Bus Protection Relay

June 22, 2024 Luciano Bertene

The main function of a bus protection relay is to quickly detect fault currents occurring within the bus system. These include short circuits, ground faults, overcurrents and other abnormal electrical conditions. When the relay detects a fault, it quickly trips protective devices, such as circuit breakers, to isolate the faulty section from the rest of the system. This immediate response helps prevent the fault from spreading and causing further damage, ensuring the safety and reliability of the entire power distribution network.

Kirchhoff's current bus protection law

Bus protection relays are essentially based on Kirchhoff's current law, which states that the help of current supplied to any node must be equal to the help of currents flowing through it.

The figure above shows the current direction of two devices connected to a similar bus in the normal state (without faults). In this illustration, one of the devices acts as the bus power and the other acts as the load. They have the same current intensity and opposite direction. In this case, the current difference through the differential relay is zero (i.e. zero) and the relay is stable and does not work.

Principle and functionality Differential relay

The principle of differential protection is based on the concept that the sum of the currents entering a protected zone must be equal to the sum of the currents leaving this zone. This principle is derived from Kirchhoff's current law, which states that the algebraic sum of currents at a node in a circuit is zero.

External faults in the bus protection relay

How Bus Protection Relays Work During External Faults: Bus protection relays use several techniques to effectively detect and respond to external faults. Its main principle is the measurement of current and voltage signals within the protected zone. When an external fault occurs, currents and voltages within the bus are affected, providing signals that can trigger the relay's protective functions.

Current measurement

Bus protection relays continuously monitor currents entering and leaving the bus. In the event of an external fault, an imbalance in these currents is detected. The relay compares the strength and direction of winds and initiates protective measures if the imbalance exceeds a predefined threshold.

Voltage measurement

Stress measurements are also crucial for detecting external faults. Bus protection relays monitor voltage levels at the bus terminals. An abrupt change or drop in voltage indicates the appearance of a superficial scar. The relay analyzes the magnitude and duration of the voltage disturbance to confirm the fault and trigger the appropriate protection scheme.

Fault location and isolation

Once an external fault is detected, bus protection relays use fault location algorithms to determine the location of the fault in the power system. This information helps isolate the faulty section from the rest of the network by opening the appropriate circuit breakers. By quickly isolating the faulted area, the relay helps minimize the impact of the fault and facilitates restoration of unaffected parts of the system.

This case is comparable to the permeable healthy point. Here the two currents can reach very high values, but still have the same strength and opposite direction. Therefore, the relay does not work due to zero current in the differential relay coil as shown above.

Internal error in the bus protection relay

Busbar protection relays are important components of electrical power systems that detect and correct internal busbar faults. This article examines the nature of internal bus defects and how protective relays effectively detect and respond to them.

Understanding internal faults: Internal bus faults refer to defects that occur within the protected zone of the bus itself. These errors can occur for a variety of reasons, such as: B. due to insulation breakdown, short circuits between conductors or device failures in the bus system. Internal defects pose a significant risk to the bus and the entire power system. Its timely detection and isolation are crucial to prevent damage and ensure system stability.

In the case of internal error shown in the figure, there are two expected situations:

The non-radial system, i.e. I2P, has a non-zero value. In this case, the differential current is sufficient (I1S+I2S), which is enough to operate the relay and connect all devices.

The redial system, so I2P = 0

In this case, the differential current I1S is sufficient and this value is also sufficient to operate the relay and trip all devices connected to the bus.

Station automation theory

Note:- All equipment current transformers must have a constant transformer size ratio. If completely different conditions exist, suitable current transformers must be used to compensate for these differences. These adjustments can be made internally in the relay (tap) or externally through similar current transformers.

There are different types of bus differential protection relays. Relays with low impedance and high electrical resistance are also considered. There are some differences between them. However, this is not taken into account in this study.

Station automation components

IEDs are intelligent devices that perform various functions in a substation. These devices include protective relays, measuring devices, and programmable logic controllers (PLCs). IEDs are equipped with sensors and communication functions to collect and transmit data to the substation automation system.

Communication networks:

Communication networks are crucial for substation automation as they facilitate the exchange of data between different IEDs and the central control system. These networks can be based on protocols such as Ethernet, IEC 61850, DNP3 or Modbus. They enable real-time monitoring, control and coordination of devices within the substation.

Central Control System

The central control system serves as the brain of station automation and is responsible for monitoring, analyzing and controlling station operations. It collects data from IEDs, performs data processing and analysis, and initiates appropriate actions based on predefined logic and algorithms. The control system provides operators with a user interface to view and interact with station components and data.

Conclusion

Additionally, bus protection relays provide valuable diagnostic information that helps identify and correct faults. By monitoring and analyzing electrical parameters, operators can quickly determine the location and nature of faults, enabling efficient maintenance and repair work. This proactive approach not only improves system reliability, but also reduces maintenance costs and improves the overall performance of the bus system.