Tech Tips
![Image](data:image/svg+xml,%3Csvg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 100 100"%3E%3C/svg%3E){kind=icon}
Power quality topics and solutions
The information and diagrams presented herein are for general educational purposes only, and should not be relied upon as instructions for customer self-wiring. Customers should at all times seek the assistance of qualified electricians or utility personnel for all wiring projects.
SYMPTOMS
- Case 1: Demand recorder gaining time
- Case 2: Digital clocks gaining time
CAUSES
- Case 1: Line notching from rectifiers
- Case 2: Faulty air conditioner
SOLUTIONS
- Case 1: Apply harmonic filter
- Case 2: Repair/replace air conditioner
Most digital clocks look for a pure sine wave cycle and will count two zero crossings. After the completion of 60 cycles, the clock will advance one second. Sometimes other nearby electrical equipment may interfere with this process and cause clocks to speed up. Case 1 refers to a clock that counts four 70-volt threshold crossings.
CASE 1: DEMAND RECORDER GAINING TIME
A steel company reported an energy demand recorder was gaining time at their facility. The recorder was gaining from a few minutes to a day each month.
At this location, the voltage waveform was distorted from line notching caused by the customer's 6-pulse rectifier loads. The line notching was causing the clock to count extra 70 volt crossings. The waveform is shown in Figure 1.
Figure 1: Input voltage to recorder before filtering
A filter was designed and installed to clean the voltage waveform at the demand recorder. The output of the filter is shown in Figure 2. Since the installation of the filter, the demand recorder has stopped gaining time.
Figure 2: Input voltage to recorder after filtering
CASE 2: DIGITAL CLOCKS SPEEDING UP
A residential customer was plagued with digital clocks gaining time through the day. During a site visit, a voltage waveform analyzer was plugged into the clock’s electrical outlet. The waveform indicated extra zero crossings caused by line notching (Figure 3).
Figure 3: Voltage causing clock to speed up
It was also noted that when the clock was unplugged momentarily, the reset 12 o’clock numbers flashed near twice the expected 60 times per minute. Moving the clock and switching individual breakers off, the notching was believed to be generated outside the home. After talking with neighbors, it was determined the three houses served off a common distribution transformer experienced similar problems. Using the clock as a trouble-shooting tool, a defective air cleaner was isolated next door. Replacing the air cleaner solved the problem.
SYMPTOMS
- Adjustable frequency drives shut down for no apparent reason
- Drive diagnostics report over voltage condition
- Failures often occur about the same time each day
CAUSES
- Capacitor switching transients
- Drives too small compared to available short circuit capacity
SOLUTIONS
- Install line reactors or isolation transformers ahead of the drive
- Use less sensitive drives
A manufacturing facility installed new adjustable frequency drives to improve production. Unfortunately, the new drives started tripping off line for no apparent reason. This often happened on clear days with no visible power interruptions or other problems. Only the new drives experienced the problem.
As the problem continued, the plant electrician noticed the trip outs often occurred about the same time each day. Furthermore, the electrician checked the drive diagnostics and always found an over voltage indication.
Examination of the situation revealed the customer was served from a 12.47-kV distribution feeder that had switched capacitors for voltage and var control. Also, the drive horsepower ratings were very small compared to the short circuit capacity available at the drive terminals. Testing confirmed that the drives would trip off almost every time a certain capacitor was switched on.
A capacitor creates a momentary short circuit at the instant it is energized. Current surges into the capacitor and line voltage collapses for a few millionths of a second. As the current surge declines, it causes a ringing transient with overshoot on the peak voltage. Since the dc bus voltage is a function of the peak voltage, the overshoot causes the dc bus voltage to exceed the over voltage setting, which results in the drive tripping off line.
The solution in this application was to install a properly sized line reactor in the plant's electrical supply to the drive. The reactor's high impedance to the transient cuts the voltage overshoot and lowers inrush current to the drive. (Isolation transformers also do this, but they cost more.) Experience has shown that a 3% impedance reactor is effective at protecting against nuisance tripping. In order to achieve the 3% impedance, it is necessary to specify the voltage and horsepower or KVA rating for the drive it is to be installed on. Voltage recordings below show the overshoot reduction achieved by the installation of reactors. This transient reduction was enough to prevent the drives from tripping.
Figure 1: Voltage on the source side of the reactor
Figure 2: Voltage on the load side of the reactor
This also shows that all drives are not created equal. Many drives in the plant survived capacitor switching with no problems. Careful selection of new drives helps prevent production outages from normal utility operations.
SYMPTOMS
- Computer system lock ups
- Standby uninterruptible power supply (UPS) "beeps"
CAUSES
- UPS has highly distorted output voltage
SOLUTIONS
- Replace with an on-line uninterruptible power supply (UPS) with sine wave output voltage
- Adjust sensitivity setting for UPS
A standby uninterruptible power supply (UPS) is designed to provide battery backup for the normal 120V AC supply in the event there is an interruption of electrical service. This feature is designed to prevent loss of computer data and protect the load from power line disturbances.
A distributing company reported occasional computer lockups when one of their UPS units "beeped". A computer operator noted that the UPS usually "beeped" when she turned on one specific computer.
The UPS user's manual claimed a "Pseudo Sine Wave" output. A snapshot of the actual output waveform is shown in Figure 1. Replacing the UPS with an on-line UPS featuring a sine wave output was recommended.
The line-to-neutral peak was 200 volts, roughly 18% above the nominal peak of 170 volts. The higher voltage and nonsinusoidal output voltage waveform may have contributed to the computer lock-ups.
Figure 1: Transfer to battery backup
Employees of a fast food restaurant reported enduring computer system lockups a few times a day. These lockups would halt the electronic cash registers until the computer system could be restarted. The output waveform of their UPS is shown in Figure 2.
The sensitivity setting of the transfer mechanism caused the UPS to switch to battery backup several times each day. The random computer lockups ceased following replacement of the standby UPS with an on-line, pure sine wave output UPS.
Figure 2: Transfer to battery backup
For some UPS systems it is possible to lower the voltage threshold setting that causes the unit to switch to the backup supply. This may be done by flipping mini-switches on the back of the unit or by software that is supplied with the unit.
SYMPTOMS
- Customer complaints about momentary interruptions.
- Frequent instantaneous operations on circuit.
- Many instantaneous breaker operations associated with blown fuses.
- Larger substation transformer with higher short circuit current capacity.
CAUSES
- Instantaneous relay trying to clear temporary faults before fuse blows.
- High short circuit current blows fuse before breaker can open.
