7th International Symposium on H.V. Engineering - ISH '91 Dresden, Germany

DR. STRAUSS

Abstract 

The paper describes EMC tests of a digital H.V. control and measuring system especially by application of electric and magnetic fields ln conformity with the IEC Draft 42(Central 0fflce)43. Further tests in a practical arrangement with breakdowns in air and in SF6 have shown the necessity of such tests in addition to the IEC tests.

1.  Introduction

The use of digital electronic systems as transient recorders, digital oscilloscopes or computers for H.V. tests is connected with the problem of high electromagnetic interferences /1/. Especially if a breakdown is occuring, the stress caused by the high frequent electromagnetic fields and  voltages is extremly high.

The IEC draft 42(C.0.)43 /2/ gives recommendations for the test of the electromagnetic interferences by means of electric and magnetic fields. In the following the EMC test of a H.V. impulse measuring system (TR-AS 100-8 /3/) which was extended by a control system (IMS 12 /4/) is described. Furthermore the system will be tested under hard operation conditions in a H.V. laboratory.

2.  IEC-Test by Application of Electric and  Magnetic Fields

2.1. Realized Arrangement

The Figure 1 (corresponding to Fig. A.2 in /2/) shows schematically a simulator for the application of electric and magnetic fields. A realized arrangement, using a sphere gap with 125 mm diameter and a cylindrical con

The impedance Z of the line was determined by /6/

Flg. 2  Realized arrangement for testing the electromagnetic behaviour of the impulse control and measuring system

An ohmic impulse voltage divider 770 kV, 10 kOhm (type SMR 10/770 /7/) was connected with the end of the line. Its response time has a value of 10 ns. The rise time achieves about 20 ns.
During the test the impulse control and measuring system must be arranged in a horizontal position, because its height is larger than 1 m.

2.2. Electric and Magnetic Field Tests

The tests were carried out in the following way:

(1) Preliminary measurements

· registration of the waveform at the end of the line using the transient measuring system TR-AS 100-8 and a digital oscilloscope type NICOLET 4094 with a sampling rate of 500 MHz

· both devices were arranged outside of the electromagnetic field

· the line was open at is end respectively terminated by it surge impedance (R = Z) or short-circuited

The Figures 3a, 4a and 5a show some results of the preliminary measurements.
The rise time of 53 ns (Fig. 3a) for the voltage impulse as well as the damped oscillation of 0,45 MHz (Fig. 4a) for the current in the short-circuited case meet the requirements given in /2/.
A high frequency initial oscillation of about 18 MHz is visible in Figure 5a.

Fig.1:  Simulator for the application of electric and   magnetic fields /2/
Z  =  characteristic impedance
C  =  20 nF, l₁ = 5 m. l₂ = 1 m
U_O  =  40 kV at R = Z
U_O  =  100 kV at R = 0

Analogous to these papers an arrangement shown in Fig. 2 was choosen to test the electromagnetic behaviour of the transient measuring system. A sphere gap in

Fig. 3:   Voltage at the end of the line
Charging voltage +40 kV, line  open, measuring divider  SMR 770/10

1. Preliminary measurement
   
2. During the EMC test
   

Fig. 4:  Current at the earthed end of the line
Charging voltage -100 kV, line short-circuited, damped oscillation of the main discharge current

1. Preliminary test
   
2. During the EMC test
   

(2) Main Test

· the test object was placed in a horizontal position below line near its the end (Fig. 2)
the measuring points were the same as during the preliminary tests

· the EMV-test took place with negative and positive polarity of the charging voltage

· the line was open at its end, respectively short-circuited

· for each condition (-, +, open, short-circuited) ten discharges with 40 kV  respectively 100 kV charging voltage  were applied and registrated

· after each strain an operation checkout of the whole system followed


The Figures 3b, 4b and 5b show the aquivalent oscillograms to the measurements at undisturbed conditions (Figures 3a, 4a and 5a).
There were no remarkable differences recognized. During all the tests the impulse control and measuring system operated without faults.

Fig. 5:  Current at the earthed end of the line

Charging voltage -100 kV, line short-circiuted,   initial oscillation at the first half wave of the current
a) Preliminary test
b) During the EMC test

3.  Test in a H. V. Laboratory
The EMV-tests under defined conditions as recommendated in /2/ and described above are very necessary. They allow to refer to an international acknowledged standard for contracts and acceptance tests.
But for practice, it is furthermore interesting to know the behaviour of the electronic devices in real arrangements of H.V. test circuits. Very often the earthing conditions are not perfect and additional overvoltages arise in the cabling.
For that reason the digital impulse control and measuring system was testet at an impulse generator 1 MV/20 kJ with full and chopped lightning voltages. Figure 6 shows the very compact installation of the several components in the testing room.

Fig.7:  Chopped lightning impulse in air

Fig.8:  Chopped lightning impulse in SF6

Fig. 6: Geometrical installation of the several components during the test in a H. V. laboratory
G Impulse generator 1 MV, 20 kJ
F  Front capacitor
CH Chopping gap in air, 50 cm  diameter
P  Pressure gas capacitor MCF  120/150 /8/, filled with  SF6
D  Resistive Divider SMR 770/10  /7/
C,M  Control and measuring system

A stronger strain is achievable by chopping the lightning impulse by means of a SF6 gap. For this purpose a compressed gas capacitor was fllled with SF6 (component P in Fig. 6). Its disruptive lightning lmpulse voltages amount between 350 kV (positive polarity) and 420 kV (negative polarity).
At the beginning of the tests an unsufficiency ln the system was discovered and eleminated. The following tests with about two hundred chopped impulses were without failure. The Figures 7 and 9 show oscillograms of

Conclusion
For the description of the electromagnetic compatibility of a H.V. impulse control and measuring system the EMC test, recommended by the new IEC Draft /2/, is a valuable tool, but there are practical arrangements in which higher stresses must be expected. Therefore additional EMC tests in a practical arrangement, as shown in Fig. 6, are useful and necessary. The
development of both systems TR-AS 100-8 and IMS 12 was closely related to both types of EMC tests.
References
/1/  Malewski, R. et al.:  Electromagnetic Interference Field Induced by Discharges of H.V. Irnpulse Generators. Paper 64.10. 4th ISH Athen (1983).
/2/ IEC Publ. 1083-1, Draft 42(Central 0ffi.ce)43 (1990-03-07). Digital Recorders for lMasuremens in High Voltage Impulse Tests, Part 1 -Requirements for Digital Recorders.