Capacitor Application Issues - PDF Free Download

Capacitor Application Issues Thomas Blooming, P.E. Daniel J. Carnovale, P.E. IEEE Industry Applications Society 53 rd Pulp & Paper Industry Technical Conf. Williamsburg, Virginia June 24-29, 2007

Introduction Capacitors provide well-known benefits Power factor correction Voltage support Release of system capacity Reduced system losses Many application issues Some basic, some complex Summarized in this paper Make reader aware of the issues and pitfalls

Outline Capacitor Sizing Power Factor Penalties Capacitor Ratings Code Requirements and Protection Capacitor Selection Capacitor Placement Harmonics (To Filter Or Not To Filter) Capacitor Switching Transients Capacitors and Fault Current

Capacitor Sizing How much kvar do I need? Not necessarily an easy question Power factor not calculated consistently Utility power factor penalties vary Not possible to cover all variations Need to know general concepts Be aware that pf calculation affects the kvar needed calculation Read your utility s published rate

Capacitor Sizing Power factor calculations During 15 or 30 minute demand interval Coincident with peak kw or kva load More difficult to meet utility pf target Accumulated kvar-hours and kw-hours Month-long average pf Easier to meet utility pf target Do not need to meet pf target at all times Just be above pf target enough of the time Credit for leading kvar-hours? Possible with some utilities, prohibited by others

Capacitor Sizing kvar needed calculation Gather past utility bills, if possible Do multiple monthly calculations Easy to do many calculations quickly with a spreadsheet Do not average kw/kva and pf to do the calculations Can lead to erroneous results Examples shown in the paper

Capacitor Sizing TABLE 1 EXAMPLE POWER FACTOR CALCULATIONS, USING MONTHLY DATA,.95 PF TARGET KW PF KVAR NEEDED 1200.78 583.8 1000.83 343.3 1100.80 463.4 950.73 577.2 700.65 588.3 850.70 587.8 Probably choose 600 kvar

Capacitor Sizing TABLE 2 EXAMPLE POWER FACTOR CALCULATIONS, USING SUMMARIZED DATA,.95 PF TARGET MAXIMUM KW MINIMUM PF KVAR NEEDED 1200.65 1008.5 MAXIMUM KW AVERAGE PF KVAR NEEDED 1200.75 671.9 AVERAGE KW MINIMUM PF KVAR NEEDED 966.7.65 812.4 AVERAGE KW AVERAGE PF KVAR NEEDED 966.7.75 541.3 Possibly choose 1000 kvar, with most conservative approach!

Power Factor Penalties Not possible to cover all penalties Table 3 shows some typical methods User should determine which type of penalty (if any) the utility is using Then investigate further Sometimes utility uses multiple penalty methods

Power Factor Penalties Most penalties are straightforward Easy to calculate and analyze Power factor penalty may be hidden kva billing No explicit pf penalty, but you will pay more if you have a low power factor (higher kva) Utility rebate for maintaining a certain power factor or higher Not a penalty, but functionally equivalent

Power Factor Penalties Usually not practical to correct all the way to unity (1.0) power factor Diminishing benefit per incremental cost May cost more than will be saved in pf penalty reduction, even with kva billing (1.0 pf target) More long run savings, but lower return on the project Leading power factor concerns, at light load Some utilities bill for leading power factor, too May require switched capacitor bank

