Variable Window RMS Protection for Doubly Fed Induction Machines
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Solution Overview
Problem
Conventional protection relays for doubly fed induction machines are inadequate for providing protection during extremely low frequency currents and voltages, especially during the start-up and braking phases, due to limitations in frequency tracking and measurement accuracy at low frequencies, which can lead to safety issues.
Innovation Solution
A protection system utilizing non-conventional instrument transformers and a protection relay with a constant sampling rate, frequency determination module, and RMS-based protection algorithms, capable of measuring low frequency signals from 0.1 Hz to 6 Hz, and implementing protection algorithms using root mean square values within a variable size rolling window.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If Fourier methods with frequency tracking are used in conventional protection relays, then protection can be provided for standard frequency ranges, but the protection execution rate slows down at low frequencies and cannot provide fast protection for extremely low frequency signals
Solution Approach 1:
The patent changes the fundamental parameter of frequency measurement from Fourier transform methods to RMS-based methods. This allows the protection relay to operate effectively across a wide frequency range including extremely low frequencies (0.1 Hz to 6 Hz) without the execution rate degradation that plagues Fourier-based systems at low frequencies.
Solution Approach 2:
The patent replaces the conventional Fourier transform mathematical mechanism with an RMS-based protection mechanism. This substitution eliminates the frequency tracking dependency and enables fast protection execution regardless of the actual frequency, solving the contradiction between measurement precision and execution rate.
Data Source
AI summary
A protection system for an electrical variable speed doubly fed induction machine (IM) having a stator (S) and a rotor (R), comprising: - instrument transformers (CTR1, CTR2, CTS1, CTS2, VTR1, VTR2) capable of measuring low frequency signals from the rotor and/or the stator; - a protection relay (R1, R2) configured to receive measurement values from the instrument transformers sampled at a constant sampling rate, wherein the protection relay comprises a frequency determination module for determining the frequency of the rotor or stator low frequency signals, and at least one protection module configured to implement an algorithm making use of root mean square values each calculated from a series of rotor or stator sampled measurement values within a variable size window size of which depends upon the determined frequency of the rotor or stator low frequency signals.