Dc bias of transformer core 2019 - big danger
30 15:05:22 dc transmission system in a highly symmetrical or unipolar runtime, there will be some direct current flow through dc system grounding into the earth, ranging from the earth surface potential at the same time, the dc current may through iron core transformer neutral point into the transformer winding, formed in the iron core transformer dc current, transformer magnetic close working point deviation, make the transformer core produces dc bias, which cause serious damage to the safe operation of power system.
At present, guangdong power grid, jiangsu power grid, guizhou power grid and ningxia grid practical operation experience shows that into the ground current of dc power transmission project has adverse effects on the regional power grid safe operation.
Dc bias magnetic harm basically has the following several aspects: 1, transformer core, noise and vibration increase due to the reason of the transformer core hysteresis expansion, dc bias occurs when the transformer core, core scale, vibration amplitude will increase, resulting in increase noise;
At the same time, due to the hysteresis expansion of non-sinusoidal vibration and the noise contains a variety of harmonic component, when a component with the transformer components resonance occurs, the noise will be bigger, could lead to a loose transformer internal parts, insulation damage.
On March 3, 2004 TianGuang dc and three HVDC unipolar polarity at the same time the earth loop runs, ling ao dc current main transformer neutral point reached 43 a, the main transformer has found that different levels of ling ao nuclear power station core belt loose, core column bending, laminated iron chip series of problems such as;
Two transformers also due to the serious bending core columns and laminated iron chip string piece, causing internal winding short-circuit, had to return to factory maintenance.
In December 2002, longquan & ndash;
Jiangsu ZhengPing 500 kv HVDC project debugging, changzhou Wu Na two sets of 500 kv main transformer neutral point reached 12 dc current.
8 a, noise of 91.
Chaoyang on February 4, 2009 for main transformer substation noise is too big, cause the surrounding community people complain and laid siege to transformer substation.
2, voltage waveform distortion due to the dc bias magnetic transformer core, serious when core might work in saturated zone, so as to increase the magnetic flux leakage field of a transformer core, the voltage waveform distortion.
In May 2004, guizhou high & ndash;
Guangdong zhaoqing dc transmission under monopole earth return operation way, guangdong power grid 220 kv spring city stands the main transformer neutral point reached 34 dc current.
5 a, noise reached 93.
9 db, total harmonic voltage distortion rate was 2.
3, plus or minus harmonics in the half-wave symmetric periodic excitation current contains only odd harmonics.
Due to the dc bias effect, make half wave depth in the saturation of transformer core excitation current harmonics.
At this point, the transformer is ac harmonic sources in the system and may cause the harmonic amplification even resonance compensation capacitor Banks, endanger the safe operation of capacitor Banks.
In three HVDC transmission system monopole earth return operation occurs, 500 kv substation in huizhou guangdong power grid monitoring to the main transformer low voltage side 2, 3 groups of capacitor for four harmonic current effective value up to 200 a.
Because of the harmonic current exceeds bid badly, 2, 3 groups of capacitor 5 times the explosion accident had happened.
4, transformer increase reactive power loss caused by transformer dc bias magnetic saturation, field current increase greatly, increase the loss of transformer and reactive power, it can make the system voltage drop, serious when can make the power grid collapse.
5, relay protection system failure caused by transformer dc magnetic biasing waveform distortion seriously, will lead to part of the relay protection device can't correct operation, the zero sequence of harmonics (
Such as 3/6/9 harmonics)
May lead to zero sequence voltage or current start of the relay protection misoperation.
This is transformer transformer core dc bias has five major hazards may be produced.