Capacitors. Dominik Pieniazek, P.E. VI Engineering, LLC Nicholas A. Losito Jr. Castle Power Solutions, LLC

Transcription

1 Capacitors Dominik Pieniazek, P.E. VI Engineering, LLC Nicholas A. Losito Jr. Castle Power Solutions, LLC

2 Outline Day 1 Basic Power Calculations Capacitor Fundamentals Capacitor Ratings Capacitor Application Capacitor Protection Day 2 Harmonics Capacitor Bank Design Considerations

3 Shunt Capacitors Medium Voltage Substation Applications Power Factor Correction

4 Basic Power Calculations

5 Basic Power Calculations Common Questions: What are VARs? Why do we care about VARs?

6 Basic Power Calculations Most plant loads (motors, transformers, etc) are INDUCTIVE and require a magnetic field to operate. The magnetic field is necessary, but produces NO USEFUL WORK. The utility must supply the power to produce the magnetic field and the power to produce USEFUL work. The ACTIVE component produces the USEFUL work, the REACTIVE component produces the magnetic field.

7 Basic Power Calculations VA VAR W An analogy that most can understand. Mug Capacity of equipment (i.e. xfmr, cable, swgr, etc) Beer Stuff that you want Foam (Head) Stuff which h prevents you from maximizing the amount of beer that you get

8 Basic Power Calculations V: Reference voltage I R : Resistive load I L : Inductive load I C : Capacitive load

9 Basic Power Calculations V ref I res Power = Re(V I*)

10 Basic Power Calculations V ref I lag Power = Re(V I*) Note that due to phase shift (30 degrees) only 86.66% of current is applied to calculate work.

11 Basic Power Calculations V ref I lead Power = Re(V I*) Regardless whether leading or lagging, power calculation yields similar results

12 Basic Power Calculations V ref I 90 Power = Re(V I*) In the case where current leads or lags the voltage by 90 degrees, P = 0

13 Basic Power Calculations Power Factor [ PF ] = Cos = P / S S (kva) Q (kvar) P (W) Power Triangle The relationship between S, P, and Q. This figure represents a lagging power factor. If Q is negative, leading power factor.

14 Basic Power Calculations So how do we know if our current is lagging or leading the voltage, and what can we do to correct it? Consider this example,

15 Basic Power Calculations 5500 kw 3400 kvar 6466 kva Incoming service is at 99.5% capacity 2000 kw 1000 kw 2500 kw 1000 kvar 800 kvar 1600 kvar

16 Basic Power Calculations 0.85 lag Power Factor [ PF ] = Cos = P / S S (kva) Q 3400 (kvar) kvar 31.8 deg P (W) 5500 kw Power Triangle Even though our facility require only 5500 kw to perform Real Work, our incoming service must be sized for 6466 kva.

17 Basic Power Calculations VAR We have a 6500 kva mug that is holding 5500 kw and 3400 kvar. W VA With the existing configuration the facility cannot add any loads without upgrading the incoming service.

18 Basic Power Calculations 5500 kw 0 kvar 5500 kva Incoming service is at 85% capacity 3400 kvar 2000 kw 1000 kw 2500 kw 1000 kvar 800 kvar 1600 kvar

19 Basic Power Calculations 1.0 unity Power Factor [ PF ] = Cos = P / S S (kva) Q 0 (kvar) 0 deg P (W) 5500 kw Power Triangle The cap bank is providing 3400 kvar, so our service is now providing only 5500 kva (reduction from 6466 kva.

20 Basic Power Calculations COS [ ] = 0.67 COS [ ] = 0.95 Q2 = Q1 + Qc 1,500 kva Q1 = 1,118 kvar 1,053 kva Q2 = 330 kvar 1,000 kw Required Apparent Power Before and After Adding a Power Capacitor Bank Qc = 788 kvar An example of how to calculate the size of a cap bank based on a target power factor

21 Basic Power Calculations But wait, there s more.

22 Basic Power Calculations 5500 kw 3400 kvar 6466 kva Odds are that the utility is charging you for penalties for a low power factor = 0.85 Typically, penalties are applied for power factor less than 0.95% kw 1000 kw 2500 kw 1000 kvar 800 kvar 1600 kvar

23 Basic Power Calculations 5500 kw 3400 kvar 6466 kva Volt Drop = 5.8% There are also voltage considerations. Assuming typical values: Source impedance = 9% at 6500 kva The expected voltage drop at the main bus will be close to 6%! 2000 kw 1000 kw 2500 kw 1000 kvar 800 kvar 1600 kvar