SOLUTIONS
- Reduce causes for insulation failures.
- Coordinate fuses with breaker clearing time.
- Remove instantaneous trip from service.
BACKGROUND
A warehouse and distribution center complained about too many momentary interruptions. The power would go off and then back on often for no apparent reason. This caused problems for computer inventory systems, file servers, voice mail and etc.
The 12.47kV circuit feeding this customer was short, just two or three miles long. The supply substation had a 30 MVA transformer with about 8,000 amperes of short circuit current available. Most tap line fuses were no larger than about 125 amperes.
The substation log and interruption reports showed that nearly every instantaneous operation at the substation was associated with a blown tap line fuse. There were very few occasions where the instantaneous breaker operation cleared temporary faults without blowing a fuse. A disturbance analyzer confirmed one event where the tap line fuse blew just before the circuit breaker opened. Figure 1 shows the sequence of events.
Figure 1: Fuse Blows Before Breaker Opens
SOLUTION
Analysis of the circuit showed it was very difficult for the substation breaker to open before short circuit current damaged tap line fuses. The higher current blew the fuse faster than the breaker's 0.05-0.08 second minimum clearing time. Tap lines near the substation had larger fuses, but even large fuses would blow first. This case suggested removing the instantaneous relay from service and changing the reclosing.
DISCUSSION
One job for the instantaneous relay is to prevent permanent tap line interruptions for temporary faults such as lightning. The price for this feature is a momentary interruption for all customers on the feeder. Most industrial and large commercial customers today have electronic equipment that shuts down for momentary outages. It has become more desirable to allow tap fuses to blow before the substation circuit breaker trips by removing the instantaneous trip on substation circuit breakers.
It may make sense to retain instantaneous tripping on certain circuits such as circuits supplying mostly residential customers. If an instantaneous trip is kept on a circuit, remember to include the breaker clearing time when coordinating fuses with instantaneous relays. Consider how many permanent tap line interruptions will be prevented against momentary interruptions for the entire circuit. The better solutions consider line protection, fuse coordination, and the number of temporary tap line insulation failures.
SYMPTOMS
- Applying low voltage capacitors much smaller than high voltage caps
- Nuisance drive shutdowns
- Damage to sensitive equipment
CAUSES
- Magnification of high voltage capacitor switching transients
SOLUTIONS
- Determine likelihood of overvoltage problems (contact Technical Services for assistance)
- Apply capacitors as a tuned filter
BACKGROUND
A rate study performed for a sewage treatment facility revealed that installing power factor correction capacitors would be beneficial. Calculations showed installing 100 kVAR of capacitance would improve the customer's power factor to near unity, thereby reducing their bill.
Analysis of potential steady-state harmonic concerns revealed there was little danger of exciting a resonance characteristic of the customer's adjustable speed drive systems (5th, 7th, 11th, 13th, etc.). However, a study of capacitor switching transients revealed that energizing a 12 kV capacitor with the proposed 100 kVAR low voltage capacitance could cause transient overvoltages at the customer's 480 volt bus 50% higher than without low voltage capacitance. This could easily damage some electronics.
The transient magnification phenomenon occurs when the series resonance formed by the low voltage capacitor in series with the distribution transformer is on the same order of that formed by the 12 kV capacitor in series with the system impedance. Nearly equal resonant frequencies, combined with a low voltage capacitor much smaller than the high voltage capacitor, in theory can cause transient overvoltages as high as 6 per unit.
Figure 1: Equivalent Capacitor Switching Circuit
To avoid matching the high voltage resonance, some new installations may allow for installing a different amount of low voltage capacitance. However, in this case, installing 70 kVAR would bring the power factor up to 0.98 lagging but could still cause severe amplification of 12 kV capacitor switching transients.
SOLUTION
One effective method of reducing the transient at the low voltage bus is to install the low voltage capacitors as a harmonic filter. The filter is usually tuned to the lowest characteristic harmonic (typically the 5th) to mitigate potential harmonic concerns in addition to preventing capacitor switching transient magnification.
SYMPTOMS
- Distortion of characters on computer screen
CAUSES
- Magnetic fields (EMF) affecting screen
- Electric fan near screen
- Screen is near power transformer or other "heavy" electrical distribution
SOLUTIONS
- Relocate monitor away from EMF source
- Relocate EMF source
- Shield monitor from fields
- Change monitor refresh rate
BACKGROUND
An individual reported a computer monitor screen was "jumping around". An investigation revealed that an Electromagnetic Field (EMF) of 11 mG was interfering with the screen. The distortion appeared as a wavy screen and oscillating of text primarily around the edges of the screen. A similar screen is illustrated in figure 1. Measurements in the area suggested that the EMF was due to electrical distribution circuits running under the floor.
Magnetic field interference to monitor screens does not usually occur until a level of roughly 10 mG. Some high resolution screens have detected image distortion at 3-4 mG.
SOLUTION
Magnetic field strength decreases rapidly as you move away from the source. Making use of this principle may assist in eliminating monitor screen distortion without the need for shielding.
Additional measurements revealed that the magnetic field strength was about 7-8 mG near an empty workstation. Relocating the monitor to this table eliminated the screen distortion.
Magnetic field strength will vary with the amount of current flowing in the source of the EMF. Relocating sensitive monitors may not always provide a permanent solution. In some cases it may be necessary to install a shield made of a ferromagnetic material around the monitor. A shield for a typical monitor costs approximately $800.
When applicable, changing the monitor refresh rate to 60 Hz has decreased image distortion. The change will place the monitor’s CRT in synchronization with the 60 Hz current induced magnetic field.
Figure 1: Wavy screen caused by an electric fan held close.
SYMPTOMS
- Installing capacitors
- Capacitor fuse blowing
- High levels of harmonic distortion
- More than 20% of load is static power converters
POTENTIAL PROBLEMS
- Capacitors resonate with system inductance
SOLUTIONS
- Install capacitors as harmonic filter
- Relocate capacitor bank
- Install capacitors as harmonic filter
- Size capacitor, capacitor step sizes to avoid characteristic harmonic load currents
BACKGROUND
To operate power systems more efficiently, utilities and their customers are installing numerous capacitor banks. Many of the capacitor banks are being installed within customer's premises. This Tech Tip will review the analysis necessary to prevent steady-state harmonic resonances when installing capacitors on a utility system or inside a customer facility. Note that further investigation is required to prevent other potential problems such as capacitor transient amplification.
DETERMINING THE RESONANT FREQUENCY
Calculating the resonant frequency created with a capacitor and system inductance is surprisingly simple. As shown in Equation 1, simply calculating the square root of the three phase short circuit MVA divided by the capacitor MVAR will provide the resonant harmonic, hf, for the system under study.
