In an internally fused capacitor bank, each element of the unit has a fuse link in series that isolates it when it fails. The unit has many elements in parallel and few series groups, and the bank uses few parallel units and more series groups than an externally fused one. Because a blown link cannot be seen, IEEE C37.99-2000 requires unbalance protection to detect the lost elements.
Short answer
In a bank with internal fuses (internally fused), the fuse is not outside but inside the unit, in series with each element. If an element fails, the current through its link rises sharply, the link blows and the element is isolated. The unit stays in service with one element fewer. IEEE C37.99-2000 (cl. 4.2.2, 4.3.2, 4.4.2, 7.1.3 and 8.4.1) points to two design consequences: the bank uses few parallel units and more series groups, and unbalance protection is mandatory because a blown link cannot be seen.
What a unit with internal fuses looks like
The construction is the opposite of the externally fused unit:
| Unit of an externally fused bank | Internally fused unit | |
|---|---|---|
| Series groups inside the unit | Many | Few |
| Parallel elements per group | Few | Many |
| Fuse | One per unit, outside | One link per element, inside |
| Unit size | Smaller | Normally large: a complete unit is not expected to fail |
An element is the set of two electrodes separated by a dielectric, that is, the smallest part of the capacitor. Units with internal fuses are normally large because a complete unit is not expected to fail.
- An element fails. Its current rises far above normal
- The link of that element blows. It happens in a few cycles, which reduces the risk of successive failures
- The group is left with one element fewer. The remaining elements of the group take more voltage
- The unit stays in service. There is no visible indication; unbalance detects the change
How the bank design changes
Compared with an externally fused bank, the internally fused one uses fewer parallel units and more series groups (cl. 4.2.2). Criteria of clause 4.3.2:
- Higher-power units. They reduce the number of units, simplify the structure and allow the largest number of failed elements before unbalance has to trip.
- At least two parallel units per series group. If many internal fuses operate in one unit, the other unit helps keep the voltage low at the terminals of the affected one.
- Maximum parallel units. Set by the parallel-energy capability of the internal fuses. If there are too many parallel units, the fuse may not operate properly. The manufacturer must state how many units can be placed directly in parallel.
- If there is too much parallel energy, the number of series groups is changed or the bank is reorganized into several wyes (cl. 5.1.2: the energy of the parallel group must not exceed what the internal fuses can interrupt).
With the fuse outside, the design is different; compare it with the externally fused bank. The fuse type of each design is in capacitor fuses: expulsion or current-limiting.
What the internal fuse must meet
Clause 7.1.3 requires the link to isolate the failed element properly and to withstand, without operating, conditions that are not a fault. It must be able to interrupt overvoltages, including system transients and the higher voltage left on the unit by fuses already blown. It must withstand external faults and inrush and outrush currents without operating throughout the life of the unit.
The current-limiting behavior of the link depends on the available fault current, on the energy discharged by the parallel elements (of the order of 1 kJ) and on the voltage across the element at the instant of the fault. The standard asks for the fastest fuse that meets those requirements.
Why unbalance is mandatory
No unit of this type shows at a glance that it has lost elements (cl. 5.3). Unbalance protection is the way to know. The guide repeats it at three points:
- In a grounded-wye bank with internal fuses and one unit per phase, unbalance protects against excessive voltage on the elements and trips on severe unbalances (cl. 5.1.1).
- In a delta bank, unbalance serves to detect the unit with failed elements (cl. 5.3).
- The signal indicates the total number of failed elements inside a unit. The exact number is only determined by measuring all the units of the bank, which is recommended when there is an alarm or trip (cl. 8.4.1).
The unbalance setting must consider the capability of the internal fuses, the transient overvoltage the elements withstand, and the consequences of a fault to the case or of a link that fails to operate, in addition to the rule of not exposing healthy units to more than 110% of their rated voltage.
Our worked example. Take a unit with 14 parallel elements per group and 3 series groups (the dimensions of the guide’s diagram), with constant terminal voltage. If links blow in the same group, the voltage on the elements of that group rises relative to normal as follows: with 1 link blown, 3·14 / (14 + 2·13) = 105%; with 2, 3·14 / (14 + 2·12) = 110.5%. This is a simple voltage division: in the real bank, the voltage at the unit terminals also rises and adds to this effect.
The expected signal when an internal fuse blows is calculated as shown in unbalance calculation with internal fuses.
Advantages and limits
Advantages. The link isolates an element in a few cycles, which reduces the risk of successive failures. A small bank can use large units and a simple structure (cl. 5.1.2).
Limits. Without a visible signal, detection depends entirely on the protection and its calibration. Healthy elements are left with more voltage. And the number of parallel units is bounded by the energy the link can interrupt.
To see unbalance protection in detail, read the capacitor bank unbalance protection. The full overview is in capacitor bank protection, and the equipment and its tests in capacitor bank.
The voltage limit on the elements that stay in service is explained in capacitor unit limits: 110% voltage and 135% power.
What we do
Deciding how many elements can be lost before alarming and before tripping depends on the exact design of the unit the manufacturer supplies. At Tiza Group we define it within 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
- Externally fused capacitor bank: design and limits
- Unbalance calculation in an internally fused bank
- Capacitor bank trips
- Medium-voltage fuse
References
- IEEE Std C37.99-2000, IEEE Guide for the Protection of Shunt Capacitor Banks: cl. 4.2.2, 4.3.2, 4.4.2, 5.1.1, 5.1.2, 5.3, 7.1, 7.1.3, 8.4.1 and Figure 23.
Frequently asked questions
How do you know an internal fuse has blown?
It cannot be seen from outside. The link is inside the unit, so the warning comes from the unbalance protection, which detects the change in capacitance. The actual number of lost elements is only known by measuring the capacitance of the units.
Why must there be at least two parallel units per group?
Because if one unit loses many elements, the other parallel unit helps keep the voltage low at the terminals of the affected unit. IEEE C37.99 describes this as desirable in 4.3.2.
Does an internally fused bank need unbalance protection if it is connected in delta?
Yes. The guide states that, if the units have internal fuses, unbalance detection is needed to find a capacitor with failed elements, because there is no visible indication of a blown fuse.