24 Basic Power Calculations 5500 kw 0 kvar 5500 kva Capacitor bank reduces the voltage drop at main bus by 5%! Volt Drop = 1.0% 3400 kvar 2000 kw 1000 kw 2500 kw 1000 kvar 800 kvar 1600 kvar

25 Basic Power Calculations 5500 kw kvar 5728 kva Note that the capacitor bank can also raise the bus voltage above the nominal value. Volt Drop = -1.5% 5000 kvar 2000 kw 1000 kw 2500 kw 1000 kvar 800 kvar 1600 kvar

26 Basic Power Calculations 3000 kw kvar 4386 kva The voltage continues to rise if the capacitor bank remains connected and the load is reduced. Volt Drop = -3.6% 5000 kvar Out of service 2000 kw 1000 kw 0 kw 1000 kvar 800 kvar 0 kvar

27 Capacitor Fundamentals

28 Capacitor Fundamentals C = e o A / d for a parallel plate capacitor, where e o is the permittivity of the insulating material (dielectric) between plates.

29 Capacitor Fundamentals We recall that we can add series capacitances to obtain an equivalent capacitance. 1/C eq = 1/C 1 +1/C 2

30 Capacitor Fundamentals Similarly, we can add parallel capacitances to obtain an equivalent capacitance. C eq = C 1 + C 2

31 Capacitor Fundamentals but we typically do not have much use for capacitance values. So we convert capacitance to impedance: 1 X C C fc

32 Capacitor Fundamentals Z R jx c Z R jx c Z X C Z V 2 S V 2 S X C

33 Capacitor Fundamentals V [ kv ] S [ kvar ] *1000 Z[ ] 2 Example: The capacitance of a capacitor is 6.22 F and the nameplate voltage is 8000 V. Calculate the power rating. X C 1 2(3.14)(60)(6.22x [ ] 6 ) S 2 (8) , [ kvar]

34 Capacitor Fundamentals

35 Capacitor Fundamentals

36 Capacitor Fundamentals

37 Capacitor Fundamentals

38 Capacitor Fundamentals

39 Capacitor Fundamentals Note that IEEE Std 18 requires the discharge resistor to reduce the terminal voltage to 50 V in the time frame as specified in the table below. Discharge resistor

40 Capacitor Ratings

41 Capacitor Ratings Medium-voltage capacitors are available in many different styles. The main points of differentiation are listed below: Voltage rating kvar rating Single bushing or dual bushing Internally fused, externally fused, or fuseless

42 Capacitor Ratings IEEE 81 defines the ratings for capacitors Voltage, rms (terminal to terminal) Terminal-to-case (or ground) insulation class Reactive power Number of phases Frequency

43 Capacitor Ratings IEEE 18 provides capacitor tolerances The capacitance shall not vary more than -0% to +10% of nominal value based on rated kvar, voltage, and frequency measured at 25 deg C. This means that a new 150 kvar unit can range anywhere from 150 kvar to 165 kvar.

44 Capacitor Ratings IEEE 18 states the capacitor is intended to operate at or below rated voltage. Capacitors shall be capable of continuous operation given that none of the following limitations are exceeded: 110% of rated rms voltage (temporary overvoltage parameters will be discussed later) 120% of peak voltage, including harmonics but excluding transients 135% of nominal rms current based on rated kvar and rated voltage 135% of rated kvar

45 Capacitor Ratings * Impulse tests shall be applied between terminals and case, with the terminals connected together. For capacitors having bushings with two different BIL ratings, this test shall be based on the bushing with the lower BIL. The nameplate shall show both BIL ratings, e.g. 150/95 kv BIL. ** Not applicable to indoor ratings

46 Capacitor Application

47 Capacitor Application Power factor correction capacitor banks are typically installed in the following ways : Pole top Metal-Enclosed / Pad-Mount Open rack Terminal end at equipment

48 Capacitor Application Pl Pole Top Installation tllti

49 Capacitor Application Transient inrush reactors Pl Pole Top Installation tllti

50 Capacitor Application Pad-Mounted Installation

51 Capacitor Application Metal-Enclosed Substation Installation

52 Capacitor Application Three-phase iron core harmonic filter reactor Metal-Enclosed Substation Installation

53 Capacitor Application Open Rack, Medium-Voltage Substation Installation

54 Capacitor Application Open Rack, High-Voltage Substation Installation

55 Capacitor Application Installation in Equipment

56 Capacitor Application Power factor correction capacitor banks can be configured in the following ways : Delta Wye - Solidly Grounded Wye - Ungrounded A common misconception is that the capacitor bank should be connected Delta since it is being applied to a delta or highimpedance grounded system. This is NOT true.