POWER FACTOR PENALTIES RATE TYPE DESCRIPTION OF PF PENALTY EXAMPLE kva (demand) rates PF (kva) adjustment PF ratio (kw demand) adjustment PF magnitude (kw demand) adjustment PF multiplier (PFM) kvar demand charge Penalty for < 1.0 pf; generally applied as a $/kva When the pf is less than X%, the demand may be taken as X% of the measured kva If the pf is < X%, the demand will be adjusted by the following: X%/actual pf * actual demand = adjusted demand. PF adjustment increases or decreases the net (kw) demand charge X% for each Y% the pf is above or below the utility specified pf Demand is increased (or decreased) by a calculated multiplier determined by a utility table or by a formula $X per kva of reactive demand in excess of Y% of the kw demand Demand = 800 kw; pf=80%; kva=1000; demand charge = $10/kVA pf penalty = (1000 800)*$10 = $2000/month When the pf is less than 90%, the demand may be taken as 90% of the measured kva pf=80%; kva=1000; demand charge = $10/kVA Billed demand = 0.90*1000 = 900 kw pf penalty = (900 1000*0.80)*$10 = $1000/month If the pf is < 85%, the demand will be adjusted by the following: 85%/actual pf * actual demand = adjusted demand. Demand = 800 kw; pf=80%; demand charge = $10/kW Adjusted demand = (0.85/0.80)*800=850kW pf penalty = (850-800)*$10 = $500/month Where the pf is < 85%, the net demand charges shall be increased 1% for each whole 1% the pf is < 90%; likewise, where the pf is higher than 95%, the demand charges will be reduced by 1% for each whole 1% the pf is above 90%. Demand = 800 kw; pf=80%; demand charge = $10/kW Up to 90%, demand adjustment = 800*10%=80kW (from 80% to 90%) = net demand of 880 kw If pf is corrected to 1.0, pf adjustment (reduction) = 800*10%=80kW (from 90%-100%) = net demand of 720kW Correcting pf from 80% to 100%, potential net savings is (880-720)*$10/kW = $1600/month Demand = 800 kw; pf=80%; PFM = 1.086; demand charge = $10/kVA pf penalty = 800*$10*(0.086) = $688/month $0.45 per kva of reactive demand in excess of 50% of the kw demand Demand = 800 kw; pf=80%; kvar demand = 600; excess kvar demand = 600 800*0.50 = 200 kvar pf penalty = 200 kvar*($0.45/kvar) = $90/month kvarh charge $X per kvarh $0.000835 per kvarh kvarh = 500,000 pf penalty = 500,000*0.00835 = $417/month kwh adjustment (note that this often applies where the kw demand is first adjusted) $P/kWh for first Q*kWh*demand $R/kWh for next S*kWh* demand $X/kWh for next Y*kWh demand $Z/kWh for all additional $0.040/kWh for first 100 kwh*demand $0.035/kWh for next 150kWh*demand $0.025/kWh for next 150kWh*demand $0.020/kWh for all additional kwh Actual demand = 800 kw; Adjusted demand = 1000 kw; kwh measured = 500,000 With penalty 100*1000=100,000 kwh @ 0.04/kWh=$4000 150*1000=150,000 kwh @ 0.035/kWh=$5250 150*1000=150,000 kwh @ 0.025/kWh=$3750 (500,000-100,000-150,000-150,000)*$0.02/kWh = $2000 Total = $15,000 Without penalty 100*800=80,000 kwh @ 0.04/kWh=$3200 150*800=120,000 kwh @ 0.035/kWh=$4200 150*800=120,000 kwh @ 0.025/kWh=$3000 (500,000-80,000-120,000-120,000)*$0.02/kWh = $3600 Total = $14,000 Penalty = $15,000 - $14,000 = $1,000/month (in addition to demand penalty)

Capacitor Ratings Capacitors are built with tolerances above nameplate ratings Applicable standards IEEE Std 18-2002, IEEE Standard for Shunt Power Capacitors Older versions in 1992 and 1980 IEEE Std 1036-1992, IEEE Guide for Application of Shunt Power Capacitors

Capacitor Ratings IEEE Std 18-2002 continuous overload limits, intended for contingencies and not for a nominal design basis 110% of rated rms voltage 120% of rated peak voltage (including harmonics) 135% of rated rms current (nominal current based on rated kvar and voltage) Was 180% in IEEE Std 18-1992 135% of rated reactive power

Capacitor Ratings Short time overvoltage limits IEEE Std 18-1992 IEEE Std 18-1980 IEEE Std 1036-1992 Capacitor may be expected to see 300 such overvoltages in its service life without superimposed transients or harmonic content.

