Tools to dispel myths about your IT system's power quality.
Poor electrical power quality is easy to blame for problems you are experiencing with your organization's IT systems because most people don't know much about it and aren't responsible for its generation, distribution, or the natural or human events that cause disruptions.
But once power quality is cited as the culprit, the IT professional often has to take action. To help you get started, this article will provide some basic information about electrical power quality.
To conduct a power quality study, you need a power quality analyzer for testing the integrity of electrical power distribution systems and for locating faults.
The analyzer will give you the ability to validate power quality coming from your supplier; detect problems external and internal to your room or building; help categorize and diagnose problems; uncover hidden or intermittent issues; verify electrical system capacity; and measure energy usage.
The Fluke 430 Series Power Quality Analyzer, AEMC 3945 Power Quality Analyzer and the Dranetz-BMI Power Quality Analyzer offer the features for a comprehensive power quality study and cost under $10,000.
Surface Combat Systems Center (SCSC) in Wallops Island, Va., uses the Dranetz-BMI PowerVisa 440D with Dran-View 6 software (DADMS ID #49679).
But a word of caution: Power analyzers and other electrical test equipment should be connected by a certified electrician.
Resolving the Problem
There are four steps to satisfactorily resolve your power quality problems: planning, monitoring, evaluating and mitigating. Don't bypass the first three steps because they are as important to successfully mitigating power quality problems as properly preparing a surface is to a satisfactory paint job.
If you suspect a power quality problem, carefully plan where to connect the analyzer. SCSC has two analyzers which are often used in pairs. One is located at the electrical panel closest to the equipment experiencing the problem, and the other is located closest to the source of power for the building. You may be surprised to learn there are more power quality problems generated inside your building than coming from the source of electricity.
After connection, install the memory card and set up the instrument configuration. The monitor we use provides automatic, wizard, upload or manual setup options. Use the automatic setup option and then check the analyzer phasor diagram to make sure the electrician connected each probe to the corresponding A-B-C neutral leg.
[FIGURE 1 OMITTED]
To capture intermittent events and collect enough data for analysis, we recommend 60 days for power quality studies. During the collection period, the technicians need to keep a log with the date, time and description of equipment problems. This will help focus the data analysis effort. Data are recorded on a compact flash card. A 256 megabit compact flash card will hold approximately 30 days of data. If the card fills up in a day or so, you probably have installed the unit incorrectly.
Once you have collected enough power quality data, it is time to analyze it. The analyzer software allows you to view the information as event lists, graphs, or you can create custom reports.
[FIGURE 2 OMITTED]
We initially run a report that summarizes all disturbances by category and magnitude. If we find measurements outside industry standards or customized limits, we compare notes and any other available information, including anecdotal, about the circumstances of the power quality event and the effects on the equipment.
We also copy the event list into Microsoft Excel to feed into our command metrics dashboard. A Pareto chart (Figure 1) and a line chart (Figure 2) are used to visually communicate complex information to managers in a simple, concise manner.
The Pareto chart clearly shows the categories of the top 20 percent of the power quality issues, and the line chart shows the trend for those top issues.
Power Quality Problems
There are five major categories of power quality problems: interruptions, sags, surges, transients and harmonics.
* Interruptions occur when the line voltage is reduced to zero. Interruptions can be momentary (less than two seconds) or sustained.
* Sags are a short duration (less than two seconds) decrease in the line voltage.
* Surges are short duration increases in line voltage.
* Transients are very short duration but significant deviations in line voltage (usually high voltage spikes).
* Harmonics are distortions in the shape of the alternating current (AC) waveform.
Depending on the intensity and duration of the power quality problem, unprotected IT equipment may be damaged.
Since discussing the causes of power quality problems are beyond the scope of this article; we will, instead, focus on the tools for determining if you have a power quality problem and in what ways your power is nonconforming to your equipment's power requirements.
Determining Power Quality
Probably, no organization has "perfect" electrical power quality, so you need to determine if your electrical power quality is "good enough" to effectively operate your IT system. It is important to know what the threshold is between acceptable or unacceptable power quality; otherwise, your solution may either not fully resolve the problem or may exceed your actual requirements and thus be too complex or costly.
