The consideration of power supplies has become critical to the success of converting analog instrumentation and control systems to digital control systems (DCSs). Careful planning is particularly necessary for nuclear power plants, where instrumentation systems are required for safely shutting down a reactor, mitigating the consequences of an accident, and performing post-accident analysis.
With the conversion of existing analog instrumentation and control (I&C) systems to digital control systems (DCSs), careful consideration and planning of power supply systems has become critical. This article addresses the most important issues that must be considered in the design, operation, and maintenance of power supply systems for nuclear facilities, specifically: power quality, redundancy and diversity of supply, good planning prior to installation, and maintenance of the equipment over its lifespan.
Pursue Power Quality
Most DCSs are provided with power conditioning; however, the equipment selected to provide power for these systems should meet an established set of minimum requirements for voltage regulation and noise suppression to ensure that the output of the uninterruptible power supply (UPS) meets the manufacturer’s requirements for the DCS.
According to the Electric Power Research Institute’s (EPRI) Report TR-1016731, the most commonly reported causes of failure for nonsafety-related DCSs are electromagnetic interference (EMI)-induced disturbances and power supply transients and failures. Power supply transients can include supply voltage surges or spikes, frequency deviations, and noise induced by harmonic content.
Voltage and Frequency Stability. Results of testing documented in the EPRI Report TR-1001072 reveal that programmable logic controllers (PLCs) are most susceptible to the depth, duration, and likelihood of voltage sags. Disturbances to the power system voltage and frequency can result from lighting, short circuits, and switching of loads such as capacitor banks. All of these disturbances can cause transient voltage surges and spikes.
The duration of a voltage sag affects PLC performance and can result in premature system shutdown in some cases. Even momentary voltage surges can result in malfunction of input and output circuits, especially if the PLC power supply is also used to provide sensing voltage for the inputs. For example, a UPS inverter produced a square wave output that was incompatible with three of five PLCs tested. In contrast, a UPS inverter that produced a true sine wave output effectively mitigated voltage sags.
Regulation, Monitoring. The addition of a constant voltage transformer to the PLC’s power supply was found to be very effective in improving the ability of the PLC to ride through such voltage surges and transients. UPS systems that supply equipment that includes a DCS should be designed to produce a true sine wave output to reduce the impact of harmonics.
Battery chargers are not designed to operate as a stand-alone rectifier for an inverter and, therefore, must be always connected in parallel with a set of batteries for the UPS inverter to properly compensate for any transients.
Electromagnetic Interference. EMI and power surges can affect the performance of safety-related equipment in nuclear power plants. This is especially true for DCSs, where noise from EMI can be misinterpreted as legitimate logic signals. The U.S. Nuclear Regulatory Commission’s Regulatory Guide (RG) 1.180 addresses this issue. DCS equipment must be tested in accordance with the standards recommended in this guideline. UPS equipment should meet the minimum standards for radiated and conducted emissions to ensure electromagnetic compatibility with the control systems to which they supply power.
EPRI TR-102323 provides guidelines for data collection, testing, and practices to ensure electromagnetic compatibility in DCSs in nuclear power plants and also to meet the requirements specified in RG 1.180. In addition, surge suppression should be installed on all power and control circuits surrounding these systems to eliminate unwanted disturbances or noise.
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