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Tests of the DELPHI Barrel RICH electrical hv insulator
Nuclear Instruments and Methods in Physics Research A283 (1989) 37-42 North-Holland, Amsterdam
TESTS OF THE DELP
BAIL EL
37
CH ELECTRICAL HV INSULAT
G. KALKANIS *, J. DREES * *, W. KLEMPT and E. ROSSO CERN, Geneva, Switzerland
Received 1 March 1989 and in revised form 29 May 1989 The expected lifetime of a cylindrical high voltage test insulator is estimated from accelerated do high voltage test results; a method to evaluate the failure probability and the by lifetime of insulators with similar geometry and insulating material is described; the failure probability and the expected lifetime of the electrical insulator of the planned DELPHI Barrel RICH (Ring Imaging Cherenkov) detector are evaluated. The insulating structure consists of mylar ribbons wound helically in layers, staggered and glued together with a polyurethane adhesive . 1. Introduction
sation potential mixed into the drift gas. The electrons drift in the drift gas (ethane + methane + TMAE) to the detector, where each photon conversion point is reconstructed in three dimensions. The Barrel RICH (BR), enclosed by the Inner Cylinder (IC), the Outer Cylinder (OC) and the End Flanges, is axial-symmetric with respect to a plane through the interaction point, perpendicular to the circulating beam (fig . 1). The Inner and Outer Cylinders have a length of about 3.1 m and 3.8 m, respectively, and a radius of about 1 .3 m and 2.0 m. A Middle Wall (MW) divides the BR into two halves; each half contains 24 liquid radiator tubes, 24 drift tubes, 24 field shaping wire frames, the gas radiator and 24 x 6 parabolic mirrors. The maximum drift voltage foreseen is 150 kV for the drift tubes, 1 .5 m long. This voltage will be applied by high voltage (hv) cables, attached to the central circular conductive plate of the MW . To achieve the
The Barrel RICH (Ring Imaging CHerenkov) detector of the LEP-DELPHI experiment [1] - now under construction - is designed to provide fr, K and p identification, in the momentum range of 0.3-30 GeV/c, over a laige solid angle. It is based on the following principles [2]: the Cherenkov light generated in a liquid and in a gas radiator by the traversing charged particles is focused - in the case of the gas radiator mirrors are used - into a drift tube, where the photons are converted into electrons by photoionisation of TIME (tetrakis dimethyloamino ethylene), a gas with a low ioni* On leave from Physics Department, University of Athens, Greece. * * On leave from Physics Department, University of Wuppertal, FRG.
GAS RADIATOR FIELD SHAPING Vh RES \ DRIFT TUBES I MvVPCS LIQUID RADIATOR TUBES ELECTRICAL INSI. LATOR INNER CYLINDER
BEAM IINTERACTI)N P01 JT
.. . .. .
r
....
. . . . .. .. . ,
Fig. 1 . Longitudinal and transverse cuts of the design of t-ie DELPHI Barrel RICH . 0168-9002/89/$03 .50 © Elsevier Science Publishers B.V . (North-Holland Physics Publishing Division)
38
G. Kalkanis et al. / The DELPHI Barrel RICH electrical by insulator
10-4 Ell, required electric field homogeneity, E_L -< Barrel RICH are severe constraints to the design of the distribution of necessary, such as: (a) an accurate direcalong the drift equipotential parallel electrodes tion on all surfaces (drift tube walls - inside and outside -, liquid radiator tube walls, high voltage cables, inner cylinder electrical insulation), (b) field shaping thin wires on the mirror side, and (c) a radial degradation of the potential on the nonconductive part of the middle wall, from the central plate to the earthed outer cylinder [3] . Between the inner cylinder and the liquid radiator tubes an electrical insulator is needed. The insulator has to be a cylinder, with axis parallel and coincident to the axis of the BR, having a radius of about 1.3 m, a length of about 3.1 m, and a thickness of no more than 15 mm; the thickness is restricted by the design of DELPHI. The insulation is provided by a number of layers of mylar, constructed by winding helically a 0.2 m wide mylar ribbon on the IC over its full length, staggered and glued together with a polyurethane adhesive [4,5] . Two linear voltage degraders, constructed with equidistant conductive strips traced on both sides of a thin kapton printed circuit board and connected to a high stability resistor chain, will ensure a constant degradation of the electric field from the middle to both ends of the cylinder and will prevent : (a) distortions of the homogeneity of the electric field inside the drift tubes, (b) sparks from the liquid radiator tube walls to the insulation, and (c) surface discharges (flashover) along the inner cylinder [3] . The required and expected lifetime of the insulator under the design conditions is so long that testing under real conditions was completely out of the question . An accelerated test on the lifetime of test cylinders had to be performed to yield information within a reasonable time on its capability to withstand a specific do high voltage over a certain period. Accelerated tests are achieved by subjecting the test units (samples) to conditions that are more severe than the realistic ones. The results have to be interpreted and analyzed in terms of a model, and then extrapolated to realistic conditions . Thus, a number of small and thin cylinders, having a length of 0.56 m and a radius of about 0.2 m, were constructed, identically to the large cylinder, by winding helically a 0.1 m wide mylar ribbon in a number of ...3 and -. .a glueda togtogether with aL layers, Staggered layers, a poly _1_reta4_haÎie adhesive, to study various construction details, to investigate and optimize the characteristics of the degrader needed, and to fix the test conditions. Finally, a test cylinder, 2.2 mm thick in total (mylar + glue), was subjected to a voltage as high as 200 kV and its time to failure was measured . Then, the failure probability and the by lifetime of insulators with the same and similar geometry (e.g. the IC insulator) are evaluated . The evaluations are based on results obtained during a study
