October 18, 2004
Protection of Distribution Pole-Top Capacitor Banks.
This is the first in a series of five articles on the protection of distribution pole-top capacitor banks. This article will describe how capacitor units fail, and the current and voltage escalations that occur during the capacitor-unit failure process.
Future articles will discuss how to select the ampere rating and speed characteristic of the capacitor bank fuse to:
- withstand energizing inrush currents (Unit 2);
- accommodate anticipated capacitor bank current and operate quickly in response to an evolving capacitor unit failure (Unit 3);
- protect capacitor units against case rupture (Unit 4); and
- withstand transient outrush currents (Unit 5).
General
The function of a capacitor bank fuse is, in general, to provide system protection as well as capacitor bank protection. With respect to system protection, the capacitor bank fuse should detect a potentially damaging overcurrent condition and operate promptly to isolate the phase leg containing the faulted capacitor unit, thereby avoiding an unnecessary outage of the entire feeder. For capacitor bank protection, the fuse should operate promptly to remove the phase leg containing the faulted capacitor unit from the circuit before the unit’s case ruptures, spilling dielectric fluid which may ignite and burn, and possibly damage adjacent capacitor units or other equipment. To best achieve these objectives, the capacitor bank fuse should operate as promptly as possible in response to an evolving capacitor-unit failure within the capacitor bank, while avoiding needless operation in response to transient inrush and outrush currents.
Capacitor-Unit Failure Mode
Capacitor units employed in overhead distribution capacitor banks typically consist of a number of series groups of parallel-connected packs. Regardless of the internal construction and dielectric materials used (i.e., paper-film or all-film), capacitor-unit failure usually begins with the dielectric breakdown of one pack, which in effect shorts out that series group of packs (see Figure 1). As a result, the voltage across each of the remaining series groups of packs increases by a factor related to the total number of series groups of packs in the capacitor unit and to the capacitor bank connection. The capacitor unit current increases by this same factor. The increased voltage across each of the remaining series groups of packs will eventually lead to failure of another pack, shorting out another series group of packs and causing a new step-wise increase in voltage across each of the remaining groups, and an increase in the current through them. If left to continue, this process will result in all of the series groups of packs being shorted, at which time the faulted capacitor-unit current will escalate to the available phase-to-ground fault-current level in a grounded-wye connected capacitor bank, to three times the pre-failure phase current in an ungrounded-wye connected capacitor bank, or to the available phase-to-phase fault-current level in a delta connected capacitor bank. The faulted capacitor unit’s case may or may not rupture before the last series group fails, but certainly the probability of case rupture increases with the number of series groups of packs shorted. The actual probability of case rupture, however, cannot be determined in absolute terms because the case-rupture phenomenon is, of itself, extremely difficult to quantify — particularly in the early stages of the failure process when the faulted capacitor-unit current is relatively low. Moreover, capacitor units of different manufacture and design respond differently to evolving series group failures. For example, the older paper-film dielectric capacitors tend to arc and produce gas when a pack is shorted. If allowed to persist long enough, this arcing may produce sufficient pressure to rupture the faulted unit’s case. Good field experience with bank fusing of these units, however, tends to suggest that case ruptures due to this cause do not occur very often. It has been suggested that, because of arcing, the failure of subsequent series groups of packs occurs fairly quickly, resulting in sufficient phase current escalation to operate the bank fuse before the gas buildup actually ruptures the faulted unit’s case.
Figure 1.
Schematic representation of a typical high-voltage capacitor unit.
The newer all-film dielectric capacitors, by comparison, do not arc and produce gas nearly as much as paper-film capacitors when a series group of packs is shorted, since a puncture in the dielectric film tends to result in a non-arcing contact between the shorted foils. While case ruptures due to internal gassing are thus unlikely, ruptures can nevertheless occur due to other causes. Because of the reduced arcing and gassing, the escalating failure process takes place much more slowly than for paper-film capacitors, with the result that the increased current flow can lead to increased temperature and pressure inside the faulted unit’s case, which can ultimately increase to the point where the case ruptures. Such ruptures, however, are usually nonviolent in nature, with damage typically limited to a cracked seam or bushing seal.
As will be discussed later, the ampere rating and speed characteristic of the capacitor bank fuse should be selected to isolate the phase containing the failing capacitor unit as promptly as possible, that is, with the fewest series groups of packs shorted. In making this selection, the capacitor bank fuse time-current characteristics should be compared with capacitor-unit case-rupture curves, which are published by the various capacitor manufacturers, and which illustrate the probability of case rupture for various time and current relationships. In so doing, it is necessary to consider that the capacitor bank fuse responds to phase current, which may or may not be the same as the current flowing in the capacitor units themselves, depending on the capacitor bank connection and configuration.
Current and Voltage Escalation Due to Evolving Capacitor-Unit Failure
For wye-connected capacitor banks having only a single capacitor unit in each phase, the phase current and the capacitor-unit current are, of course, the same. If, however, the capacitor bank is wye-connected with multiple capacitor units in parallel in each phase or is delta connected, the phase current and the capacitor unit current are not the same. Moreover, the phase current and the capacitor-unit current escalate at different rates depending on the number of capacitor units in parallel in each phase and on the capacitor bank connection. In order to determine how well the capacitor bank fuse protects the individual capacitor units against case rupture, the escalating phase currents and capacitor-unitcurrents must be known.
