by May 11, 2002 0 comments



Before implementing a large UPS system, it’s important to get your electrical installation analyzed by a power consultant. This will help you determine wiring faults, the quality of equipment used, and plan the capacity for the system. For instance, in electrical wiring, the neutral wire takes three times more current than the live one. So if your neutral wiring is thinner, it could overheat and burn out, unless you use 3-phase input power. Similarly, other electrical equipment like MCBs (Miniature Circuit breaker) need to be of the right class. The fastest ones (magnetic breakers) can switch at a fraction of the mains input cycle.

There are many types of disturbances in the electricity besides power cuts that can damage your equipment. The most common ones are voltage surges, sags, brownouts, spikes, harmonic distortion and frequency variation, most of which have the potential to damage equipment. Let’s understand what they mean.

Lightning strikes are a major cause of spikes in the mains voltage. These spikes are very short, but their peak values can be high enough to damage equipment

Power cut. This doesn’t need an explanation, as it’s a frequent occurrence in most places in this country. A power cut is simply the loss of mains voltage.

Power surge. This is a sudden increase in the main voltage. It usually occurs when heavy motors and machinery are switched off. These draw a lot of power, so when they’re turned off, they cause a sudden rise in the voltage, which usually lasts 1/20th of a second. 

Spike. Similar to a surge, only this one is usually caused by lightening strikes, or when utility power comes back after a failure.

Sag/brownout. A dip in the mains voltage. If it lasts for less than a second, it’s called a sag, else a brownout. While surges occur when heavy machinery is turned off, brownouts occur when it’s turned on. 

Frequency variation. The mains frequency is fixed at a particular value, which is different for different countries. In India, this is 50 Hz. Any variation from this frequency can damage electrical and electronic equipment. Please check the frequency sensitivity of the UPS you’re going to buy. It should be able to withstand some amount of frequency variations.

Harmonic distortion. The mains voltage is in the form of a perfect sine wave alternating at a frequency of 50 Hz in India. Harmonics are multiples of this frequency that get superimposed on this sine wave and distort it from its original form. Various types of electrical and electronic equipment can cause this. 

THD (Total Harmonic Distortion). The ratio of the sum of the powers of all harmonic frequencies above the fundamental frequency to the power of the fundamental frequency. It’s usually expressed in dB, and the measurements for calculating THD are made at the output of a device. 

Noise. This could be any kind of disturbance that would affect the mains voltage waveform. A motor running on a mains line, for instance, can cause spikes in the line voltage due to the sparks that occur in its brushes. These spikes would get superimposed on the sine wave, and distort it. 

Knowing the most common power problems in your area can help you take the necessary measures to protect your equipment from it. For instance, excessive EMI and THD can distort the main voltage waveform, and affect the quality of power being fed to your load. The THD can actually reduce the power factor (explained later in this article). THD distorts the input sine wave the voltage at its peak. A computer draws its power close to the peaks, so a distorted sine wave will not be able to deliver the peak current that an ideal sine wave would. Therefore, the computer may get a lower voltage than it actually should. For a computer type load, a THD of around 10% is acceptable.

Capacity planning
Once you’ve analyzed the power situation in your area, you should do power planning and decide upon the UPS capacity you really need. An important aspect here is the PF (Power Factor). In electrical terms, this is the ratio of the effective power (watts) to the apparent power (volt-amps or VA). Mathematically put: 

To put it differently: 

PF = Effective Power (Watts)/ Apparent Power (VA)

Watts = PFxVA

Resistive load, such as light bulbs have unity power factor, so its apparent power VA is equal to the effective power in Watts. Computer load, however, is non-linear, so its power factor is less than unity. If a lower Power Factor rating is plugged into the above equation, the Effective power consumed by the load is lower than its apparent power. Since most UPS ratings are in VA, you must know the actual power consumed by your load and its power factor to determine what UPS rating to actually go for. Usually, a Power Factor value of 0.7 is good enough for computer load. 

Power Factor can be measured using a Power analyzer. Ask your UPS vendor to use one to measure it for your total load. If your projected load is 100 VA, then the actual power consumption for it is around 70 Watts. This will help you determine the actual battery ratings you need, so that you don’t end up paying extra for higher rated batteries. 

Anil Chopra

Case Study: UPS Installation at Max New York Life

We saw a 60 kVA Powerware UPS (Powerware 9315) installation at Max New York Life Insurance in
Guragon. It’s been running there for the past one year, and currently powers a 20 kVA computer load. The UPS electronics is mounted inside a sleek rack and is connected to the mains and the battery pack. (You can buy the battery pack as racks from Powerware or use your own batteries. The disadvantage in the latter case is the absence of certain sensors that monitor the battery pack temperature and interact with the alarms in case of overload and heating up of batteries.) 

Some specs of the UPS include an output voltage regulated between +/-1%, Voltage THD (Total Harmonic Distortion) of less than 3% for non-linear load, a transient response of less than 5% for 100 % load. 

The front panel of the UPS housing has an LCD panel to display all the requisite parameters. The display details are navigable through a set of buttons under the LCD display. Certain operational parameters like input and output voltages’ lower and upper cutoffs can be managed from here. They are calibrated at the installation time and can be edited later (password protected) if needed. LEDs show the status of operation, say, battery mode, bypass mode or mains. The UPS records logs of alarms (up to last 8 alerts) along with the date and time of occurrence. 

The UPS has a communication interface to monitor and control the UPS from a PC through a direct cable (RS-232) link or a network link. The software for the purpose is called
PowerVision. You can put an internal modem (powered by the UPS) to send alerts through a dedicated phone line to a pager or dial in to a remote PC. The UPS also supports SNMP for integration with a network-management system.

Contact: Invensys, Powerware International, Delhi. Tel: 011-6499418. E-mail:
skv_iesi@vsnl.net

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