which had been undertaken as a complementary investigation, with small, planar, insulator samples - constructed in the same way as the insulation of the test and inner cylinders - and represented by a mathematical model, the Inverse Power Law Model, with a Weibull lifetime distribution. 2. Test conditions - measurements 2.1. The insulating material A laminated insulating structure is very well suited in our case, because of its ease of fabrication and its capability to eliminate the effects of local imperfections of the insulating material. If, in addition, the solid multilayer insulator is made of thin film ribbons, staggered and glued together by a lower resistive adhesive, used as impregnant, the current tends to follow the adhesive, thereby reducing the possibility of the development of electric field or voltage stress concentrations in existing small gas pockets or voids, which could promote the formation of a destructive ionization. The insulating laminated structure used consists of 0.1 m wide, 125 lim thick, mylar ribbon wound helically in layers, staggered and glued together with 10 lim thick Hexalite 6103 polyurethane adhesive (see fig. 2). The 1 mm wide joints between two ribbons were offset in successive layers in order to avoid weak regions and to provide the longest possible path for the current throupyh the glue (resistivity of mylar >- 2 X 102° SZ m, resistivity of glue = 3 X 1015 SZ m). With reference to the joints of the first (v = 1) layer, the joints of the odd layers were offset by (v - 2) X 12.5 mm, (v > 1), and those of the even layers by 50.0 + (v - 1) X 6.25 mm in the same direction . The insulation had to be cured !or a period of one week at 75° C, for polymerisation of the glue according to the specifications . 2.2. The test cylinder. Degradation The test cylindrical insulator was constructed by winding helically mylar ribbon around a 0.56 m long
v,
v,
Vz
VI
V,
Vo V,
vo
V,
Vz V?
V) V3
ELECTROOES KAPTON ° ELECTRODES/GLUE MYLAR - GLUE
v,
Fig. 2. Cross section of the mylar-polyurethane larninated insulating structure and the degrading electrode configuration (arbitrary scale).
G. Kalkanis et al. / The DELPHI Barrel RICH electrical by insulator
aluminium cylinder, of a radius of 0.2 m, over its full length, like the inner cylinder of the BR. The length of the test cylinder was restricted by the size of the winding machine which was used. Two linear voltage degraders were provided by 108 equidistant copper electrode strips (1.53 mm wide, 125 p m thick), on each side every 2.53 mm, traced on both sides of a 125 ILm thick kapton printed circuit board, from the middle of the cylinder to its two earthed ends. The kapton printed circuit board was glued on the insulator before curing . The strips were staggered and electrically connected as shown in fig . 2, to eliminate punchthrough effects in the kapton due to high electric fields, leading to damage or breakdown of the insulator . The central electrode, where the by was applied, was wider on both sides, 6.59 mm wide on the outside surface and 4.06 mm wide on the inside surface. The internal, overlapped, end of the windings around the cylindrical insulator kapton foil had the strips etched away over different distances on the two sides, in order to avoid carbonization of the kapton due to corona effect at the edges of the cut strips, as had been experienced. The external, free, end of the kapton foil was extended for about 0.1 m, and the strips of both sides were connected to two high stability resistor chains with a resistivity of (108 x 18 MSZ) = 2 GO each. The resistors were soldered, not on the kapton printed circuit board, but on special connectors, where the electrode board was introduced; solderings had caused severe carbonization of the kapton, and consequently short circuit after some hours with 2 kV voltage difference between two succesive electrode strips . Tests of the resistor chain itself, performed in a freon-12 environment, proved that practically no corona appeared on the resistor chain if the chain was immersed inside silicolloid (RTV-121, Rhodia) and covered by a semiconductor thermally shrunk sleeve . Inside our test setup (see subsection 2.3), which was limited in space, visible corona discharges had been seen, in the case of uncovered resistors, not only on sharp edges of the soldering material, but also on the bodies of the resistors of the degrader, resulting in a strange nonlinear voltagecurrent relationship, with a knee appearring at higher current values (see fig. 3), instability of the current and some time sparks . The situation was improved progressivelly, first by replacing the originally used degrading electrode wires by strips, next by immersing Cide reslstol chain in silicolloid, and finally, by covering its surface with a thermally shrunk semiconductor sleeve . The improvements and the final dissappearance of the corona, resulting in a linear voltage-current relationship, are apparent in fig . 3. 9
al_
2.3. The test setup The test cylinders, with the voltage degrader on and connected to the resistor chains, were installed in a
I
I
I
I
39
~--,-,-I~ ~
WIRE ELECTRODES STRIP ELECTRODES " SEMICO~CTOR SLEEVE + SILICOLLOID + SILICOLLOID " SEMICONDUCTOR SLEEVE
.200
1M
0
O
0 o0aa o o~ :~a IC
.100
u
0
20
40
60
80
100
120
HV (kV)
140
160
180
100
Fig. 3. The changes in the voltage-current relationship, while reducing the formation of corona discharges progressively, by replacing the electrodes of the degrader and by covering the resistor chain with silicolloid or/and a semiconductor sleeve. gas-tight, light-transparent and electrically safe vessel (1 mxl mx1 m). A high voltage power supply (Wallis OLH 200) that could provide up to 200 kV do was connected to the test setup, equipped with digital voltage and current meters or/and a computer readout system, an adjustable limit tip-off system, a chronometer and recording facilities . The high voltage was applied to the central strip at the middle of the cylinders via a special, 0.35 m long electrode, covered by a conically shaped, oil-filled . ceramic insulation. The test setup was also equipped with a freon-12 circulating and monitoring system, an automatic heating and temperature monitoring system to keep the temperature inside the vessel stable at 50 ° C ( ± 2 ° C, and a specially designed system to protect the monitoring electronic devices and recorders (fig. 4). 2.4. Meast
A number of extensive and finally destructive tests were performed, and numerous samples were sacrificed in order to understand all the mechanisms, to investiHV
10707171
108MMS?