DISTRIBUTION LINE EXAMPLE
Consider a case where a 1200 KVAR capacitor is to be installed on a 12.47 kV system at a location where the three phase short circuit current is 2800 amps.
Resonant conditions near the 3rd, 5th, 7th, 11th, and 13th harmonic are often the most troublesome since these are common load currents found on the distribution system. Installing a 900 KVAR capacitor at this location would move the resonance to the 8.2nd harmonic, farther away from the characteristic harmonic load currents.CUSTOMER FACILITY EXAMPLE
Consider a case where installing 600 KVAR capacitors in a customer facility is necessary to improve the customer's power factor to 95%. The capacitor will be switched in four steps of 150 KVAR each. The three phase short circuit current is 18000 amps at 480 volts.
Resonance calculation steps (600 KVAR energized):
Installing these capacitors could cause resonance problems if the customer has six pulse rectifier loads (adjustable speed drives, arc furnaces, etc.) These loads require significant 5th, 7th, 11th, and 13th harmonic currents. With 600 KVAR energized, the 5th harmonic load current would resonate with the system inductance and 480 volt capacitor. With two steps on-line (300 KVAR), the 7th harmonic current would resonate with the system inductance and 480 volt capacitor. Installing these capacitors as a harmonic filter or altering the amount of capacitance would be viable alternatives.
Preventing Damage Due to Ground Potential Differences
SYMPTOMS
- Television, phone, computer system damage during storms.
POTENTIAL PROBLEMS
- Improper bending.
- Separate grounding electrodes. Long distances between grounding electrodes.
SOLUTIONS
- Bond phone and CATV grounding conductors to the power grounding electrode conductor at the service entrance.
BACKGROUND
Most people have suffered sensitive equipment damage during lightning storms, or know someone who has. On the surface, voltage surges appear to be the culprit and usually receive the blame for this damage. The power system can carry high voltage surges (lightning), but there is a more likely cause.
Many "voltage surges" are actually voltage differences in the earth that reach sensitive equipment because of bonding errors. Lightning and faults on the distribution system can cause a very large ground potential difference. Sensitive equipment that references multiple grounding systems that are not bonded together, can be exposed to very high voltage differences.
VOLTAGE POTENTIAL DIFFERENCE BETWEEN POWER AND CATV GROUNDING ELECTRODES
Televisions often reference two or more grounding electrode systems, significantly increasing the possibility of damage during a storm or line fault. In addition to the power grounding, the electrode system television may also reference cable television (CATV), an antenna system, or satellite dish grounding electrode system. If these grounds are not solidly referenced to the power grounding electrode system at the house at the house service, very high ground potential differences may appear inside the television. Figure 1 shows a simplified schematic of a television tuner and the power and CATV grounds. The power and CATV grounds may be at significantly different voltages during a storm if they are not bonded at the main power service entrance.
Figure 1: Tuner, CATV and Power Grounding Electrodes
Figure 2 shows three configurations that occur frequently. The left example is an example of incorrect grounding. The middle sketch is correct, but not preferred due to the long bonding juniper, grounding. Power, phone and CATV bonds are connected with No. 6 copper or larger. The right sketch is correct and preferred, with the power and communication bonded with a very short bonding conductor. Many new homes are constructed to bring all utilities to one location.
Figure 2: House Grounding Electrode Configurations
WHAT THE CODE SAYS
The 1993 National Electric Code sets the requirements for bonding the communication, radio, and television antenna and CATV grounds to the power ground in Articles 800-40d, 810-21j and 820-40d. The code requires a minimum No. 6 copper bonding conductor between these ground electrodes and the power grounding electrode, where separate electrodes are used. Please see these articles and Article 250 of the National Electric Code for further detail on the proper grounding of low voltage electrical systems.
Low Neutral to Earth Voltages Reduce Stray Voltage Problems
Note: The term “stray voltage” must “NOT” be confused with the term “contact voltage”. The term “stray voltage” is associated with elevated neutral to earth voltages caused by the flow of return currents in the earth. Stray voltages are generally in the range of 10 volts or less and not considered a threat to human life. On the other hand, the term “contact voltage” represents a potentially hazardous situation where an energized conductor has made contact with a normally de-energized surface. Stray voltage sources can be difficult to identify and resolution may be time consuming. In contrast, contact voltage sources are easily identified and can be corrected immediately.
SYMPTOMS
- Tingling or shocking sensation when touching metal objects
- Tingling or shocking sensation in swimming pools
- Tingling or shocking sensation when touching docks while in the water
- Farm animals behaving strangely when contacting water troughs, stanchions or other metal surfaces
- Dairy cattle symptoms include: reluctance to enter a milking parlor, uneasiness inside a milking parlor, uneven milk output, reduced milk production and health and breeding problems
- Normal operating distribution system
- Customer wiring or equipment problems
- High resistance utility system neutral connections
- High utility system neutral currents
- High resistance grounds
- High efficiency devices that create triplen harmonics
- Multiple circuit interaction
- COMMON SOLUTIONS
- Correct wiring and equipment problems
- Repair neutral connections
- Reduce neutral current
- Lower resistance to ground
- Bond grounds of different potential
- Neutral Isolation devices listed for the purpose
Electrically grounded equipment normally has a small voltage with respect to earth even with proper wiring and grounding. Sometimes the voltage is high enough to cause tingling or shocking sensations for people and animals. This only occurs when the person or animal becomes part of an electrical path between the grounded object and "remote" earth.
Outdoor electric distribution systems typically use neutral wires that are grounded and connected to the earth, in many locations. This is done for safety reasons. The many connections to earth allow some neutral current to flow through the earth instead of the neutral wire. Voltage drop occurs on the neutral and ground due to the current flowing through the neutral and ground impedances. This results in a voltage difference between the neutral and "remote" earth. Neutral to Earth Voltage, or NEV, appears at pole grounds and customers' main panels. Some neutral to earth voltage appears at grounded equipment because equipment grounding conductors are bonded to the neutral in the main panel.
A person or animal touching the earth and an electrically grounded object may experience a tingling or shocking sensation if the neutral to earth voltage is sufficient enough to be felt. One or two volts may be perceptible in certain situations. Water faucets, swimming pools, docks and dairy parlors are examples of locations typically associated with NEV issues.
Figure 1 shows a voltmeter measuring NEV in a swimming pool. Touching a hand rail, concrete deck or metal coping while in the water can result in a tingling or shocking sensation if perceptible neutral to earth voltages are present. Installing a pool in accordance with the 2005, or later, editions of the National Electrical Code will generally prevent both contact voltage issues and NEV issues in swimming pools.
