Protection schemes for commercial systems play a crucial role in ensuring the reliability and safety of electrical power networks. These schemes are designed to identify and isolate faults occurring within the network, preventing damage to equipment and minimizing any disruptions to the supply of power. In this overview, we will discuss the key elements of protection schemes for commercial systems, emphasizing their application in high-voltage transmission and distribution networks.
To understand protection schemes, we first need to appreciate the concept of faults. A fault refers to any abnormal condition in an electrical system that can result in current surges or abnormal voltage levels. Faults can occur due to various reasons, such as short circuits, ground faults, or equipment failures.
Relay protection is the primary means of detecting and isolating faults in commercial systems. Protective relays are devices that monitor electrical quantities, such as current, voltage, and frequency, and actuate circuit breakers to isolate faulty sections of the network. These relays operate based on predetermined settings and functions, which are optimized for the specific requirements of the network.
One common protection scheme used in commercial systems is the overcurrent protection scheme. This scheme utilizes current relays, which monitor the current flow in each section of the network. If the current exceeds a pre-defined threshold, indicating a fault, the relay will actuate the circuit breakers to isolate the faulty section. Overcurrent protection schemes are effective in detecting faults caused by short circuits and excessive loads.
Another essential protection scheme is the differential protection scheme. This scheme utilizes current transformers (CTs) to compare the currents entering and leaving a particular section of the network. The principle behind differential protection is that the sum of currents entering and leaving a section should be zero, assuming no fault exists. If a fault occurs within the protected zone, the currents will become unbalanced, and the differential relay will initiate the isolation of the faulty section. Differential protection schemes are particularly suitable for protecting transformers, generators, and motors.
Ground fault protection is also critical in commercial systems. Ground faults occur when a conducting path comes in contact with the earth, resulting in an abnormal current flow. Ground fault protection schemes use current transformers and ground fault relays to detect the presence of ground faults. These relays monitor the current in the system and compare it to a pre-set threshold. If the detected current exceeds the threshold, the ground fault relay will initiate the isolation of the faulty section.
To illustrate the practical application of protection schemes, let’s consider a high-voltage transmission system with a protection scheme that combines overcurrent and differential protection. The system consists of two generators connected in parallel through a busbar. The protection scheme is designed to detect faults within the busbar section and actuate the corresponding circuit breakers.
The overcurrent protection relays are set to operate at a current of 1200 A. If the current exceeds this value, the relay will trip the circuit breakers and isolate the faulty section. The differential protection scheme uses CTs to compare the currents entering and leaving the busbar section. In normal operating conditions, the currents should be balanced, resulting in a zero-sum at the differential relay. However, if a fault occurs within the protected zone, the currents will become unbalanced, prompting the differential relay to trip the circuit breakers.
In addition to setting the relay thresholds, coordination should also be considered to ensure proper operation of the protection scheme. Coordination involves setting the relay time-delay characteristics, ensuring that the relays closest to the fault operate first, while those further away operate with delay. This coordination prevents unnecessary tripping and improves the selectivity and speed of the protection scheme.
It is worth noting that protection schemes in commercial systems should adhere to international standards such as those provided by the Institute of Electrical and Electronics Engineers (IEEE) and the International Electrotechnical Commission (IEC). These standards specify the requirements for coordination, relay settings, and testing, ensuring the safe and effective operation of protection schemes in commercial systems.