~ZH
0 ...
108x18MQ
Fig. 4. Schematical configuration of the by electrical insulator test setup.
G. Kalkanis et al. / The DELPHI Barrel RICH electrical by insulator Table 1 Data of the test runs . Length : 560 mm, diameter including mylar : 201 .4 mm, thickness of mylar+glue ; 2 .2 mm Run 1 Run 2 Run 3
Time
Hv [kV]
E [kV/mm]
526 h 3 h 20 min 40 min
150 175 200
68 .1 79 .5 90 .9
tic constants of the material and the test method ; since the inverse of the stress is raised to the n th power, eq. (2) is called the inverse power law . Under the last assumption, eq. (1) yields the fraction F(t ; E) of units failing by time t under a constant stress E as
F(t ; E) = 1-exp[ -(cE"t)fl ],
(3)
or gate various construction details, to investigate, check and optimize all the parameters of the described degrader configuration and to fix the test conditions . The test cylinder, constructed as described above and consisting of 16 layers of mylar, was tested in three successive rums (see table 1) . When the voltage was applied for the first time on the cylinder, the rise time rate was 1 .5 kV/min. After the first time, the up and down rate was 10 kV/min. Between runs the test sample was subjected to voltage up and down gymnastics to simulate as realistic conditions as possible . Times were recorded from the maximum run voltage application to the moment of starting to decrease the voltage, or - in the case of !Run 3 - to the moment of failure. During the high voltage runs, freon-12 gas was continuously circulated inside the test setup, while the temperature was currently monitored and kept at 50 " C (±2°C) .
3.
e Weibull derived inverse power law
3.1 . The inverse power law model The inverse power law model for accelerated lifetime tests, with the electric field or stress (voltage/ thickness) as the accelerating variable, is applicable if the underlying cumulative distribution function of the lifetime t can be described, according to Weibull, by : F(t)=1-exp [ - (t/,)"],
t>>-0,
where F(t) is the failure probability at time t . Two additional assumptions are needed with respect to the two parameters P and T : (a) the shape parameter P is a constant, independent of the stress ; (b) the scale parameter T, which is the time corresponding to F = 1 - 1 /e = 0 .632 (the 663 .2th percentile of the distribution, also called the characteristic life time), is an inverse power function of the stress, E, i .e. T(E)
= 1/(cE "),
where c and r, are pcsitive parameters, characteris-
F(t ; E) =1- exp(-c'E"t,@),
(3')
where y = n P . Taking as reference a voltage stress Eo , for which the measured characteristic lifetime is T, the probability of a breakdown after a time t at stress E is - according to eq . (3') -
F(t ; E)=1-exp[- (E/EO)"(t/T)'6] . 3.2. The Weibul! shape parameters
P
(4)
and y
A large number of thin circular test samples of the described insulating structure, with five different thicknesses, located between planar electrodes, were sub jected to a constant voltage as high as 150 kV in all cases, and their times to failure were measured [5] . In these measurements the area of the electrodes and of the insulator remained constant, whilst the electric field E was varied from 361 .4 to 100 .3 kV/mm (by varying the thickness of the insulator from 0 .415 to 1 .495 mm) . The time at which a certain percentage of samples failed is the expected lifetime with a probability equal to the percentage. The statistical treatment of the data by a combination of graphical and analytical methods that complement each other showed [5] that the underlying lifetime distribution is of the Weibull type with common values for the shape parameters at different stresses, thus proving the validity of the data and the model. Additionally, it was found that the data are well reproduced by eq . (4) with P=0 .91±0 .07,
y=5 .7±0 .2 .
(5)
The shape parameters ß and y are determined by the properties of the insulating material ; the errors of P, y are highly correlated .
