IEC 61378-2:2001 pdf download

IEC 61378-2:2001 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 of lEG 60354. It should be noted that the hot-spot temperature rise determined by this procedure may not be the same as the value likely to be obtained in normal service because the extra losses in service due to the harmonic current are mainly concentrated at the ends of the windings whereas those occurring during the test are distributed throughout the winding. As a result, the hot-spot generated during the test may be less than that developed in normal convertor operation. In order to compensate for this difference in performance, an enhancement factor for eddy losses will be calculated for the specific harmonic content of the load current.
8 Insulation levels
8.1 Line windings
The insulation levels for the line terminals, windings and neutrals are defined in clause 5 of
lEG 60076-3:
— highest voltage for equipment applicable to a transformer winding. Lm.
— rated insulation level: rated lightning and switching impulse withstand voltages (phase to earth);
— separate source a.c. corresponding to insulation level of neutral terminal.
NOTE Ills important itial the transferred vollage does not .icceed the specified switching imputs. level of the valve winding.
8.2 Valve windings
The purchaser shall specify the lightning and switching impulse levels in accordance with tables 2. 3 and 4 of lEG 60076-3.
NOTE The insulation levels should be coodinaled in Such a way thai there I$ an adequate margin WI eacess Of the d.electrlc 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 earth and across the winding(s).
If chopped wave tests are required. the test voltage shall be as prescribed in clause 14 of
IEC 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 normally low-voltage windings. and there is no possibility for transferred switching impulse from the d.c. system.
The valve windings are subjected to a transferred switching impulse during the test of the line winding.
8.2.3 DC separate source-voltag, withstand level
These tests shall be applied to the terminals of the valve winding(s). The test voltage shall be
given by U = 1,5 ((N — 0.5) t1dm + 0.7 U).
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 UP, = 1.25 ((N — 0,5) Udm + 0.35 Uvm).
For an explanation concerning the polarity-reversal test. see IEC 6 1378-3.
8.2.5 AC separate source-voltage withstand level
All terminals of the valve windings shall receive an a.c. separate source-voLtage withstand lest. The test level is given by
8.3 Induced voltage level with partial discharge measurements
With the lransormer connected and excited as in service, a routine induced voltage test as described in clause 12 of lEG 60076-3. based on the highest voltage for equipment, tjm’ shall be performed.
9 Sound level
9.1 General
Unlike ac. power transformers which normally exhibit a sound-power level under service conditions virtually the same as the one obtained at no-load during factory tests, the sound- power level of a convertor transformer cannot be determined easily for the load condition at site and is likely to be substantially greater than that derived from tests at no-load. This is due to the harmonic content of the load current and to the d.c. bias currents in the windings connected to the valves.
9.2 Guaranteed sound-power levels
Guaranteed levels shall be clearly based on sinusoidal no•load excitation or sinusoidal loadcurrent excitation at the time of placing the order. In recognition of the difference between this sound•power level and that which is measured on site under load conditions. (see 9.3), the purchaser shall also specify the individual and aggregate maximum sound•power levels of the main Iransformer and cooling equipment determined at the factory at noload. or agree on the values with the supplier before contract.