IEC 61378-2 pdf download Convertor transformers — Part 2: Transformers for HVDC applications
7.5 Determination of hot-spot temperature
The procedure to determine the hot-spot temperature shall be in accordance with section 2 ofIEC 60354. lt should be noted that the hot-spot temperature rise determined by this proceduremay not be the same as the value likely to be obtained in normal service because the extralosses in service due to the harmonic current are mainly concentrated at the ends of thewindings whereas those occurring during the test are distributed throughout the winding.As aresult,the hot-spot generated during the test may be less than that developed in normalconvertor operation. In order to compensate for this difference in performance, anenhancement factor for eddy losses will be calculated for the specific harmonic content of theload current.
8Insulation levels8.1 Line windings
The insulation levels for the line terminals，windings and neutrals are defined in clause 5 ofIEC 60076-3:
– highest voltage for equipment applicable to a transformer winding,Um;
– rated insulation level: rated lightning and switching impulse withstand voltages(phase to earth);
separate source a.c. corresponding to insulation level of neutral terminal.
NOTElt is important that the transferred voltage does not exceed the specified switching impulse level of thevalve winding.
8.2 Valve windings
The purchaser shall specify the lightning and switching impulse levels in accordance withtables 2,3 and 4 of lEc 6o076-3.
NOTE The insulation levels should be coordinated in such a way that there is an adequate margin in excess ofthe dielectric stresses that appear in actual service.
8.2.1 Lightning impulse levels
The lightning impulse level of the valve winding shall be specified for each terminal to earthand across the winding(s).
lf chopped wave tests are required， the test voltage shall be as prescribed in clause 14 ofIEc 60076-3.
8.2.2 Switching impulse levels
The switching impulse level of the valve winding shall be specified for each terminal to earth.
However, windings for back-to-back schemes should not be tested, because they are normallylow-voltage windings,and there is no possibility for transferred switching impulse from thed.c. system.
The valve windings are subjected to a transferred switching impulse during the test of the linewinding.
8.2.3 DC separate source-voltage withstand level
These tests shall be applied to the terminals of the valve winding(s). The test voltage shall be given by U dc = 1 ,5 (( N – 0,5) U dm + 0,7 U vm ).
Positive polarity shall be used.
8.2.4 Polarity-reversal level
These tests shall be applied to the terminals of the valve winding(s).
The test voltage is given by U pr = 1 ,25 (( N – 0,5) U dm + 0,35 U vm ).
For an explanation concerning the polarity-reversal test, see IEC 61 378-3.
8.2.5 AC separate source-voltage withstand level
All terminals of the valve windings shall receive an a.c. separate source-voltage withstand test. The test level is given by