C_ ~ala~e®lati~es9c
4.1 . Evaluation of the lifetime of the test cylinder According to eq . (4), taking as reference an electric field Eo for which the measured lifetime is t o , the failure probability after at time t at an electric field E is given by F(t ;
E) = 1-exp[- (EIEO)Y(tlto)ß] .
G. Kalkanis et al. / The
DELPHI Barrel RICH electrical hv insulator
By using the scaling law (E1Eo) Y(tlto) ß=1
(6)
it is found that Run 2 (see table 1) corresponds to a time of 8.7 h spent at 150 kV, and Run 3 corresponds to 4 h spent at 150 kV . It is therefore concluded that the test cylinder survived a time corresponding to 539 h at 150 kV, which yields a lifetime for the test cylinder of -r1 =22±8d . 4.2. The failure probability for insulators with the same geometry
From eq. (4) one can now calculate the failure probability for insulators with the same geometry of the test cylinder at any field E and time t, in a rough way . For cylinders of different dimensions but the same shape the ratio of the electric fields corresponding to the same failure probability is given by [6] Et/E2
= Rl/Y ,
R = A2/A1,
where R is the ratio of the area A 2 of cylinder 2 to the area A1 of cylinder 1 over which the high electric field is applied. Let us recall that y has been determined from probes with varying thickness, therefore we scale with the area. Eq. (4) then gives F(t ; E) = 1 - exp [ - (E/El) Y (A2/A1)(t/T l) ß I
(9)
for the probability of a breakdown after a time t of a cylinder 2, having an area A 2 over which the high electric field E is applied, while Tl is the measured lifetime at El of a test cylinder having an area A1 . An alternative method would be to scale geometrically different objects with the ratio of the insulator volumes V2/VI throughout. In that case y has to be replaced by y = y + 1 and eq. (8) by El/E2 = (V2/v1)
1/f .
The exponent of the Weibull distribution eq. (9) would be replaced by (E/El) v ( V2/V,)(t/ T1) ß = (U/U1)(E,/E1)Y(A2/A1)(t/T1)ß,
where U, Ul are the high voltages applied to the cylinder 2 and the test cylinder, respectively. Since the high voltage is the same in all cases, the numerical values of F remain unchanged . 4.3. The Barrel RICH Inner Cylinder With the parameters ß and y of the insulating material, the lifetime Tl (eq. (7)) of the test cylinder at electric field El = 68.1 kV/mm, and the geometrical
41
characteristics of the test (see table 1) and the inner (length = 3.1 m, radius = 1.3 m) cylinders, which yield A 1 and A2 respectively, one obtains from eq. (9) the following result for evaluating the failure probability of the BR inner cylinder F(t ; E)= 1 - exp[ - ( Emax/El) Y (A2/AI)(t/Tt) '6 1 =1 - exp[ -aEm.tß1 ,
(10)
where a =1.665 x 10 -10 , ß = 0.91 ± 0.07 (eq. (5)), Y = 5.7 f 0.2 (eq . (5)) E... = maximum electric field (stress) of IC in kV/mm, t = time in days. If 150 kV at maximum would be applied, which means an electric field E.,,,=10 kV/mm (the Inner Cylinder insulator thickness is 15 mm), the failure probability is F = (2 ± 1)% after one year and the lifetime r = 84 yr. Decreasing the by to 120 kV [7] would increase the lifetime by a factor of about 4 (T = 340 yr). 5. Conclusions Extensive accelerated tests were performed on cylindrical by insulators . The technical fea ibility of various construction details was checked ; essential characteristics of the degrader were investigated and optimized ; parameters which cannot be simulated by computers, such as by rise time ratios, stability, microcorona discharges etc ., were studied . Finally, the time to failure of a test cylinder, subjected to voltages as high as 200 kV, was measured . From the lifetime distribution - checked to be of the Weibull type - of the laminated insulating material used and the values of its shape parameters and the measured lifetime of the test cylinder, a method was described to evaluate the failure probability and the by lifetime of the Barrel RICH Inner Cylinder insulator and of insulators with similar geometry. Acknowledgement This work would not have been possible without the excellent technical work done by J.C. l'Abbe. References [1] DELPHI Technical Proposal . CERN/LEPC/83-3 (1983) ; DELPHI Progress Report, CERN/LEPC/84-16 (1984); DELPHI Progress Report, CERN/LEPC/86-11 (1986) ; DELPHI Notes 83-86, 84-60 GEN-11, 85-44 GEN-31, 85-55 RICH-8A, 86-82 RICH-22 .
42
G. Kalkanis et al. / The DELPHI Barrel RICH electrical by insulator
[2] J. Seguinot and T. Ypsilantis, Nucl . Instr. and Meth . 142 (1977) 377. [3] G. Kalkanis and E. Rosso, to be published in Nucl . Instr. and Meth. (1989) . [4] ALEPH Note 86-145 (1986); M. Price, private communication.
[5) G. Kalkanis, W. Klempt and E. Rosso, Nucl . Instr. and Meth . A275 (1989) 267. [6) International Electrotechnical Commission, Report 15B (Secretariat) 97 (1984). [7] J. Drees and G. Kalkanis, DELPHI 87-64 RICH 27 (1987).
