Distributed Measuring System for Predictive Diagnosis of Uninterruptible Power Supplies in Safety-Critical Applications
2. Critical Analysis of the State-of-the-Art for UPS Predictive Diagnosis
- An increase of the cost and the size of the diagnostic system.
- Poor system scalability with an increase of the effort for its installation and maintenance.
3. Innovative Power Supply System for Safety-Critical Applications
- AC/DC rectifier stage plus input transformer. This unit converts the AC voltage of the line to a DC voltage used to charge the batteries and to feed the following DC/AC inverter. The rectifier is realized through a 6-branch 3-phase silicon controlled rectifier (SCR) bridge and is sized to supply simultaneously the inverter, in conditions of maximum load, and the battery at the maximum charging current. To reduce the distortion produced by the network rectifier and the ripple to the battery, an isolation and voltage adaptation transformer is used, together with inductors placed at the exit of the bridge conversion.
- Energy storage. The energy storage unit is organized as a complex array of valve regulated lead acid (VRLA) rechargeable cells. This technology is used due to its low purchase cost and low maintenance cost/effort vs. other battery technologies. Thanks to the autonomy of the energy storage subsystem, a static UPS reduces the carbon emissions related to operating an EG. The latter converts in electric energy the mechanical energy produced with an internal combustion engine. During a brief power outage, the VRLA cells provide current to the load, eliminating the need to start the EG. Battery modules with high capacity lead to a reduced number of EG starts per year. This way the fuel consumption, the CO2/NOx emissions and the warm-up and cooldown phases of the EG, are reduced.
- DC/AC stage with inverter transformer. The inverter converts the DC voltage Vdc supplied by the AC/DC rectifier or from the battery into AC voltage, stabilized in terms of frequency and in terms of amplitude. The inverter output voltage is generated through a switching strategy with pulse width modulation (PWM) driving insulated gate bipolar transistor (IGBT) power devices. The use of a high carrier frequency for the PWM, and of a dedicated filter circuit constituted by the AC transformer and capacitors, ensures minimal distortion of the output voltage. A THD lower than 2% can be achieved (1.5% with a linear load in Table 1). As discussed in , if input and output transformers are both present, then the inverter transformer is redundant.
- Bypass stage with AC/AC regulation and Bypass Switch. In normal operating mode, the inverter guarantees the power distribution. In case of malfunction or overload in the inverter, the bypass line supplies the load, through the main static switch. The possibility of a manual bypass is also foreseen. A 3-phase voltage stabilizer maintains the nominal value of the output voltage within a 2% compared to a variation of the input voltage range between ± 10% of the nominal value of the line.
- Diagnostic supervisor. This block in Figure 1 and Figure 2, is in charge of managing for the UPS the testing and periodic checks, ensuring the operation of the equipment and generating alarms in case of anomalies. This block implements a predictive diagnostic strategy that exploits the network (see Figure 2) of monitoring units for faults in the battery modules and faults in the power transformers.
4. Battery Degradation and Specifications for the Monitoring System
5. Degradation in Power Transformers and Specifications for the Monitoring System
- They allow a diagnosis of the fault when is too late and the transformer is already damaged.
- As proved by experimental measurements carried out on real power systems, in the operating environment where such systems are used the background acoustic noise level is at least in the range of 60 dB/70 dB, with frequencies from few Hz to kHz. The acoustic effects of vibration degradation of the power components are detectable only after a big damage has occurred.
- An accurate visual inspection often requires a stop of the DUT, thus causing a denial of service.
6. Predictive Diagnostic System Implementation
- Three capacitive accelerometers, capable of 3-axis measurements with a bandwidth up to 1 kHz, a sensitivity of 0.5 mg, and a dynamic range configured to ±2 g. The LIS3DH sensors from STMicroelectronics, realized using micro-electro-mechanical-systems (MEMS) integrated technology, have been adopted. To easy their mechanical connection with the DUT, each accelerometer is encapsulated within a dedicated case.
