Assessment of transient stability of cable based transmission grids with reactive power compensation

Underground transmission cables are gaining popularity due to its applications near cities and aesthetic purpose. For example in Denmark, the transmission power grid is changing significantly as many conventional overhead lines (OHL) are replaced by cables and more is expected over the coming years.

Due to the difference in physical geometry and materials, the electrical line parameters are different between the two. The shunt capacitance in particular, is substantially higher in cables as compared to OHL. As opposed to traditional cables found in distribution voltage levels, the amount of reactive power generated by the capacitance of HVAC transmission cables is significantly large posing a greater challenge for grid operators to maintain system stability. The excessive reactive power alters the bus voltages and excitation level of synchronous generators (SGs) which results in a different equilibrium operating point during both steady and transient states. It is necessary to use reactive power compensation devices such as shunt reactors (SRs) to mitigate these effects, with respect to grid codes and operation limits, and if no reactive compensation is applied, SGs will operate in severe under-excitation mode. For transient stability, such condition results in a loss of synchronizing torque to oppose the rotor acceleration and power angle deviation following a large disturbance, making the system more susceptible to instability.

The main purpose of this paper is to investigate the effects of undergrounding on the transient stability of transmission systems and how reactive power compensation can influence the performance. This is achieved by assessing and comparing Critical Clearing Time (CCT) between OHL and an equivalent compensated cable case studies following a permanent fault. The study is done through a series of sensitivity analysis with respect to the cables compensation degree. A separate case of disconnecting the SRs of the faulted line is also carried out. The tendencies are initially observed and explained for smaller systems, Single-Machine Infinite Bus (SMIB) and 9-bus system, and subsequently shown on the larger IEEE 39-bus system. The investigation concludes that fully compensated equivalent cable system performs consistently better than the OHL system, though without a general pattern. On the other hand, it is observed that the system stability limit can be improved further by switching out the corresponding SRs of the faulted line when the line is taken out of service. Over-compensation without SR switching, also leads to a positive effect on the transient stability. However, over-compensation improves the excitation condition of the SGs at the expense of voltage stability, hence the improvement seen can be expected only up to a certain range of compensation degree.