TESTS OF THE DELP
BAIL EL
37
CH ELECTRICAL HV INSULAT
G. KALKANIS *, J. DREES * *, W. KLEMPT and E. ROSSO CERN, Geneva, Switzerland
Received 1 March 1989 and in revised form 29 May 1989 The expected lifetime of a cylindrical high voltage test insulator is estimated from accelerated do high voltage test results; a method to evaluate the failure probability and the by lifetime of insulators with similar geometry and insulating material is described; the failure probability and the expected lifetime of the electrical insulator of the planned DELPHI Barrel RICH (Ring Imaging Cherenkov) detector are evaluated. The insulating structure consists of mylar ribbons wound helically in layers, staggered and glued together with a polyurethane adhesive . 1. Introduction
sation potential mixed into the drift gas. The electrons drift in the drift gas (ethane + methane + TMAE) to the detector, where each photon conversion point is reconstructed in three dimensions. The Barrel RICH (BR), enclosed by the Inner Cylinder (IC), the Outer Cylinder (OC) and the End Flanges, is axial-symmetric with respect to a plane through the interaction point, perpendicular to the circulating beam (fig . 1). The Inner and Outer Cylinders have a length of about 3.1 m and 3.8 m, respectively, and a radius of about 1 .3 m and 2.0 m. A Middle Wall (MW) divides the BR into two halves; each half contains 24 liquid radiator tubes, 24 drift tubes, 24 field shaping wire frames, the gas radiator and 24 x 6 parabolic mirrors. The maximum drift voltage foreseen is 150 kV for the drift tubes, 1 .5 m long. This voltage will be applied by high voltage (hv) cables, attached to the central circular conductive plate of the MW . To achieve the
The Barrel RICH (Ring Imaging CHerenkov) detector of the LEP-DELPHI experiment [1] - now under construction - is designed to provide fr, K and p identification, in the momentum range of 0.3-30 GeV/c, over a laige solid angle. It is based on the following principles [2]: the Cherenkov light generated in a liquid and in a gas radiator by the traversing charged particles is focused - in the case of the gas radiator mirrors are used - into a drift tube, where the photons are converted into electrons by photoionisation of TIME (tetrakis dimethyloamino ethylene), a gas with a low ioni* On leave from Physics Department, University of Athens, Greece. * * On leave from Physics Department, University of Wuppertal, FRG.
GAS RADIATOR FIELD SHAPING Vh RES \ DRIFT TUBES I MvVPCS LIQUID RADIATOR TUBES ELECTRICAL INSI. LATOR INNER CYLINDER
BEAM IINTERACTI)N P01 JT
.. . .. .
r
....
. . . . .. .. . ,
Fig. 1 . Longitudinal and transverse cuts of the design of t-ie DELPHI Barrel RICH . 0168-9002/89/$03 .50 © Elsevier Science Publishers B.V . (North-Holland Physics Publishing Division)
38
G. Kalkanis et al. / The DELPHI Barrel RICH electrical by insulator
10-4 Ell, required electric field homogeneity, E_L -< Barrel RICH are severe constraints to the design of the distribution of necessary, such as: (a) an accurate direcalong the drift equipotential parallel electrodes tion on all surfaces (drift tube walls - inside and outside -, liquid radiator tube walls, high voltage cables, inner cylinder electrical insulation), (b) field shaping thin wires on the mirror side, and (c) a radial degradation of the potential on the nonconductive part of the middle wall, from the central plate to the earthed outer cylinder [3] . Between the inner cylinder and the liquid radiator tubes an electrical insulator is needed. The insulator has to be a cylinder, with axis parallel and coincident to the axis of the BR, having a radius of about 1.3 m, a length of about 3.1 m, and a thickness of no more than 15 mm; the thickness is restricted by the design of DELPHI. The insulation is provided by a number of layers of mylar, constructed by winding helically a 0.2 m wide mylar ribbon on the IC over its full length, staggered and glued together with a polyurethane adhesive [4,5] . Two linear voltage degraders, constructed with equidistant conductive strips traced on both sides of a thin kapton printed circuit board and connected to a high stability resistor chain, will ensure a constant degradation of the electric field from the middle to both ends of the cylinder and will prevent : (a) distortions of the homogeneity of the electric field inside the drift tubes, (b) sparks from the liquid radiator tube walls to the insulation, and (c) surface discharges (flashover) along the inner cylinder [3] . The required and expected lifetime of the insulator under the design conditions is so long that testing under real conditions was completely out of the question . An accelerated test on the lifetime of test cylinders had to be performed to yield information within a reasonable time on its capability to withstand a specific do high voltage over a certain period. Accelerated tests are achieved by subjecting the test units (samples) to conditions that are more severe than the realistic ones. The results have to be interpreted and analyzed in terms of a model, and then extrapolated to realistic conditions . Thus, a number of small and thin cylinders, having a length of 0.56 m and a radius of about 0.2 m, were constructed, identically to the large cylinder, by winding helically a 0.1 m wide mylar ribbon in a number of ...3 and -. .a glueda togtogether with aL layers, Staggered layers, a poly _1_reta4_haÎie adhesive, to study various construction details, to investigate and optimize the characteristics of the degrader needed, and to fix the test conditions. Finally, a test cylinder, 2.2 mm thick in total (mylar + glue), was subjected to a voltage as high as 200 kV and its time to failure was measured . Then, the failure probability and the by lifetime of insulators with the same and similar geometry (e.g. the IC insulator) are evaluated . The evaluations are based on results obtained during a study
