Zhejiang Rockwill Energy Technology Co., Ltd.

Sometimes, UFCL-limiter is the only solution

As shown in Figure 1 below, the UFCL-limiter is installed in the bus-tie section and is series-connected to the bus coupling circuit-breaker (CB). In the event of a short-circuit in the outgoing feeder, the prospective short-circuit current flowing through the outgoing feeder CB (Ik") may reach 80kArms, which is equivalent to a peak current of 200kAp. This exceeds the ratings of the CB (40kArms and 100kAp). In other words, the CB is unable to provide protection against this high peak short-circuit current and the operation speed of the CB is too slow. This will lead to serious mechanical and thermal stress and eventually equipment failure.

However, thanks to the high operation speed and current limiting capabilities of the UFCL-limiter, it is possible to resolve this issue without upgrading all the equipment in the system. By installing the UFCL-limiter on the strategic position of the bus-tie, the short-circuit current i2 contributed by T2 is limited at the rise of the first cycle and interrupted before the prospective current i2 reaches its peak. The total (peak) short-circuit current flowing through the CB of the fault circuit is then kept below 100kAp (i1 + i2 <100 kAp), which is the rated peak withstand current of the CB. Therefore, the CB can withstand the fault current and trip to clear the fault safely.

In comparison with complex conventional solutions, the UFCL-limiter has both technical and economic advantages when used in transformer or generator feeders, in switchgear sectionalizing and connected in parallel with reactors. There is no need for customers to upgrade all the switchgear, bus-bars cables, etc.

The advantages of the use of a UFCL-limiter in a network are:

• Reduction of the short-circuit current of the system (compared to the short-circuit current with closed tie circuit breaker)

• Reduction of voltage sags and flicker due to the lower total source impedance

• Reduction of harmonics due to the lower total source impedance

• Higher system availability due to the parallel connection of the feeding generators and transformers

• Higher loads possible in a sub-system (higher than the ratings of the feeding generators and transformers in that sub-system)