Figure 1
Neutral to earth voltages around docks can be measured in much the same way as in swimming pools. Installing a dock in accordance with the National Electrical Code will protect swimmers in the vicinity of the dock from contact voltage. It is extremely important that all electrical circuits on a dock be protected by Ground Fault Current Interrupters, or GFCIs, because GFCIs help protect against contact voltage. GFCIs must be installed in accordance with the manufacturer’s instructions. They must also be maintained and tested regularly to ensure proper operation. Installing a dock in accordance with the NEC will not address stray voltage, or NEV, issues.
Stray voltage can make farm animals reluctant to drink from grounded water troughs. Dairy production may be affected if cattle feel a tingling or shocking sensation from metal stanchions at milking time. Figure 2 shows a voltmeter measuring NEV and the electrical path to an animal.
Figure 2
Recommendations
If a contact voltage issue or an NEV issue is suspected, a qualified electrician or the local utility should be contacted to investigate the situation and take appropriate action. Contact voltage issues should be resolved immediately. NEV issues should be further investigated and addressed appropriately by the local utility and the facility owner.
Neutral to earth voltages should be kept below perception levels when practical. Stray voltages above one volt should be reduced in dairy parlors. Eight volts or more of NEV should be reduced where practical. The need to reduce the voltage increases with higher NEV values.
Customer wiring problems and equipment grounding errors should be identified and corrected.
Wiring problems or short circuits on systems owned by nearby customers should also be identified and corrected.
Swimming pools should be installed in accordance with the 2005, or later, editions of the National Electrical Code. Bonding should be improved on existing pools when possible.
If possible, unplug all power to a dock when swimming in the vicinity of the dock. This is best accomplished by using cords that disconnect the hot wire, neutral wire, and ground wire when unplugged. New power installations around water should include an isolation transformer, where allowed.
High resistance neutral connections should be replaced. A clamp-on amp meter will show if current is not flowing through poor connections. A digital volt meter directly across connections or adjacent pole grounds is another way to detect bad connections.
Phase current balance is important. In general, balanced circuits have less neutral current and lower neutral current means lower NEV. Balance circuits and reduce neutral currents when beneficial. Be aware that balancing a circuit may not resolve an NEV issue if significant amounts of triplen harmonic currents are present.
If possible, convert 120 volt single phase loads to 240 volts to help reduce NEV caused by secondary neutral current.
Install an equipotential plane, ground ring or additional ground rods as the situation dictates. Equipotential planes can help by keeping conductive materials in a given area at, or near, the same potential. This is a must where farm animals are present. Ground rings can be used to intercept ground currents and divert them directly to the system neutral. Additional ground rods may reduce NEV by lowering the net resistance to neutral currents.
Stray voltage issues can be difficult to diagnose and solve. Additional information is available in EPRI Report TR-113566, dated September 1999 and titled “Identifying, Diagnosing and Resolving Residential Shocking Incidents” or the “Agricultural Wiring Handbook Eleventh Edition 1996”, published by the National Food and Energy Council, Columbia, Missouri.
Note: The information and diagrams presented herein are for general educational purposes only, and should not be relied upon as instructions for customer self-wiring. Customers should at all times seek the assistance of qualified electricians and/or utility personnel for all wiring projects.This Tech Tip briefly describes harmonic limits recommended by the recently revised standard IEEE 519. If you have questions regarding harmonic problems or for assistance in applying IEEE 519, please contact Power Quality.
BACKGROUND
IEEE 519 was upgraded from a Guideline to Recommended Practice in 1992 and is now available from IEEE. IEEE 519 was developed to establish goals for the design of power systems which include linear and non-linear, or harmonic, loads. It defines acceptable voltage quality and limits distortion current users may demand. The recommended limits are applied at the interface between sources and loads (the Point of Common Coupling or PCC).
CURRENT DISTORTION LIMITS
IEEE 519 current distortion limits are based upon the ratio of the size of the supply system to the customer load. This ratio, the Short Circuit Ratio, is calculated at the PCC between the consumer and utility. Loads that are small when compared to supply capacity may require higher distortion.
Table I contains limits for total harmonic distortion (THD) and individual harmonics. The letter h represents the individual harmonic number. For example, h equals 5 for current at 300 Hertz (5 x 60 Hertz). Note as plant load increases on a given supply system, the ratio will decrease and the limits may tighten.
Table I: IEEE 519 Current Distortion Limits
To illustrate, consider the following data:
Short circuit capacity: 108 MVA (5000A @ 12.47 kV) Customer Load: 1.50 MV Short Circuit Ratio: 108/1.50 = 72
For ratios between 50 and 100, the total harmonic distortion (THD) limit is 12%.
VOLTAGE DISTORTION LIMITS
IEEE 519 establishes maximum voltage distortion at the PCC with each consumer. Supply voltage quality should stay within limits provided the current limits are satisfied. Table II shows the voltage distortion limits.
Table II: IEEE 519 Voltage Distortion Limits
COMMUTATION VOLTAGE NOTCHING LIMITS
Voltage notching is common on many industrial power systems, especially those with large dc drives. When rectifying ac to dc, current is transferred from one phase to another. Since current does not transfer instantly, there is an overlap period during which two devices are conducting. During the overlap, a transient ac short circuit occurs through the two conducting devices causing a voltage notch. Six short circuits, or voltage notches, occur per 60 Hertz cycle or 360 notches per second.
IEEE 519 notch limits are divided into two parts: notch depth and notch area. These are illustrated in Figure 1.
Figure 1: Notch Depth and Notch Area
Table III contains recommended limits, where Special Applications include hospitals and airports, and a Dedicated System is exclusively dedicated to the converter load.
Table III: IEEE 519 Commutation Notching Limits
SYMPTOMS
- Nuisance ground fault interrupter trips (may occur at regular intervals)
- GFI monitors neutral-to-ground bond
- GFI set with no time delay
- Nearby switched, grounded-wye capacitor
CAUSE
- Capacitor switching transient causes current surge in customer neutral-to-ground bond
SOLUTIONS
- Coordinate GFI with transient current
- Use GFI that monitors phase and neutral conductors for ground faults
BACKGROUND
An industrial customer called his local utility representative to report nuisance trips of his ground fault interrupter (GFI) on his main panel. The customer initially had his GFI set for trip at 200 amps with no time delay. To reduce the nuisance trips, the customer had decreased the GFI sensitivity to 400 amp trip with 18 cycle (0.3 second) delay. He had no GFI trips at this level but requested assistance in locating the cause for earlier nuisance shutdowns.