57 Capacitor Application The driving factor which determine the configuration for the given application is COST. Voltage considerations IEEE 1036 suggests that only banks rated 2400 V and below should be Delta connected. This is mainly because standard voltage ratings for wye connected banks may not be available. Cost of phase-to-phase vs phase-to-neutral rated capacitors at higher voltages tends to point installations towards wye connected banks for larger bank installations.

58 Capacitor Application Delta Lower voltages (<= 2400 V) Standard capacitors are typically not available at 1380 V Distribution systems (pole top) Units are configured with a single series group of capacitors with capacitors rated phase-to-phase. Therefore, unbalance detection is not required.

59 Capacitor Application Wye Solidly Grounded Initial cost of the bank may be lower since the neutral does not have to be insulated from ground. Capacitor switch recovery voltages are reduced High inrush currents may occur in the station ground system The grounded-wye d arrangement provides a low-impedance fault path which may require revision to the existing system ground protection scheme. Typically ynot applied to ungrounded systems. When applied to resistance-grounded systems, difficulty in coordination between capacitor fuses and upstream ground protection relays (consider coordination of 40 A fuse with a 400 A grounded system). Typical for smaller installations (since auxiliary equipment is not required)

60 Capacitor Application The most common capacitor bank configurations for larger substation applications are Wye-Ungrounded Three of the most common unbalance protection schemes are shown. Discussion of the protection schemes will be presented later.

61 Capacitor Protection Fusing Fuseless It Internally Fused Externally Fused

62 Capacitor Protection Bank kprotection ti Summary

63 Capacitor Protection Fuseless Capacitors Constructed of small capacitor elements which are arranged in series and parallel. The elements are constructed of aluminum foil with a dielectric of electrical grade polypropylene. This design provides a safe failure mode. In the event that the dielectric fails, the energy in the resulting small arc punctures many layers of the thin film and foil within the element. The arc causes the film layer to receded allowing many layers of the aluminum foil electrodes to touch and weld together forming an electrically stable electrical joint. This results in an entire series section being shorted.

64 Capacitor Protection Example of Fuseless Installation

65 Capacitor Protection Internally Fused Capacitors Constructed such that each element is protected with a series connected current limiting fuse. The design is such that isolated fusing prevents potential damage to the adjacent elements and fuses. The current limiting mode chops the fault current to prevent the energy stored in the parallel connected elements from being discharged into the faulted element.

66 Capacitor Protection Group Fusing Individual dvdu Fusing

67 Capacitor Protection Group Fusing Considerations for Selecting Fuse (typical for distribution pole mounted racks) Continuous Current Transient Current Fault Current Tank Rupture Curve Coordination i Voltage on Good Capacitors

68 Capacitor Protection Continuous Current For wye-solidly grounded systems: Fuse > = 135% of rated capacitor current (includes overvoltage, capacitor tolerances, and harmonics). For wye-ungrounded systems: Fuse > = 125% of rated capacitor current (includes overvoltage, capacitor tolerances, and harmonics). Care should be taken when using NEMA Type T and K tin links which are rated 150%. In this case, the divide the fuse rating by 1.50.

69 Capacitor Protection Transient Current Capacitor switching (specifically back-to-back switching) Lightning surges Back-to-back is typically y not a factor for pole mounted capacitors banks. High frequency lightning surges: High frequency lightning surges: Use NEMA T tin links for ampere ratings up to 25 A. Use NEMA K tin links for ampere ratings above 25 A.

70 Capacitor Protection Fault Current Ensure that the fuse can interrupt the available fault current Tank Rupture Coordination Ensure that the fuse maximum clearing TCC curve for the fuse link is plotted below the capacitor tank rupture curve. In cases of high fault currents, the tank rupture curve should be compensated for asymmetry. Voltage on Good Capacitors For an ungrounded ddsystem, a fault on one phase results in a 1.73 times overvoltage on the un-faulted phases. Ensure that the fault is cleared before the second capacitor failure.

71 Capacitor Protection Problems with Fusing of Small Ungrounded Banks Consider a kv, 1500 kvar cap bank made of three (3) 500 kvar single-phase units. 1500[ kvar] [ kv ] 69.44[ A] [ A] 100[ A] Fuse If a capacitor fails, we will expect approximately 3x line current. It will take a 100 A fuse approximately 500 seconds to clear this fault (3 x A = A). The capacitor case will rupture long before the fuse clears the fault. The solution is using smaller units (explanation to follow).

