There are several issues to designing a successful surge suppression system. The first part is to understand the nature of surges, that they can be generated externally or internally.

External surges are generally more severe than internal surges while internal surges generally occur more frequently (about 80 percent of all surges are internal).

• External surges are frequently caused by storms and normal utility company switching operations
• Internal surges occur when equipment within the building is cycling on and off

There are “whole building” surge protectors, offering protection at the service entrance. While this will minimize problems from the 20 percent of surges originating outside a building, the commercial or industrial facility will still need protection from the 80 percent of surges originating within the building. For best performance, a surge suppressor must be close to, but not within the protected equipment, since surges may involve very high voltages and currents. These high currents can readily inject into sensitive circuits, thereby causing damage that would not be prevented by a “whole building” surge protector. Surges within a building can be very large, up to 6,000 volts and 3,000 amperes according to some industry estimates.

For more information, here is a comprehensive List of Surge Suppressor Manufacturers, which may help you in looking for equipment specific to your needs. BPU does not endorse or recommend any manufacturer; this list is provided for your convenience. 

Eliminate Voltage Unbalance

Voltage unbalance degrades the performance and shortens the life of three-phase motors. Voltage unbalance at the motor stator terminals will cause phase current unbalance far out of proportion to the voltage unbalance. Unbalanced currents lead to torque pulsations, increased vibrations (120 Hz) and mechanical stresses, increased losses, and motor overheating, which result in a shorter winding insulation life.

Voltage unbalance is defined by the National Electrical Manufacturers Association (NEMA) as 100 times the absolute value of the maximum deviation of the line voltage from the average voltage on a three-phase system, divided by the average voltage.

For example, if the measured line voltages are 462, 463 and 455 volts, the average is 460 volts.

The voltage unbalance is: (460-455) /460 x 100 = 0.65%

According to ANSI Standard C84.1, Electric Power Systems and Equipment – Voltage Ratings (60 Hertz), about 98 percent of surveyed electrical power-supply systems are within 3 percent of voltage unbalance, with 66 percent of the systems at 1 percent or less. Common causes of voltage unbalance include:

• Faulty operation of power factor correction equipment
• Unbalanced or unstable utility supply 
• Unbalanced transformer bank supplying a three-phase load that is too large for the bank
• Unevenly distributed single-phase loads on the same power system
•  Unidentified single-phase to ground faults
•  An open circuit on the distribution system primary

It is recommended that the voltage unbalances at the motor terminals not exceed 1 percent, though NEMA standards are at or below 3 percent. Unbalances over 1 percent require derating of the motor.

Voltage Unbalance in Percent	Derate Motor to These Percentages of the Motor's Rating
1%	98%
2%	95%
3%	88%
4%	82%
5%	75%
>5%	Do not operate!

Note: These unbalance numbers are general rules of thumb. Motor manufacturers should be contacted directly to provide unbalance tolerance limits for specific motors.

The efficiency of a rewound, 1800-rpm, 100-hp motor is given as a function of voltage unbalance and motor load in the table. The general trend of efficiency reduction with increased voltage unbalance is observed for all motors at all load conditions.

Motor Efficiency* Under Conditions of Voltage Unbalance

Voltage Unbalance
	Motor Efficiency, %
Motor Load % of Full	Nominal	1%	2.5%
100	94.4	94.4	93.0
75	95.2	95.1	93.9
50	96.1	95.5	94.1

Suggested Actions 

• Regularly monitor voltages at the motor terminals to verify that voltage unbalance is maintained below 1 percent
• Check your electrical system single-line diagrams to verify that single-phase loads are uniformly distributed
• Install ground fault indicators as required and perform annual thermographic inspections
• Derate the motor to ensure long life

In general, companies should consider phase monitors that can be set for independent programmable trip settings for under-voltage, over-voltage and phase imbalance.

Current Unbalance

Voltage unbalance causes extremely high current unbalance. The magnitude of current unbalance may be 6 to 10 times as large as the voltage unbalance. For the 100-hp motor in this example, line currents (at full-load with a 2.5 percent voltage unbalance) were unbalanced by 27.7 percent.

Temperature Rise

A motor will run hotter when operating on a power supply with voltage unbalance. The additional temperature rise is estimated with the following equation:

Percent additional temperature rise = 2 x (percent voltage unbalance) 2

For example, a motor with a 10° C temperature rise would experience a temperature increase of 8° C when operated under conditions of 2 percent voltage unbalance. Winding insulation life is reduced by one-half for each 10° C increase in operating temperature.