GFIs are used at the main panel of many industrial and commercial systems to protect against ground faults. They should not be confused with ground fault circuit interrupters which are often used on 120 volt circuits.
GFIs usually protect for ground faults using one of two methods. One method is to monitor all three phase conductors and neutral to detect ground fault current. Another is to monitor the neutral-to-ground bond for ground current surges at the customer main panel. Monitoring the neutral-to-ground bond can make the GFI susceptible to neutral current surges.
FINDINGS
Since this customer has a GFI that watches current in the neutral-to-ground bond, a disturbance analyzer was used to monitor the bond. Steady-state current in the neutral-to-ground bond was low. However, the disturbance analyzer recorded a 5 millisecond (1/3 cycle) current surge when switching a nearby 12.47 kV capacitor bank. Figure 1 shows the current in the neutral-to-ground bond as each phase of the capacitor energized.
Figure 1: Current Surge in Neutral-Ground Bond
Consider Figure 2, a simplified model of the system. Random closing of the capacitor switches causes a transient current imbalance and surge in neutral current. Some of the surge current will flow through the service transformer and the customer main GFI.
Figure 2: Surge Current Path Thru GFI
SOLUTION
Adding at least three cycles time delay should be sufficient to eliminate nuisance trips on GFIs that monitor current in the neutral-to-ground bond. In this case, moving the GFI setting back to 200 amp trip and 6 cycles delay (the next shortest delay) improved ground fault sensitivity without suffering nuisance trips.
Your Neutral Wire May Be Working Overtime
CONDITIONS FOR A PROBLEM
- 120/208 or 277/480 volt system
- Loads are connected line-neutral
- Loads require 3rd harmonic current
- Single phase rectifiers
- Personal computers
- Some electronic ballasts
CAUSE
- 3rd harmonic current adds in the neutral
SOLUTIONS
- For full neutrals, keep distortion below 30%
- Use double-size neutral
- Use separate neutral for each phase wire
- Specify low harmonic equipment
BACKGROUND
Overloaded neutral wires are most common in office areas since these locations usually have 120/208 wye distribution with many computer loads connected at 120 volts. Reduced neutrals make the problem even worse. A typical scenario is addition of or converting to equipment that requires high harmonic current.
On a power system, if all phase currents are equal, it is common to assume the neutral wire is carrying little or no current. However, with the recent proliferation of single phase power supplies, this may no longer be true. Neutral current can sometimes be higher than phase currents. This is very important in facility wiring systems because neutral wires have no overload protection.
CANCELLATION -- OLD LOAD CURRENT
To understand cancellation of return current in the neutral, consider Figure 1. At each point in time, the neutral current is the sum of the phase currents. For the old load current, summing up the three phase currents at each point in time gives zero current in the neutral. For example, adding the Old Load Current values for each phase current at the vertical dotted line, we find the neutral current at that point in time is zero amps.
ADDITION -- PERSONAL COMPUTER LOAD CURRENT
The waveform for personal computer and other 120 volt electronic loads is very different. Figure 1 shows a typical waveform for a personal computer load. Adding the phase currents at each point in time, we find that the phase currents do not cancel to zero in the neutral. The third harmonic current in the personal computer load current adds in the neutral. For this load current, the neutral current is higher than the phase currents.
CONDITIONS TO BE CONCERNED ABOUT
The following three conditions must be present before becoming concerned about overloading neutrals due to harmonic loads. 1. The system is wye-connected 2. Most loads are connected line-neutral 3. Loads require 3rd harmonic current
Be sure to use true rms meters when measuring current, especially neutral current.
SOLUTIONS
Since there are usually no overcurrent devices to protect the neutral, we must rely on good engineering design to prevent neutrals from becoming overloaded. A common design practice is to use double-sized neutrals or a separate neutral for each phase conductor when supplying single phase electronic loads from a wye system. Using distribution panels with double-rated neutral bus bars is also recommended.
The information and diagrams presented herein are for general educational purposes only, and should not be relied upon as instructions for customer self-wiring. Customers should at all times seek the assistance of qualified electricians or utility personnel for all wiring projects.
CONDITIONS FOR A PROBLEM
- 120/208 or 277/480 volt system
- Loads are connected line-neutral
- Loads require 3rd harmonic current
- Single phase rectifiers
- Personal computers
- Some electronic ballasts
CAUSE
- 3rd harmonic current adds in the neutral
SOLUTIONS
- For full neutrals, keep distortion below 30%
- Use double-size neutral
- Use separate neutral for each phase wire
- Specify low harmonic equipment
SYMPTOMS
- Adjustable Speed Drives Trip Off-line
- Programmable Logic Controller Shuts Process Down
- Lights Blink
- Utility Customer Perceives Poor Reliability
CAUSES
- New Sensitive Electronic Equipment
- Short Circuits on Electrical System
SOLUTIONS
- Specify Less Sensitive Equipment
- Reduce Problems on the Electrical System
- Apply Mitigation at the Equipment
Sensitive electronic equipment can increase production losses and generate complaints about declining reliability. Plant personnel notice the lights blink just as production quits. They think they had a power interruption when, in fact, it was a voltage sag. A few considerations when purchasing equipment can prevent thousands of dollars in production losses and frustration from overly sensitive to voltage sags.
VOLTAGE SAGS
Voltage sags are brief reductions of the system voltage. A sag may last from one hundredth of a second to more than a second. The magnitude of a sag is commonly between eighty (80%) percent and ninety (90%) percent of the normal operating voltage.
Voltage sags are a common part of the electric supply system. A short circuit on one part of the system may cause voltage to sag many miles away. Even faults on a neighboring utility may cause sensitive processes to quit. Short circuits in the plant or in a nearby plant may also cause nuisance shut downs from sags.
Your local Cinergy company can perform a sag analysis to determine the magnitude and frequency of sags to be expected from the utility system.
PROCESS EQUIPMENT SELECTION
Production losses increase dramatically with small increases in the equipment sensitivity. Consider two different brands of equipment that do the same job. Brand A will shut down if the voltage sags to 70% or lower. Brand B is more sensitive shutting down for sags to 90% or lower voltage. The shaded areas in Figure 1 show a hypothetical example of the areas of vulnerability for the two brands of equipment and how the sensitivity translates to production losses. When a fault (short circuit) occurs anywhere within the area of vulnerability, the equipment will shut down due to the resulting voltage sag. In this case, Brand B is much more vulnerable and may suffer 6 times more production losses than Brand A.