Capacitor Ratings Short time overvoltage limits IEEE Std 18-1980 3.00 per unit rms voltage for 0.0083 seconds 2.70 per unit rms voltage for 0.0167 seconds IEEE Std 18-1992 and -1980 2.20 per unit rms voltage for 0.1 seconds 2.00 per unit rms voltage for 0.25 seconds 1.70 per unit rms voltage for 1 second 1.40 per unit rms voltage for 15 seconds 1.30 per unit rms voltage for 1 minute 1.25 per unit rms voltage for 30 minutes

Capacitor Ratings Study engineers Compare measured or calculated voltages and currents against capacitor tolerances Different durations for different cases Harmonics: steady state Compare peak voltage stress on capacitors with continuous limits Transformer energization inrush: < 1 second Compare peak voltage stress on capacitors with certain short time limits Significant inrush is < 1 second, therefore compare against 1.70 pu

Capacitor Ratings Heavy duty capacitors Some manufacturers claim tolerances above those required by standards Really just higher voltage capacitors with a de-rated nameplate Not a substitute for good engineering, especially in harmonic filters

Capacitor Ratings Capacitor motor starting Apply capacitors at voltage above rating Produce more kvar (square of voltage ratio) Short periods of time, limited times Large motors, typically medium voltage Large voltage drop when starting Switch on capacitors for a short time during starting Minimize kvar draw and voltage drop May be economical, even if shorter life

Code Reqs. and Protection National Electric Code, Article 460 Guidance for installation and protection More information for low voltage capacitors Conductor and Disconnect Sizing NEC Article 460.8 (A), for LV capacitors: Ampacity of conductors shall not be less than 135% of rated capacitor current NEC Article 460.8 (C) (3), for LV caps: Rating of the disconnecting means shall not be less than 135% of rated capacitor current

Code Reqs. and Protection Conductor and Disconnect Sizing (cont.) Why 135%? Not practical to set capacitor overcurrent protection for overloads Caps draw more current during overvoltages Caps are attractive path for harmonic currents Caps have little/no diversity on all the time Capacitor switching transients could lead to nuisance trips Without tight overload protection, it is necessary oversize the conductors

Code Reqs. and Protection Overcurrent protection Protect system: Isolate the fault from the system Protect capacitor: Prevent more energy from reaching a failed capacitor, avoid can rupture NEC Article 240.3 (Overcurrent protection) For capacitors, Article 460 applies

Code Reqs. and Protection Overcurrent protection (continued) NEC Article 460.8 (B), for LV capacitors: The rating or setting of the overcurrent device shall be as low as practicable. Possible to choose an overcurrent device that does not protect the cable against overloads See recommended wire and fuse/breaker charts for given capacitor sizes published by capacitor manufacturers Set overcurrent devices as low as possible Avoid nuisance trips Realize we do not always protect cable

Code Reqs. and Protection Overcurrent protection (continued) Bussman SPD Electrical Protection Handbook recommendations for cap fuses Generally, size dual-element, current-limiting fuses at 150% to 175% of the capacitor rated current and size non-time-delay, fast-acting, current-limiting fuses at 250% to 300% of the capacitor rated current. Large difference? Oversized fuses? Look at time-current characteristics (TCCs) Fuse size based on high current, short time behavior, not by nominal (long time, overload) ratings

Code Reqs. and Protection Overcurrent protection (continued) 200 ka interrupting capacity currentlimiting fuses in low voltage cap banks Often specified by consultants Therefore, used in many standard designs Very fast-acting fuses Time current characteristics require a larger nominal fuse size to avoid nuisance tripping Fuses appear to be oversized but are not

