Power System Protection & Power Swings

Erstwhile stand-alone Power Systems in India had been interconnected to form large integrated Regional Grids, around 30–40 years ago. These Regional Power Pools had been operating with a few asynchronous ties with the neighboring Power Pools, till 2003. These Regional Power Pools have since been synchronously interconnected with each other, with a number of 765/400/220 kV interconnectors. The last asynchronous region in the mainland India has also been recently connected synchronously, to form a large single synchronous system. Large scale synchronous interconnections are bound to bring forth Power System Stability issues. Whereas, Power System Stability as an issue is well understood, treatment of the same and the defense mechanism against a possible state of instability are not so well understood. Power Swings, which arise as the natural consequence of readjustment of the system to a disturbance can, if not adequately damped, lead to instability and loss of synchronism.

Power Swings occur in an ac power system, as a natural consequence of the process of restoring the dynamic balance in the system following any system event, disturbing the balance of the system. The large amplitude Power Swings could be caused by system events like a severe fault duly cleared, cleared with some delay (say due to Breaker failure), switching out of a heavily loaded system element etc. Even normal events like the outage of generators, disconnection of loads, outage of a transmission element and so on also introduces Power Swings, of course normally of relatively smaller amplitudes.

These Power Swings which are caused by the natural response of the system to regain a new equilibrium can be recoverable (when the system is stable) or irrecoverable (when the system is unstable). If however, during the tenure of the Power Swing, if another system element were to be switched out, by the protection system of another element, operating upon incorrectly sensing the Power Swing condition as a fault, the situation may get aggravated and may trigger a cascade and eventual loss of synchronism. In the absence of adequate defense mechanisms in place, the whole system may collapse. Such a situation might have been avoidable, had the concerned protection device not made the unwanted tripping decision sensing Power Swing as a fault. Such a possibility needs to be prevented. In other words, it is extremely important to prevent protective devices from operating and tripping, otherwise healthy system elements, during a Power Swing situation.

The most widely used transmission line protection system, broadly classified as ‘Distance Protection’ are susceptible to sensing the Impedance presented to it during a Power Swing as a three phase short circuit, unless it is prevented from doing so by supplementary logic. All distance protection relays are hence complimented by Power Swing Blocking (PSB) logic, for this purpose, using the rate of change of impedance, as a distinguishing characteristic. Choice of application of PSB logic is left to the user.

The application of PSB logic in distance protection in India however, has not kept pace with the growing needs of the system development. The nearly half a century old principle of blocking tripping in back up zones while allowing tripping in primary zone (Zone-1) is still being followed, in our country. The CBIP publication 274 of 1999, which happens to be the only comprehensive document on Power System Protection application also did not cover this aspect, leaving the choice to the respective user. It will be remembered that in the devastating events of 30/31 July 2012 also, several of the ER-NR-WR interconnecting lines had tripped on Power Swing. There is a fair chance that had these lines been prevented from tripping, the system could have survived. There is no way of being positively sure, about this possibility, since these lines had their distance protection Zone-1 tripping allowed by the PSB option.

Though it is possible to block distance protection relays from tripping by PSB logic, a supplementary logic for positive tripping to split the system in the event of irrecoverable Power Swings (Instability, from which recovery is not possible, without positive system splitting) is also absolutely essential. This can be achieved by strategically applied Out Of Step (OOS) protection. In India OOS tripping is yet to be applied to transmission lines, though this has by now become an established practice globally. With the advent of numerical relays OOS protection is a low cost add-on and can be provided easily.

The paper discusses the phenomenon of Power swing in some detail. Distance Protection principle and its susceptibility to sensing a Power Swing as a three phase fault is explained. The physical characteristic of Power Swings, used to discriminate between a swing and a fault is explained and the PSB logic and blocking options available are discussed. The possibility of distinguishing between recoverable and irrecoverable Power Swings (a Loss of Synchronism situation) and the distinctly conflicting requirements in the two situations are brought out. Finally, a comprehensive logic of blocking Distance Protection tripping in all zones during Power Swing and use of OOS tripping for system splitting on a pre-selected axis to allow recovery from Loss of Synchronism is proposed. There is an urgent need for adoption of such a treatment in the Indian Power System, which is all the more susceptible to instability, now than ever before.