One of the parameters used in calculating the escalating phase currents and capacitor-unit currents is the number of series groups of parallel-connected packs in the capacitor units under consideration. Unfortunately, the number of series groups of packs in high-voltage capacitor units is not published and varies from manufacturer to manufacturer. However, it is generally the case that capacitor units are designed with the fewest number of series groups of packs possible, and that the voltage across any given series group (in a fully functional capacitor unit) is limited to between 1 kV and 2.5 kV. Accordingly, in the discussion which follows, a total of three different numbers of series groups of packs will be assumed for purposes of illustration, on the basis that the voltage across any individual series group is within the voltage limits specified above. For example, a 13.8-kV wye-connected capacitor bank will employ 7.97-kV capacitor units, having either 4, 5, or 6 series groups of parallel connected packs, with 1.99 kV, 1.59 kV, or 1.33 kV respectively, normally impressed across each series group. Similarly, a 13.8-kV delta-connected capacitor bank will employ 13.8-kV capacitor units, having either 6, 7, or 8 series groups of parallel-connected packs. Escalating phase currents and capacitor-unit currents are described individually in the following sections for grounded-wye, ungrounded-wye and delta-connected overhead distribution capacitor banks.
Grounded-Wye Connected Capacitor Banks.
When a series group of parallel-connected packs is shorted, the current through the faulted capacitor unit escalates to 
per unit, where n is the total number of series groups of parallel-connected packs and x is
the number of series groups shorted. In similar fashion, the phase current escalates to 
per unit, where m is the number of capacitor units connected in parallel in each phase. Using the above formulas, the current escalation in the faulted capacitor unit and in the bank fuse can be calculated as a function of the number of series-connected groups and of the number of groups shorted. Per-unit current escalation values are shown in Table 1 for a typical grounded-wye connected overhead distribution capacitor bank utilizing two capacitor units connected in parallel per phase. As noted earlier, when all of the series groups of packs are shorted, the currents escalate to the available phase-to-ground fault-current level, as is indicated by the symbol 
in Table 1.
| Number of Series Groups of Packs Shorted |
Current, Per Unit |
|||||
|---|---|---|---|---|---|---|
| 4 Series Groups of Packs | 5 Series Groups of Packs | 6 Series Groups of Packs | ||||
| Faulted Capacitor Unit |
Bank Fuse (Phase Current) |
Faulted Capacitor Unit |
Bank Fuse (Phase Current) |
Faulted Capacitor Unit |
Bank Fuse (Phase Current) |
|
| 1 | 1.33 | 1.17 | 1.25 | 1.13 | 1.2 | 1.1 |
| 2 | 2.0 | 1.5 | 1.67 | 1.33 | 1.5 | 1.25 |
| 3 | 4.0 | 2.5 | 2.5 | 1.75 | 2.0 | 1.5 |
| 4 | |
|
5.0 | 3.0 | 3.0 | 2.0 |
| 5 | — | — | |
|
6.0 | 3.5 |
| 6 | — | — | — | — | |
|
One per-unit nominal capacitor current is 25.1 amperes; one per-unit nominal phase current is 50.2 amperes.
Note that the per unit phase current is less in each case than the per-unit capacitor current, until the faulted unit is fully shorted. The “diluting” effect becomes more pronounced with increasing numbers of capacitor units connected in parallel per phase, as is clearly shown in Table 2. For this reason, capacitor bank fusing is generally most effective when the smallest number of capacitor units per phase are used — a very practical concept today with the availability of 200- and 300-kVAR (single-phase) capacitor units.
| Number of Series Groups of Packs Shorted |
Phase Current, Per Unit | ||
|---|---|---|---|
| 200-kVAR Units (2 per phase) |
100-kVAR Units (4 per phase) |
50-kVAR Units (8 per phase) |
|
| 1 | 1.13 | 1.06 | 1.03 |
| 2 | 1.33 | 1.17 | 1.08 |
| 3 | 1.75 | 1.38 | 1.19 |
| 4 | 3.0 | 2.0 | 1.5 |
| 5 | |
|
|
Based on capacitor units having 5 series group of packs.
Ungrounded-Wye Connected Capacitor Banks.