72 Capacitor Protection Individual Fusing Considerations for Selecting Fuse (typical for substation capacitor banks) Continuous Current Transient Current Fault Current Tank Rupture Curve Coordination Voltage on Good Capacitors Energy Discharge into Faulted Unit Outrush Current Coordination with Unbalance Detection System

73 Capacitor Protection Continuous Current Fuse > = 135% of rated capacitor current (includes overvoltage, capacitor tolerances, and harmonics) Care should be taken when using NEMA Type T and K tin links which are rated 150%. In this case, the divide the fuse rating by 1.50.

74 Capacitor Protection Transient Current Lightning surges Capacitor switching (specifically back-to-back switching) High magnitude, high frequency lightning gsurges are typically ynot a concern for substation installations. Back-to-back switching is typically controlled with pre-insertion closing resistors or current limiting reactors. h f i ll i h ll l f ill h h By the nature of installation, the parallel fuses will share the transient current and will not be a factor.

75 Capacitor Protection Fault Current Ensure that the fuse can interrupt the available fault current. In substation banks with multiple series groups, fault current will not flow through a failed capacitor unit unless other units experience a simultaneous failure. For this reason expulsion fuses are commonly used rather than current limiting fuses Tank Rupture Coordination Ensure that the fuse maximum clearing TCC curve for the fuse link is plotted below the capacitor tank rupture curve. In cases of high fault currents, the tank rupture curve should be compensated for asymmetry. y

76 Capacitor Protection Example of a Definite Tank Rupture Curve. Th i b h The time between the rupture curve and the fuse maximum clear curve is the coordination margin.

77 Capacitor Protection Example of a 10% and 50% Rupture Curve for a 100 kvar Capacitor. Probability based tank rupture curves are developed when there is too much variance in rupture test data. Based on the 10% and 50% curves, one can extrapolate the curves for any probability. bili

78 Capacitor Protection Voltage on Good Capacitors When a short-circuit on one unit occurs, an overvoltage results on the un-faulted phases. Ensure that the fault is cleared before the second capacitor failure. A table summarizes this voltage rise on the un-faulted units Per Unit Voltage on Un-failed Capacitors

79 Capacitor Protection Energy Discharge Into a Failed Unit When a capacitor failure occurs, the stored energy in the parallel connected capacitors can discharge through the failed capacitor and its fuse. The total calculated parallel stored energy should not exceed the energy capability (Joule rating) of the capacitor and fuse. If the energy capabilities are exceeded, a failure of the fuse and/or rupture of the capacitor tank can result. Typical rating of film capacitors is 15,000 Joules (4650 kvar in parallel) and 10,000 Joules (3100 kvar in parallel) for paper- film capacitors. Expulsion fuses are typically rated 30, Joules. Current limiting fuses are required if ratings are exceeded. (1 Joule = 1 W x sec, use 0.2 cycle clearing time for calculation)

80 Capacitor Protection Outrush Current When a capacitor failure occurs, the parallel connected capacitors can discharge high frequency current into the failed capacitor. The fuses of the un-failed capacitors should be able to withstand the high frequency discharge currents. These calculations and measurements are complex and are determined by the manufacturer. Coordination with Unbalance Detection Scheme The individual fuse must clear the fault before the unbalance protection scheme trips the entire capacitor bank.

81 Capacitor Protection Fusing Recommendations by McGraw Edison

82 Capacitor Protection Recall Problem with Fusing of Small Ungrounded Banks kv, 1500 kvar cap bank made of three (3) 500 kvar units 1500[ kvar] [ kv ] 69.44[ A] [ A] 100[ A] Fuse It will take a 100 A fuse approximately 500 seconds to clear this fault (3 x A = A). The capacitor case will rupture long before the fuse clears the fault. The solution is using smaller units with individual fusing. Consider five (5) () 100 kvar capacitors per phase, each with a 25 A fuse. The clear time for a 25 A A is below the published capacitor rupture curve.

83 Capacitor Protection Why is the Current 3 x Nominal Line Current for a Phase-to- Neutral Fault on a Wye-Ungrounded Capacitor Bank? B B A N A N V NG C C Since V = I*Z, where Z is constant (assuming f = 60 hz) If voltage across capacitor is increased by 1.732, the current also increases by factor of t t( i f 60h ) I A = p.u.

84 Capacitor Protection Minimum Conductor Size It was noted that capacitors are rated 135% of rating. This requires the conductor to be sized 135% of the nominal capacitor rating.

