Breaker Failure Protection

The requirements for improvements in the quality of power supplied to electric utility customers result in changes of the way distribution feeder protection is designed and applied. Many protection schemes that in the past have only been used at the transmission level today are common at the distribution level. One of the reasons is that they are available as some of the numerous functions in a multifunctional distribution feeder relay. One of these schemes is the Breaker Failure Protection. Breaker failure results in prolonged exposure of industrial customers to low voltages and of electrical equipment to large short circuit currents, and may lead to damage of equipment and complete shut down of the manufacturing process. This is the reason that Breaker Failure Protection has gained popularity at the distribution level of the system. The most common Breaker Failure Protection is based on monitoring of the current in the protected circuit. After a fault is detected and the relay issues a trip signal, it will also initiate the timer of the Breaker Failure Protection function. If the breaker trips as expected, the current in all three phases will go to zero, which will reset the undercurrent element used to detect the correct breaker operation. Since at the distribution level the feeder is protected by a single relay, the Breaker Failure Protection function is usually started by a built-in protection function in the protection relay.

Fuse Saving Scheme

The problem with fuse protection of distribution transformers is that it does not allow automatic restoration of the power supply and requires a crew to be sent to the location to replace the fuse, leading to long supply interruption. Since most short circuit faults have a temporary nature, attempting to clear the fault before the fuse burns has become a standard practice in many utilities using a Fuse Saving Scheme. It uses a low set instantaneous overcurrent element to trip the breaker in the substation immediately after the fault occurs. There is no coordination of the instantaneous overcurrent element with the downstream fuses. The breaker is tripped before the fuse protecting the faulted element will start to melt. After the reclosing, the low set instantaneous element is disabled and overcurrent elements that coordinate with the downstream protective devices are used.

The advantage is that in case of a temporary fault the fuse is not going to melt, i.e. it will not require a replacement and will result in a short interruption of the load during the dead interval of the reclosing sequence. This can be very important, especially in cases where the fuse is at a remote location.

The disadvantage is that all the customers supplied from the feeder will be affected by the interruption during the reclosing cycle. That is why the decision to apply the Fuse Saving Scheme should be made based on the sensitivity of the loads to voltage sags or interruptions.

Selective Backup Tripping

Protection of distribution feeders today is commonly provided by a single multifunctional relay. If the relay fails and at the same time there is a fault, the protection is typically provided by time-delayed overcurrent elements of the transformer protection that trips the transformer breaker. This has the negative result of first, delayed operation and second, the tripping of the source breaker leading to a voltage interruption of all feeders connected to the distribution bus.

A significant improvement can be achieved by the selective backup tripping from the transformer relay. If it receives a signal that the feeder relay has failed, when a fault is detected and there is no blocking signal from any of the healthy feeder relays, the transformer relay will first send a trip signal to the breaker of the feeder with the failed relay. If the fault is on that feeder, it will be cleared, thus eliminating the need for tripping the transformer breaker and causing the voltage interruption for all feeders.

Load shedding in substations

Load shedding in substations can be executed based on several different principles. It also can be triggered by different system events and can serve different purposes. Load shedding is executed locally in the distribution substations and triggered by the drop of frequency below the pre-defined thresholds in accordance with the defensive plans. Load also can be shed in order to prevent the separation of the system. It is triggered by criteria implemented in a system integrity protection scheme (SIPS) and executed in substations when they receive commands for load shedding from the SIPS. Distributed load shedding is a relatively new concept since it requires each individual distribution feeder to be equipped with an IED that measures the frequency and can perform the load shedding function. This means also that each of the feeder relays need to have voltage inputs. Such a device also provides measurements, recording and other functions.

In case of distributed load shedding each individual relay belongs to a specific step of the load shedding system and usually has a more limited functionality compared to specialized IEDs used in centralized systems.

The fact that each feeder can be controlled by a separate step in the load shedding system with a different setting, allows the implementation of a more flexible system that will shed load closer to the requirement for balancing load and generation in the area separated after a disturbance.

Load shedding functions in multifunctional protection IEDs can be achieved using different methods and their combination in complex schemes using the programmable scheme logic that such devices typically have.