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Floating Wye Metal Enclosed Capacitor Banks
 
With the advent of larger rated capacitor units (in KVAR), it becomes more economical to use these larger units to construct capacitor banks. In addition, capacitor banks now can hold more KVAR in the same space as the older style banks. This is very desirable for older factories and substation with space limitations.

The newer all-film capacitors designs allow higher stress on the internal packs and the capacitor fluid is more flammable than the PCB fluids used in older units. These factors have led to installation and design problems that need to be addressed. At the heart of these application issues are the removal of faulty capacitors from the bank before tank ruptures, or improper fuse failures occur. Careful attention will also need to be paid to the number of units applied in parallel due to over-voltage concerns when any unit fails. The object of proper fusing is to remove the failed capacitor unit from service as quickly as possible before case rupture and still maintain upstream co-ordination.

Before addressing capacitor bank connections, fusing and protection specifically, we should review some basics in the application of capacitor banks. Capacitor banks may be applied in different arrangements or, configurations. How the banks are constructed depends on the users preference for operating the bank and on the electrical system to which the banks are applied.

Generally Medium Voltage Systems are Supplied in Four Different Configurations, or Methods of Connection

(a) Delta system - three phase, three wire ungrounded.

(b) Ungrounded WYE System - three phase, four wire, 4th wire is carried as a neutral.

(c) Unigrounded WYE System - three phase, four wire, solidly grounded with the 4th wire carried as a neutral and only grounded at the source.

(d) Grounded WYE (multi-grounded) three-phase four wire.

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The delta connection is a three phase, three-wire system

The ungrounded WYE system can be a three phase, three-wire system or, a three phase, four wire system with the 4th wire carried as a neutral.

The uni-grounded system is grounded at the substation and the fourth wire is carried as a neutral and is not grounded again.

The multi-grounded system has the fourth wire as a ground and is grounded at intervals along its path.

Capacitor banks can only be connected delta or, ungrounded (floating) WYE when applied on delta, ungrounded WYE and uni-grounded systems. For the multi-grounded electrical system capacitor banks can be connected in any of the configurations, delta, ungrounded, or grounded WYE. Only the multi-grounded system will allow you to connect your capacitor bank grounded WYE (with the capacitor neutral connected to the system ground.)

Floating (ungrounded) WYE connections for capacitor applications can be used on three-phase/three wire, three-phase/four wire, uni-grounded and three phase/four wire multi-grounded systems. For capacitor/filter applications the banks are typically connected ungrounded, WYE.

There are both advantages and disadvantages in a floating WYE connection of capacitors. One of the advantages is the fault current in an internally faulted capacitor is limited to three times the line current.

There are several ways to prove the above statement. See appendix 1.

Improper Fusing Practices of Floating WYE Connected Capacitors

The previous discussion outlined bank and electrical system configurations. Once the user has selected the general application needed and knows the size of the bank, the concern is now the proper selection of individual capacitor and fuse size. The larger capacitor ratings available require larger fuse sizes in order to protect the capacitor from tank rupture. Normal current limiting fuse selections for metal enclosed banks may not be adequate in a floating WYE configuration if the proper engineering practices are not applied.

For example in a 12.47 KV, 1500 KVAR bank connected floating WYE, we can use one 500 KVAR, 7.2 KV capacitor per phase. A 100 amp fuse 8.3 KV current limiting fuse should be selected.

500KVAR/7.2 KV = 69.44 amps * 1.5 = 104 amps (It is a normal industry practice to use a 1.5 factor for medium voltage applications for current limiting fuse selection)

Looking at the three phase current rating of the 1500 KVAR bank connected floating WYE on a 12.47 KV system will have a line current of 69.44 amps.

1500KVAR/(12.47KV * sqrt(3)) = 69.44 amps

However, when a capacitor fails internally, in a floating WYE configuration, and the fuse has not yet operated, the fault current, as described above, will be three times the line current or, approximately 208 amps.

The maximum clearing curve for a 100-amp/8.3KV fuse shows that the fuse can handle 208 amps for approximately 500 seconds (or, more than 8 minutes.) The capacitor may have ruptured long before the fuse clears.

Now, what is occurring within the capacitor unit that leads to case rupture? Internally, these capacitors can have multiple parallel packs connected in series groups. As each pack fails, an over-voltage develops in the remaining packs of the capacitor. Assume a capacitor has three series groups. If one group fails, this will develop a 30% over-voltage on the remaining packs. Per IEEE Std. 18-1992 section 8.3.2.1 the capacitor can handle this over-voltage for one minute. As pointed out above the fuse can handle 208 amps for more than 8 minutes. The capacitor packs will continue to fail and eventually, if not cleared, will fail the major insulation. As this is occurring the capacitor survival time decreases rapidly. There is a very good chance for a tank rupture.