85 Capacitor Protection Unbalance Protection As single-phase units in a multiple unit/phase installation fail and are removed from service, the remaining units are experience an overvoltage condition. IEEE Standard 1036 provides overvoltage limitations. Duration Max Voltage (x rated RMS) 6 cycles cyclescles s sec min 1.30 An unbalance protection scheme must by implemented to prevent the failure of the overvoltaged units.

86 Capacitor Protection

87 Capacitor Protection Neutral Voltage Unbalance with Unbalance Compensation

88 Capacitor Protection V A V NG V G V N Normal Conditions V N = V G V AN = V BN = V CN V C V B

89 Capacitor Protection Ungrounded or Impedance Grounded System V A V NG V G V N Normal Conditions V N = V G V AN = V BN = V CN = 1.0 p.u. V C V B

90 Capacitor Protection V NG Phase to Neutral Fault Phase to Neutral Fault V NG = V LN V AN = V BN = V LL = p.u.

91 Capacitor Protection V A V NG V G V N One Can Removed V NG = 0.2 p.u. V CN =12pu 1.2 p.u. V C V B

92 Capacitor Protection Ungrounded or Impedance Grounded System V A V NG V G V N Normal Conditions V N = V G V AN = V BN = V CN = 1.0 p.u. V C V B

93 Capacitor Protection Ungrounded or Impedance Grounded System Gnd V A V NG V N Ground Fault V NG = V LN V AG =V BG =V LL = pu p.u. V C V C =V G Ground Fault at Cap Bank or Anywhere on the System V B

94 Capacitor Protection Wye-Ungrounded: Voltage Between Capacitor Bank Neutral and Ground vs. Percentage of Capacitor Units Removed from Series Group

95 Capacitor Protection Wye-Ungrounded: Voltage on Remaining Capacitor Units in Series Group vs. Percentage of Capacitor Units Removed from Series Group

96 Capacitor Protection Wye-Grounded: Neutral Current vs Percentage of Capacitor Units Removed from Series Group

97 Capacitor Protection Wye-Grounded or Delta: Voltage on Remaining Units in Series Group vs. Percentage of Capacitor Units Removed from Series Group

98 Capacitor Protection Double Wye-Ungrounded, Neutrals Tied Together: Neutral Current vs. Percentage of Capacitor Units Removed from Series Group

99 Capacitor Protection Double Wye-Ungrounded, Neutrals Tied Together: Voltage on Remaining Capacitor Units in Series vs. Percentage of Capacitor Units Removed from Series Group

100 Capacitor Protection A phase B phase C phase P: Number of units in group (P=6) S: Number of series groups (S=4) Reference Figure for Calculations to Follow

101 Capacitor Protection # of Series Groups Grounded Y or Delta Ungrounded Y Split Ungrounded Y (equal sections) and over Minimum recommended number of units in parallel per series Group to limit voltage on remaining units to 110% with one unit out

102 Capacitor Protection Many more configurations and calculations shown in IEEE C37.99

103 Day 2

104 Capacitor Fundamentals Further discussion on capacitor voltage ratings: On a ungrounded or impedance grounded system, a ground fault on one phase will cause the other two phases will be elevated by Does this mean that capacitors must be rated phase-to-phase? Certainly nothing wrong with this, but cost will be significantly higher.

105 Capacitor Fundamentals Recall: S V X 2 X C This means that a 150 kvar, V unit applied at 7200 V will provide only 50 kvar. S NEW [ V ] 12470[ V ] S NEW 50[ kvar ] 2 150[ kvar]

106 Capacitor Fundamentals Using V capacitors on a V Ungrounded or Resistance-Grounded System will require 3x more cans. It should be noted that the V cans will also be larger than the 7200 V cans. Results in a much larger and more costly installation. This solution would be required if a ground fault could be maintained for extended periods of time.

107 Capacitor Fundamentals Perhaps a 150 kvar or 200 kvar, 7620 V or 7960 V units applied at 7200 V would be a better solution. S NEW 7200[ V ] 7620[ V ] S NEW 134[ kvar] OR S NEW 7200[ V ] 7960[ V ] S NEW 123[ kvar ] [ kvar] 150[ kvar] S NEW 7200[ V ] 7620[ V ] S NEW 178[ kvar] OR S NEW 7200[ V ] 7960[ V ] S NEW 163[ kvar ] 2 200[ kvar] [ kvar] Note that the 7620 V unit provides an additional 6% The 7960 V unit provides an additional 11%

108 Explusion Fuses: Capacitor Fundamentals Provides a means of disconnecting a failed capacitor from the circuit by melting a tin-lead low current link. The shorted capacitor unit causes a large increase in the current through the fuse. The current is limited only by the power system reactance and the other capacitor units in series with the failed capacitor unit. The pressure is generated by the hot arc making contact with the fiber lining of the fuse tube. The link is cooled and stretched as it is forced out the tube. The fuse continues to conduct until a natural current zero occurs. The current zero is caused by the power system fault current crossing zero. If other capacitors are connected in parallel with the failed unit, all the stored energy in these capacitors will be absorbed in either the fuse operation or the failed capacitor unit. Most of the energy is absorbed in the failed capacitor.