Code Reqs. and Protection Unbalance protection Large, MV caps can have many capacitors in series and parallel groups Individually fused capacitors Capacitor failure (removed by fuse) results in unbalance in the bank, stressing some of the remaining capacitors Unbalance protection not required by code Used to detect (alarm) and protect (trip) against overvoltage conditions resulting from unbalance

Code Reqs. and Protection Capacitor discharge Capacitors are energy storage devices Not safe to retain this energy indefinitely NEC specifies automatic means to discharge NEC Article 460.6 (A), for LV capacitors: Reduce charge to 50 V or less within 1 minute NEC Article 460.28 (A), for caps > 600 V: Reduce charge to 50 V or less within 5 minutes Switching before discharge can result in greater than normal switching transients

Capacitor Selection Capacitor selection issues (besides size) Utility penalties Installed cost and payback of equipment Load variability kw losses Self excitation of motors Harmonic resonance Voltage regulation Load requirements (flicker requirements)

Capacitor Selection Equipment cost > include installation Payback is main criterion for approval Rough estimate of costs given in the paper Higher $/kvar for multiple small capacitors Lower $/kvar for large capacitors Fixed caps: Caps are only 1/3 of total cost Protective device (breaker or fuse) Installation labor and material Switched caps: Cost of capacitor is typically a larger proportion of the cost

Capacitor Selection TABLE 4 INSTALLED COST COMPARISON OF POWER FACTOR CORRECTION EQUIPMENT TYPE OF CORRECTION INSTALLED COST, $/KVAR Fixed (LV motor applied) $15 Fixed (LV) $25 Fixed (MV) $30 Switched (LV) $50 Switched (MV) $50 Static Switched (LV) $75 Switched Harmonic Filter (LV) $75 Switched Harmonic Filter (MV) $60 Active Harmonic Filter (LV) $150

Capacitor Selection Nominal size and configuration Caps available in a variety of sizes Smaller kvar increments in smaller kvar sizes 50 or 100 kvar increments in larger sizes Low voltage caps Typically internally connected as a three-phase delta configuration Medium voltage caps Single-phase or three-phase capacitors connected in ungrounded wye or delta configurations

Capacitor Selection Fixed versus switched capacitors Power factor correction needs Billed over whole month > fixed Billed in demand interval > switched Load variation More variation > switched Personal preference Operate without user attention > switched Paper has further description about switched capacitors

Capacitor Selection Switched capacitors Automatically switches on needed kvar If sized large enough No human input required Harmonics More likely to tune system to critical frequency Switching devices Mechanical contactors Most common, inexpensive (LV) Static switches (LV only, at present time) For loads needing instant response, such as welders

Capacitor Selection Switched capacitors (continued) Controller Senses system power factor Voltage measured within cap bank System current measured with external CT(s) Adds/removes capacitor steps Meet target power factor (programmable) Time delays (programmable) Not necessary for static-switched cap bank Avoid hunting for rapidly varying loads Allow trapped charge to dissipate» 1 minute for LV caps» 5 minutes for MV caps

Capacitor Selection Switched capacitors (continued) MV versus LV design MV banks typically have fewer & larger steps than LV MV contactors are more expensive and require more space

Capacitor Selection Switched capacitors (continued) Stages versus steps Reduce contactors with different size stages Often done in MV systems, sometimes in LV systems Stages: Physical capacitor groups Steps: Connected kvar values possible Example: 1500 kvar MV bank One 300 kvar stage and two 600 kvar stages Five steps in 300 kvar increments, three contactors Disadvantage: Stages not used equally With equal size stages controller can equalize duty Stages, esp. first stage, may be switched more often

Capacitor Selection Static switched capacitors For loads requiring very fast kvar compensation High impact loads, such as spot welders Benefits beyond power factor correction Minimize sudden voltage drops and flicker Often configured as harmonic filters standard Power electronic switches (SCR/thyristors) Can switch very quickly (within one cycle, 16.7 msec) Precisely control when capacitors are switched Match system voltage with capacitor voltage Capacitor switching transients largely eliminated Disadvantage: Cost

