Introduction
Power system protection is a specialized field within electrical power engineering that focuses on safeguarding electrical power systems from faults. Its primary goal is to disconnect faulty components from the rest of the network to maintain system stability. The aim of a protection scheme is to isolate only the affected components, allowing the remaining parts of the network to continue operating. Therefore, protection schemes employ a practical and cautious approach to promptly rectify system faults. The devices responsible for safeguarding power systems from faults are commonly referred to as protection devices.
Components
Protection systems typically consist of five essential components:
- Current and voltage transformers: These transformers lower the high voltages and currents of the electrical power system to levels that can be effectively handled by the relays.
- Protective relays: These devices detect faults and trigger a trip or disconnection command.
- Circuit breakers: They are responsible for opening or closing the system based on commands received from the relays and autoreclosers.
- Batteries: These provide power in case of a disconnection of the main power supply.
- Communication channels: These channels enable the analysis of current and voltage at remote terminals of a line and allow for remote tripping of equipment.
In certain parts of a distribution system, fuses are capable of both detecting and disconnecting faults.
Failures can occur in any of these components, such as insulation failure, fallen or broken transmission lines, incorrect operation of circuit breakers, short circuits, or open circuits. Protection devices are installed with the purpose of safeguarding assets and ensuring a continuous supply of energy.
Switchgear refers to a combination of electrical disconnect switches, fuses, or circuit breakers used for controlling, protecting, and isolating electrical equipment. Switches can be safely opened under normal load conditions (although some switches may not be safe to operate under abnormal conditions), while protective devices are designed to be safe under fault current. Critical equipment often has completely redundant and independent protective systems, while less significant branch distribution lines may have simpler and more cost-effective protection measures.
Types of Protection
High-Voltage Transmission Network
The protection system for high-voltage transmission and distribution networks serves dual purposes: safeguarding the infrastructure and ensuring the safety of the public, including employees. At its core, the protection mechanism is responsible for disconnecting equipment that undergoes an overload or experiences a short circuit to the ground. Certain components within substations, such as transformers, may necessitate supplementary protection measures to account for factors like temperature and gas pressure, among other considerations.
Generator Sets
In a power plant, protective relays play a vital role in safeguarding alternators and transformers against potential damage caused by abnormal operational conditions, internal failures, insulation issues, or regulation malfunctions. These failures are typically rare occurrences, emphasizing the importance of reliable and effective protective relay systems. When a protective relay fails to identify a fault, the resulting damage to the alternator or transformer can lead to expensive repairs or replacement of equipment, as well as financial losses due to the inability to generate and sell energy.
Overload and Back-Up for Distance (Overcurrent)
Overload protection is a crucial aspect of electrical systems, ensuring safe operation by monitoring the current in a circuit. This protection mechanism relies on a current transformer to measure the current accurately. There are two primary types of overload protection: instantaneous overcurrent (IOC) and time overcurrent (TOC).
Instantaneous overcurrent protection triggers the operation of a circuit breaker when the current surpasses a predefined threshold. It immediately interrupts the circuit to prevent further damage or hazards. On the other hand, time overcurrent protection functions based on a current vs. time curve. By comparing the measured current against a predetermined level, it determines if the current has exceeded this level for a specific duration. If the criteria are met, the circuit breaker or fuse will actuate, interrupting the circuit to mitigate risks.
To gain a more comprehensive understanding of both types of overload protection, the concept of “Non-Directional Overcurrent Protection” can be explored in detail. There is a useful video available on YouTube that explains the functioning and significance of these protection mechanisms.
Earth Fault/Ground Fault
Earth fault protection involves the use of current transformers to detect an imbalance in a three-phase circuit. In a properly functioning system, the three-phase currents are generally balanced, meaning they have similar magnitudes. However, if one or two phases become inadvertently connected to the ground through a low impedance path, their magnitudes will experience a significant increase, leading to an imbalance in the currents. When this imbalance surpasses a predetermined threshold, a circuit breaker is designed to activate. Another variation of earth fault protection is known as restricted earth fault protection, which specifically targets earth faults occurring between two sets of current transformers. This type of protection is limited to that specific zone.
Distance (Impedance Relay)
Distance protection, implemented through an impedance relay, effectively detects both voltage and current in a power circuit. During a fault occurrence, such as a short circuit, the voltage level experiences a drop. By measuring the ratio of voltage to current at the relay terminals, which represents the impedance, the relay can determine if the impedance falls within a pre-set threshold. If it does, the circuit breaker will be triggered to operate. This type of protection is particularly suitable for relatively long transmission lines exceeding 10 miles, as it relies on the characteristics of the line itself.
When a fault arises on the line, the relay compares its impedance setting to the apparent impedance of the line from the relay terminals to the fault location. If the relay setting is determined to be lower than the apparent impedance, it signifies that the fault lies within the protected zone. However, if the transmission line is relatively short, less than 10 miles in length, coordinating distance protection becomes more challenging. In such cases, it is advisable to utilize current differential protection as the preferred method of safeguarding the system.