109 Capacitor Fundamentals Current Limiting Fuses: Uses a long uniform cross section element. This configuration makes the fuse a current chopping fuse. The fuse develops a back voltage per inch of element across the entire length of the element. When this voltage exceeds the available voltage across the fuse, the fuse forces the arc to extinguish. The result is that a trapped voltage may and probably will remain on the other capacitors in the series group. The fuse by its design avoids absorbing all of the available energy on the series group. This fuse is used for capacitor banks with a large number of parallel l capacitors. It can be used on applications with essentially infinite it parallel l stored energy, as long as sufficient back voltage can be developed to force the current to extinguish. This is the fuse we apply to series, large shunt, and DC banks. Because of the high back voltage that is developed, this fuse must be used with several capacitors in parallel to limit the voltage build up or a flashover may occur elsewhere in the capacitor rack.

110 Capacitor Fundamentals Current Limiting Fuses vs Expulsion Fuses: Expulsion Fuse Current Limiting i i Fuse

111 Capacitor Fundamentals Current Limiting Fuses vs Expulsion Fuses: Expulsion Fuses Operate mechanically and provide a visual indication Require additional space for operation Typically applied for outdoor application due to ionized gas release. Combination expulsion with current limiting iti characteristic ti fuses can be used in indoor metal-enclosed equipment. Less expensive

112 Capacitor Fundamentals

113 Capacitor Fundamentals Current Limiting Fuses Do not emit ionized gases during operation. Ionized gases are undesirable because they can cause bushing and insulator flashovers that result in additional damage. Do not require ventilation. Fast current-limiting operation High interrupting capacity, noiseless operation Can be specified for indoor and outdoor applications. No pressure build-up, therefore, no vents or special reinforced compartments are required. More expensive

114 Capacitor Fundamentals Note no pigtail and blown fuse indication

115 Capacitor Fundamentals Current Limiting with Expulsion

116 Capacitor Fundamentals What about arresters? How and where should they be applied? Depending on application, environment, exposure to switching, etc, arresters may be necessary. We recall that when a travelling wave meets a high impedance, the wave can double in size. For this reason, arresters (if used) should be installed as close to the capacitor bank as possible. Installation of arresters at the breaker feeding the capacitor bank will not do much for protection of the capacitor bank.

117 Capacitor Protection A basic three (3) arrester method is shown below. This is typical for solidly grounded systems and wye-grounded capacitor banks.

118 Capacitor Protection Depending on type of installation, system parameters and level of protection required, a six(6) arrestor method may be applied.

119 Capacitor Protection For an ungrounded system or a high-impedance grounded system, a four (4) arrestor grounding method might be considered an wye ungrounded bank. Phase to Neutral Fault V NG = V LN V AN = V BN = V LL = p.u. Ground Fault V NG = V LN V AG = V BG = V LL = p.u. If faults can be maintained, V LL V LL Arr PH Arr PH must be rated V LL Arr N must be rated V LN Arr N The effective arrester MCOV is V LL + V LN

120 Capacitor Protection Note that if a basic three (3) arrester method is applied to an ungrounded bank, the arresters must be rated high enough to sustain a temporary overvoltage condition during a phase-toground fault on the system. This may not provide an adequate level of protection for the capacitors. Phase to Neutral Fault V NG = V LN V AN = V BN = V LL = p.u. Ground Fault V NG = V LN V AG = V BG = V LL = p.u. If faults can be maintained, Arresters must be rated V LL Arresters do not provide protection across the capacitor bushings. Note that the BIL applies to bushing-to- case insulation. V LL V LL

121 Capacitor Protection Good Presentations on Capacitor Arrester Applications Guidelines for Selection of Surge Arresters for Shunt Capacitor Banks ABB Technical Information Surge Arrester Application of MV-Capacitor Banks to Mitigate Problems of Switching Restrike CIRED 19 th International Conference on Electricity Distribution, Vienna, May B th fth l dd h t h t Both of these papers also address phase-to-phase arrester connections.