Capacitor Selection Overvoltage considerations Can be caused by leading power factor Possible with fixed capacitors and light load Generators have trouble with leading pf Voltage rise Depends on system stiffness (MVA SC ) and capacitive kvar Other factors (don t just blame capacitors!) Transformer tap selection Utility voltage variation» High voltage at night or on weekends

Capacitor Placement Physical location At motor terminals Great, if power factor is only concern Caps switched as needed, with each motor Reduce overall system losses Harmonics in system may complicate this application Consult tables to choose capacitor size Self-excitation concerns Don t oversize capacitor

Capacitor Placement Physical location At motor terminals Self-excitation Sufficient kvar at motor terminals to fully supply motor reactive power needs Load with some inertia Can cause damaging overvoltages Solution: Do not apply too much capacitance» Tables based on not fully correcting motor pf to unity, to avoid self-excitation

Capacitor Placement Physical location If needed for power factor compensation Apply anywhere downstream of the meter If needed for kva or loss reduction Apply at or near the loads Loss reduction 1-2% of overall kw is possible with distributed capacitors Some may claim more Payback is generally 10 years or more Typically not enough to justify cost to add capacitors

Capacitor Placement Physical location Avoid placement downstream of a motor starter in such a way that the capacitor would see sudden voltage changes Can be downstream of main contacts (steady voltage) Avoid placement downstream of softstarts Unless connected via contactor and switched on after softstart is in bypass

Capacitor Placement Physical location Harmonic concerns Capacitor on a motor and upstream cable can create a tuned circuit Harmonic filter: just L & C in series This filter can attract harmonics from the system and overload the capacitor Possible result: One or a few capacitors on certain motors keep failing while others are completely unaffected Safer design choice: Apply capacitors at main distribution or MCCs if harmonics are a concern

Capacitor Placement Electrical location: LV vs. MV Many LV unit substations? One MV bank may be more cost effective Physical space available? Where? Single MV bank may be more compact than multiple LV banks If room in substation or electrical room

Capacitor Placement Electrical location: LV vs. MV Customer preferences? Some companies avoid MV, electricians not trained Harmonics? May be possible to avoid a harmonic resonance situation at LV by applying an MV bank Need to pay greater attention to system/background harmonics with an MV bank

Capacitor Placement CT location Important when applying automatically switched capacitors, LV or MV Place CT where where it will measure the full load needing power factor correction Including the capacitor itself, so the capacitor bank can see its impact on the system CT orientation (polarity) is also critical to proper operation of an automatic bank

Capacitor Placement CT location Assumption: Load is balanced Concerned about 3-phase pf on bill LV: 3-phase capacitors and contactors MV: Could be three-phase or three single-phase units Usually not possible to separately compensate power factor on different phases, therefore: Measure one phase current and one phase voltage Switch all steps as a three-phase group High-end products may require 2 or 3 CTs Static switched capacitor bank or active filter

Capacitor Placement CT location Field errors: CT on the breaker serving the capacitor bank This does nothing to measure the system pf CT on the middle or end of busbar in LV switchgear Does not measure full load CT on the wrong phase or wrong polarity Controller won t calculate pf or switch steps correctly Can be compensated for by making wiring changes» No need to shut down system to move the CT» Change voltage input to controller to match CT phase and polarity

Capacitor Placement CT location Commissioning sanity checks Verify that the controller reads a realistic power factor Not just.8, for example, but.8 lagging, not leading some people miss the second part Can often be checked against system metering Confirm that the power factor improves when a step is energized Add steps manually during commissioning, if necessary Pf should get closer to unity from a lagging pf

Capacitor Placement CT location Double-ended substations Capacitor bank(s) may be placed on either/both of the secondary buses CTs must be in a shared (additive) arrangement to ensure that both transformer loads are compensated for Capacitors may be placed on both halves of the double-ended substation if each capacitor bank has summed CTs on its main and on the tie How many CTs if automatic capacitor banks on both sides of a double-ended sub?

