A fuseless bank has no internal or external fuses: the units, with two bushings, are connected in series strings from phase to neutral, and the protection relies on the failed element being left shorted. The healthy elements of the string absorb its voltage: with E elements in series they rise by a factor E/(E−1). That is why it is used at 34.5 kV or above, with more than 10 elements per string, under IEEE C37.99-2000.
Short answer
A fuseless bank connects the units in series strings between phase and neutral, or between two line terminals in a delta or single-phase bank. There is no internal or external fuse. IEEE C37.99-2000 (cl. 4.2.3, 4.3.3, 4.4.3 and 8.5.1) explains that the protection relies on the failed element being left shorted, and on the healthy elements of the string absorbing its voltage. It is normally used at 34.5 kV or above.
What the bank looks like
- The units normally have two bushings, with the elements insulated from the case.
- Each string is a series of units from phase to neutral. Several strings can be placed in parallel if the bank is large.
- Each unit is, inside, a series of groups of elements. The number of series elements in the string is the number of series units times the number of series element groups in each unit.
- The discharge energy is small: there are no units directly in parallel, so no group discharges into the failed unit.
A bank with a different construction, with units whose elements are connected in series, has at times been offered as fuseless. The guide warns about it: it requires a much more sensitive unbalance protection and the guide does not cover it.
What happens when an element fails
- An element fails. It is left shorted by the weld that forms at the fault point
- The healthy elements of the string take its voltage. The applied voltage is divided among fewer series elements
- The bank stays in service. Without fuses, the protection watches how many elements were lost
- Unbalance trips. When the voltage on the remaining ones is excessive
The standard states that the failure of one element in a group of parallel elements also shorts the other elements of that group (cl. 8.5.1: in the example of 10 groups of three elements, the failed element shorts itself and the two in parallel with it).
The E/(E−1) factor
If the string has E elements (or groups) in series and one is shorted, the voltage across each of the remaining ones is approximately the previous voltage multiplied by E/(E−1) (cl. 4.4.3 and 5.1.2).
Example from the standard (cl. 8.5.1). A string of six units, each with eight series groups of elements, has 6 × 8 = 48 series elements. If one fails, the remaining 47 carry 48/47 of the voltage they had: 2.1% more.
Our worked example. With the same 48 elements, if k elements are lost, the voltage on the remaining ones is 48/(48 − k) of the previous: 104.3% with 2 elements lost, 109.1% with 4 and 111.6% with 5. With 5 elements lost the 110% of rated voltage that the guide uses as the reference for taking the bank out is exceeded.
Why more than 10 series elements
The guide asks that each string normally have more than 10 series elements, so that the bank does not have to leave service because of the failure of a single element (cl. 4.3.3 and 5.1.2). This can be checked with the formula: with 10 elements, the factor is 10/9, that is 111%, above 110%. With 11 elements it drops to 110%. This condition is only achieved at high system voltages, and it is the reason the fuseless design is not normally applied below about 34.5 kV.
If the current through the string exceeds the continuous capability of the unit, the guide says to add strings in parallel (cl. 5.1.3).
Effect on the protection
- No coordination with fuses. Unbalance can trip with a short delay, because there are no fuses to coordinate with (cl. 4.2.3 and 5.1.3).
- The signal indicates how many elements failed in the string. It does not indicate which ones. The location is determined by measuring the capacitance of the strings pointed out (cl. 8.5.1).
- Calculation. It is done with the number of failed elements (element groups) and the total number of series elements. If the manufacturer does not give the number of series elements per unit, the guide suggests estimating it by dividing the unit voltage by 2,400 V and rounding up, because each element typically has between 1,800 and 2,400 V. A 7,960 V unit would have about 4 series elements.
The estimate serves for bank protection, and it does not replace the nameplate or manufacturer data sheet value when available.
The calculation of the unbalance signal for this design is in unbalance calculation for fuseless and unfused banks, and the readings that can mislead the relay in ambiguous indications and faults that unbalance cannot see.
Difference from the unfused bank
The unfused bank has no fuses either, but it connects the units in parallel groups placed in series, the conventional arrangement. There the voltage on the remaining elements rises more than in the fuseless design, because there are fewer series elements between phase and neutral. The discharge energy is also higher than in the fuseless design, though modest, because there are units directly in parallel.
The other fuse-free design is covered in unfused bank.
When it is chosen
The guide presents it as suitable for banks of 34.5 kV or above, with an advantage in the small discharge energy and in the short unbalance delay. On the other side, unbalance protection is the only way to know that elements have failed, and the voltage on the remaining ones rises with every element lost, with no fuse to isolate it. To compare it with the other types, see capacitor bank protection and capacitor bank unbalance protection.
The connection (wye, double wye or H) that goes with each design is compared in bank connection: grounded wye, ungrounded wye or delta.
What we do
The number of series elements, the string arrangement and the unbalance signal are defined with the manufacturer’s actual data. At Tiza Group we resolve it in the bank’s engineering design and studies and quote the study according to the scope.
Related guides
- IEEE C37.99: shunt capacitor bank protection guide
- Unfused capacitor bank: series-parallel without fuses
- Unbalance calculation in fuseless and unfused banks
- How to size a capacitor bank
- Capacitor bank trips
References
- IEEE Std C37.99-2000, IEEE Guide for the Protection of Shunt Capacitor Banks: cl. 4.2.3, 4.3.3, 4.4.3, 5.1.2, 5.1.3, 7.1 and 8.5.1 (Figures 32 and 33).
Frequently asked questions
Why is a fuseless bank not used below about 34.5 kV?
Because it needs more than 10 elements in series in each string. With fewer, the failure of one element would raise the voltage on the remaining ones too much (E/(E−1)) and force the bank out of service for a single failure. At low voltages there is no room for that many series units.
What happens when an element fails in a fuseless bank?
It is left shorted by the weld that forms at the fault point, and its voltage is shared among the healthy elements of the series string. The bank stays in service until unbalance indicates that the voltage on the remaining ones is already excessive.
How are the failed elements located?
Unbalance indicates how many elements failed in a string. The location is determined by measuring the capacitance of the strings the protection points to.