As shown in the Appendix, when a capacitor unit is shorted in one phase of an ungrounded capacitor bank with one series group, the remaining phases will see 1.73 pu over-voltage. This is also specified in IEEE Std 1036-1992 table 7. Table 6 in section 5.1.2 states that a capacitor should handle this over-voltage for one second. Refer again to the example given above of the 1500 KVAR bank at 12.47 kV, and recalling that the ungrounded fault current is three times the line current, for a fault current of approximately 210 amps, a 30 amp fuse will clear 200 amps at one second. This implies that the largest acceptable fuse size for this application to meet the standard is a 30-amp fuse. However, a 100 KVAR capacitor at 7.2 kV requires a 20amp fuse. Therefore, to meet this standard for a 1500 KVAR capacitor bank, floating WYE connected, we would need five -100-KVAR units per phase.

Two Parallel Connected Unequal Size Capacitors Per Phase in a Floating WYE Bank

In a floating WYE connection, a similar problem develops when a larger capacitor is placed in parallel with a smaller unit (two units in parallel.) For example, consider the construction of a 1350 KVAR bank on a 12.47 KV system, which would require 450 KVAR per phase. A 450 KVAR unit is not a standard unit. Using standard production units we could use one 300 kvar and one 150 KVAR capacitor per phase rather than three 150 KVAR units to reduce cost and enclosure size.

The 300 KVAR unit requires a 65-amp fuse and the 150 KVAR unit requires a 30-amp fuse. The line current for the 1350 KVAR bank is 62.5 amps. The fault current would be 3 times 62.5 amps is equal to 187.5 amps when a capacitor is shorted. For a 300 KVAR capacitor failure, the 65-amp fuse will take approximately 25 seconds to clear this current. When the 300 KVAR capacitor goes to a complete short, the 150 KVAR capacitor will "dump" its stored energy into the 300 KVAR capacitor. This energy should not exceed the energy capability, or joule rating, of the capacitor and/or the fuse. However, the 150 KVAR capacitor and the 30-amp fuse has to handle this out rush as well. The minimum melt curves for each fuse shows that the 65-amp fuse has six times the joule (energy) rating of the 30 amp fuse. The 30-amp fuse will have to handle the parallel energy for the time frame that it takes the 65-amp fuse to clear. Therefore, there is a very good chance that the 30-amp fuse will operate first. It is very likely that the 300 KVAR capacitor can fail but the 65-amp fuse is still good while the 30-amp fuse on the good 150 KVAR capacitor fails (1/2CV2 {energy stored} versus I2t*R of the fuse).

The inductance between the two capacitors will determine the amount of energy that will "dump" from one unit to the other. Normally, this inductance is negligible since units are mounted side-by-side, with very short spans of bus and wiring. So the energy of this "dump" could be high enough to damage the good fuse giving the user a false indication to the problem.

On floating WYE connections it is recommended to use unbalance detection schemes to protect from additional capacitor failures. If an unbalance detection scheme is employed in the example given above the unbalance protection will be activated when the 30-amp fuse fails first since the 65-amp fuse has not had enough time to clear.

According to IEEE Std. C1036-1992 section 5.1.2 it is recommended that a minimum of four units per phase be used in parallel to avoid the above conditions. Four units are recommended so that after a loss of one unit, voltage on the remaining units in that phase will not exceed the capacitors maximum voltage rating and the proper energy contribution from the good units will assist in clearing the fuse on the bad unit. Unbalance detection schemes can eliminate overvoltage concerns after the loss of a capacitor unit.

Therefore, in this example, using three 150 KVAR units in parallel is a better solution. This would give the 30-amp fuse a better chance to clear a failed capacitor before tank rupture. A 30-amp fuse can handle 187.5 amps for approximately 1 second. In addition, the energy from the other two capacitors will assist in clearing the fuse on the failed unit. However, as stated above, in a floating WYE connection, when a capacitor has a complete short it can handle 1.73 over-voltage for only approximately 1 second. Therefore, even using three 150 KVAR units per phase is marginal based on the requirement as stated in IEEE 1036-1992, but it is a lot better than one 450 KVAR per phase, or one 150 KVAR and one 300 KVAR per phase. Since the recommended minimum number of units per phase is not applied, the unbalance protection scheme would have to trip the bank on the loss of one unit.

