An unfused bank has its units in a series-parallel connection with no internal or external fuse. When an element shorts, the voltage on the remaining elements rises more than in a fuseless bank, and it can cause a cascading failure under system overvoltages. That is why IEEE C37.99-2000 asks for protection such that the healthy units do not exceed 110% of their rated voltage or the level the manufacturer recommends.
Short answer
An unfused bank uses a series-parallel connection of the units, like an externally fused bank, but with no internal or external fuses. When an element shorts, the voltage on the remaining elements of that unit rises more than in a fuseless bank. For that reason IEEE C37.99-2000 (cl. 4.2.4, 4.3.4, 4.4.4 and 8.6.1) asks for an unbalance protection that does not expose the healthy units to more than 110% of their rated voltage, nor to the level the manufacturer recommends.
What it is and when it is used
The guide defines an unfused bank as any bank without fuses, internal or external (cl. 3.24), and the unfused unit as a unit without internal fuses (cl. 3.25). It is distinguished from the fuseless bank by how the units are connected:
| Fuseless | Unfused | |
|---|---|---|
| Arrangement | Strings of units in series, from phase to neutral | Groups of parallel units, connected in series |
| Voltage of use | High: 34.5 kV or above | Normally below 34.5 kV, or at higher voltages with low parallel energy |
| Voltage on the remaining elements | Rises little | Rises more |
The guide explains the use: the unfused design is normally applied below 34.5 kV, where series strings are not practical, or in higher-voltage banks with modest parallel energy. It does not require as many parallel units as an externally fused bank (cl. 4.2.4).
These banks are normally of modest size. That is why neither fuses nor subdividing the bank into several strings are needed to limit the energy into an internal fault (cl. 8.6.1).
Its fuse-free neighbor, with series strings of elements, is the fuseless bank.
What happens to the voltage
Since there is no fuse, a failed element is left shorted and its voltage is shared among the element groups remaining in series inside the unit. The guide explains that this overvoltage must be taken into account in the protection, in addition to protection against external arcing and the 110% rule for healthy units.
Our worked example. A unit with 4 series element groups (the guide uses as an example a 7,960 V unit, which it estimates at about 4 series elements) is left, after one group shorts, with 3 groups sharing the unit voltage. Each goes from 1/4 to 1/3 of that voltage, that is 4/3 ≈ 133% of what it had. This is a maximum value, because in the real bank the voltage across the parallel group of units drops somewhat as its capacitance increases. Even so, it clearly exceeds 110%. In a fuseless bank of 48 series elements, the same failure raises the voltage by 2.1%.
The 110% limit on healthy units is in capacitor unit limits: 110% voltage and 135% power.
Why the risk is a cascading failure
With the remaining elements already stressed, a system overvoltage can start a violent cascading failure (cl. 8.6.1). Clause 4.3.4 turns this into the trip criterion: the bank must come out when
- the voltage on the healthy units exceeds 110% of rated, or
- the overvoltage on the remaining elements of a failed unit is so high that a cascading failure under a transient system overvoltage is likely.
In small banks, the guide allows the trip to be based on the failure of a single element, because the probability of a failure is low when there are few elements.
What unbalance measures in an unfused bank
The unbalance signal indicates the number of failed element groups inside a unit. To know how many elements failed in the whole bank, all the series groups must be measured. The guide recommends doing it only when there is an alarm or trip, and leaves it to maintenance judgment whether it is part of the regular program.
To calculate the overvoltage on the remaining groups, you need to know how many element groups each unit has. The manufacturer usually gives it on the nameplate or data sheet. If not, the guide allows estimating it by dividing the unit voltage by 2,400 V and rounding up. For a 7,960 V unit, 7,960 ÷ 2,400 = 3.31, which gives 4. If the unit had 5, the difference in the signals would be small and a setting made with 4 would still protect.
The count of remaining elements is shown in unbalance calculation for fuseless and unfused banks.
Limits of unbalance protection
Unbalance protection does not act fast enough to prevent a serious failure caused by the high system short-circuit currents inside a unit (cl. 8.1). In a grounded-wye bank with a single series group per phase, or with the cases grounded, a fault inside a unit can carry system short-circuit current. For those cases the guide notes that an external fuse may be advisable.
Nor does it detect a defective connection. That fault can grow until it produces internal pressure and a unit rupture before unbalance operates (cl. 7.1.4).
Other faults this scheme does not see are grouped in ambiguous indications and faults that unbalance cannot see.
What we do
Deciding whether a bank can be unfused depends on the system voltage, the bank size and the margins of each unit. At Tiza Group we define it within the bank’s engineering design and studies: we compare the bank types with your data and quote the study according to the scope.
For the protection overview, see capacitor bank protection and the capacitor bank unbalance protection.
Related guides
- IEEE C37.99: shunt capacitor bank protection guide
- Fuseless capacitor bank: series strings and unbalance
- Unbalance calculation in fuseless and unfused banks
- Capacitor bank trips
- Capacitor bank: equipment, parts and configurations
References
- IEEE Std C37.99-2000, IEEE Guide for the Protection of Shunt Capacitor Banks: cl. 3.24, 3.25, 4.2.4, 4.3.4, 4.4.4, 5.1.1, 7.1.4, 8.1 and 8.6.1.
Frequently asked questions
What is the difference between a fuseless and an unfused bank?
Both lack fuses. The fuseless bank connects the units in series strings from phase to neutral, with more than 10 series elements, and is used from about 34.5 kV. The unfused bank uses the conventional arrangement of groups of parallel units connected in series, and is normally used below 34.5 kV.
Why can an unfused bank trip on a single failed element?
Because in small banks there are few series elements, and the failure of one raises the voltage on the remaining ones a lot. In the smallest banks the guide says the trip can be based on the failure of a single element.
How do you know how many elements have failed?
Unbalance indicates the number of failed element groups inside a unit. The actual number in the whole bank is only known by measuring all the series groups, and doing so is recommended when there is an alarm or a trip.