DC-Biased Transformer in High-Voltage DC Power Transmission System
Investigation and Suppression of DC Bias Phenomenon
The DC bias is that DC component appears in transformer-exciting currents, which is an abnormal running state. There are mainly four causes leading to DC bias:
- (1) The operation of high-voltage direct current transmission systems (HVDC).
- (2) The solar storm and the consequent geomagnetically induced current (GIC).
- (3) AC/DC corridor-sharing high voltage power transmission systems.
- (4) The unbalanced triggering angle of the converter valve.
- (1), (3) and (4) are all related to the HVDC.
In order to increase power transmission capacity, provide stability for long-distance power delivery, and decrease transmission line losses, HVDC are utilized in China. Several ± 500 kV DC transmission lines have been built from west to east, each of them almost 1000 km long. However, the use of the HVDC also creates problems of its own. For example, in a monopolar HVDC system, the earth acts as the return path of the DC current, and a large direct current will flow in the earth, which will bring great potential differences in a large area. Thus, direct currents will flow through the transformers in the AC substations if their neutral points are grounded, and the transformers may be under DC bias.
As shown in Figure 2.18, in the normal state, the transformer works in a linear region of the magnetizing curve and the magnetic flux is sinusoidal. When the direct current flows in the windings, the transformer works in an abnormal state, where the direct current generates DC flux, which raises the waveform of the magnetic flux. Consequently, the transformer core is saturated significantly, and the exciting current produces the amount of harmonics under DC bias, due to the nonlinearity of the transformer core. Furthermore, the DC bias may lead to acute vibration, great noise and local overheating of the transformer, and may even make the protection fail. The generated harmonics in the exciting current will increase the reactive power loss of the power grid.
Since the current of the DC grounding electrode is very large, it is difficult to prevent a transformer located in the substations near the grounding electrode being affected. Therefore, it is vital to choose the appropriate grounding site, considering the current distribution in the soil. Some practical and effective methods are required to eliminate or suppress the DC bias phenomenon.
The direct current can be eliminated by connecting a condenser to the neutral point of the transformer, as shown in Figure 2.19. The characteristic of obstructing the direct current is utilized to cut off the invasive passage of bias current. Such obstructing equipment were applied for a set of 240 MVA transformers in 1996 in China.
The neutral point of transformer and the ground can also be linked by the linear or nonlinear resistance. The direct current flowing through the neutral point can be limited in a specific range by adjusting the magnitude of the resistance equipment. Although the
Figure 2.18 Schematic diagram of the DC bias phenomenon
Figure 2.19 Inpouring reverse current for compensation
direct current is not eliminated thoroughly, the transformer will operate normally with slightly biased current.
The above two methods are effective in suppressing the influences of DC bias phenomena. However, possible side-effects as a result of the installed condenser or resistance should be considered, in order to protect the power grid from new hazards.
Another alternative method is to compensate the direct current by an additional power source. The device injects reverse current with the same strength as the biased current to the system. This method is comparably reliable and safe, and has been adopted in a substation in Jiangsu province, China.