Operation of a motor when the unbalance exceeds 5 percent is not recommended. When unbalanced voltages exceed 5 percent, the temperature rise is so fast that protection by derating is not practical.

Example:

3% Unbalance = 18% Overheating
5% Unbalance = 50% Overheating 
7% Unbalance = 98% Overheating

Because energy-efficient motors have lower losses than standard-efficiency motors, they do not run nearly as hot during a voltage unbalance.


Voltage Sags, a Closer Look

While not as much of a household name as “power surges,” voltage sags are arguably the most common occurrence affecting power quality and can be extremely costly. Computer equipment and machinery used in industrial settings (adjustable speed drives, programmable logic controllers and robotics equipment) have all become increasingly susceptible to voltage sags.

Voltage Sags: What Causes Them and How to Guard Against Them

Voltage sag is a reduction in voltage for a short period of time. In technical terms, the sag is a reduction of 10 to 90 percent of the normal RMS (Root Mean Square) voltage at 60 Hz. By definition, a voltage sag occurrence lasts between 8 milliseconds and 1 minute, though the most common voltage sags last no more than 1 second.

Causes of Voltage Sag

Exact sources of voltage sags can be difficult to pinpoint, and can be on either side of the electric meter. In fact, they can frequently be caused by equipment within an industrial plant or commercial facility. Large motors starting up in a plant, for example, can cause voltage sags. Short circuits at a facility or a nearby plant may also cause nuisance shutdowns from sags. External causes of voltage sags include switching operations, as well as natural causes such as wind, lightning, traffic accidents, trees falling on wires, and animals such as squirrels or raccoons. The cause of the situation does not need to be close by. Voltage sag on a power grid can have an impact on electricity users for more than a one hundred mile radius.

Detecting Voltage Sags

The most common way for detecting voltage sags is the use of a power quality monitor or power analyzer. They work by measuring power as it enters a facility and comparing that power to current standard power quality curves such as ITIC or ANSI. Basic power quality monitors allow graphing of RMS voltages on a daily, weekly or monthly basis, while monitors that are more sophisticated use software to track voltage sags and other power quality disturbances compared with standard power quality curves.

Reducing the Impact of Voltage Sags

One way to minimize the impact of voltage sags is to specify and purchase electrical equipment that is more tolerant to voltage variation. Equipment can be specified that can handle sags down to 90 percent of nominal voltage, while an alternative might be to purchase a similar piece of equipment that can still operate without shutting down at 70 percent of nominal voltage. Ride-through times also vary, and this can be a significant feature since about 80 percent of all voltage sags are gone within 0.2 seconds due to the operation of the utility's protective equipment. If this sensitive equipment can maintain the load for 0.2 seconds during a voltage sag, the production downtime will likely be greatly reduced.

The most common way for detecting voltage sags is the use of a power quality monitor or power analyzer. They work by measuring power as it enters a facility and comparing that power to current standard power quality curves such as CBEMA, ITIC or ANSI. Basic power quality monitors allow graphing of RMS voltages on a daily, weekly or monthly basis, while monitors that are more sophisticated use software to track voltage sags and other power quality disturbances compared with standard power quality curves.

Power quality curves are an empirical set of curves that represent the intensity and duration of voltage disturbances. The CBEMA (Computer Business Equipment Manufacturers Association) initially created the curves as a realistic, at-the-equipment, maximum allowable voltage that equipment can withstand, without damage or upset. The power quality curves in common use today are the ANSI (American National Standards Institute), CBEMA and the ITIC (Information Technology Industry Council) curves.

The ANSI curves plot the deviation from nominal voltage as a percentage of nominal voltage compared to the duration or the maximum length of time the voltage is permitted to reach. As an example, the limit for voltage excursions greater than 1-second duration might be ± 10 percent.

The ITIC and CBEMA curves plot voltage with respect to duration, but as a percentage of absolute voltage. Electronic equipment can typically withstand high voltages provided they last for less than 1 millisecond in duration, but voltages greater than +10 percent or -20 percent for between 0.5 seconds and 10 seconds duration will likely create problems.

Note: These curves are merely guidelines, and some electronic equipment may require higher power quality conditions than those represented in these standards.


