The process of providing reliable power starts with good design, followed by quality control during construction and constant monitoring throughout a facility's lifetime. Part of this continuous monitoring process involves conducting periodic power quality site surveys (PQSSs). These surveys help pinpoint and resolve problems that can lead to unreliable performance, equipment damage, hazardous conditions, and lowered productivity and profitability. This article takes an up-close look at the components of a PQSS — something your facility may need in the not-too-distant future.
The “Recommended Practice for Powering and Grounding Electronic Equipment,” published by the Institute of Electrical and Electronics Engineers (IEEE), describes three levels of PQSSs. Each level encompasses more fact-finding tasks. Level I surveys include a visual inspection, plus testing and analyses of AC distribution and grounding systems. Level II encompasses all aspects of Level 1, along with applied AC-voltage and load-current monitoring. Level III includes all Level II aspects, plus monitoring of the site's environmental parameters. All three surveys consist of the same basic steps described below.
Planning and Preparation
Successful planning and preparation begin by developing a schedule that details all aspects of the PQSS. This schedule should list all the areas requiring investigation, including the date and time the investigations will occur and the length of time necessary to complete them. You'll also need time to review and implement the recommended corrective actions.
At a minimum, gather the following preliminary information to develop a schedule:
- Floor plans that identify areas experiencing trouble
- Panel schedules
- Riser diagrams
- Transformer information
- Lighting plans
- Computer network topology
During the survey, you'll use a variety of power quality measuring instruments. They should — at the least — provide true rms voltage and current measurements and harmonic content for voltages and currents through the 50th harmonic. As part of your preparation, make sure you have these basic instruments on hand:
- Power quality monitor
- True rms multimeter
- Infrared scanner
- Hall-effect current probes
- Video camera
- Tape recorder
- Circuit tracer
- Transient analyzer
- Ground resistance meter
- Insulation tester
A site survey begins with the investigative team examining the outside of the facility and the immediate vicinity. This gives them a complete perspective on the utility service area, including the type of electrical service, utility power factor correction capacitor banks, neighboring facilities, nearby substations, apparent modifications to the system, and ongoing construction in the area.
Next comes a visual inspection of the electrical distribution systems inside the facility. Inspectors look for anything that could cause interference: broken or corroded conduits, noisy transformers, or damaged wiring insulation associated with photocopiers, air compressors, and UPS systems. Special attention is given to critical loads near known harmonic sources.
The top sources of poor power quality are loose connections and connections with poor conductivity and/or inadequate grounding. The best thing to do is resolve any connectivity problems prior to the survey.
Monitoring the Power
Inspectors typically follow a three-step process to monitor points at the service entrance, transformers, distribution panels, and suspect equipment. In the first step, they use the power quality monitor's scope mode to view voltage and current magnitudes and waveshapes. Next, the time-interval setting is used to record background events. Finally, they employ limit- and sensitivity-threshold settings to record any disturbance or event that may affect the equipment or process being monitored.
Because electrical loads fluctuate throughout the business cycle, the inspectors measure the system under a variety of loads and operating conditions. They take voltage, current, harmonic-distortion, ground-resistance, and bonding-resistance measurements.
The primary indicator of harmonic problems is the measurement of current distortion followed by voltage distortion. The acceptable limits for a system's harmonic content are set by manufacturers and IEEE's Standard 519-1992. If this is the first site survey at your facility, the measurements taken by the inspectors will be considered the baseline data. This data will be compared with newly acquired data when future problems arise.
Analyzing the Data
After taking the necessary measurements, the inspectors perform a methodical analysis of the data and summarize their findings in a clearly written, well-documented report. This report should include appendices that contain all pertinent calculations.
The inspectors begin by scanning the data to see if any recorded power quality events correlate with particular equipment malfunctions. Next, they'll check to see if any of the recorded PQ events are outside the electrical system's normal operating range. They also will compare the recorded events to equipment maintenance or site logs, and they'll look for correlations between equipment symptoms and problems found during the physical inspection. In the end, the inspectors should be able to identify the source of a problem as a voltage sag, ground or neutral event, transient, or voltage distortion.
When you set out to resolve a problem, start with the simplest solutions first. They're typically the least expensive. These solutions should include a comprehensive check of the power distribution system and verification that transient protection is zoned.
Up to 80% of all power problems involve the distribution system. Approximately 35% of these problems stem from incompatible loads, overloaded circuits, and wiring errors. In addition, 30% are caused by inadequate or missing grounding, 25% from loose wires and connections, and 10% from failed or unplugged loads. A qualified electrical contractor can identify most, if not all, of these problems by checking actual loads in relation to wire size.
Contractors also can identify and correct the following:
- Open hot/neutral/ground connections
- Missing neutral-to-ground bonding at the main panel and transformers
- Connection to the building reference ground
- Hot/neutral/ground reversals
- Neutral/ground shorts
- Two hot conductors to a 120V outlet
- More than one building ground reference
- All loose connections throughout the system
As for sensitive electronic loads, they should be segregated from nonsensitive ones. In addition, sensitive loads should have dedicated circuits with oversized power and neutral conductors and a ground conductor. All of these should be installed in metallic conduit feeds from a panel board with an oversized neutral bus. The neutral bus's source should emanate from the main service entrance.
One last distribution factor to consider is limiting voltage drops to no more than 1% for branch circuits and 2% for feeder circuits. Remember, any distribution measures you implement must, at all times, meet or exceed the minimum requirements detailed in the National Electric Code (NEC).
Approximately 80% of transient problems can be attributed to sources inside a facility. Furthermore, office equipment accounts for 60% of internally generated transients, and adjacent building equipment (such as HVAC systems) account for 20% of them. That's why it's important to install transient voltage surge suppressors (TVSSs) that provide zones of protection.
Since the impedance of lead lengths dramatically affects the cutoff voltage of a TVSS, it is preferable to have each TVSS integral to the power panel it serves. If this is not an option, then lead lengths must be kept as short as possible. A general rule of thumb to calculate increases in cutoff voltage due to lead length is to add 300V for every 14 in. of lead length.
Once you've checked the power distribution system and verified the presence of transient protection zones, you can then proceed to implement more complex recommendations. These may include the following:
- Balancing loads
- Oversizing transformers
- Derating transformers
- Using k-rated transformers
- Installing zigzag transformers to protect distribution transformers
- Installing harmonic filters/traps
- Relocating the source of power disturbance
- Decreasing the internal impedance of power sources
- Installing power conditioning equipment
Understanding the process behind a PQSS is the first step in conducting a successful one. Following the guidelines outlined here should help you, provided you tailor them to your unique objectives. If you maintain a vigilant monitoring program and follow through by implementing the appropriate corrective measures, you'll be one step ahead in the never-ending battle for power quality.
John C. Gray is an electrical engineer with Williams Communications Group in Bridgeton, Mo. You can reach him at [email protected].
Additional sources include the Cutler-Hammer Online University, Dranetz-BMI, the Electric Power Research Institute (EPRI), Leviton, and Utilicorp United.