Another consideration is how long these devices can ride-thru a voltage sag. About eighty (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 performance will be greatly enhanced.
Careful selection of new equipment can reduce production losses. Selection should not be based purely on initial cost since the cost of production losses may be very significant. The additional cost of a less sensitive device may be recovered through avoided production losses. Specifications for equipment should include voltage sag survivability to reduce down time and financial losses.
Figure 1: Area of vulnerability for sensitive equipment.
Mitigation for existing systems could include a UPS (uninterruptible power supply), CVT (constant voltage transformer), or a motor-generator set for a number of loads to provide sag ride-thru. The optimal location for the UPS or CVT would be at the controls of the sensitive equipment such that a smaller more economical unit can be purchased.
RULE OF THUMB
Voltage Sags cannot be eliminated, but here are a few rules of thumb to take care of most voltage sag problems. Undervoltage trip setting should be 70% or less of the equipment rated voltage. Power Loss ride through should be at least 0.2 seconds. Equipment manufacturers can provide information on Auto Restart and/or Flying Restart options and their applications.
SYMPTOMS
- Shock injury to humans or animals
- No equipment grounding conductor
- No neutral-to-ground bond at main panel
CAUSES
- Earth is only ground fault return path
- High-impedance ground fault path
SOLUTION
- Use equipment grounding conductors to provide low impedance path for fault current
One reason for grounding electrical equipment is to prevent injury and death when equipment insulation fails. Proper grounding and bonding increases safety by allowing failed equipment to be quickly de-energized. Common grounding and bonding mistakes can be a hazard.
GROUNDING CONDUCTOR GROUND FAULT RETURN
Consider the properly grounded system shown in Figure 1. With a ground fault at the equipment as shown, the only impedances limiting the fault current through the breaker are the impedances of the conductors, splices and terminations. If the total impedance is assumed at one ohm, the 20 ampere breaker will conduct 120 amps and trip the circuit quickly to extinguish the fault.
Figure 1: Properly Grounded System
EARTH GROUND FAULT RETURN
Now consider a case where earth is the only return for ground fault current (Figure 2). The total circuit impedance is that of the conductors, splices, terminations and the earth ground impedance. If the total circuit impedance is assumed at 10 ohms, only 12 amps will flow through the breaker. The breaker will not trip and the motor frame will remain energized at or near line voltage (120 volts) indefinitely. Anyone contacting this motor frame could suffer severe injuries or even death. Note the assumed impedance may be much lower than reality. Utilities often assume a driven ground rod has an impedance of 25 ohms. This breaker will conduct less than 5 amps with a circuit impedance of 25 ohms.
Figure 2: Improperly Grounded System
Since the earth grounds may have impedances 10 to 100 times higher than equipment grounding conductors, significant shock hazards can occur when relying on the earth to return for ground fault current. Using the earth rather than an equipment grounding conductor has little effect on the normal operation of most equipment. Therefore, hazards due to using the earth as the sole fault return path often go unnoticed for years.
SUMMARY
Maintaining a low impedance path for ground fault current is vital to the proper operation of breakers and fuses. Using equipment grounding conductors will help establish this low impedance path. Using earth as the only ground return path will severely limit the amount of current the breaker will conduct and inhibit its ability to clear ground faults.
SYMPTOMS
- Data lines run between buildings
- Computer system lock-ups
- Data card damage
POTENTIAL PROBLEMS
- Ground potential difference between building grounding systems
- Data signal upset or component damaged
SOLUTIONS
- Use fiber optic data cable
- Optical isolation
- Equalize voltage between grounding systems
BACKGROUND
Frequent unexplained lock-ups of computer systems, and component damage during lightning activity are two problems that may be caused by ground potential differences. Differences in voltage between two separate building grounds often exist in locations that share data or communication equipment. Those differences can occur in large buildings or in completely separate buildings. These locations are often served by separate electrical services.
Monitoring at one location showed computer lock-ups directly correlated with transient differences in voltage between the office and plant grounding systems. The building ground voltage difference upset the data signal ground reference and interfered with normal data transmission. This voltage difference rose to as much as 10 volts (zero-peak) several times each day. Most computer systems operate on DC voltage levels of 5V or less
Lightning currents in the thousands of amps can cause ground voltage differences many times larger than those caused by normal load switching disturbances. Therefore, it is reasonable to expect significant lightning damage to computer systems, or communication systems if the ground potential difference situation is not corrected.
SOLUTIONS
One way to prevent ground voltage difference problems is to attempt to equalize ground voltages by bonding the building grounding systems together with a suitable copper conductor.
The best way to prevent ground voltage differences from causing computer system problems is by using fiber optics. This will completely isolate the two buildings and will do a good job of preventing damage due to lightning. Fiber optic links would also eliminate any “electrical noise” that may be present on the data lines. The choices may range from converting the entire data link to fiber, or to simply installing optical isolators at both ends of each data cable. If optical isolators are installed, surge protection should also be used for each end use device.
Figure 1: Block Diagram of System
The information and diagrams presented herein are for general educational purposes only, and should not be relied upon as instructions for customer self-wiring. Customers should at all times seek the assistance of qualified electricians or utility personnel for all wiring projects.
SYMPTOMS
- TV, VCR or other remote controls quit or do not work correctly, i.e. channel switching
- Loss of control from infrared detectors such as automatic doors or position sensors
- Compact fluorescent bulb or high efficiency fluorescent lighting located in the room
CAUSE
- Fluorescent lamp emits high frequency infrared light on infrared remote receiver
SOLUTIONS
- Relocate lamps to reduce their light from hitting the receiver
- Replace lamps and/or ballasts with different brand or model
BACKGROUND
Compact fluorescent light, CFL bulbs and some fluorescent ballasts increase lighting efficiency by driving the lamp at frequencies higher than the 60Hz utility power. They produce light that looks constant to the human eye. Careful examination of the light with instruments shows some ripple at twice their switching frequency.
Some of the light from all lamps is infrared and not visible to humans. The infrared light from these lamps also has ripple characteristics at twice the switching frequency.
Most remote controls for TVs, VCRs, and CATV controllers use infrared light to communicate. The remote control receiver must be able to detect very low light levels from the hand-held remote control. This sets the stage for problems.
The infrared light ripple from the lamps sometimes is close to the carrier frequency of the remote control. Problems occur when the high frequency infrared light overpowers or confuses the receiver as in Fig 1.
Figure 1: Fluorescent Light Interference with Remote Controls
This causes loss of control, volume drift, channel changing, and related problems for TVs VCRs. Similar problems occur for other consumer remote control products.