Power Quality Solutions for Offices, Retail Stores and Warehouses

Harmonics

Harmonics are generated by non-linear devices that repeatedly switch current on and off, such as transistors, diode bridges and SCRs. On a 3-phase, 4-wire power system supplying power to single-phase switch-mode power supplies (computer power supplies, for example) or fluorescent lighting, significant harmonics (all odd harmonics, generally) flow on the phase conductors as a result of the non-linear current drawn by the loads.

These harmonics can overload the neutral conductors, connections in panel boards and transformers if the situation is not addressed. The neutral current can approach 175 percent of the phase conductor current.

Line-Side Computers/Fluorescents Solutions

• Recirculate Back to Load
  Zigzag transformer- The application of a zigzag transformer (zero-sequence traps) or a delta/zigzag distribution transformer simply provides an alternate path for the 3rd harmonic currents to flow and does not allow the current to flow back through the main step-down transformer.
  
  
• Alter the Waveform
  Series-connected neutral blocking filter - a capacitor and reactor combination that is connected in series with the neutral conductor.
  
  Harmonic-Mitigating/Phase-Shifting Transformers - Multiple power suppliers or ballasts of equal load are phase shifted using both a delta/wye and delta/delta transformer.
  
  
• Live With It
  K-Rated Transformer - Same as for VFD motor drives.
  
  Double neutrals - increase the neutral conductor size to twice the size of the phase conductor in any circuits where a “shared neutral” is used.
  
  Derated transformers - When switch-mode power supplies represent over 40 percent of total load, transformers are usually derated at least 50 percent.

Voltage Interruptions

A surge/swell is an over-voltage and sags are an under-voltage (a minimum of ± 10 percent) that lasts from about 8 milliseconds (one-half cycle) to one minute. Sags below 10 percent of voltage supply are usually classified as interruptions.

Line-Side Solutions for Interruptions

• Battery UPS - An uninterruptible power supply (UPS) can range from a 300 VA unit to protect a single PC, up to 1,000 kVA system to protect clusters of equipment or entire facilities. There are three types of battery-based UPS: 
  
       1. Off-Line (or Standby Power Supply), 
       2. Line-Interactive (or Hybrid System), and
       3. Online (or True UPS).
   
• Flywheel UPS - During power disturbances, the flywheel continues to spin for a period of time (from 10 seconds on up to many minutes) and up to 75 percent of the energy stored in the flywheel is used to continue the rotation of the generator and supply uninterrupted power to the load.

Power Transients

Transients are non-repetitive brief over-voltages or waveform distortions in an electrical system. A spike is an over-voltage transient that lasts from nanoseconds to milliseconds. Spikes can originate outside of your facility from lightning or from neighboring plants or work areas that start or stop large equipment (motors, presses, etc.). An average bolt of lightning carries a current of 30,000 amperes, transfers a charge of 5 coulombs (amp-seconds), has a potential difference of about 100,000,000 volts, dissipates 500 megajoules (enough to light a 100 watt light bulb for two months), and lasts a few milliseconds. Transients can disrupt, damage or destroy electronic equipment.

Spike Solutions:

• Transient Voltage Surge Suppressor/Arrestor (TVSS) - A fast-acting transient device that can be used for lower-voltage transient attenuation. It acts by clamping the line voltage to a specific value and then conducting any excess impulse energy to the safety ground, regardless of frequency. The energy shunting capability of a TVSS is expressed by its joule rating, which determines the amount of energy the device can handle.
  
  
• Residential/commercial class surge suppressor panels, receptacles and power strips utilize solid-state metal oxide varistors (MOVs) as the suppression element. MOVs are grouped by their voltage rating and energy handling ability. The voltage surge energy is absorbed by the MOV and converted into heat. Repetitive, high-energy surges have been reported to degrade MOVs. When equipment has signal/data connections in addition to AC connections (fax machine, TV set with cable, PC with LAN connection, multi-function copiers), damage can come not only from the AC (power) side, but from the signal wires as well. Many manufacturers offer commercial protectors that combine AC and signal line protectors. The arrangement is called “surge reference equalization”, “portable ground window”, “bubble of protection”, multi-port surge protection, or a number of other names.
  
  
• Most wall outlet surge suppressors come with six to eight outlets and various indicators and are readily available at local electronics stores.
  