Applications Which Will Not Meet The Standard

There are situations where the minimum number of units per phase as specified by IEEE 1036-1992 section 5.1.2 cannot be met. This will be the case for the application of small banks. One example is a 150 KVAR bank which would use one 50-KVAR unit per phase. With the 50 KVAR single-phase capacitor being the smallest standard capacitor available it appears that the user does not have much of a choice. This bank would be better connected in a delta with group fusing.

Another example is a 600-KVAR ungrounded bank, with one 200 KVAR unit per phase. The user chooses to use these types of designs for economic reason, i.e. fewer units of larger KVAR will save money and physical equipment size and will result in less expensive equipment. However, to comply with this standard to meet the maximum proper energy contribution and co-ordination, we would have to use four 50-KVAR units. However, this would make any bid using four 50-KVAR capacitors per phase compared to using 200 KVAR capacitor units per phase noncompetitive. The user needs to be aware of these potential problems and carefully consider proper specification requirements.

Using the 200 KVAR capacitors on a system of 12,470/7200 volts the current rating of the bank will be approximately 28 amps. Therefore, 40 amp current limiting fuses would be used to protect the metal enclosed bank. This fuse will show that it will meet the tank rupture curve of the capacitor. However, as stated the fault current will be limited to three times the fault current or, approximately 84 amps. The 40-amp current limiting fuse will handle this current for 200 seconds. As stated above the standards require a capacitor to handle 1.70% over-voltage for one second. There is a very good chance the capacitor will rupture long before the fuse operates.

Delta Connected Banks May Be The Answer On Smaller Banks

What is a solution for this dilemma? We could connect the bank delta. IEEE 1036-1992 section 5.2 states that delta connected capacitors are ". . . generally only used at . . ." lower voltages, e.g., 2400 volts, and delta connected banks are more complicated and less economical than WYE connected banks. It does not state delta connections cannot be used at higher voltages. Table 5 of the same standard states that there is no minimum number for units in parallel for delta connected banks. Unfortunately, delta connected banks have their own fusing problems.

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Delta connected banks can be fused in two different arrangements. First an " in line" or, "group fuse" method (outside the delta see figure 2a) of the circuit. The second method uses " branch" or "individual fusing" (inside the delta, Figure 2b) of the circuit.

Three phase capacitors use fuses in the line because they are connected delta Internally. Normally branch fuses are used for single-phase capacitors connected delta. However, on the smaller banks mentioned above, the single phase capacitors could be connected delta and fused outside the delta (In the line.) On small banks that have only one capacitor per phase, this should be the method of choice when the neutral of the capacitor bank is not grounded.

When the bank has higher KVAR ratings and units are placed in parallel, the in line fusing becomes large, and may not coordinate with the tank rupture curve of the capacitor and the upstream co-ordination may not be possible.

For example, consider both fusing methods for a 450 KVAR, 4160 volts delta connected bank, using 150 KVAR per phase, will require the following fusing:

In line fusing or group fusing

450 KVAR/(4.16 KV / sqrt(3)) = 62.45 amps * 1.5 = 93.68 amps (a 100-amp fuse is required).

In branch fusing

150 KVAR/4.16 KV = 36 amps * 1.5 = 54 amps (a 50-amp fuse can be used).

There are other potential problems in fusing a delta-connected bank with "in branch" fusing. It is a normal practice utilized in metal enclosed banks to install two bushing capacitors connected phase to phase with the capacitor tank grounded to the frame. In some cases, the user only applies one fuse per phase. This could be dangerous. When a capacitor starts to fail and the fuse operates the capacitor is still in the circuit via the second bushing. The failure within the capacitor is fed thru this connection and eventually the major insulation of the can will fail and the capacitor tank will rupture.

The other method is to use two fuses, i.e. one per bushing. This gives the user a false sense of security. In this case both fuses would have to operate before the failed capacitor can be effectively removed from the system. Normally only one of the fuses operates, which will be the one nearest the faulted packs. The other bushing remains connected to the system via the good fuse. The result is still an eventual major insulation failure if the bank is not removed from service.

The burning between packs could possibly continue due to the second bushing still being energized via the second fuse. During this condition a low energy fault could be developed. The current limiting fuse still in the circuit will be getting warm while the capacitor could be boiling. Eventually the major insulation will be breached grounding the faulted capacitor through the tank to frame, and there will be a race between the capacitor and the fuse to see if the fuse will clear before the capacitor ruptures.

