer of epoxy resin for air cushion transportation.
In some cases, it should be considered whether the shielding of the whole test field is necessary or the application of a shielded cabin (Fig. 9.32) is sufficient. Inside such a self-carrying cabin the lowest PD noise levels can be reached. If the space of such a commercially available cabin is sufficient, no expensive shielding of the whole test area is required. A similar effect can be reached with a metal-enclosed test system (Fig. 3.41). A perfect shielding must be completed with a perfect filtering of all voltages penetrating into the shielded area (Sect. 9.2.3).