Wiring Mistakes in Telecommunications Systems Can be Costly

If you have experienced surges and spikes in telecommunications systems that have led to the destruction of electronic equipment, gradual circuit deterioration resulting in premature equipment failure and intermittent operation, or unexplained incidences of data and software corruption, the problem may be rooted in inappropriate design and wiring practices. Improper grounding techniques are the chief cause of problems relating to these circumstances, with shielding problems (isolation) running a close second. A simple review of the basics and examples of the most frequent mistakes are provided as a quick “spot check” for the telecommunications user.

Before making any wire and cable decisions, building and office managers should evaluate the requirements of their building:

In general, communications cables shall be separated from electric light, power, class 1, non-power-limited fire alarms, and medium power network broadband communications circuits in raceways, compartments and boxes. Exceptions include situations where an approved barrier is used, and for those instances when the electric conductors are introduced solely for power supply to the communications equipment, and meet required separation clearance distances.

Underground communication wires and cables should be separated from electric light or power conductors by means of brick, concrete, tile partition or other suitable barriers. Where practical, the communications wire and cable shall be located below the electric light or power conductors. Communications wires and cables shall not be attached to a cross arm that carries electric light or power conductors. A bonding jumper should be connected between the communications grounding electrode and the power grounding electrode system at the building or structure served where separate electrodes are used.

Secondary protectors can provide a means to limit the current to less than the current carrying capacity of listed communication wires and cables, telephone lines, etc. Any over-voltage protection, arrestors or grounding connections should be connected on the equipment terminals side of the secondary protector current limiting means.

An “online” UPS provides some “natural” protection against surges, spikes and transients. Some UPS designs are virtually transparent to short-duration events, so attached equipment such as surge suppressors may be required. In addition, UPS systems will not protect phone lines, network cables and other conductors.

There are also considerations for the telecommunications room itself as follows:

• The telecommunication rooms should be properly ventilated and not shared with electrical feeders, branch circuits or transformers. Generally, carpet is prohibited and floors should have anti-static properties.
• Telecommunications outlet boxes and conduit pathways are for low-voltage telecommunication wiring only, and should not be shared with other wiring.
• A riser cable system in each building links the floors together. Ideally, riser cables should be distributed in one or more riser shafts enclosed in a series of vertically-aligned closets beginning in the basement and extending throughout the height of the building. These shafts should be aligned vertically to facilitate cable pulling.

All telecommunications rooms should have a Telecommunications Grounding Bus Bar (TGB). A TGB provides a central ground attachment point for telecommunications systems, computers and other equipment located in the telephone/data room. All cable tray, ladder rack, access floors and telecommunications racks and/or cabinets contained within the telecommunications room should be grounded/ bonded together using the appropriate size and grade of copper wire or bus bar. The first choice for a ground is a bus bar connected to the building ground; the second choice is a bus bar connected to a water pipe.

It cannot be overemphasized how important proper grounding procedures are to the proper operation of telecommunications systems. The first item designed, constructed and installed in a telecommunication facility should be the ground system. The purpose of the ground system is the equalization of potentials across the system at relatively low impedance. Meshed bonding networks, common bonding networks, ground planes and ground rings are examples of how large data centers ensure that the voltage potential across their ground systems is equalized at the lowest possible impedance.

It is important to ensure that low-impedance grounding and bonding connections exist among the telephone and data equipment, the AC power system's electrical safety-grounding system, and the building grounding electrode system. This is in addition to any male grounding electrodes, such as the lightning ground ring. Failure to observe any part of this grounding requirement may result in hazardous potential being developed between the telephone (data) equipment and other grounded items that personnel may be near or might simultaneously contact.


Harmonic Resonance Solution with Power Factor Correction

Harmonics are generated by non-linear devices that repeatedly switch current on and off, such as transistors, diode bridges and SCRs. This includes any type of static power converter, such as an uninterruptible power supply, DC drives, inverter welding power supplies or adjustable-frequency controllers (AFCs) like those used for Variable Frequency/Adjustable Speed (VFD/ASD) drives and UPSs. It also includes internal power supplies, such as for computers, copiers and electronic ballasts for lighting.

