Slingshot Disconnection Understanding the Effect of Disconnecting an Un-Stabilized Ungrounded Source

The IEEE C37.234 [1] guide is important for protection and control engineers looking to implement bus protection. Within this framework, Section 8.3 addresses bus protection and details using a broken delta-configured PT and stabilizing resistors to mitigate large overvoltage on ungrounded systems and to assist in detecting ground faults. This paper investigated the sizing and impact of these elements on ungrounded systems using an Electro-Magnetic Transient (EMT) program. This paper also explored the phenomenon we call the “voltage slingshot” effect. Unbalanced capacitance within a power system can lead to asymmetric voltage distribution and transient instability. The incorporation of stabilizing resistors can be used to mitigate these issues. Section 8.3 of IEEE C37.234 [1] describes the impact of broken delta-configured PTs connected to a stabilizing resistor and the existing industry practice for sizing them. We investigated the efficacy of this setup using EMT software with different magnitudes of capacitance imbalance on a system. We concluded those situations that warranted or precluded a broken delta/stabilizing resistor as an effective solution to system imbalance. We created an equation describing the relationship between capacitance imbalance and zero sequence voltage/stabilizing resistor current flow. Furthermore, this paper explored the voltage slingshot effect, which arises during rapid breaker reclosing onto a capacitive system. Such actions can cause voltage levels to surge well beyond 5 per unit, potentially compromising the insulation and protective devices. Understanding the mechanisms behind this effect is paramount to ensuring the reliable operation of power systems. In conclusion, this paper highlights the significance of Section 8.3 in IEEE C37.234 [1], emphasizing the need to address the impacts of broken delta and stabilizing resistors on systems with imbalanced capacitance. It underscores the importance of safeguarding against the voltage slingshot effect to maintain system stability and reliability in the face of electric transient disturbances.