Capacitor Application Issues

Transcription

1 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

2 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

3 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

4 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

5 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

6 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

7 Capacitor Sizing TABLE 1 EXAMPLE POWER FACTOR CALCULATIONS, USING MONTHLY DATA,.95 PF TARGET KW PF KVAR NEEDED Probably choose 600 kvar

8 Capacitor Sizing TABLE 2 EXAMPLE POWER FACTOR CALCULATIONS, USING SUMMARIZED DATA,.95 PF TARGET MAXIMUM KW MINIMUM PF KVAR NEEDED MAXIMUM KW AVERAGE PF KVAR NEEDED AVERAGE KW MINIMUM PF KVAR NEEDED AVERAGE KW AVERAGE PF KVAR NEEDED Possibly choose 1000 kvar, with most conservative approach!

9 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

10 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

11 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

12 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

13 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 = ( )*$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 = ( *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 = ( )*$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 ( )*$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 = *0.50 = 200 kvar pf penalty = 200 kvar*($0.45/kvar) = $90/month kvarh charge $X per kvarh $ per kvarh kvarh = 500,000 pf penalty = 500,000* = $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 $0.020/kWh for all additional kwh Actual demand = 800 kw; Adjusted demand = 1000 kw; kwh measured = 500,000 With penalty 100*1000=100, /kWh=$ *1000=150, /kWh=$ *1000=150, /kWh=$3750 (500, , , ,000)*$0.02/kWh = $2000 Total = $15,000 Without penalty 100*800=80, /kWh=$ *800=120, /kWh=$ *800=120, /kWh=$3000 (500,000-80, , ,000)*$0.02/kWh = $3600 Total = $14,000 Penalty = $15,000 - $14,000 = $1,000/month (in addition to demand penalty)

14 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

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

16 Capacitor Ratings IEEE Std 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 % of rated reactive power

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

18 Capacitor Ratings Short time overvoltage limits IEEE Std per unit rms voltage for seconds 2.70 per unit rms voltage for seconds IEEE Std and 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

19 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

20 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

21 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

22 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

23 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 (A), for LV capacitors: Ampacity of conductors shall not be less than 135% of rated capacitor current NEC Article (C) (3), for LV caps: Rating of the disconnecting means shall not be less than 135% of rated capacitor current

24 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

25 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 (Overcurrent protection) For capacitors, Article 460 applies

26 Code Reqs. and Protection Overcurrent protection (continued) NEC Article (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

27 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

28 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

29 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

30 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 (A), for LV capacitors: Reduce charge to 50 V or less within 1 minute NEC Article (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

31 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

32 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)

33 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

34 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

35 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

36 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

37 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

38 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

39 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

40 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

41 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

42 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

43 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

44 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

45 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

46 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

47 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

48 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

49 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

50 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

51 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

52 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

53 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

54 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

55 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?

56 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

57 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

58 Harmonics (To Filter or Not) Why do we care about harmonics? IEEE Std , 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.

59 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

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

61 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

62 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

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

64 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

65 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

66 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

67 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

68 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

69 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

70 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.

71 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

72 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

73 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

74 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

75 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

76 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

77 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

78 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

79 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

80 Capacitor Switching Transients Voltage Figure 6. Capacitor Energization Transient Voltage (per unit) Current (A) Current Hz Degrees FIGURE 6. CAPACITOR ENERGIZATION TRANSIENT

81 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

82 Capacitor Switching Transients Voltage Voltage (per unit) Current (A) Capacitor #2 Current Hz Degrees FIGURE 7. BACK-TO-BACK CAPACITOR SWITCHING TRANSIENT

83 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

84 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

85 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)

86 Capacitor Switching Transients FIGURE 8. VOLTAGE MAGNIFICATION CIRCUIT

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

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

89 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

90 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

91 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

92 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

93 The End