Application of Artificial Neural Networks - Digital Signal Processing in Power System Protection and Control - page 259

- Optimisation tasks in power plants,
- Signal analysis,
- Process control and automation.
The authors of this topic have also developed a number of ANN-based solutions
for protection purposes, among which the following can be named [ 27 , 29 - 37 ]:
• Out-of-step protection of synchronous machines,
• Transformer protection with neural inrush detection,
• Detection of high impedance faults,
• Current transformer saturation detection and correction.
For the reader's convenience selected example of ANN application for current
transformer saturation detection is described in more detail. This example is
representative enough and may serve as an illustration of design and successful
application of ANN solution for a protection task [ 31 , 33 ].
12.3.1 Example of ANN Application for CT Saturation Detection
and Compensation
The overall performance of a protection relay is also a function of quality of the
analog signal pre-processing path including current and voltage transformers,
analog anti-aliasing filters and A/D converters. One of the most seriously deteri-
orating impacts on protection operation may be observed when traditionally used
induction-type current transformers become saturated due to high AC fault current
and/or DC current components. It is obvious that protection criterion values cal-
culated on the basis of saturated CT secondary signal may fall quite distantly from
their correct values, which might have been determined if the CT primary
unsaturated signal was available. Erroneous measurement may in consequence
lead to false decisions (e.g. underreaching of overcurrent relays, overestimation of
fault loop impedance in distance relays) and protection maloperation [ 16 ].
Several approaches may be found in the literature to mitigate or eliminate the
impact of CT saturation on protection operation. A large number of papers, which
dealt with the CT saturation detection and correction issues, may be divided into
four groups:
• No CT current correction is performed, but the information on CT saturation is
used for other purpose, e.g. for adaptation of protection settings [ 37 ] or per-
forming calculations during CT unsaturated periods only [ 2 ].
• Correction performed is based on the information extracted from the secondary
current during the saturation interval, which reproduces the primary current
waveshape [ 12 - 14 ].