Abstract
In China, a structure in which multiple line-commutated converter based high-voltage direct current (LCC-HVDC) systems feed into the same AC system has been formed. For the multiple LCC-HVDC infeed system, the electrical strength and real operating condition evaluation of the receiving AC system are of great significance for system operation. The multi-infeed short circuit ratio (MISCR) is generally utilized to assess the strength based on the rated parameters; hence, it cannot analyze the real operating condition under the non-rated status. In this paper, an LCC-HVDC system is equivalent to a grounded impedance without changing the external characteristics. The multi-infeed operating short circuit ratio (MIOSCR) is then deduced from the equivalent impedance and equivalent nodal analysis. Relationships between the MIOSCR and electrical strength indices of AC and AC/DC systems are analyzed, and the MISCR is deduced as a special case of the MIOSCR under the rated status. The multi-infeed operating margin coefficient (MIOMC) is finally proposed, which can be utilized to evaluate the operating margin of the LCC-HVDC system and provide guidance for designing operating strategies. The proposed MIOSCR and MIOMC overcome the previous shortcomings of inaccuracy about operating status assessment by the MISCR. The results of the case study demonstrate the effectiveness of the MIOSCR and the MIOMC for assessing the operating condition and margin of converter buses.
Highlights
The line-commutated converter based high-voltage direct current (LCC-HVDC) transmission system plays an important role in interconnecting power grids due to its fast and controllable power regulation and the economics of longdistance transmission
It can be seen that only the rated DC power of LCC-HVDC systems are considered in the expression of the multi-infeed short circuit ratio (MISCR); the assessment under real operating conditions by the MISCR may lead to inaccuracies
The multi-infeed operating short circuit ratio (MIOSCR) is defined based on the ratio between the bus self-impedance and equivalent HVDC impedance, and it considers the converter bus voltage and reactive power consumed under the non-rated status, avoiding the deficiency of the indicator defined based on the rated active power ratio
Summary
The line-commutated converter based high-voltage direct current (LCC-HVDC) transmission system plays an important role in interconnecting power grids due to its fast and controllable power regulation and the economics of longdistance transmission. For the MIDC system, the LCC-HVDC systems are connected to different AC buses, and the interactions between converter buses cannot be ignored In this case, optimistic results of the electrical strength will be obtained when using the SCR since the interactions between inverters are not considered [9]–[13]. CIGRE proposed the multi-infeed short circuit ratio (MISCR) indicator to evaluate the electrical strength based on the SCR expression [14]. The generalized SCR proposed in [17], [18] is based on the correspondence between the voltage stability limit and the determinant of the Jacobian matrix of the power flow equations This indicator is difficult to calculate; its application is restricted. The MISCR value of converter buses can be obtained based on network parameters and the rated DC power of HVDC systems
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