Back-Up
The goal of protection measures is to isolate and remove only the affected portion of a plant or system, without causing unnecessary disruptions. However, there are instances when a circuit breaker or protection relay may fail to function properly. In critical systems, if the primary protection fails, backup protection mechanisms come into play. Remote backup protection is designed to remove both the affected and unaffected components of the plant in order to clear the fault. On the other hand, local backup protection specifically targets and removes only the affected components to resolve the fault.
Low-Voltage Networks
In low-voltage networks, which typically operate at lower electrical potentials, an essential aspect of ensuring safe and reliable power distribution involves managing overload and earth faults. To accomplish this, the network incorporates the use of protective devices such as fuses and low-voltage circuit breakers.
Fuses are safety devices designed to interrupt the flow of current when the current exceeds a specified threshold. When an overload occurs, causing the current to rise beyond the fuse’s rated capacity, the fuse element melts, breaking the circuit and preventing further damage to the network. This safeguard prevents excessive current from damaging equipment, causing fires, or posing risks to personnel.
Cybersecurity
The bulk system, a vast interconnected electrical network comprising transmission and control systems, faces an ever-increasing barrage of cybersecurity threats on a daily basis (Electric Grid Cybersecurity, 2019). Primarily targeting the control systems within the grid, these attacks exploit their internet connectivity, granting hackers an easier entry point. Such malicious actions can result in equipment damage and impede the utility professionals’ ability to effectively manage the system.
Coordination
Protective device coordination refers to the process of determining the optimal timing for interrupting current during abnormal electrical conditions. The primary objective is to minimize the extent of an outage. In the past, protective device coordination was carried out using translucent log-log paper, but modern methods involve detailed computer-based analysis and reporting.
Protection coordination is achieved by dividing the power system into protective zones. Each zone is responsible for taking necessary actions to isolate any faults that occur within its boundaries, effectively separating the affected zone from the rest of the system. These zones are defined based on generators, buses, transformers, transmission and distribution lines, and motors. Key characteristics of the protective zones include overlapping regions denoting circuit breakers, and in the event of a fault, all circuit breakers within the affected zone will open sequentially to isolate the fault. Overlapping regions are created by employing two sets of instrument transformers and relays for each circuit breaker, ensuring redundancy to eliminate unprotected areas. However, the design of these overlapping regions aims to keep them as small as possible. This way, when a fault occurs within an overlap region and the two zones encompassing the fault are isolated, the portion of the power system that loses service remains minimal despite the isolation of two zones.
Disturbance-monitoring Equipment
Disturbance Monitoring Equipment (DME) is utilized to monitor and record crucial system data related to faults. DME serves three primary purposes:
- Model validation: It ensures that the system models used for analysis and simulation accurately represent the observed behavior during disturbances.
- Disturbance investigation: DME helps in analyzing and investigating disturbances or faults that occur within the system, providing valuable insights into their causes and effects.
- Assessment of system protection performance: By monitoring and recording system data, DME allows for the evaluation of the performance of protective devices and schemes during fault conditions.
DME devices encompass various components, including:
- Sequence of Event Recorders: These record the response of equipment to specific events, capturing the sequence and timing of actions taken by different system elements.
- Fault Recorders: They capture the actual waveform data of primary voltages and currents within the system during fault occurrences, enabling detailed analysis and diagnosis.
- Dynamic Disturbance Recorders (DDRs): These devices record incidents that showcase power system behavior during dynamic events, such as low-frequency oscillations (0.1 Hz – 3 Hz) and abnormal frequency or voltage excursions. DDRs provide valuable information for understanding system dynamics and identifying potential instabilities.
In summary, Disturbance Monitoring Equipment plays a crucial role in validating models, investigating disturbances, and assessing the performance of system protection. Its various components, including sequence of event recorders, fault recorders, and dynamic disturbance recorders, contribute to capturing and analyzing data essential for maintaining a stable and reliable power system.
Performance Measures
Protection engineers evaluate the performance of protection systems based on two key measures: dependability and security. Dependability refers to the system’s ability to operate correctly for faults that occur within its designated zone. On the other hand, security refers to the system’s ability to avoid operating for faults that occur outside its designated zone. Both dependability and security are crucial aspects of reliability in protection systems. To assess the relative reliability of different protection schemes, engineers often utilize fault tree analysis, a useful tool in their toolkit. Quantifying the reliability of protection systems is essential for making informed decisions on system improvements, managing tradeoffs between dependability and security, and achieving optimal outcomes with minimal expenditure. In the competitive utility industry, a quantitative understanding of performance is particularly vital.
Performance and Design Criteria
When evaluating system-protection devices, several performance and design criteria come into play. These criteria include reliability, selectivity, speed, economy, and simplicity.
Reliability: The devices must consistently function as intended when fault conditions arise, even if they have been idle for extended periods, spanning months or years. Reliability is crucial to prevent costly damages resulting from system failures.
Selectivity: The devices must exhibit the ability to distinguish between legitimate faults and false alarms, minimizing unwarranted trips or operations.
Speed: The devices must respond promptly to faults, swiftly mitigating equipment damage and reducing the duration of faults. Intentional time delays, if any, should be precise and well-calibrated.
Economy: The devices should offer effective protection while keeping costs to a minimum. Maximizing protection while minimizing expenses is a primary objective.
Simplicity: The devices should employ protection circuitry and equipment in an efficient and streamlined manner, minimizing complexity wherever possible.
