Battery Enclosures (Cabinet or Rack) - Which is best for my batteries?

A battery cabinet is an enclosed cabinet used to house batteries for Inverter, UPS or other DC storage applications while a battery rack is an open frame (usually metallic) designed and fabricated for the same purpose. Generally, battery cabinet are preferred over battery racks because they provide better aesthetics with the cabinet usually designed to match the UPS in terms of outlook thereby giving a homogeneous outlay for the installation.

The above is an Eaton Battery Cabinet in line and match with an Eaton UPS. On the other, a battery rack is simply a rigid frame designed to carry the batteries, openly, and as such does not offer the same level of aesthetics with the battery assembly and the backup equipment(UPS, Inverter, Rectifier etc) often looking out of place when placed side by side.

Apart from aesthetics, the battery cabinet, if properly installed, offers better electrical isolation between the user and the high DC voltage usually present on batteries thereby providing a safer operating environment. For this reason (and combined with the aesthetic value derivable from the line and match configuration), battery cabinets are usually the preferred choice for data centres with moderate requirements in terms of backup time. On the other hand, battery racks are usually considered when extended backup times are required with the rack preferably located away from the application area. The choice of which is normally best is a function of so many factors. To help decide, we make a comparison between the available factors.

From the comparison above, it is clear that the choice of enclosure depends on application and specific considerations of the designer or user. As for me, I would gladly trade off aesthetics but cannot do same for overall safety. So, rack it is for me! I would rather design my enclosure as neat as possible with some form of transparent shielding or terminal caps to help with environmental isolation but for you, your choice depends solely on you!

Harmonics in electrical systems

What are harmonics?

Harmonics are unwanted currents that are integer multiples of the fundamental line frequency (50 or 60 Hz). For a 50Hz supply (fundamental frequency), 2^nd harmonic is 100Hz, 3rd is 150Hz and so on.

Causes of harmonics

Nonlinear loads cause harmonics to flow in the power lines. Some loads cause the current to vary disproportionately with the voltage during each half cycle.

A large portion of non-linear loads is constituted by SMPS-based power supplies. These supplies draw current in high-amplitude, mostly non-sinusoidal, short pulses which create distortion that travels back into the power supply and can affect other loads connected to the same source. Their current and voltage have waveforms that are non-sinusoidal, containing distortions, whereby the fundamental frequency (50 or 60Hz) has numerous additional frequencies superimposed upon it, creating multiple frequencies within the normal sine wave. The multiple frequencies are harmonics of the fundamental frequency.

Linear loads - loads where the voltage and current follow one another without any distortion to their pure sine waves). Examples of linear loads are resistive heaters, incandescent lamps, and constant speed induction and synchronous motors.

Examples of non-linear loads

• Switch-mode power supplies;
• Ballast-based fluorescent lighting;
• Variable speed motors and drives;
• Photocopiers;
• Personal computers;
• Laser printers;
• Fax machines;
• Battery chargers;
• Some UPSs

Some effects of harmonics

• Harmonic currents can overload cables and protection circuit breakers due to increased current, thereby causing false tripping of breakers.
• Increased current flowing in the neutral conductor of a circuit - especially due to the 3^rd, 9^th and 15^th harmonics (triple harmonics) which can cause neutral current to rise to as much as 1.73 times the phase current, even under balanced load conditions.
• Equipment and generator malfunctions and/or failures due to excessive voltage distortion.
• Overheating of electrical distribution equipment, cables, transformers, standby generators, motors etc. which in extreme cases can cause fire.

(In transformers and motors, hysteresis loss and eddy current increases with frequency and since harmonics increase the frequency seen by the equipment, these losses lead to increased heating in induction equipment).

In cables, the effective impedance of the cable increases at high frequencies thereby limiting the current carrying capacity of the cable leading to overloading and overheating.

• Equipment malfunctions due to excessive voltage distortion
• Reduction in the speed of electric motors - at the 5th harmonic, counter electromotive force (CEMF) which acts in opposition to the direction of rotation may eventually cause motor speed to decrease.
• Increase cost of utility power to the end user due to metering error and lower system power factor problems.
• Increased internal energy losses in connected equipment, causing component failure and shortened life span
• Poor crest factor and its related problems.

Classes of harmonics

Harmonics can broadly be categorized as:

Positive Harmonics:

These are harmonics with the numbers 1, 4, 7, 10, 13, etc. (note a common difference of 3 starting from the fundamental frequency harmonic 1). They produce magnetic fields and currents rotating in the same direction as the fundamental frequency harmonic.