which had been undertaken as a complementary investigation, with small, planar, insulator samples - constructed in the same way as the insulation of the test and inner cylinders - and represented by a mathematical model, the Inverse Power Law Model, with a Weibull lifetime distribution. 2. Test conditions - measurements 2.1. The insulating material A laminated insulating structure is very well suited in our case, because of its ease of fabrication and its capability to eliminate the effects of local imperfections of the insulating material. If, in addition, the solid multilayer insulator is made of thin film ribbons, staggered and glued together by a lower resistive adhesive, used as impregnant, the current tends to follow the adhesive, thereby reducing the possibility of the development of electric field or voltage stress concentrations in existing small gas pockets or voids, which could promote the formation of a destructive ionization. The insulating laminated structure used consists of 0.1 m wide, 125 lim thick, mylar ribbon wound helically in layers, staggered and glued together with 10 lim thick Hexalite 6103 polyurethane adhesive (see fig. 2). The 1 mm wide joints between two ribbons were offset in successive layers in order to avoid weak regions and to provide the longest possible path for the current throupyh the glue (resistivity of mylar >- 2 X 102° SZ m, resistivity of glue = 3 X 1015 SZ m). With reference to the joints of the first (v = 1) layer, the joints of the odd layers were offset by (v - 2) X 12.5 mm, (v > 1), and those of the even layers by 50.0 + (v - 1) X 6.25 mm in the same direction . The insulation had to be cured !or a period of one week at 75° C, for polymerisation of the glue according to the specifications . 2.2. The test cylinder. Degradation The test cylindrical insulator was constructed by winding helically mylar ribbon around a 0.56 m long
v,
v,
Vz
VI
V,
Vo V,
vo
V,
Vz V?
V) V3
ELECTROOES KAPTON ° ELECTRODES/GLUE MYLAR - GLUE
v,
Fig. 2. Cross section of the mylar-polyurethane larninated insulating structure and the degrading electrode configuration (arbitrary scale).
G. Kalkanis et al. / The DELPHI Barrel RICH electrical by insulator
aluminium cylinder, of a radius of 0.2 m, over its full length, like the inner cylinder of the BR. The length of the test cylinder was restricted by the size of the winding machine which was used. Two linear voltage degraders were provided by 108 equidistant copper electrode strips (1.53 mm wide, 125 p m thick), on each side every 2.53 mm, traced on both sides of a 125 ILm thick kapton printed circuit board, from the middle of the cylinder to its two earthed ends. The kapton printed circuit board was glued on the insulator before curing . The strips were staggered and electrically connected as shown in fig . 2, to eliminate punchthrough effects in the kapton due to high electric fields, leading to damage or breakdown of the insulator . The central electrode, where the by was applied, was wider on both sides, 6.59 mm wide on the outside surface and 4.06 mm wide on the inside surface. The internal, overlapped, end of the windings around the cylindrical insulator kapton foil had the strips etched away over different distances on the two sides, in order to avoid carbonization of the kapton due to corona effect at the edges of the cut strips, as had been experienced. The external, free, end of the kapton foil was extended for about 0.1 m, and the strips of both sides were connected to two high stability resistor chains with a resistivity of (108 x 18 MSZ) = 2 GO each. The resistors were soldered, not on the kapton printed circuit board, but on special connectors, where the electrode board was introduced; solderings had caused severe carbonization of the kapton, and consequently short circuit after some hours with 2 kV voltage difference between two succesive electrode strips . Tests of the resistor chain itself, performed in a freon-12 environment, proved that practically no corona appeared on the resistor chain if the chain was immersed inside silicolloid (RTV-121, Rhodia) and covered by a semiconductor thermally shrunk sleeve . Inside our test setup (see subsection 2.3), which was limited in space, visible corona discharges had been seen, in the case of uncovered resistors, not only on sharp edges of the soldering material, but also on the bodies of the resistors of the degrader, resulting in a strange nonlinear voltagecurrent relationship, with a knee appearring at higher current values (see fig. 3), instability of the current and some time sparks . The situation was improved progressivelly, first by replacing the originally used degrading electrode wires by strips, next by immersing Cide reslstol chain in silicolloid, and finally, by covering its surface with a thermally shrunk semiconductor sleeve . The improvements and the final dissappearance of the corona, resulting in a linear voltage-current relationship, are apparent in fig . 3. 9
al_
2.3. The test setup The test cylinders, with the voltage degrader on and connected to the resistor chains, were installed in a
I
I
I
I
39
~--,-,-I~ ~
WIRE ELECTRODES STRIP ELECTRODES " SEMICO~CTOR SLEEVE + SILICOLLOID + SILICOLLOID " SEMICONDUCTOR SLEEVE
.200
1M
0
O
0 o0aa o o~ :~a IC
.100
u
0
20
40
60
80
100
120
HV (kV)
140
160
180
100
Fig. 3. The changes in the voltage-current relationship, while reducing the formation of corona discharges progressively, by replacing the electrodes of the degrader and by covering the resistor chain with silicolloid or/and a semiconductor sleeve. gas-tight, light-transparent and electrically safe vessel (1 mxl mx1 m). A high voltage power supply (Wallis OLH 200) that could provide up to 200 kV do was connected to the test setup, equipped with digital voltage and current meters or/and a computer readout system, an adjustable limit tip-off system, a chronometer and recording facilities . The high voltage was applied to the central strip at the middle of the cylinders via a special, 0.35 m long electrode, covered by a conically shaped, oil-filled . ceramic insulation. The test setup was also equipped with a freon-12 circulating and monitoring system, an automatic heating and temperature monitoring system to keep the temperature inside the vessel stable at 50 ° C ( ± 2 ° C, and a specially designed system to protect the monitoring electronic devices and recorders (fig. 4). 2.4. Meast
A number of extensive and finally destructive tests were performed, and numerous samples were sacrificed in order to understand all the mechanisms, to investiHV
10707171
108MMS?