Commercial and industrial buildings can experience similar problems. TV stores are especially susceptible. Sometimes motion detectors for automatic doors are affected. Infrared position sensors may also be affected.
SOLUTIONS
Unfortunately, it is difficult to predict problems without detailed technical information. There is little coordination between lighting and remote control manufacturers to prevent problems. Some lights are more likely to interfere than others. Some remote controls are more sensitive than others.
For residential problems with TVs, the offending bulb may have to be moved so it's light does not land on the TV. A different lamp shade might help. Another solution is to try a different lamp brand or model. These may prevent unnecessary service calls from TV repair shops.
Commercial and industrial applications should consider possible impacts in advance. Most lighting catalogues will show the switching frequency of the high frequency ballasts. The infrared light frequency will be twice the switching frequency. Check with controller specifications to avoid problems.
Understanding Voltage Unbalance
SYMPTOMS
- Motors overheating
- Motor fuses blowing
- Motor unbalance protection operates
- Unbalanced motor currents
CAUSES
- Motor problems
- Unbalanced voltages
SOLUTIONS
- Use higher rated motors
- Adjust protection circuits
- Reduce voltage and load unbalance
BACKGROUND
New technology, high efficiency motors, and competitive pressures to reduce motor costs cause more problems with voltage unbalance. Some adjustable frequency drives are often far more sensitive to voltage unbalance than a standard induction motor. Designs for high efficiency motors tend to be more sensitive to voltage unbalance. Motors manufactured with less tolerance for unbalance can sometimes have a slightly lower selling price.
The increased sensitivity to unbalance increases the number of frustrating events between electric utilities and customers. Voltage unbalance that meets national and international standards may be considered unacceptable to the motor or to the adjustable speed drive.
VOLTAGE UNBALANCE
Voltage unbalance normally results from uneven loading or uneven impedance in the electric supply system. This causes a different voltage drop on each of the three phases. This difference between phase to phase voltages is called voltage unbalance.
The correct way to calculate percent unbalance is to use phase to phase voltages in the equation below.
For example, phase-to-phase voltages of 470, 479, and 482 average to 477. The maximum deviation from average is the phase with 470 volts. The deviation between 470 and 477 is 7 volts. The percent unbalance is
MOTOR AND DRIVE RESPONSE TO UNBALANCE
Induction motors and adjustable frequency drives respond to the voltage unbalance with much greater current unbalance. For example, a 1% voltage unbalance can easily translate to 5% to 9% current unbalance on induction motors. High efficiency motors are usually closer to the 9% current unbalance for 1% voltage unbalance. Current unbalance in an adjustable frequency drive can exceed 15 times the voltage unbalance.
Higher levels of current unbalance may cause nuisance fuse blowing, motor overheating, and a variety of other problems. People who use motor saving protection circuits are often encouraged to set the current unbalance level at no more than 5%. As little as 1% voltage unbalance can cause motor protection circuits to trip.
RECOMMENDATION
Where practical, improve voltage balance by balancing other single phase loads. Remember that voltage and current unbalance normally can be tolerated so long as motor currents stay below safe levels. Increase the unbalance trip setting, but keep phase current overload settings at a safe level. Consider a slightly oversized motor or reduce load a few percent to tolerate the unbalance. For adjustable speed drives, line reactors will help current unbalance and provide many other benefits.
Some motor and equipment manufacturers want less than 1% voltage unbalance. International standards suggest 2% voltage unbalance is acceptable. The National Equipment Manufacturers Association and many other organizations recently approved the following language in the appendix to ANSI Std. C84.1. "Electric supply systems should be designed and operated to limit the maximum voltage unbalance to 3 percent when measured at the electric-utility revenue meter under no-load conditions."
SYMPTOMS
- Motors overheating
- Motor fuses blowing
- Motor unbalance protection operates
- Unbalanced motor currents
CAUSES
- Motor problems
- Unbalanced voltages
SOLUTIONS
- Use higher rated motors
- Adjust protection circuits
- Reduce voltage and load unbalance
BACKGROUND
New technology, high efficiency motors, and competitive pressures to reduce motor costs cause more problems with voltage unbalance. Some adjustable frequency drives are often far more sensitive to voltage unbalance than a standard induction motor. Designs for high efficiency motors tend to be more sensitive to voltage unbalance. Motors manufactured with less tolerance for unbalance can sometimes have a slightly lower selling price.
The increased sensitivity to unbalance increases the number of frustrating events between electric utilities and customers. Voltage unbalance that meets national and international standards may be considered unacceptable to the motor or to the adjustable speed drive.
VOLTAGE UNBALANCE
Voltage unbalance normally results from uneven loading or uneven impedance in the electric supply system. This causes a different voltage drop on each of the three phases. This difference between phase to phase voltages is called voltage unbalance.
The correct way to calculate percent unbalance is to use phase to phase voltages in the equation below.
For example, phase-to-phase voltages of 470, 479, and 482 average to 477. The maximum deviation from average is the phase with 470 volts. The deviation between 470 and 477 is 7 volts. The percent unbalance is
MOTOR AND DRIVE RESPONSE TO UNBALANCE
Induction motors and adjustable frequency drives respond to the voltage unbalance with much greater current unbalance. For example, a 1% voltage unbalance can easily translate to 5% to 9% current unbalance on induction motors. High efficiency motors are usually closer to the 9% current unbalance for 1% voltage unbalance. Current unbalance in an adjustable frequency drive can exceed 15 times the voltage unbalance.
Higher levels of current unbalance may cause nuisance fuse blowing, motor overheating, and a variety of other problems. People who use motor saving protection circuits are often encouraged to set the current unbalance level at no more than 5%. As little as 1% voltage unbalance can cause motor protection circuits to trip.
RECOMMENDATION
Where practical, improve voltage balance by balancing other single phase loads. Remember that voltage and current unbalance normally can be tolerated so long as motor currents stay below safe levels. Increase the unbalance trip setting, but keep phase current overload settings at a safe level. Consider a slightly oversized motor or reduce load a few percent to tolerate the unbalance. For adjustable speed drives, line reactors will help current unbalance and provide many other benefits.
Some motor and equipment manufacturers want less than 1% voltage unbalance. International standards suggest 2% voltage unbalance is acceptable. The National Equipment Manufacturers Association and many other organizations recently approved the following language in the appendix to ANSI Std. C84.1. "Electric supply systems should be designed and operated to limit the maximum voltage unbalance to 3 percent when measured at the electric-utility revenue meter under no-load conditions."
