High-voltage direct current (HVDC) technology has been applied in many countries. Due to power imported via HVDCs, many synchronous generators have been shut down in receiving-end grids. Under this circumstance, simultaneous commutation failures (CFs) of multi-infeed HVDCs are likely to occur under AC faults that may cause a blackout. Thus, it is crucial to enhance dynamic var support in receiving-end grids. Many studies mainly focus on fast action of power electronic dynamic var sources and synchronous machine (SM) AVRs’ control, while spontaneous var output (SVO) of SMs may be underestimated. However, due to time delay of power electronic and AVR control, initial CFs are still likely to occur under short-circuit faults. To address this problem, this paper analyzes the mechanism of SVO and its impact on initial CFs. Then, the concept of Volt-VAR secure operational range (VVSOR) is proposed to ensure sufficiency of SVOs for preventing initial CFs. An optimization model for evaluating the VVSOR is established accurately without iteration with time-domain simulations. A heuristic solution method based on the decomposition algorithm is proposed. Finally, validity and efficiency of the evaluation method are verified based on a modified IEEE 39-bus system and a real system in eastern China.

State estimation (SE) is essential for combined heat and electric networks (CHENs) to provide a global and self-consistent solution for multi-energy flow analysis. This paper proposes an SE approach for CHEN based on steady models of electric networks (ENs) and district heating networks (DHNs). A range of coupling components are considered. The performance of the proposed estimator is evaluated using Monte Carlo simulations and case studies. Results show that a relationship between the measurements from ENs and DHNs can improve the estimation accuracy for the entire network by using the combined SE model, especially when ENs and DHNs are strongly coupled. The coupling constraints could also provide extra redundancy to detect bad data in the boundary injection measurements of both networks. An analysis of computation time shows that the proposed method is suitable for online applications