~ZH
0 ...
108x18MQ
Fig. 4. Schematical configuration of the by electrical insulator test setup.
G. Kalkanis et al. / The DELPHI Barrel RICH electrical by insulator Table 1 Data of the test runs . Length : 560 mm, diameter including mylar : 201 .4 mm, thickness of mylar+glue ; 2 .2 mm Run 1 Run 2 Run 3
Time
Hv [kV]
E [kV/mm]
526 h 3 h 20 min 40 min
150 175 200
68 .1 79 .5 90 .9
tic constants of the material and the test method ; since the inverse of the stress is raised to the n th power, eq. (2) is called the inverse power law . Under the last assumption, eq. (1) yields the fraction F(t ; E) of units failing by time t under a constant stress E as
F(t ; E) = 1-exp[ -(cE"t)fl ],
(3)
or gate various construction details, to investigate, check and optimize all the parameters of the described degrader configuration and to fix the test conditions . The test cylinder, constructed as described above and consisting of 16 layers of mylar, was tested in three successive rums (see table 1) . When the voltage was applied for the first time on the cylinder, the rise time rate was 1 .5 kV/min. After the first time, the up and down rate was 10 kV/min. Between runs the test sample was subjected to voltage up and down gymnastics to simulate as realistic conditions as possible . Times were recorded from the maximum run voltage application to the moment of starting to decrease the voltage, or - in the case of !Run 3 - to the moment of failure. During the high voltage runs, freon-12 gas was continuously circulated inside the test setup, while the temperature was currently monitored and kept at 50 " C (±2°C) .
3.
e Weibull derived inverse power law
3.1 . The inverse power law model The inverse power law model for accelerated lifetime tests, with the electric field or stress (voltage/ thickness) as the accelerating variable, is applicable if the underlying cumulative distribution function of the lifetime t can be described, according to Weibull, by : F(t)=1-exp [ - (t/,)"],
t>>-0,
where F(t) is the failure probability at time t . Two additional assumptions are needed with respect to the two parameters P and T : (a) the shape parameter P is a constant, independent of the stress ; (b) the scale parameter T, which is the time corresponding to F = 1 - 1 /e = 0 .632 (the 663 .2th percentile of the distribution, also called the characteristic life time), is an inverse power function of the stress, E, i .e. T(E)
= 1/(cE "),
where c and r, are pcsitive parameters, characteris-
F(t ; E) =1- exp(-c'E"t,@),
(3')
where y = n P . Taking as reference a voltage stress Eo , for which the measured characteristic lifetime is T, the probability of a breakdown after a time t at stress E is - according to eq . (3') -
F(t ; E)=1-exp[- (E/EO)"(t/T)'6] . 3.2. The Weibul! shape parameters
P
(4)
and y
A large number of thin circular test samples of the described insulating structure, with five different thicknesses, located between planar electrodes, were sub jected to a constant voltage as high as 150 kV in all cases, and their times to failure were measured [5] . In these measurements the area of the electrodes and of the insulator remained constant, whilst the electric field E was varied from 361 .4 to 100 .3 kV/mm (by varying the thickness of the insulator from 0 .415 to 1 .495 mm) . The time at which a certain percentage of samples failed is the expected lifetime with a probability equal to the percentage. The statistical treatment of the data by a combination of graphical and analytical methods that complement each other showed [5] that the underlying lifetime distribution is of the Weibull type with common values for the shape parameters at different stresses, thus proving the validity of the data and the model. Additionally, it was found that the data are well reproduced by eq . (4) with P=0 .91±0 .07,
y=5 .7±0 .2 .
(5)
The shape parameters ß and y are determined by the properties of the insulating material ; the errors of P, y are highly correlated .
