System Integration & Control Systems Design


Monitoring of power supply quality

December 2000 System Integration & Control Systems Design

Electrical power is most often generated a long distance from the final user. The further the distance, the more switching and transforming that occurs en route, the greater the likelihood of the quality being affected.

Lightning, sudden load changes and fluctuating power factors can plague those requiring clean, reliable power. Data centres, telecommunications centres, high-tech manufacturing facilities, hospitals, laboratories, and other users with critical electronic loads cannot afford to have 'dirty power' serving their sensitive equipment and systems.

When a major telecommunications company in the western United States searched for new solutions to increase the uptime of the power distribution system at one of its major Denver-area data centres, it provided a PC-based power monitoring system based on LabVIEW and data acquisition (DAQ) boards from National Instruments. Previous monitors installed at the US West facility in Denver provided only localised 'snapshots' of power problems.

The information gathered was often inadequate to pinpoint the source of the problem and its overall effect on the facility. Because building engineers needed a complete picture of how the entire power distribution system behaves under abnormal conditions - such as how it reacts to a changeable power source and to continually changing nonlinear loads - a facility-wide monitoring system was needed. US West selected a systems integrator (SI) offering a design that was based on National Instruments' supplied hardware and software. Deciding factors were the design's flexibility, performance and economical implementation.

Hardware design

While designing a comprehensive yet economical solution for US West, an integrated set of hardware and software with extended capabilities was required. LabVIEW software, analog and digital signal conditioners, and high-speed DAQ boards from National Instruments provided the data measurement, display and analysis capabilities needed to achieve the project objectives.

The SI developed a distributed, PC-based data collection system to capture voltage and current waveforms, breaker status, uninterruptible power supply (UPS) operating status, UPS battery voltages, and low-speed data from resistance-temperature detectors (RTDs), humidity sensors, and watt and VAR transducers (for measuring real power and reactive volt-amperes, respectively). With the high-speed data capture capability of LabVIEW, the system continuously monitors waveform voltages and currents and saves to disk any out-of-tolerance waveforms at 12 kilosamples per second, per channel.

It has 12 data collection stations distributed over eight floors of computer rooms and two levels of subbasements. The system is interconnected via Ethernet with a common relational database. It monitors computer rooms for power consumption, temperature and humidity. It monitors the four main power buses and UPS outputs at the site for power quality using advanced waveform capture routines developed in LabVIEW. The SI selected to use a separate PC and dynamic data exchange (DDE) technology to manage alarms from any input and annunciate them on remote workstations.

There is an autodialler to provide realtime alarming system-wide and to remote locations. All data collected is deposited into a separate Pentium file server running Windows NT. The system extracts power waveforms and other related data from the database and sends it to remote workstations for analysis with LabVIEW.

Each of the eight high-speed data collection stations is a 486-66 PC with an AT-MIO-16F-5 multifunction DAQ board. Using the circular buffer structure of the 16F board and unique LabVIEW virtual instruments (VIs) results in a 99,5% continuity in data collection while simultaneously storing data to hard disk. Ahead of the A/D card, 5B Series signal conditioning modules were selected based on cost, medium bandwidth capability, flexible configurations, wide input types, high common-mode voltages and input/output electrical isolation. The four low-speed data collection stations are 486-66 PCs with AT-MIO-64F-5 DAQ cards. Again, 5B modules accept the 4 to 20 mA inputs and 24 V d.c. contact closure inputs representing power, VAR, degrees F, relative humidity, UPS operating status and power breaker status.

Software design

Special VIs (Virtual Instruments) were created in LabVIEW to fit the software to the application. For example, power disturbances - transients, harmonics, voltage sags or swells and dropouts - may remain undetected by many power monitors because they can occur on any phase of the power distribution system, on voltage or current. The SI developed special LabVIEW routines, such as multichannel simultaneous triggering and sine-wave tracking, to ensure that all power disturbances are collected and stored to disk. Complex data reduction VIs were also incorporated into the software to prevent repetitive harmonic waveform data from piling up in the database, wasting space.

All workstations use the data display and analysis functions of LabVIEW. Using DDE routines, the system can send prioritised alarms to the database, workstations and modems within seconds after detection of a disturbance. Custom routines were developed for remote alarm annunciation via auto-diallers and for access on remote workstations. The designers provided simplified human-machine interface menus so that nontechnical personnel, such as security guards, can monitor alarms and electrical system performance. For in-depth investigation and analysis, more complex menus were created for the maintenance and engineering staff. Although all workstations are configured identically, they have a keylock system that allows employees to access the system only at levels appropriate to their job functions.

Each LabVIEW workstation has comprehensive algorithms installed to calculate and display frequency spectra, true power factor, distortion power factor, phase imbalance, alarm summaries, power consumption trends, and other advanced functions not normally found in off-the-shelf disturbance analysers. The SI wrote VIs for all power calculations and spectrum analyses, as well as to provide true power factors at 60 Hz and at each harmonic. (Would be 50 Hz in SA)

The system can analyse up through the 70th harmonic. The user can extract data from the database to analyse each power disturbance as it occurs, build historical trends, map power usage around the facility or test electrical system equipment during periodic maintenance.

The power monitoring system project at US West won first place in the special projects category of the American Consulting Engineers Council of Colorado's 1995 Excellence Awards. The system is installed permanently and runs continuously. The US West technical staff can now understand and address power problems more completely and more efficiently than previously possible. The distribution system performs more reliably and adjustments can be made to accommodate future changes in computer loads.

PC-based data collection stations, which include National Instruments' DAQ boards and signal conditioners, are distributed throughout the facility and are connected to a common relational database by a dedicated Ethernet link
PC-based data collection stations, which include National Instruments' DAQ boards and signal conditioners, are distributed throughout the facility and are connected to a common relational database by a dedicated Ethernet link

National Instruments

082 877 8530

ni.southafrica@ni.com

www.ni.com





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