Requirements and Practical Experience for Generator Protection Specific Functions

Generator protection is the most complicated part of protective relaying. It includes both impedance based and differential based protection functionality. However, in addition it often requires very specific protection functionality which is not present at all in any other parts of the power system.

The goal of this paper is to present and explain end user requirements and possible practical solutions for such specific generator protection functionality. For example, the following functionality will be discussed:

1. Shaft overcurrent protection has traditionally been using magnetic core CT. Such solution had a lot of practical problems regarding positioning and mounting of the shaft CT. A modern solution utilizing a Rogowski Coil for shaft overcurrent protection will be presented. It has been successfully in operation for several years in a large hydro power plant and several new installations of such scheme are on its way.

2 .Injection based 100% Stator ground fault is often used on large machines. It poses its own challenges for the protection equipment and sometimes is quite difficult to achieve due to various reasons. A possible solution of using a dedicated stator injection transformer will be presented in the paper.

3. Synchronizing of the generator to the grid is an important functionality and requires dedicated equipment to achieve high quality synchronizing. “Bad synchronizing” or “Out-of-phase” synchronizing can be harmful for both the generator and the grid stability. Tripping after such an event can even be harmful for the circuit breaker. Due to that reason special functionality is developed to detect “bad-synchronizing” of the machine to the network. Principles and operation of this scheme will be discussed in the paper.

4. In Norway islanding operation of a certain part of the grid is quite common. Due to that reason there are grid requirement for generators, but as well for associated line protection, to withstand and operate for large frequency deviation from the rated frequency of 50Hz. For example, in some parts of the grid a frequency excursion up to 70Hz during 10s shall be possible. Such islanding conditions are even tested live, and some records and practical experience will be shared in the paper.

Paper will explain the requirements, application and experience for these dedicated generator protection functionalities. Actual field recordings will be used in the paper to demonstrate protection behavior.