44 / 2018-08-17 14:51:14

Harmonic Characteristics of Core Grounding Current of Converter Transformer in High Voltage Direct Current Transmission Systems

,Converter Transformer,Harmonic Characteristics,Core Grounding Current

Converter transformers are the key equipment in high voltage direct current (HVDC) transmission systems and the transformer core is required to be one-point grounded. As one of the most vulnerable components in transformers, if the transformer core is multi-point grounded, the loop current between the core inside and the ground appears. This large current will result in overheat of the transformer core and may finally lead to certain catastrophic power accidents. In China, the measurement of core grounding current of converter transformer still follows the relevant regulations of condition-based maintenance & test in domestic industry standard for traditional AC power transformers. However, the operation condition of converter transformers is quite different from that of traditional AC power transformers, especially the valve side which withstands both AC, DC, harmonic and impulse voltages.
This will inevitably result in some differences and contribute to the total core grounding current. It is sometimes found that the core grounding current of a converter transformer in a ±800 kV converter station exceeds 300 mA. Likewise, the core grounding current of a converter transformer in a ±500 kV converter station may well exceed 100 mA. Actually, the large core grounding current of a converter transformer does not necessarily indicate a grounding fault. Therefore, this prompts us to wonder if it is reasonable to merely set such an alerting value of total current for converter transformers. The alarm value for core grounding current of conventional AC power transformers and converter transformers could be differentiated. In the meanwhile, question rises whether the relatively large core grounding current of converter transformers derive from the harmonics considering the special operation condition of converter transformers compared to conventional AC transformers.
In order to reveal the basic harmonic characteristics of core grounding current of converter transformers, a signal detection and analyzing tester that can analyze the harmonic characteristics of the on-site core grounding current signal of converter transformer was developed in this paper. Then the core grounding current signal of converter transformer in a ±500 kV converter station was obtained using the signal detection and analyzing tester. Finally, the harmonic characteristics of core grounding current of converter transformers were obtained, and its harmonic distribution and corresponding peak ratio to the fundamental harmonic were obtained.
The results showed that the core grounding current of converter transformers has multiple harmonics. Their harmonic characteristics exhibit some differences considering the connection of the transformer windings. However, for the same winding connection, the differences in harmonic characteristics between different polars are not obvious. For the Y/Y connection converter transformers, the distribution of harmonics with the order of 2 to 20 are more even compared with that of Y/∆ connection converter transformers and few distinct large harmonics (with the ratio to the fundamental harmonic greater than 0.5) can be found except the third harmonic. For the Y/∆ connection converter transformers, the harmonics with the odd order are quite obvious. Among those typical harmonics are the 3rd, 7th, 9th, 17th and 19th harmonic. Their ratios to the fundamental harmonic all exceed 50 percent. The peak value of some harmonics is even equal to or greater than that of the fundamental harmonic.