        Typical symptoms of harmonics include:

• Overheated transformers/motors
• Overheated neutral conductors 
• Electrical fires
• High motor loss and vibration
• Transformer or motor audible noise
• Capacitor failures
• Nuisance trips of protective devices
• Timing circuit/digital clock errors

An AFC has a converter section, which converts AC line power to DC, and an inverter section, which converts DC to adjustable frequency AC.

1. Input line harmonics are caused solely by the converter section and are usually referred to as line-side harmonics.
2. Output line harmonics are caused solely by the inverter section and are called load-side harmonics. They are completely isolated from each other.

Thus, load-side harmonics only affect the equipment driven by the AFC, while line-side harmonics affect the whole power system. Typically, three-phase power converters produce 5th, 7th, 11th, 13th, etc., harmonics. These line-side harmonic currents flow back through the impedance of the power system and create voltage distortion at these same frequencies. Single-phase power converters like computer power supplies produce all of the odd harmonics including large magnitudes of 3rd harmonic current.

Harmonic Resonance

Harmonic-generating equipment on systems with large amounts of capacitance in parallel with inductance, such as systems with power-factor correction capacitors or capacitive welders can result in resonance condition. At some given harmonic frequency in any system where a capacitor exists, there will be a crossover point where the inductive reactance equals the capacitive reactance. Parallel resonance causes problems only if a source of harmonics exists at the frequency where the impedances match. This is typically called harmonic resonance. It is extremely unlikely that these two impedances are identical, but near resonance can be very damaging.

When installing power factor correction capacitors, the resulting parallel resonant frequency, or harmonic order, can be estimated using the following equation: Hr = Ö(Tr /(Z * Cr)

Where,

Hr is the parallel resonant harmonic (i.e., 5th, 7th, etc.)

Tr is the transformer rating, kVA

Z is the transformer impedance, %

Cr is the three-phase load of the capacitor bank in kVa

For example: A 1,500 kVa transformer that has a 5.75 percent impedance connected to a capacitor load of 600 kVa, the capacitor bank will be resonant with that source impedance around the 7th harmonic.

Hr = (1,500 /(0.0575 * 600) = 6.59

Therefore, if any magnitude of 7th harmonic current flows on the power system at that bus, the effect could be catastrophic.

Harmonic resonance can occur if both of the following are true:

• Harmonic producing loads are operating on the power system.
• At a specific location in a power system, a capacitor, or a group of capacitors, and the source impedance have the same reactance (impedance) at a frequency equal to one of the characteristic frequencies created by the loads.

Unfortunately, harmonic resonance is said to be a “self correcting problem” – most times, capacitor fuses open, capacitor cans fail or the source transformer fails. Power capacitors must withstand a maximum continuous RMS over-voltage of 110 percent and an over-current of 180 percent based on the nameplate rating. This over-voltage and over-current includes both the fundamental frequency and harmonic contributions. Also, the VA rating of the capacitor cannot exceed 135 percent. In a best-case scenario, harmonics will cause the electrical control equipment to act erratically. In the worst case, harmonic resonance can result in explosions and equipment damage may occur.


Harmonic Resonance Solutions

Therefore, if the selected capacitor is going to cause resonance with the system, you have two choices:

1. Apply another method of kVAR compensation (harmonic filter, active filter, synchronous   condenser, etc), or 
2. Change the size of the capacitor bank to overcompensate or under-compensate for the required kVAR and live with the ramifications.

Also, keep in mind:

• Always consider harmonic resonance even if applying a “small” capacitor on a “large” system.
• Do not convert 480 V capacitors to 480 V filters – continuous over-voltage may damage the capacitors.
• Make sure to account for actual kVAR when applying higher (voltage) rated capacitors on a lower (voltage) rated system (i.e., applying 600 V capacitors on a 480 V system yields 64 percent of rated kVAR).
  

Power Quality Solutions for Harmonics Generated by Computers and Fluorescent Lights

Harmonics are generated by non-linear devices that repeatedly switch current on and off, such as transistors, diode bridges and SCRs. This includes internal power supplies, such as computers, copiers and electronic ballasts for lighting. Typical symptoms of harmonics include:

• Overheated transformers/motors
• Overheated neutral conductors
• Electrical fires
• High motor loss and vibration
• Transformer or motor audible noise
• Capacitor failures
• Nuisance trips of protective devices
• Timing circuit/digital clock errors

A switch-mode power supply has a converter section, which converts AC line power to DC. Electronic fluorescent ballast has both a converter and an inverter section, which converts DC to adjustable high frequency AC. Input-line harmonics are caused solely by the converter section and are usually referred to as line-side harmonics. Output-line harmonics are caused solely by the inverter section and are called load-side harmonics. They are completely isolated from each other. Thus, load-side harmonics only affect the equipment driven by the inverter, while line-side harmonics affect the whole power system. Single-phase circuits are especially vulnerable to harmonics because there is no natural balance effect from three phases.

