Special Protection System Advantages and Disadvantages

Quite a few yrs. ago around 1988, I was a Protection and Control Engineer at a large utility in the SE. We were doing our planning to bring the final unit of a large 4 unit plant on line, when it was discovered that we could encounter some unusual instability scenarios. The funny thing was that with all units on-line and above a certain MW output, all that would need to occur would be opening of a remote 500kV breaker on one of the particular lines and the event could trigger, eventually bringing ALL 4 units at the plant out-of-step and tripping off all of the generation in just a few minutes (3600 MW).

The studies were performed numerous times by internal and external experts but the results were always the same. The key problem seemed to be the existing network configuration of 4 units and only 3 transmission lines. Adding a 4th 500kV line from the plant seemed to cure the problem under all conditions, including close in 3 phase faults with breaker failure. Unfortunately, the cost and timeline to build a new t-line was a real challenge!

In order to proceed with commissioning the 4th unit and remediating any scenarios for tripping all generation, a Special Protection System (SPS) was developed. A transfer trip channel was installed at the remote substation, keying on the breaker contact opening. At the plant, a Unit Trip scheme was installed that had a MW meter supervising tripping of any one unit selected by the plant operator (U1-U2-U3-U4). If all units were on line and generation was above 2500 MW (margin of safety added), then a receipt of remote breaker opening would trip the selected unit to avoid having all units cascade into out-of-step condition.

Advantages: Clearly, this Special Protection System saved the day, and bought time until an additional line was added 4 years later.

Disadvantages: The downside was the challenge of installing and testing such a complicated scheme with the potential for mis-operation. I don't recall any mis-operations occurring, but it was still a bit "dicey". I have been at that same plant during a full load unit trip (Generator differential) and it was an "exciting" experience to say the least! While I did recommend that we conduct a "live test" to see what would really happen and perhaps test our system BLACK START procedures, this suggestion was not well received by management (LOL).

This was my only encounter with such a special protection system scheme in my 35 years of utility work, but it was very interesting to be involved with this project.