Capacitor Placement CT location Multiple LV services compensated by one capacitor bank, on one of the LV busses Summing CT to aggregate the CT currents from each low voltage bus Need same type of transformer connection (for proper phasing) on each LV CTs at each bus need to be on the same phase LV bus with capacitor needs to be able to handle leading pf, and possible overvoltage When the capacitor bank is doing significant compensation for the other busses

Harmonics (To Filter or Not) Why do we care about harmonics? IEEE Std 519-1992, Section 6.5: A major concern arising from the use of capacitors in a power system is the possibility of system resonance. This effect imposes voltages and currents that are considerably higher than would be the case without resonance. The reactance of a capacitor bank decreases with frequency, and the bank, therefore, acts as a sink for higher harmonic currents. This effect increases the heating and dielectric stresses. The result of the increased heating and voltage stress brought about by harmonics is a shortened capacitor life.

Harmonics (To Filter or Not) Series resonance Inductance (transformer, cable, reactor) in series with capacitance (pf correction capacitor) At some frequency the series combination will be equal and will sum to nearly zero (ignoring R) Very attractive path for harmonic current at that frequency Harmonic filters are purposely series resonant at a fixed frequency to attract harmonic currents Uncontrolled series resonance can result in nuisance fuse operations or breaker trips, as well as capacitor failures

Harmonics (To Filter or Not) Utility Transformer VFD PF Cap 100 HP FIGURE 2. SYSTEM SHOWING SERIES RESONANCE

Harmonics (To Filter or Not) Series Resonance Equivalent Circuit Transformer and Cable PF Cap Harmonic Current Source FIGURE 3. SERIES RESONANCE, EQUIVALENT CIRCUIT, RESULTS IN HIGH HARMONIC CURRENT

Harmonics (To Filter or Not) Parallel resonance Capacitors tune the system to a certain harmonic Known as parallel resonance between the capacitors and the source (including transformer) inductance This frequency is the crossover point at which the inductive and capacitive reactances are equal This frequency can be easily calculated (estimated) Parallel resonance presents a high impedance to harmonics at or near the resonant frequency Amplifying harmonics at these frequencies Causes problems if a source of harmonics exists at or near that frequency Problem more likely if multi-step capacitor bank Several possible resonant frequencies

Harmonics (To Filter or Not) Utility Transformer PF Cap VFD 100 HP FIGURE 4. SYSTEM SHOWING PARALLEL RESONANCE

Harmonics (To Filter or Not) Parallel Resonance Equivalent Circuit PF Cap Source Impedance Harmonic Current Source FIGURE 5. PARALLEL RESONANCE, EQUIVALENT CIRCUIT, RESULTS IN HIGH HARMONIC CURRENT

Harmonics (To Filter or Not) Harmonic filters Configure a capacitor bank as a filter Put inductance in series with capacitance Supplies reactive power Counter intuitively, more kvar WITH the reactor Filters harmonics: passive harmonic filter Avoids parallel resonance problem Parallel resonance below tuning of the filter Typical LV filter tuning is 4.7 th harmonic System parallel resonance around 3.8 th, give or take» Well below any harmonics of concern, except in very rare instances

Harmonics (To Filter or Not) When is a harmonic filter needed? Rule of thumb If kvar > 25% of transformer kva and Harmonic-producing load (e.g. drives) is greater than 40% of the transformer kva Below 15% and 25%, respectively, should be okay Better to do resonance calculations Possible problem if both: Significant amount of harmonic load and Resonance point near a characteristic harmonic of harmonic loads (5 th, 7 th for drives) If either or both are in a gray area then a study may be needed

Harmonics (To Filter or Not) Harmonic filters Manufacturer explicitly chooses the series tuning point of the filter Typically 4.7 th for LV and small MV banks Won t change, regardless of the system in which the filter is installed Parallel resonance is determined by both filter and the system 4.7 th filter in one system may cause parallel resonance at 3.8 th in one system, 4.1 st in another

