Based on the damping torque analysis (DTA), the dominant instability mechanism of grid-forming (GFM) converters under a stiff grid is identified. With the increase of short-circuit ratio (SCR), the frequency of the low-frequency oscillation (LFO) dominated by the outer active power control loop (APCL) increases, resulting in the inner voltage control loop (VCL) bandwidth unable to counteract the negative damping effect introduced by APCL. In particular, 1) it is proved that the system stability features a maximum stability stationary point with the increase of VCL bandwidth, exhibiting a “non-monotonic” characteristic in system stability. 2) It is demonstrated that the dynamics of the outer reactive power control loop (RPCL) can be neglected in the stability analysis for GFM converters across different grid strengths. Case studies are conducted to verify the effectiveness and generality of the theoretical analysis.

Synchronous condensers (SCs) are generally used at the receiving-end stations of ultra-high-voltage direct current (UHVDC) transmission systems due to their strong reactive power support and flexible regulation of reactive power according to the interconnected grids operating conditions. In this paper, different starting control schemes of a SC integrated power grid are investigated providing four main contributions: 1) The principle of reactive power support of the SC on the interconnected power grid is analytically studied, providing the establishment of mathematical models. 2) Four different starting control schemes are developed for the initialization and SC integration, i.e. in Scheme 1, a preset initial falling speed is directly utilized without initialization; in Scheme 2, a black start sequential control approach with a static frequency converter (SFC) is proposed; in Scheme 3, PI/PD/PID controllers are respectively applied for the excitation device at the speed-falling stage; in Scheme 4, a pre-insertion approach of an energy absorption component with R/L/RL is utilized to suppress the surges at the SC integration instant. 3) The dynamic behaviors of four different starting schemes at specific operating stages are evaluated. 4) The success rate of SC integration is analyzed to evaluate starting control performance. Performance of the SC interconnected system with four different starting control schemes is evaluated in the time-domain simulation environment PSCAD/EMTDC^TM. The results prove the superiority of the proposed starting control approach in Scheme 4.

Increased grid integration of photovoltaic (PV) has aggravated the uncertainty of distribution network operations. For a distribution network with PV, the impact of the PV location on the network power losses and voltage fluctuations is investigated with analytical derivations reflected by the line impedance. Optimization approaches of the PV location with consideration of two aspects, i.e., minimum network power losses and minimum voltage fluctuations, are analyzed. A particle swarm optimization (PSO) algorithm is used to synthesize an optimal compromised solution so as to determine the PV location. A 10 kV distribution network with one PV is established on the time-domain simulation environment PSCAD/EMTDC. The simulation results justify the theoretical analysis and indicate that when the active power of the PV is more/less than twice that of the overall loads/end loads, the network power losses and node voltage fluctuations are both minimum when the PV is integrated into the head/tail end of the network. When the active power of the PV is between the above two conditions, nodes $t/f$ can be identified for the integration of the PV between the head/end nodes of the network to achieve the minimum network power losses/voltage fluctuations, respectively. The effectiveness of the proposed optimization approach is verified and can provide a reference for selecting the PV location in the distribution network.