122 Harmonics

123 Harmonics Recall that the impedance of a capacitor is inversely proportional to the system frequency. X C 1 1 C 2 fc Harmonics flow to the point of lowest impedance. The higher the harmonic, the lower the impedance of the capacitor. As capacitors absorb harmonics, the capacitor heats up and the life expectancy is reduced. d The voltage harmonics stress the capacitor dielectric and reduce the life expectancy of the capacitor.

124 Harmonics

125 Harmonics Where do harmonics come from? Power Electronics (drives, rectifiers, computer power supplies, etc) Arcing Devices (welders, arc furnaces, florescent lights, etc) Iron Saturating Devices (transformers) Rotating Machines (Generators) Parallel Resonance (between cap bank and power equipment) IEEE Std 519 provides recommended limits of harmonic distortion at the point-of-common-coupling (PCC) with the utility.

126 Harmonics

127 Harmonics

128 Harmonics Resonance When a number of harmonic current sources are injecting currents into the supply and the frequency of one of the harmonics coincides with the resonant frequency of the supply transformer and Power Factor Correction capacitor combination, the system resonates and a large circulating harmonic current is excited between these components. The result of this is that a large current at this harmonic flows in the supply transformer, this resulting in a large harmonic voltage distortion being imposed upon the load voltage.

129 Harmonics A study should be performed to determine levels of harmonics on a system to determine if any filters are necessary when installing a capacitor bank. Care should be taken to determine if the filtered capacitor bank will introduce any resonance problems. If resonance problems exist, the filter design must be adjusted. d

130 Harmonics An example of a 13.8 kv harmonic filter

131 Capacitor Bank Design Considerations

132 Design Considerations So how do we size a capacitor bank? Dt Determine your primary goal Voltage support Lower utility bill (avoid penalties) Increase capacity of system It can be all three, or any combination of the above. Note that correcting to unity power factor at maximum load is costly and may not be necessary.

133 Design Considerations For a 20 MVA load at 0.88 power factor (17.6 MW, 9.5 MVAR) To achieve 95% power factor, a 3.72 MVAR bank is required To achieve 98% power factor, a 5.93 MVAR bank is required To achieve unity power factor, a 9.50 MVAR bank is required

134 Design Considerations Determine if current limiting reactors or tuning reactors are required. Harmonics and resonance may dictate tuning reactors Back-to-back switching may require current limiting reactors (unless another method is used to mitigate the switching surges, i.e. pre-insertion closing resistors/reactors, zero-crossing breakers, etc)

135 Design Considerations Determine the proper voltage. Capacitors are very susceptible to voltage transients and harmonics. Increasing the rated voltage increases the protective margin on the insulation. The voltage at the capacitor terminals will be higher than bus voltage if reactors are utilized. It is important t to account for this voltage difference. Determine the voltage swing of the system. Will the capacitors remain on-line while the facility is lightly loaded.

136 Design Considerations We listed some reasons for specifying higher than bus nominal rating of capacitors. However, care must be taken to ensure that the kvar rating is properly adjusted as a result. Three (6) 150 kvar, 7960 V wye-connected capacitors provide a nominal 901 kvar when connected to a 13.8 kv bus. Three (6) 150 kvar, 8320 V wye-connected capacitors provide a nominal 825 kvar when connected to a 13.8 kv bus

137 Design Considerations Determine optimal size and number of stages. Depending on swing in plant tload, a single bank sized dfor full plant capacity may not be the answer. IEEE 1036 recommends limiting the voltage change to 2-3%. The delta voltage can be estimated by: V MVAR 100% MVA SC Switch of a capacitor applies high stresses to the insulation. Limiting the number of stages and limiting the frequency of switching will increase the life. Ideally, a capacitor is switched on and left on.

138 Design Considerations Determine best location for the installation. The most effective placement for power factor correction capacitor banks is at the load. However, this is not always practical or cost effective. Typically, a capacitor bank is installed on each bus of a main-tie- main switchgear. If capacitors are installed at the motor pecker head (running capacitors), ensure that the capacitor VAR rating does not exceed 90% of the motor no-load VAR. Otherwise, it is possible to damage the motor by overexcitation.

139 Design Considerations Use caution when sizing motor running capacitors. Logic would suggest that installation of a power factor correction capacitor at the motor terminals sized to provide unity ypower factor makes sense. THIS IS NOT THE CASE. Do not exceed 90% of the motor no-load kvar demand. Exceeding this value can result in damage to the motor insulation as a result of overexcitation.