Harmonics (To Filter or Not) Harmonic filters: Why 4.7 th If tuned right on nominal frequency (5 th ), it may attract excessive harmonic current In excess of component ratings Component tolerances Component drift over time As capacitors age, tuning rises (closer to 5 th ) Good to start a safe distance from 5.0 th tuning MV banks, with many caps: 4.9 th may be okay

Harmonics (To Filter or Not) De-tuned banks Harmonic filter, just tuned to 4.2 nd or 4.1 st Good if pf correction is main goal Reduced filtering effect Less concern about overloading filter More of a plug and play solution Small but significant amount of 5th harmonic current will still be filtered Will still filter some 7 th, 11 th, 13 th, etc. Other custom tuning points may be chosen

Harmonics (To Filter or Not) Other considerations Multiple capacitors at different locations can cause multiple resonance points Example: Capacitors on multiple motors Distributed capacitors not recommended on system with harmonic sources Assumption: Harmonic filters attract all harmonic currents >>> NOT TRUE! Most filters are 4.7 th tuned, not 5.0 th Absorb roughly half (ballpark: 30-70%) of the harmonic current on a typical system.

Harmonics (To Filter or Not) Other considerations (continued) Fixed filter applied to an individual load Intention: Filter only harmonics from that load Can attract harmonics from elsewhere Place a reactor upstream of the filter and load Harmonic loads with high pf (e.g. drives) High pf means little kvar needed kvar determines 60 Hz current Filter impedance determines harmonic current» Can sink a large amount of harmonic current, regardless of kvar size Easy to overload Generally ok if just part of the overall load

Harmonics (To Filter or Not) Other considerations (continued) Automatic filter banks and harmonic loads Desirable to switch on the filter steps quickly If many harmonic loads switched on at once and If filter steps switch on too slowly One small filter step may be left to sink a lot of harmonic current until the next steps switch on Can also occur with a filter bank with a few fixed steps Recommendation: Avoid fixed steps in a filter

Harmonics (To Filter or Not) Other considerations (continued) Specify a harmonic filter First choose 60 Hz pf correction needed Specify tuning point (4.7 th or 4.2 nd ) Harmonic filtering needed? Performing a harmonic study?» If not, consider 4.2 nd Determine harmonic current in filter Easy to determine component values (mh, kvar) More difficult to rate components when designing» Measurements, estimates, studies» LV filters: Oversized, for typical harmonic loads» MV filters: More likely to require studies

Harmonics (To Filter or Not) Other considerations (continued) Multiple filters with different tuning Avoid higher order filter amplifying lower order harmonics (parallel resonance!) Switch lowest order on first Switch highest order off first E.g. 5 th and 7 th filters: Switch 5 th on, then 7 th Switch 7 th off, then 5 th 7 th could amplify 5 th harmonic if left online alone» Possible parallel resonance near the 5 th

Harmonics (To Filter or Not) Other considerations (continued) Filter capacitors: higher voltage rating Typically 550 V or 600 V caps in 480 V filters Steady-state voltage rise on caps due to reactor» 60 Hz effect, regardless of harmonics Higher peak voltages on caps due to harmonics Cannot convert straight cap bank to filter without properly rated caps Straight banks with higher voltage caps Additional margin, more reliability Possible to convert to a filter later in some designs

Harmonics (To Filter or Not) Other considerations (continued) Automatic filter bank Each step must be configured as a filter Each step needs its own tuning reactor LV filter kvar Nominal kvar based on nameplate kvar Actual effective kvar is higher (typically 5% or so) Filter caps specified in kvar at applied voltage Higher voltage filter caps: nameplate kvar at 480 V MV filter kvar Effective kvar, including effect of filter reactors