Power Factor Correction and Capacitor Issues
This Tech Tip briefly describes issues related to the installation and operation of power factor correction capacitors. If you have questions regarding the issues presented here or for further assistance, please contact Power Quality.
Background
Power factor is the ratio of the real power (kW) to the apparent power (kVA) that a system draws. Power factor is an indication of the inefficiency of a load or loads. Many utilities charge an additional amount for low power factor to compensate for the additional system losses and generating capacity. By adding a source of reactive power (kVAR), such as capacitors, near the load(s), power factor can be improved and additional charges reduced or eliminated. Other benefits include voltage improvement and more available system capacity. When it is decided for financial or other reasons that power factor correction capacitors may be beneficial, the following issues should be addressed.
Capacitor Ratings
Overvoltage is the predominant cause of capacitor failure. Capacitors should not be operated at more than 110% of their rated terminal voltage. Lack of ventilation or placement with exposure to heat or sun will shorten capacitor life. The NEC requires certain discharge characteristics.
Harmonics
Harmonics are a measure of the distortion in a current or voltage waveshape. Non-linear loads such as adjustable speed drives and electronics require current that is heavy in harmonics. This current causes distortion in the voltage waveshape. Capacitors form parallel resonant circuits with system impedance. If the resonant frequency is at the same frequency as the harmonic frequency, increased voltage distortion, damaged capacitors and blown capacitor fuses can result. Solutions to harmonics problems include installing the capacitors as a filter, sizing the capacitors to avoid harmonic frequencies or relocating the capacitors.
Voltage rise
When capacitors are switched on, they will tend to raise the voltage level at the bus they are on. When there is little load on, such as overnight or on a weekend, this can lead to voltages outside the acceptable tolerance of connected equipment. Calculate the voltage rise due to capacitors on a bus on the low side of a transformer as follows:
If this calculated voltage rise added to the highest voltage is unacceptable, consideration should be given to either switching the capacitors off manually when the load goes down or installing the capacitors with automatic switching control.
Location
Capacitors can be located in one place such as at the main bus or distributed on distribution buses or at loads such as large motors. Placing capacitors at large motors can provide two benefits: they will come on when they are required and losses in the feed to the motor will be reduced. Care must be taken when applying capacitors at motors, as harmonic issues become much more difficult to address with distributed capacitors. There are several types of motor applications where capacitors should definitely not be installed.
Capacitor switching transients
A capacitor creates a momentary short circuit at the instant it is energized. Current surges into the capacitor and line voltage collapses for a few millionths of a second. As the current surge declines, it causes a ringing transient with overshoot on the peak voltage. This can create several problems with connected equipment. The overshoot can cause adjustable speed drives to trip out. Capacitors on the low side of a transformer can amplify the transient caused by a capacitor switching on the high side causing more frequent trips or equipment damage. The switching transient can also cause a surge in neutral current level. This may cause ground fault circuit interrupters to trip. Care must be taken in location and sizing of capacitors to head off these potential problems.
References
The issues presented here are easier and less expensive to deal with if addressed prior to installation of power factor correction capacitors. The following references contain more detailed information about the topics discussed here.
- IEEE Red Book Chapter 8
- IEEE Standard 519
- Power Clinic Tech Tips 2, 5, 7, 10 & 11
Power Factor Correction Capacitor Issues
This Tech Tip briefly describes issues related to the installation and operation of power factor correction capacitors. If you have questions regarding the issues presented here or need further assistance, please contact your Power Quality representative.
Background
Power factor is the ratio of the real power (kW) to the apparent power (kVA) of an electrical load. Power factor can be thought of as the electrical efficiency of the load. Many utilities have an additional charge for a low power factor to compensate for the additional system losses and generating capacity required to serve the load. By adding a source of reactive power (capacitors) near the load, power factor can be improved and any additional charges reduced or eliminated. Other benefits include improved voltage and increased system capacity. When it is decided, for financial or other reasons, that power factor correction capacitors may be beneficial, the following issues should be addressed.
Capacitor Ratings
Overvoltage is the predominant cause of capacitor failure. Capacitors should not be operated at more than 110% of their rated terminal voltage. Lack of ventilation or placement with exposure to heat or sun will also shorten capacitor life.
Harmonics
Harmonics are a way to measure of the distortion in a current or voltage waveshape. Non-linear loads such as adjustable speed drives and electronics draw current that is distorted. This distorted current waveshape causes a corresponding distortion in the voltage waveshape.
Capacitors form parallel resonant circuits with system impedance. If the resonant frequency is the same as a harmonic frequency, increased voltage distortion, damaged capacitors and blown capacitor fuses can result. Solutions to harmonics problems include modifying the capacitors to create a filter, sizing the capacitors to avoid resonance, or relocating the capacitors.Voltage Rise
When capacitors are energized, they raise the voltage level at their location. When there is little load, such as nights or weekends, voltages can rise outside the acceptable tolerance of connected equipment. Calculate the approximate voltage rise due to capacitors on the low side of a transformer as follows:
If this calculated voltage rise, when added to the highest steady-state voltage, is unacceptable, consideration should be given to either switching the capacitors off manually when the load is reduced or installing the capacitors with an automatic switching control.Location
Capacitors can be located in one place, such as the service entrance, at several locations such as sub-panels, or at specific loads such as large motors. Placing capacitors at large motors can provide two benefits. They can be energized and de-energized with the motor and losses in the feed to the motor will be reduced. Care must be taken when applying capacitors at motors, as harmonic issues become more difficult to address with distributed capacitors.
Capacitor Switching Transients
A capacitor creates a momentary short circuit at the instant it is energized. When energized, current surges into the capacitor and the voltage collapses for a few millionths of a second. As the current surge decreases, it causes a ringing transient with overshoot on the peak voltage. See Figure 1.
Sample Waveshape for Capacitor Switching
Figure 1
This transient can create several problems with connected equipment. The voltage impulse can cause adjustable speed drives to trip. Also, the initial voltage collapse can cause some equipment to trip. Additional capacitors on the low side of a transformer can amplify the transient caused by a capacitor switching on the high side causing more frequent trips or equipment damage. The switching transient can also cause a surge in neutral current level. This may cause ground fault circuit interrupters to trip. Care must be taken in location and sizing of capacitors to minimize these potential problems.References
The issues presented here are easier and less expensive to deal with if addressed prior to installation of power factor correction capacitors. The following references contain more detailed information about the topics discussed here.
- IEEE Red Book Chapter 8
- IEEE Standard 519
- Power Clinic Tech Tips 2, 5, 7, 10 & 11