140 Design Considerations As an example for a 4000 V, 4000 hp motor: 100% load current = % pf 100% load kvar = 1516 kvar No load current = % pf No load kvar = 809 kvar Max size of running capacitor is 0.90 x 809 kvar = 728 kvar

141 Design Considerations M: Motor Magnetizing Curve C1: Capacitor size at 100% motor mag current C2: Capacitor sized > 100% motor mag current C4: Capacitor sized < 100% motor mag current If the capacitive reactance of the capacitor is less less than that of the motor reactance (this occurs when to large of a capacitor is chosen). This combination of reactance will result in a resonant frequency below 60 hertz. Therefore, as the motor slows in speed, the frequency of the motor terminal voltage will decrease from a value of near 60 hertz toward zero. When the motor's terminal voltage frequency passes through the resonant frequency setup between the capacitor reactance and the motor reactance, the terminal voltage will become very high, only limited by the properties of the iron. Depending on the inertia of the motor, this resonance (or high voltage) may be present for a considerable period of time.

142 Design Considerations Determine the most optimal type of installation. Will the capacitor bank be installed within a fenced substation? Metal-enclosed, pad mount, or open rack may be good choices Will the capacitor bank be installed in a process area? Metal-enclosed or pad mount may be good choices Will the capacitor bank be pole mounted on a distribution line?

143 Bank Failures

144 Design Considerations Consider the impact to personnel safety adjacent equipment when deciding between a metal-enclosed and open-rack system. Porcelain can resemble shrapnel when a capacitor bushing fails.

145 Design Considerations Determine the most optimal configuration. Higher reliability costs more kvar, V, wye-grounded (1) 800 kvar per phase bank will be a smaller footprint and cost less than 2400 kvar, V, wye-ungrounded (8) 100 kvar per phase bank. H h li bili f h d d b k i However, the reliability of the wye-ungrounded bank is significantly higher

146 Design Considerations Determine the switching equipment When breakers are used for switching capacitors (single bank or back-to-back switching), the breakers must be rated for capacitor switching. IEEE C37.99 provides the equations for calculating the inrush current and frequency.

147 Design Considerations

148 Design Considerations Consider a single 4800 kvar wye-ungrounded bank switched (with nominal inductance from equipment): 3253A 600 Hz, the product is 0.20 x 10 7 Switching a second similar bank on the same bus without current limiting reactor: 24,058 A 7.66 khz, the product is 18.4 x 10 7 By adding a 100 mhh current limiting i i reactor, the inrush is: 7254 A 2.31 khz, the product is 1.7 x 10 7

149 Design Considerations

150 Design Considerations

151 Design Considerations Energization of a single capacitor bank.

152 Design Considerations Back-to-back kswitching of the same unit.

153 Design Considerations Example of fbreaker with pre-insertion i resistor.

154 Design Considerations Another application.

155 Design Considerations A Pre-Insertion Manufacture s Perspective.

156 Design Considerations Another concern is voltage amplification as a result of switching a second capacitor bank.

157 Design Considerations

158 Design Considerations Consider other accessories: Disconnect switch Grounding switch Kirk-key interlock Ventilation requirements Control power

159 Design Considerations

160 Design Considerations

161 Design Considerations Be aware that larger medium voltage motors may include surge packs. The surge pack will decrease the crest voltage and rate of rise of the impending surge. High rates of rise damage end turns while high crest voltage damage winding to core insulation. Typically the capacitance of the is small enough that it can be neglected, but this should be verified.

162 Design Considerations Typical Surge Pack Application

163 Design Considerations Do not confuse Harmonic Filter Banks with Power Factor Correction Banks. The voltage ratings of harmonic filter banks are substantially higher because they are connected on the back end of a tuning reactor where the voltage is substantially higher. As a result of the higher voltage, the installed kvar can be anywhere e from 25% to 40% higher than nominal design. The capacitor cans must be capable of this output.

164 Grounding of Wye Banks If multiple wye-grounded banks are in close proximity, use peninsula grounding g or single-point grounding. g Single-Point Grounding The neutrals of all banks of a given voltage are connected together with insulated cable/bus and tied to the ground grid only at one point. This prevents high-frequency currents (due to back-to-back b kswitching) )from flowing into the ground grid.

165 Grounding of Wye Banks Peninsula Grounding One or more isolated ground grid conductors are carried underneath the capacitor rack of each phase and tied to the station ground grid at one point at the edge of the capacitor area. All capacitor bank neutral connections are made to this isolated peninsula ground grid conductor.

166 Grounding of Wye Banks

167 Grounding of Wye Banks

168 References IEEE Std. 18 IEEE C37.99 NEMA CP-1 IEEE Std 1036 IEEE Std 399 (Brown Book)