In ungrounded-wye connected capacitor banks, both the faulted capacitor-unit current and the phase current escalate step-wise as increasing numbers of series groups of packs become shorted, just as happens in grounded-wye connected capacitor banks. Unlike the situation with grounded banks, however, the voltages and currents in the unfaulted phases of an ungrounded-wye connected bank also increase as series-group failure steps occur. Table 3 shows escalating per-unit current values in the faulted capacitor unit and in the bank fuse, and escalating per-unit voltage values in the unfaulted phases for a typical ungrounded-wye connected overhead distribution capacitor bank utilizing two capacitor units connected in parallel per phase.
| Number of Series Groups of Packs Shorted |
Current and Voltage, Per Unit |
||||||||
|---|---|---|---|---|---|---|---|---|---|
| 4 Series Groups of Packs | 5 Series Groups of Packs | 6 Series Groups of Packs | |||||||
| Current | Voltage Unfaulted Phases |
Current | Voltage Unfaulted Phases |
Current | Voltage Unfaulted Phases |
||||
| Faulted Capacitor Unit |
Bank Fuse (Phase Current) |
Faulted Capacitor Unit |
Bank Fuse (Phase Current) |
Faulted Capacitor Unit |
Bank Fuse (Phase Current) |
||||
| 1 | 1.26 | 1.11 | 1.03 | 1.2 | 1.08 | 1.02 | 1.16 | 1.06 | 1.02 |
| 2 | 1.72 | 1.29 | 1.08 | 1.5 | 1.2 | 1.05 | 1.39 | 1.16 | 1.04 |
| 3 | 2.67 | 1.67 | 1.2 | 2.0 | 1.4 | 1.11 | 1.72 | 1.29 | 1.08 |
| 4 | 6.0 | 3.0 | 1.73 | 3.0 | 1.8 | 1.25 | 2.25 | 1.5 | 1.15 |
| 5 | 6.0 | 3.0 | 1.73 | 3.28 | 1.91 | 1.29 | |||
| 6 | 6.0 | 3.0 | 1.73 | ||||||
One per-unit nominal capacitor current is 25.1 amperes; one per-unit nominal phase current is 50.2 amperes, and one per-unit nominal voltage is 7.97 kV.
A comparison between Tables 1 and 3 shows that the step-wise current escalation in ungrounded banks is less than that in grounded banks with an equal number of series groups of packs shorted. Table 3 also indicates that when all of the series groups in a capacitor unit are shorted, the phase current escalates to only three times the pre-failure phase current, but the voltage across each unfaulted phase escalates to 1.73 times normal. This overvoltage can lead to capacitor-unit failures in the unfaulted phases of the bank if allowed to persist for a time duration greater than that permitted. A curve illustrating the relationship between permissible capacitor-unit overvoltage and time is shown in Figure 2.
Figure 2.
Capacitor-unit power-frequency overvoltage versus time in seconds permitted
in ANSI/IEEE Std. 18, IEEE Standard for Shunt Power Capacitors. Note: This
curve applies for up to 300 applications of power-frequency overvoltages of
the magnitudes and durations illustrated. Capacitor manufacturers may publish
curves applicable to their particular units.
Because of the relatively small phase current escalation, capacitor bank fuses may not always operate fast enough to prevent overvoltage-induced damage to capacitor units in the unfaulted phases of an ungrounded-wye connected capacitor bank. Protection against such damage, however, can be provided by means of relay schemes employed to detect the voltage increase at the capacitor bank neutral (with respect to ground) that results during the capacitor-unit failure process, and initiate isolation of the bank.
Delta-Connected Capacitor Banks.
In delta-connected capacitor banks, faulted capacitor-unit current and phase current escalation is similar (but not identical) to that in grounded-wye connected capacitor banks. Furthermore, the voltages and currents in the unfaulted phases of a delta-connected capacitor bank do not change with an evolving capacitor-unit failure in another phase. Table 4 shows escalating per-unit current values in the faulted capacitor unit and in the bank fuses for a typical delta-connected overhead distribution capacitor bank utilizing two capacitor units connected in parallel per phase. As noted earlier, when all of the series groups of packs are shorted, the currents escalate to the available phase-to-phase fault-current level, as is indicated by the symbol 
in Table 4.
| Number of Series Groups of Packs Shorted |
Current, Per Unit |
|||||
|---|---|---|---|---|---|---|
| 6 Series Groups of Packs | 7 Series Groups of Packs | 8 Series Groups of Packs | ||||
| Faulted Capacitor Unit |
Bank Fuse (Phase Current) |
Faulted Capacitor Unit |
Bank Fuse (Phase Current) |
Faulted Capacitor Unit |
Bank Fuse (Phase Current) |
|
| 1 | 1.2 | 1.05 | 1.17 | 1.04 | 1.14 | 1.04 |
| 2 | 1.5 | 1.13 | 1.4 | 1.1 | 1.33 | 1.09 |
| 3 | 2.0 | 1.26 | 1.75 | 1.19 | 1.6 | 1.15 |
| 4 | 3.0 | 1.53 | 2.33 | 1.35 | 2.0 | 1.26 |
| 5 | 6.0 | 2.36 | 3.5 | 1.67 | 2.67 | 1.44 |
| 6 | |
|
7.0 | 2.65 | 4.0 | 1.8 |
| 7 | — | — | |
|
8.0 | 2.93 |
| 8 | — | — | — | — | |
|
One per-unit nominal capacitor current is 14.5 amperes; one per-unit nominal phase current is 50.2 amperes.
The next article will review the application principles that must be considered when selecting the capacitor bank fuse (e.g., ampere rating and speed characteristic) and how to confirm that the bank fuse will not operate due to energizing inrush currents. Go to Unit 2.