Negative Harmonics:

These are harmonics with the numbers 2, 5, 8, 11, 14, etc. (also note the common difference of 3 starting from the second harmonics). They develop magnetic fields and currents that rotate in a direction opposite to that positive frequency set.

Zero Sequence Harmonics:

Harmonic numbers 3, 9, 15, 21, etc.(note a common difference of 3 starting from the third harmonic). They do not develop usable torque, but produce additional losses in the machine.

Mitigating/eliminating effects of harmonics

Over-sizing the neutral conductor:

Over-sizing the neutral conductor does not eliminate harmonics but can help mitigate its effect. This is because the neutral conductor impedance is lowered and so it can carry more current which may result from harmonics but with lesser risk of overloading and overheating which may lead to fire.

Multiple Neutral conductors:

This effectively produces the same effect as above only that each phase has its own neutral conductor rather than a common one.

Over-sizing transformers:

Loading transformer to not more than 60% of its capacity helps the transformer to deal with harmonic related problems such as eddy current, hysteresis losses and copper losses.

Eddy current losses: Power dissipated due to current circulating in metallic material (core, windings, case, and associated hardware in motors, etc.) as a result of electromotive forces induced by variation of magnetic flux.

Hysteresis: The energy loss in magnet material that results from an alternating magnetic field as the elementary magnets within the material seek to align themselves with the reversing magnetic field.

Using K-rated transformers:

K-rated transformers are transformers designed with high K-factor so that they can handle the high heat resulting from harmonics.

Using equipment which generates less harmonics:

Using linear loads and equipment with linear power supply which generate little or no harmonics

Harmonic Filters:

Harmonic filters are design to pass harmonic contents to ground. They are usually designed to handle specific frequencies. With good design, they can be used to eliminate harmonics.

kW vs kVA

Do you know it is possible to appear to have correctly sized a UPS but still overload the UPS? This is because nameplate rating of some UPS(usually in VA or Watt) could be confusing when the actual usable power is not considered for the prospective load.

Example:
A 1000VA UPS. The user wants to power a 900VA server with the UPS. The server has a Power Factor of 1, so has a Watt rating of 900W and a VA rating of 900VA. Although the VA rating of the load is 900VA, which is within the VA rating of the UPS, the UPS will not power this load. That is because the 900W rating of the load exceeds the Watt rating of the UPS, which is 60% of 1000VA or around 600W which has a power factor of 0.6 which was not stated on the UPS name plate.

So what is the real power obtainable from a UPS?

The power in Watts is the real or usable power drawn by the equipment. It is the resistive component of power supplied to the equipment and is the portion that actually does useful work. Volt-Amp is called the "apparent power" and is the product of the voltage applied to the equipment and the current drawn by the equipment. the VA power is a sum of the resistive and the reactive components of power.

kVA = kW + jkVA(r)

As seen above, the smaller the reactive component of any equipment, the closer the useful power is to the apparent power.

Resistive loads e.g incandescent lamps and heaters have identical VA and Watt ratings which implies that all the power delivered to such an appliance is converted to useful power. However, for computer equipment and other partly reactive loads, the Watt and VA ratings can differ significantly, with the VA rating as seen from the expression iniial above. The ratio of the Watt to VA rating is called the "Power Factor" and is expressed either as a number (i.e. 0.7) or a percentage (i.e. 70%).

Power factor = kW / kVA

Significance of kW and kVA
The Watt rating determines the actual power purchased from the utility company or the heat loading generated by the equipment.
The VA rating NOT kW is used for sizing wiring, battery capacity and circuit breakers.

Different types of switch mode power supply
1. Power Factor Corrected supplies
2. Capacitor Input supplies.

It should be noted that it is usually not possible to tell which kind of power supply is used by physical inspection of the supply.

For PFC supplies, the Watt and VA ratings are equal (power factor of 0.99 to 1.0). Capacitor Input supplies, however, have the characteristic that the Watt rating is in the range of 0.55 to 0.75 times the VA rating (power factor of 0.55 to 0.75). This is because the capacitor introduces a reactive component into the supply.

Large computing equipment such as routers, switches, drive arrays, and servers typically use the Power Factor Corrected supply and consequently for this type of equipment the power factor is 1. Personal computers, small hubs, and personal computer accessories typically have Capacitor Input supplies and consequently for this type of equipment the power factor is less than one, and is ordinarily in the range of 0.65.