- Each MEMS accelerometer sensor has an on-chip dedicated analog to digital converter (ADC) with relevant memory buffers, managed through a first-in first-out (FIFO) policy. The digital outputs of the MEMS accelerometers have a 16-bit data size and are acquired by the Cortex M3 processing core in Figure 4 through its digital I/O interface.
- On-chip Digital to Analog Converter (DAC) with two channels, each with 12-bit resolution. A trans-conductance amplifier is mounted on the printed circuit board (PCB) to generate current waveforms at the output with peak levels up to 10 A, needed for battery impedance measures.
- On-chip 48 kSa/s ADC with 16 multiplexed inputs at 14 bits.
- Mixed-signal processing chip integrating a 32-bit ARM CortexM3 processor at 100 MHz with 256 kByte of non-volatile flash memory and 96 kByte of RAM memory plus a rich interface set: 3 SPIs, 1 CAN, 1 USB. A 32-bit 100 MHz unit is used instead of 8-bit or 16-bit microcontrollers [50,51,52] operating at few tens of MHz since local signal processing has to be implemented.
- On-board power supply regulators that provide the needed low voltage supply levels starting from an external voltage level up to 36 V.
- 10 values from 0.1 to 1 Hz with 0.1 Hz frequency resolution,
- 10 values from 1 to 10 Hz with 1 Hz frequency resolution,
- 10 values from 10 to 100 Hz with 10 Hz frequency resolution,
- 10 values from 100 Hz to 1 kHz with a 100 Hz frequency resolution.
7. Experimental Measurements
7.1. Power Transformer Degradation
7.2. Battery Degradation
- In Figure 9a, an example acquisition of the excitation current (10 A peak) and voltage response (15.5 mV peak) at 1 kHz; the module of the impedance is 762.4 µΩ and the phase shift is 30 degrees.
- In Figure 9b, the current needed to discharge in 10 h a 425 Ah cell (x axis), at 20 °C and its operating voltage (y axis). Below 1.8 V there is a rapid change in the cell behavior.
- the impedance with null imaginary part, Rs from Equation (3), usually between 500 Hz and 1 kHz;
- the impedance with a peak of imaginary part, fc from Equation (1), usually above 1 Hz.
Conflicts of Interest
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|Size (kVA)||Input Voltage (Vac)||Input Freq. (Hz)||Input Current Distortion, %||Input Voltage (Vdc)||Output Voltage (Vac)||Output Freq. (Hz)||Efficiency (%)|
|10 to 150||400 V ± 20%||50 Hz/60 Hz ± 10%||5% with 12 pulses bridge + THD filter||384 V ± 20% (e.g., from batteries)||3 × 400 V ± 5% (see inverter output data)||50 Hz/60 Hz ± 1%||88 to 93|
|SCR-Based Rectifier (AC/DC)|
|Output Current Max. (A)||Efficiency (%)|
|50 to 350||93 to 97|
|IGBT-Based Inverter (DC/AC)|
|Input Voltage (Vdc)||Efficiency (%)||Output Voltage (Vac)||Output Voltage Stability (static)||Output Voltage Stability (dynamic)||Total Harmonic Distortion (THD)|
|384 V ± 20% (min. 307 V)||95 to 96||3 × 400 V||1% static *||5% dynamic, reset to 1% in 40 ms *||1.5% linear load **, <5% with no linear load **|
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Saponara, S. Distributed Measuring System for Predictive Diagnosis of Uninterruptible Power Supplies in Safety-Critical Applications. Energies 2016, 9, 327. https://doi.org/10.3390/en9050327
Saponara S. Distributed Measuring System for Predictive Diagnosis of Uninterruptible Power Supplies in Safety-Critical Applications. Energies. 2016; 9(5):327. https://doi.org/10.3390/en9050327Chicago/Turabian Style
Saponara, Sergio. 2016. "Distributed Measuring System for Predictive Diagnosis of Uninterruptible Power Supplies in Safety-Critical Applications" Energies 9, no. 5: 327. https://doi.org/10.3390/en9050327