| Summary: | The influence of photovoltaic (PV) output with stochasticity and uncertainty on the grid-connected system’s voltage stability is worth further exploration. The long-term voltage stability of a 3-bus system with a large-scale PV power station considering the adjustment of an on-load tap changer (OLTC) was studied. In this typical system, two supercritical Hopf bifurcation (SHB) points are found using the bifurcation calculation. At the SHB point that appears first, a small sudden increase in reactive load power or a sudden increase in PV active power <i>P</i><sub>pv</sub> can eventually cause a voltage collapse after a long increasing oscillation. The long-term collapse phenomenon shows that SHB cannot be ignored in the PV grid-connected system. Meanwhile, the time constant of OLTC can affect the progress of long-term voltage collapse, but it has different effects under different disturbances. When <i>P</i><sub>pv</sub> drops suddenly at the SHB point, due to the adjustment of OLTC, the load bus voltage can recover to near the target value of OLTC after a long period of time. Similarly, the time constant of OLTC can affect the progress of long-term voltage recovery. To prevent the long-term voltage collapse when <i>P</i><sub>pv</sub> increases suddenly at the SHB point, a new locking-OLTC index <i>I</i><sub>lock</sub>, depending on the value of <i>P</i><sub>pv</sub> corresponding to the SHB point, and a locking OLTC method are proposed, and the voltage can be recovered to an acceptable stable value quickly. Compared with the system without OLTC, OLTC adjustment can effectively prevent long-term voltage oscillation instability and collapse, so that PV power can play a bigger role in power systems.
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