Harmonics (To Filter or Not) Application advice: general steps Perform a resonance calculation Necessary for each step of an automatic bank If resonance is not close to 5 th, 7 th, 11 th, 13 th, etc., apply straight caps If resonance is close, choose filters (4.7 th ), de-tuned bank (4.2 nd ), or further study If very heavy drive or rectifier loads Passive filters may become overloaded Consult the manufacturer or study further

Capacitor Switching Transients Normal system event Possible whenever a capacitor is energized Due to difference in system voltage and capacitor voltage at time of switching Cap voltage can t change instantaneously System voltage pulled down to cap voltage Inrush current as capacitor charges Voltage on capacitor rises, overshoots, and oscillates Characteristic oscillation frequency

Capacitor Switching Transients 2.5 5000 2.0 Voltage 4000 1.5 Figure 6. Capacitor Energization Transient 3000 1.0 2000 Voltage (per unit) 0.5 1000 Current (A) 0.0 0-0.5-1000 Current -1.0-2000 -1.5-3000 0 45 90 135 180 225 270 315 360 60 Hz Degrees FIGURE 6. CAPACITOR ENERGIZATION TRANSIENT

Capacitor Switching Transients Back-to-back capacitor switching Second capacitor switched in close (electrical) proximity to a previously energized capacitor High frequency transient as the previously energized capacitor shares its charge with the newly energized capacitor Second, lower frequency, transient as the pair of capacitors oscillate with the utility

Capacitor Switching Transients 2.5 10000 2.0 8000 1.5 Voltage 6000 1.0 4000 Voltage (per unit) 0.5 2000 Current (A) 0.0 0-0.5 Capacitor #2 Current -2000-1.0-4000 -1.5-6000 75 90 105 120 135 150 165 60 Hz Degrees FIGURE 7. BACK-TO-BACK CAPACITOR SWITCHING TRANSIENT

Capacitor Switching Transients Capacitor contactors/breakers Capable of interrupting capacitor current? Voltage is at a peak when when interruption occurs at a natural current zero Very important at MV Interrupting duty depends on configuration of the capacitor bank and the system Voltage across contacts can be as high as 3 per unit (of the normal line-to-neutral voltage) with ungrounded source» Even higher (3.46 pu) if there is a failed capacitor

Capacitor Switching Transients Minimizing Capacitor Transients Switch when system voltage matches capacitor voltage, even if trapped charge Precisely-controlled switching device Static switched capacitors Insert impedance in the circuit Harmonic filters, with inductors in circuit Small inductor in the circuit (often done at MV) LV: Coil wire feeding the capacitor bank Pre-insertion resistance or inductance contactors Excellent solution for unfiltered capacitor banks

Capacitor Switching Transients Utility Capacitor Switching Magnification Utility switches MV capacitor bank This transient has a natural frequency Customer has LV capacitor bank If natural frequency of LV circuit matches frequency of MV transient, overvoltage at LV bank can result Relatively rare, but does happen Avoiding voltage magnification Change natural frequency of LV system Change transformer or capacitor size Add inductance (possibly harmonic filters)

Capacitor Switching Transients FIGURE 8. VOLTAGE MAGNIFICATION CIRCUIT

Capacitor Switching Transients FIGURE 9. UTILITY CAPACITOR ENERGIZED WITH LV CAPACITOR ENERGIZED: VOLTAGE MAGNIFICATION AT 480 V BUS

Capacitor Switching Transients FIGURE 10. UTILITY CAPACITOR ENERGIZED WITHOUT LV CAPACITOR ENERGIZED: NO VOLTAGE MAGNIFICATION

Capacitors and Fault Current Include capacitors in fault studies? No, they can be ignored Capacitors do not offer a significant source power during a fault Energized capacitor will discharge into other loads (or the fault) within ¼ cycle (60 Hz) at a much higher frequency Significantly different than motors that will generate back into the faulted system and contribute to the overall fault current

Conclusions and Summary With all of these potential problems, why would anyone apply pf correction capacitors? Capacitors provide well-known benefits With care, PF correction capacitors can be applied safely and effectively

The End