Dynamic State

S. Meliopoulos, G. Cokkinides, P. Myrda, B. Fardanesh, G. Stefopoulos and M. Razanousky, USA

The solid state relays were short lived as early in the seventies, use of computers to mimic the operation of relays was timidly introduced. The introduction of the microprocessor based relay in the early 80s made the computer approach viable as a low cost but extremely powerful protection system. Numerical relays as we refer to any computer based relay, have the capability to implement the protection functions of the past as well as to improve and introduce new methods.
In addition, they offer the capability of communications and remote access opening up a new era in protection and control. Yet, with all these developments, protection systems are not 100% reliable. Relay mis-operations account for 10% of all relay operations as statistics collected by North American Electric Reliability Corporation (NERC)and others show. It is not only an American statistic but an international statistic as data from other countries show. In many cases these mis-operations can lead to wide power system disturbances and even blackouts, such as the one that happened recently in southern California.  To analyze these concerns in all NERC regions, NERC formed a national task force, the Protection System Misoperation Task Force (PSMTF), which collects and analyzes relay operations. Recently released statistics are shown in Figure 1, which shows that around 65% of misoperations are caused by settings/logic errors as well as relay and communications failures. One explanation of this statistic is that protection relaying has become very complex, leading to errors. The common characteristic of the root causes of mis-opartions is that they remain hidden until they are exposed by an event in the network. It is clear that new approaches for power system protection are needed to address these challenges.

We present and discuss a new method for zone protection based on dynamic state estimation. The approach needs very simplistic settings (that is why it was initially coined setting-less protection) and does not require coordination with any other protection schemes. It operates on sampled values and therefore breaks away from the traditional phasor based protection. We discuss the integration of the setting-less relays for all protection zones in a substation into a substation wide dynamic state estimation that enables detection of hidden failures and an algorithm to enable reliable protection even in the presence of hidden failures. These technologies are described next with examples followed by a discussion.

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