Protective devices in an electrical system offer great security to the power scheme, regardless of size and complexity, but that means conducting short circuit and protection coordination studies are necessary to stabilize the system. This includes making changes like adding loads, reconfiguring the design, or expanding the system.
Ensuring that an electrical system service is uninterrupted and efficient is a priority in designing and maintaining it. Since short circuits cannot be eliminated, exhaustively conducting studies and making the necessary coordination by a qualified engineer is always a welcome practice.
In this article, you’ll find out more about protection coordination studies and how it can help decrease short circuit conditions.
Anatomy of the Short Circuit
A power system has five operating states: normal, alert, emergency, in extremis, and restorative. Generally, a system can experience a violation of constraints, and instability in the transmission and generation, which can trigger the scheme to shift from a normal state to a faulted state. The occurrence of a short circuit indicates an abnormal condition in which the current goes beyond permissible limits several times.
The National Electrical Code defines a short-circuit current as a type of overcurrent that results from a negligible impedance fault between live conductors that have different potential while in a normal operating condition.
An instantaneous short-circuit current has two components: the steady-state AC component and transient DC component. While the AC component is symmetrical, the DC component declines exponentially, dependent on the ratio between system reactance and resistance (system X/R ratio).
The short-circuit current is dependent on the voltage source and phase angle of the fault, which consequently determines the degree of asymmetry of the fault current.
Overcurrent protection refers to the primary measures used against excessive currents due to a system fault. When a sudden and substantial increase in current happens, overcurrent protection is a simple and inexpensive way of safeguarding the continuity of service for industrial systems and electrical power distribution.
Overcurrent protection is designed for intolerable conditions such as short-circuits. It can also be put in place as a backup for power transformers or transmission lines, should there be a need to protect specified components in the primary protection scheme.
Overcurrent protective devices
An overcurrent protective device or OCPD is set up to guard against conditions wherein the current exceeds the rating of the equipment or conductor, such as in a short circuit. These devices could be:
Overcurrent relay. This is an auxiliary device connected to a switch or circuit breaker that operates when the current flowing through the circuit surpasses a preset threshold. It initiates a trip condition that activates the circuit breaker.
Power fuse. The power fuse is direct-acting and responds to an overcurrent by a current-responsive element that gets heated and severed when an overcurrent flows through it.
Low-voltage circuit breaker. This device opens a circuit automatically when a predetermined overcurrent occurs without sustaining any damage to itself. A circuit breaker is inherently three-phased and resettable, unlike power fuses that need to be replaced.
What Is Protection Coordination?
Protection coordination is the process of choosing and arranging overcurrent protective devices to lessen the probability of a short circuit to a small area. This study is used to determine relay and circuit breaker settings, fuse sizes, or to select new equipment that fits into the design.
The primary purpose of this study is to determine the proper settings for OCPDs in the system. It ensures that only the smallest affected portion of the electrical power system will be isolated and controlled, while also protecting the other equipment to prevent the fault from causing more damage.
Initially, a protection coordination study is conducted when the electrical power system is first developed. It is recommended to be done regularly, or 5-6 years if the situation is static. When making any modification or renovation to the system, it often necessitates a protection coordination study to be able to integrate new equipment and protective devices reliably.
Benefits of Protection Coordination Studies
- Reduce system downtime and avoid interruption of service
- Prevent damage to equipment or the occurrence of any failure
- Identify weak points in the system and correct any under-protected segments
- Isolate faulty circuits and ensure that no loss of power to the rest of the system happens
- Reinforce the safety of the personnel
The NEC requires careful coordination, which is the isolation of an overcurrent incident by activating the nearest upstream OCPD in the distribution system. It is done to restrict outages to the equipment or circuit affected. It is achieved by selecting and installing OCPD to handle a wide range of overcurrent, from maximum fault current to overload.
Selective coordination is necessary for systems where reliability is essential. Some examples include emergency systems, multiple elevator circuits, critical-operation power systems, legally sanctioned standby systems, data systems, and more.
Conducting a coordination study necessitates various pieces of information to ensure the output will be as reliable as possible. These data include a one-line diagram of the electrical power system or the affected portion. A short-circuit study should also be completed and time-current curves.
Does Your Electrical System Need Protection Coordination?
Maintaining the reliability of the electrical power system is crucial to ensure that the service is uninterrupted and accidents are avoided, especially in large-scale industries. At Present Group, our experts conduct reliable protection coordination studies to mitigate the devastating effects of a short circuit. Contact us today to know more.