C_ ~ala~e®lati~es9c
4.1 . Evaluation of the lifetime of the test cylinder According to eq . (4), taking as reference an electric field Eo for which the measured lifetime is t o , the failure probability after at time t at an electric field E is given by F(t ;
E) = 1-exp[- (EIEO)Y(tlto)ß] .
G. Kalkanis et al. / The
DELPHI Barrel RICH electrical hv insulator
By using the scaling law (E1Eo) Y(tlto) ß=1
(6)
it is found that Run 2 (see table 1) corresponds to a time of 8.7 h spent at 150 kV, and Run 3 corresponds to 4 h spent at 150 kV . It is therefore concluded that the test cylinder survived a time corresponding to 539 h at 150 kV, which yields a lifetime for the test cylinder of -r1 =22±8d . 4.2. The failure probability for insulators with the same geometry
From eq. (4) one can now calculate the failure probability for insulators with the same geometry of the test cylinder at any field E and time t, in a rough way . For cylinders of different dimensions but the same shape the ratio of the electric fields corresponding to the same failure probability is given by [6] Et/E2
= Rl/Y ,
R = A2/A1,
where R is the ratio of the area A 2 of cylinder 2 to the area A1 of cylinder 1 over which the high electric field is applied. Let us recall that y has been determined from probes with varying thickness, therefore we scale with the area. Eq. (4) then gives F(t ; E) = 1 - exp [ - (E/El) Y (A2/A1)(t/T l) ß I
(9)
for the probability of a breakdown after a time t of a cylinder 2, having an area A 2 over which the high electric field E is applied, while Tl is the measured lifetime at El of a test cylinder having an area A1 . An alternative method would be to scale geometrically different objects with the ratio of the insulator volumes V2/VI throughout. In that case y has to be replaced by y = y + 1 and eq. (8) by El/E2 = (V2/v1)
1/f .
The exponent of the Weibull distribution eq. (9) would be replaced by (E/El) v ( V2/V,)(t/ T1) ß = (U/U1)(E,/E1)Y(A2/A1)(t/T1)ß,
where U, Ul are the high voltages applied to the cylinder 2 and the test cylinder, respectively. Since the high voltage is the same in all cases, the numerical values of F remain unchanged . 4.3. The Barrel RICH Inner Cylinder With the parameters ß and y of the insulating material, the lifetime Tl (eq. (7)) of the test cylinder at electric field El = 68.1 kV/mm, and the geometrical
41
characteristics of the test (see table 1) and the inner (length = 3.1 m, radius = 1.3 m) cylinders, which yield A 1 and A2 respectively, one obtains from eq. (9) the following result for evaluating the failure probability of the BR inner cylinder F(t ; E)= 1 - exp[ - ( Emax/El) Y (A2/AI)(t/Tt) '6 1 =1 - exp[ -aEm.tß1 ,
(10)
where a =1.665 x 10 -10 , ß = 0.91 ± 0.07 (eq. (5)), Y = 5.7 f 0.2 (eq . (5)) E... = maximum electric field (stress) of IC in kV/mm, t = time in days. If 150 kV at maximum would be applied, which means an electric field E.,,,=10 kV/mm (the Inner Cylinder insulator thickness is 15 mm), the failure probability is F = (2 ± 1)% after one year and the lifetime r = 84 yr. Decreasing the by to 120 kV [7] would increase the lifetime by a factor of about 4 (T = 340 yr). 5. Conclusions Extensive accelerated tests were performed on cylindrical by insulators . The technical fea ibility of various construction details was checked ; essential characteristics of the degrader were investigated and optimized ; parameters which cannot be simulated by computers, such as by rise time ratios, stability, microcorona discharges etc ., were studied . Finally, the time to failure of a test cylinder, subjected to voltages as high as 200 kV, was measured . From the lifetime distribution - checked to be of the Weibull type - of the laminated insulating material used and the values of its shape parameters and the measured lifetime of the test cylinder, a method was described to evaluate the failure probability and the by lifetime of the Barrel RICH Inner Cylinder insulator and of insulators with similar geometry. Acknowledgement This work would not have been possible without the excellent technical work done by J.C. l'Abbe. References [1] DELPHI Technical Proposal . CERN/LEPC/83-3 (1983) ; DELPHI Progress Report, CERN/LEPC/84-16 (1984); DELPHI Progress Report, CERN/LEPC/86-11 (1986) ; DELPHI Notes 83-86, 84-60 GEN-11, 85-44 GEN-31, 85-55 RICH-8A, 86-82 RICH-22 .
42
G. Kalkanis et al. / The DELPHI Barrel RICH electrical by insulator
[2] J. Seguinot and T. Ypsilantis, Nucl . Instr. and Meth . 142 (1977) 377. [3] G. Kalkanis and E. Rosso, to be published in Nucl . Instr. and Meth. (1989) . [4] ALEPH Note 86-145 (1986); M. Price, private communication.
[5) G. Kalkanis, W. Klempt and E. Rosso, Nucl . Instr. and Meth . A275 (1989) 267. [6) International Electrotechnical Commission, Report 15B (Secretariat) 97 (1984). [7] J. Drees and G. Kalkanis, DELPHI 87-64 RICH 27 (1987).