Proper Voltage Ratings Of Capacitor Fuses For Floating WYE Applications

It has been the normal practice to let the voltage rating of the capacitor determine the voltage rating of the fuse. For example, a 7.96KV capacitor would use a 8.3KV fuse. However, in a floating WYE bank this could be a problem. With capacitors in parallel in a floating WYE connection, as units fail the neutral will shift putting a higher voltage on the remaining capacitors in that phase. IEEE standard 18 specifies that the maximum voltage on a capacitor is 110% of it’s rating. An unbalance protection scheme can be installed to prevent damage of the remaining capacitors when an overvoltage, or unbalance condition occurs. *See appendix (2) for overvoltage calculations.

The following table demonstrates how an increase is impressed on units as capacitors fail in that phase. as the units fail the voltage neural will shift increasing the voltage on the remaining capacitors. This table was calculated at rated voltage for the capacitor units applied.

Table 1 - CAPACITOR UNBALANCE CALCULATIONS
Capacitor voltage No. of series groups No. of parallel units/group No. of failed units %volts on remaining units Neutral shift in volts* Voltage on remaining units
7200 1 4 1 109.09 654.55 7854.55
7200 1 4 2 120 1440 8640
7200 1 4 3 133.33 2400 9600
7200 1 4 4 150 3600 10800
7620 1 4 1 109.09 692.73 8312.73
7620 1 4 2 120 1524 9144
7620 1 4 3 133.33 2540 10160
7620 1 4 4 150 3810 11430
7960 1 4 1 109.09 723.64 8683.64
7960 1 4 2 120 1592 9556
7960 1 4 3 133.33 2653.33 10613.33
7960 1 4 4 150 3980 11940

NOTE

With 4 units in parallel, the failure of one unit raises the voltage on the remaining units over 109%. However, most systems can be at least 5% above their these voltage levels. Capacitor standard IEEE 18 allows 10% overvoltage . This could raise the voltage on the remaining units above 10%, and the remaining capacitors on the effected phase will also, fail.

If this over-voltage condition is not corrected, the 8.3KV fuses will be above their voltage rating due to this overvoltage. As additional units fail the voltage on the remaining fuses and capacitors will continue to increase. IEEE Std. C37.48 states for ungrounded and delta connected capacitor applications the capacitor fuse rating should be at least 1.2 times nameplate voltage rating of the capacitor. Therefore, on the systems selected above, 12,470, 13,200 and 13,800 volts, 15KV fuses should be used.

This condition would not propagate if a proper unbalance detection scheme is used. It is recommended that an unbalance detection scheme be applied on any floating bank, especially with four, or less, units in parallel. Units will fail, fuses will be stressed and damage to the bank will occur and a possible system outage could result. Unbalance detection schemes are not part of the scope of this article.


Conclusion

Good engineering practices have been developed in the past showing a minimum of four (4) capacitors should be in parallel on floating connections. The trend now uses less then four units in parallel with the development larger KVAR units. This practice could cause capacitors to rupture before the fuses clear even if the fuses are coordinated with the tank rupture curve of the capacitors. It is recommended to use four capacitors in parallel for proper applications.

Where the application, requires the design to use one unit per phase, then the bank should be connected delta with group fusing. However, the maximum clearing curve of the fuse should be checked with the tank rupture curve of the capacitor to assure proper protection.

It is recommended in all floating WYE connected capacitor applications that a proper unbalance detection scheme should be used. A common method is to connect a PT from neutral of the bank to the system ground. This can only be applied to a three phase, four wire, multi-grounded system. If the system is not grounded the best method is to connect the capacitor bank in a split WYE configuration with a CT between the neutrals.

If an unbalance scheme is not used, in a floating WYE connected bank with multiple units in parallel, capacitors and fuses will be damaged. Also, it is recommended that the fuses should have a line-to-line voltage rating.

Floating WYE connected capacitor banks are common in the industry and have been used for years. As with any technology, it will work well if applied properly.

GILBERT Electrical Systems & Products
Neal S. Ciurro - Vice President - Gilbert Electrical Systems
Stuart Edmondson - Duke Energy

References
  • Application of the Floating WYE connection, Neal Ciurro
  • IEEE Std. 18-1992.
  • IEEE Std. 1036-1992.
  • Unbalance Protection. Large Banks Reference Data R230-30-1. McGraw-Edison article June 1970.
  • Capacitor Fusing Based on Tank Rupture Curves. Reference Data R230-90-2 McGraw-Edison Article June , 1970.
  • General Capacitor Fusing Criteria Cooper Power KVAR Briefs, March 1987.
  • IEEE Std. C37.48
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