You can view power quality problems as the difference between the quality of power supplied and the quality of the power required to reliably operate load equipment. Using this viewpoint, you can resolve problems in one of three ways:
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Reduce power supply variations.
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Improve the load equipment's tolerance to these variations.
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Insert some power conditioning equipment between the electrical supply and the sensitive loads.
Of course, practicality and economics will determine the extent to which you can use any of these options. This article will focus on the third option.
Source compatibility. Power conditioning equipment must be compatible with the intended power source to ensure its own operation and to avoid interfering with the operation of other loads connected to the same power source.
Along with the obvious considerations (input voltage levels, number of phases, and frequency), you should focus on the following subtle ones:
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Tolerance to expected sags, swells, and surges.
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Limited start-up or in-rush currents to prevent voltage sags.
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Limited harmonic input current distortion.
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Limited notching.
If you don't, the power conditioning equipment may cause disturbances for other loads. For example, while a static UPS provides interruption protection for its load, its rectifier may produce voltage notches that can cause sensitive loads in the building to malfunction.
Load compatibility. It would seem obvious that any power-applied conditioning equipment must be compatible with the sensitive load. But to ensure this, you need to know the load's requirements so that you can match them with a power conditioner with compatible output performance. For example, if the load's allowable input voltage range is +6%, -10% of nominal, then you don't need a precision voltage regulator to maintain output voltage to within ±1%.
You also must take into account fundamental considerations, such as size, operating voltage, frequency, and number of phases. Sizing depends on the type of conditioner. You size TVSS devices, which are connected in parallel with the protected circuit, according to the maximum expected surge current they may be required to conduct. On the other hand, you must size a voltage regulator to operate and conduct the load's total current.
You would often size the conditioner on the expected maximum continuous full-load current only. But to avoid undesirable load-source interactions, you'll need a more detailed load profile and an understanding of the conditioner's response to the load. A load that experiences large current variations may cause the conditioner's output voltage to vary significantly. So, you may have to oversize the conditioner or select one less affected by load current variations.
Likewise, if the load current is nonsinusoidal, you again may have to oversize the conditioner or select one specifically designed to accommodate the heating effects of nonsinusoidal current without excessive output voltage distortion. Nonlinear loads with high peak currents also require special consideration when sizing power conditioners.
Application considerations. Each power conditioning technology can be expected to protect against specific power disturbances. However, to get the best protection performance from each device, you have to understand its respective application factors.
TVSS devices. Before settling on such a device, consider the following:
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The location of the device itself influences the expected surge activity to which the TVSS will be subjected.
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Required energy rating or surge-current handling capability, which is a function of its location.
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Clamping or protection voltage levels, which depend on the load equipment's susceptibility.
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Modes of protection, which depend on the load equipment's susceptibility.
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Maximum continuous operating voltage (MCOV).
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Coordination with other TVSS devices on the system.
Isolation transformers. If you choose these power conditioning devices, you'll want to consider the following:
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Required input and output voltages of the transformer.
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Voltage compensation tap configurations.
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Operating frequency.
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KVA capacity.
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Type of interwinding electrostatic shielding, which determines the level of common mode noise attenuation.
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Nonlinear load capabilities, which determine the degree to which the transformer won't overheat.
Voltage regulators. These devices require application considerations similar to those of isolation transformers, but you must also address the following issues:
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Input voltage regulation range.
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Output voltage regulation, which must be compatible with the load's input requirements.
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Output voltage imbalance, in the case of 3-phase systems.
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Overload capability.
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Time response.
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Response to expected load current changes.
Motor-generator (M-G) sets. Make sure you take into account the following application factors:
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Output frequency, which may be different than the input frequency.
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Frequency regulation for input voltage and output load changes.
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Stored inertial energy (ride-through) capability.
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Provisions to bypass the M-G set for maintenance.
An M-G set effectively isolates the load from the utility source, and vice versa. In doing so, it appears to the load as a relatively higher source impedance in place of the normal low-impedance utility source, making load-source interactions more likely. For example, the harmonic currents of nonlinear loads are more likely to cause the generator output voltage to become distorted. Load current changes are also more likely to cause output voltage or frequency variations.
Standby power supplies (SPSs). Also called offline UPSs, these devices operate normally with the inverter in a standby mode, so that the inverter and battery provide power to the load only when the UPS senses a disturbance on the normal input power source that is outside the device's input tolerance.
Few SPS inverters generate a regulated sine wave output, but rather a square wave or quasi-square wave. Most electronic power supplies can tolerate these waveforms, particularly for the short time periods on inverter/battery operation.
An SPS can only correct for supply voltage or frequency variations by transferring to inverter/battery operation, so sites with frequent power disturbances outside the input tolerance of the SPS will experience excessive battery cycling and shortened battery life. Battery cycling can be particularly problematic because standby engine-generators are more prone to frequency variations.
You can apply smaller static UPS products, which are self-contained, with similar application considerations as outlined for voltage regulators. Larger UPS applications are more complicated and require the assistance of a qualified consultant.
Reliability considerations for critical loads often require redundant UPS systems. Continuing operation during extended power interruptions usually requires alternate power sources like a standby engine-generator with automatic transfer switchgear.
As we've shown, there is a wide variety of power conditioning products from which to choose. But no single product can solve all power quality problems. To see improvements, you must properly apply the conditioning equipment best suited to the system at hand.