There are two straightforward ways to determine what constitutes an adequate power supply.You can use the Information Technology Industry Council (ITI) CBEMA Curve or the manufacturer's equipment specifications. The ITI (CBEMA) Curve was published by Technical Committee 3 (TC3) of the ITI (formerly known as the Computer and Business Equipment Manufacturer's Association).
The manufacturer's equipment specification may not be detailed enough for all characteristics of electrical power. For example, the electrical specifications for a popular router include: input voltage of 180 to 264 volt alternating current (VAC); input line frequency of 47 to 63 hertz; and a source service requirement of 20 amps.
[FIGURE 3 OMITTED]
While you should determine if your power meets these requirements, these requirements lack the time element of momentary disturbances that could still cause problems with your IT equipment.
Another standard way to determine acceptable power quality is to use the ITI CBEMA Curve (see Figure 3). While there are many factors that contribute to power deviations, to simplify matters, the CBEMA Curve only measures the voltage deviation from the norm; it is the only factor taken into account.
The CBEMA Curve effectively encompasses all the factors involved with voltage deviations, from long term through to high-speed distortions of the waveform. The CBEMA Curve takes into account that equipment fitted with a power filter, or surge protection device, can protect against electrical noise interference and damaging power transients; and therefore, should be able to withstand greater deviations the shorter the timing of the deviation.
In the CBEMA Curve, the X-axis measures duration (time) and the Y-axis measures magnitude (voltage) of the power event. The chart has three areas: the prohibited region, the no interruption in function region, and the no-damage region.
Satisfactory power quality occurs when there are no power events outside the no interruption in function region. This chart, which can be generated by the analyzer software, provides a very simple and powerful method of determining if your power quality is sufficient to support your equipment.
Case in Point
Now let's put it all together with an example.
The Common Scenario Control Environment (CSCE) is a Versa Module Eurocard (VME) chassis-based simulator that connects to the Ship Self Defense System to drive a common scenario for combat system development testing.
The Warfare System Interoperability and Integration Testing (WSIIT) customer reported that the CSCE rebooted at irregular intervals and interrupted certification test events. The customer believed that the problem was caused by electrical power deviations, so we had an electrician connect one of the Dranetz analyzers to the electrical power panel that supplied power to the equipment rack. After a few weeks of monitoring, the customer reported a problem, along with the date and time it occurred.
After analyzing the data captured on the flash card it was determined that there were no power quality events during the time the problem occurred. The power quality events that did occur at other times were within the no interruption in function region of the CBEMA Curve.
Technicians then began to troubleshoot the equipment rack and determined that a power filter was faulty. The problem was corrected and no further reboots occurred.
Electrical power quality problems can adversely affect your IT equipment. There are different types of power quality problems, and they can originate externally or internally within your building. You should tailor your remediation efforts to the value of the data or the cost associated with the loss of service. A power quality analyzer, installed by a certified electrician, and analysis software will provide the tools to dispel any power quality myths in your organization.
By Steven Krumm and Mary Hoffken
Steve Krumm is the Surface Combat Systems Center, Combat Systems Technology division head.
Ms. Mary Hoffken is a senior systems analyst with Lockheed Martin Information Systems and Global Services.
amps -- a unit of electric current.
hertz -- is a unit of frequency. It is defined as the number of complete cycles persecond.
The volt is defined as the value of the voltage across a conductor when a current of one ampere dissipates one watt of power in the conductor.
The watt (symbol: W) is a derived unit of power in the International System of Units (SI). It measures rate of energy conversion. One watt is equivalent to 1 joule (J) of energy per second.
DATE CAUSE IN OUT 8/06 Transients 0 126 9/06 Transients 1// 42/ 10/06 Transients 73 104 11/06 Transients 2 11 12/06 Transients 0 26 1/07 Transients 0 0 2/07 Transients 23 17 3/07 Transients 6 27 4/07 Transients 42 68
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|Author:||Krumm, Steven; Hoffken, Mary|
|Date:||Oct 1, 2009|
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