Non-intrusive load monitoring (NILM) is accomplished by sampling voltage and current at high rates and reducing the resulting start transients or harmonic content to concise “signatures.” Changes in these signatures can be used to detect, and possibly diagnose, equipment and component faults associated with roof-top cooling units . Use of the NILM for fault detection and diagnosis (FDD) is important because 1) it complements other FDD schemes that are based on thermo -fluid sensors and analyses and 2) it is minimally intrusive (one measuring point in the relatively protected confines of the control panel) and therefore inherently reliable. This paper describes changes in the power signatures of fans and compressors that were found, experimentally and theoretically, to be useful for fault detection.
14 Figures and Tables
Figure 1. Five compressor starts with no pressure relief.
Table 1. Summary of electrical and electro -mechanical RTU faults detectable by the NILM alone.
Figure 2. Compressor start transients: lower traces normal, upper with 20% overcharge.
Table 2. Summary of non-electrical faults. Some of these can only be detected by the thermal measurements and some can only be detected by the NILM, but detection of most can benefit by having both types of measurements.
Figure 3. Real power start transients, mean of repetitions for each fault level: normal charge (solid), 20% undercharge (dotted), and 20% overcharge (dashed).
Figure 5. Compressor/condenser active power with bypass leakage (bottom four traces) and without (top two traces).
Figure 6. Condenser fan real power start transients, mean of repetitions for each fault level: 0% blockage (solid), 14% blockage (dotted), and 39% blockage (dashed).
Figure 7. Condenser fan pressure start transients, mean of repetitions for each fault level: 0% blockage (solid), 14% blockage (dotted), and 39% blockage (dashed).
Figure 9. Estimated pdfs of supply fan reactive power: 0% blockage (solid), 10% (…), 50% (.-), 100% (--).
Figure 10. Detail of the FFT (18-19Hz band) of condenser fan power (P1) with 0g (solid), 2.6g (dashed) and 5.3g (dotted) imbalance fault levels.
Figure 11. Detail of the FFT (18.4-19.4Hz band) of supply fan power (P1) with 0g (solid), 8g (dashed) and 16g (dotted) imbalance fault levels .
Figure 12. Mean of repetitions with no fault (solid) and 1.0 cc liquid ingestion (dotted).
Figure 13. Mean deviation of P1 for starts with 1.0 cc liquid from P1 for fault free starts.
Figure 14. Real power during steady run liquid ingestion, five repetitions.
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