On a 3-phase, 4-wire power system supplying power to single-phase switch-mode power supplies (computer power supplies, for example) or fluorescent lighting, significant harmonics (all odd harmonics, generally) flow on the phase conductors as a result of the non-linear current drawn by the loads. These harmonics can overload neutral conductors, connections in panel boards and transformers if the situation is not addressed. The neutral current can approach 175 percent of the phase conductor current.

There are three main types of solutions for line-side harmonics from computer power supplies and electronic ballasts: recirculating back to the load, altering the distorted current waveform, or simply living with it.

Line-Side Harmonics Solutions

Recirculating Back to Load

• Zigzag Transformer
  The application of a zigzag transformer (zero-sequence traps) or a delta/zigzag distribution transformer simply provides an alternate path for the 3rd harmonic currents to flow and does not allow the current to flow back through the main step down transformer. This reduces the overall voltage distortion upstream and within it, but damaging harmonic currents are still present downstream - in all phase wires, neutral wires, bus bars, connecting lugs and electrical panels. An optional line reactor is sometimes applied to reduce the current division between the original transformer and the new zigzag transformer and to force most of the 3rd harmonic current through the zigzag.

Altering the Waveform

• Series-Connected Neutral Blocking Filter
  A capacitor and reactor combination that is connected in series with the neutral conductor. These components are “parallel resonant” at the third harmonic allowing 60 Hz (normal load) current to flow but present an extremely high impedance for the third harmonic current and do not allow the load to “source” current at that frequency.
  
  
• Harmonic-Mitigating/Phase-Shifting Transformers
  Multiple power suppliers or ballasts of equal load are phase shifted by feeding one group from a delta/wye transformer, and feeding the second through a delta/delta transformer or a line reactor of equivalent impedance resulting in waveform cancellation.

Living With It

• K-Rated Transformer
  Transformers may experience extra heating in the core and windings due to harmonics. To combat potential problems, many transformer manufacturers rate their products with a K-factor (normally one to twenty, although the rating goes up to fifty) that indicates the transformer’s ability to withstand degradation from harmonic effects. K-factor rated transformers offer no means to reduce the magnitudes of harmonic current (except that they offer line reactance).
  
  
• Double Neutrals
  Increase the neutral conductor size to twice the size of the phase conductor in any circuits where a “shared neutral” is used. Note that for many newer installations every circuit includes a phase conductor and its own neutral conductor, so the only shared neutral is found in the panelboard and on the transformer. Older installations will probably have only one shared neutral for three conductors.
  
  
• Derated Transformers
  Some manufacturers simply derate their transformers to compensate for potential problems. Some transformer manufacturers will specify an oversized standard K-1 rated transformer as a K-13 or K-20 rated transformer at half its capability. This transformer may be capable of 100 KVA as a K-1 rated transformer, but rated 50 KVA as a K-13 rated transformer. This transformer is actually larger and more costly than a 50 KVA K-13 rated transformer and may not be well-suited for the job. When switch-mode power supplies represent over 40 percent of total load, transformers are usually de-rated at least 50 percent.
  

Comparison of Harmonics Solutions
Optimal Solutions	Significant Advantages	Significant Disadvantages
Zigzag/Harmonic Mitigating Transformer	"Handles" 3rd harmonics recirculating them back to the load. Can reduce other (5th and 7th) harmonics when used as phase-shifting pairs. Reduces voltage "flat-topping".	Requires fully rated circuits (and oversized neutrals) downstream to loads. Paired units must have nearly balanced loads.
K-Rated Transformer	"Live-with" harmonics.	Does not reduce "system" harmonics.
Neutral Blocking Filter	Only solution that eliminates the third harmonic current from load. Relieves system capacity and has potential for energy savings.	High cost. May increase voltage distortion.
Double Neutrals/Derated Transformer	"Live-with" harmonics - typically, least expensive.	Upstream and downstream equipment must be fully rated for harmonics.