Electrical distribution systems are vulnerable to excess current, which can be caused by overloads, short circuits, transformer inrush current, and motor starts. Techniques such as demand-side management, load shedding, and soft motor starting can be implemented to mitigate overloads under normal operating conditions.
Moreover, distribution systems are equipped with protective relays that trigger switching devices, ensuring they only respond to abnormal system conditions.
Protective relays are connected to the circuits they safeguard via current transformers (CTs) and/or voltage transformers (VTs), depending on the protection function. For instance, directional overcurrent protection requires both CTs and VTs, while earth-fault protection only uses CTs.
When the sensed current and/or voltage exceed a predetermined threshold, the protective relay triggers a trip signal to the circuit breaker. This isolates the faulted portion of the system, either by disconnecting the affected phase(s) or by removing the malfunctioning equipment. In some cases, like molded case circuit breakers (MCCBs) and fuses, the protective relay and switching element are combined into a single unit.
Note that relays require power to operate. This power can be drawn from the circuit being monitored or supplied by energy storage systems, such as capacitor trip devices (often used in smaller, low-voltage systems) or battery packs (commonly used in larger switchgear).
Powering relays directly from the monitored circuit can be unreliable due to the risk of voltage dips affecting their operation.
List of Contents
Protection schemes are classified into two primary categories:
Unit Type Protection Schemes
Non-Unit Type Protection Schemes
1.1. Unit Type Protection Schemes
Unit protection schemes provide selective protection for discrete zones or elements of the power system, such as transformers, transmission lines, generators, or busbars.
Unit protection schemes operate on the principle of Kirchhoff's Current Law, which states that the algebraic sum of all currents entering and leaving a defined zone or element within the power system must be zero.
A non-zero summation of currents within the protected zone signifies an internal fault. These schemes are inherently immune to disturbances or operating conditions external to their defined boundary. Furthermore, they must be designed to remain stable under the maximum prospective fault current that can flow through the protected zone.
1.2. Non-Unit Type Protection Schemes
Non-unit protection schemes, while designed to safeguard specific zones, do not possess strictly defined boundaries. Their protective zones may extend beyond their primary designated area and overlap with adjacent zones. While this characteristic can provide valuable backup protection, it also introduces the potential for excessive system isolation should a fault be detected by multiple overlapping non-unit schemes.
The simplest form of these schemes employs current measurement and utilizes an inverse time characteristic for protection operation, enabling faster tripping for faults closer to the source.
The Non-unit protection systems encompass the following schemes:
Time-graded overcurrent protection
Current graded overcurrent protection
Distance or Impedance Protection
1.2.1 Overcurrent Protection
Due to its simplicity, this method is a common and widely applied technique for line protection.
Its widespread use is attributed to the significant increase in current magnitude during fault conditions, often reaching several multiples of the maximum load current. However, its applicability is limited to relatively simple and low-cost equipment.
1.2.2 Ground Fault Protection
A typical configuration employs two or three overcurrent relays for phase fault protection and a dedicated overcurrent relay for ground fault protection. The inclusion of a separate earth fault relay enhances both the speed and sensitivity of ground fault protection.
The magnitude of the ground fault current is typically lower than that of the phase fault current.
Consequently, ground fault protection relays are distinct from those used for phase-to-phase fault protection.
No | Line Fault Types | Operation of Relay |
1 | Phase to Ground Fault (Earth Fault) | Ground Fault Relay |
2 | Phase to Phase fault Not with Ground | Related Phase Overcurrent Relays |
3 | Double Phase to Ground fault | Related Phase Overcurrent Relays and Ground Fault Relays |
An overcurrent relay is a protective device that initiates action when the current flowing through it exceeds a pre-defined threshold, known as the pickup or setting value.
Overcurrent protection safeguards electrical power systems from damage due to excessive currents resulting from faults, such as short circuits and ground faults. Overcurrent relays are versatile protective devices applicable to a wide range of power system components, including transmission lines, transformers, generators, and motors.
Feeder protection schemes typically employ multiple overcurrent relays to provide graded protection along the feeder's length. These relays are coordinated to ensure selective tripping, with the relay closest to the fault location operating first.
Coordination between adjacent overcurrent relays can be achieved through three primary methods: time-based discrimination, current-based discrimination, and a combination of both time and current discrimination.
Overcurrent relays provide protection against:
Overcurrent protection encompasses short-circuit protection, and short circuits can manifest as:
Phase Faults
Ground Faults
Winding Faults
Short-circuit currents typically exhibit magnitudes significantly exceeding (5 to 20 times) the full-load current. Consequently, rapid fault clearing is imperative in short-circuit scenarios.
3.1. Functional Requirements of Overcurrent Protection
Overcurrent protection systems must discriminate against transient inrush currents, permissible overload currents, and other short-duration current surges. This is achieved through the introduction of a time delay, particularly in inverse-time overcurrent relays.
The overcurrent protection system must be coordinated with adjacent overcurrent protection zones.
The overcurrent relay serves as a fundamental component within an overcurrent protection scheme.
3.2. Objectives of Overcurrent Protection
The following represent the principal objectives of overcurrent relays:
Fault Detection: Detect abnormal current conditions indicative of a fault.
Fault Isolation: Isolate the faulted section of the electrical system.
Rapid Operation: Minimize damage and hazards through rapid fault clearing.
Selectivity (Discrimination): Isolate only the faulted zone, avoiding unnecessary tripping.
Dependability: Ensure reliable relay operation when required.
Security: Prevent unintended relay operation during normal or transient system conditions.
Economic Optimization: Balance the cost of protection against the potential costs of unprotected faults.
Figure 1 – Directional Overcurrent Relay
3.3. Specifications of Overcurrent Relays
The proper functioning of an overcurrent protective device necessitates careful selection of its ratings, including voltage, current (ampere), and interrupting capacity.
An improperly selected interrupting rating presents a significant hazard to equipment and personnel.
Current limitation can be considered an additional performance characteristic of overcurrent protective devices, although it is not a mandatory requirement for all such devices.
Voltage Rating: The rated voltage of the overcurrent protective device must be equal to or greater than the circuit voltage. While a higher rating is permissible, the device's voltage rating must never be less than the system voltage.
Ampere Rating: The ampere rating of an overcurrent protective device should not exceed the ampacity of the conductors it protects. Generally, the device's ampere rating is selected to be 125% of the continuous load current.
Overcurrent protection safeguards against currents exceeding the established ratings of the protected equipment, arising from short circuits, ground faults, and overload conditions. Overload protection specifically addresses the risk of equipment overheating due to sustained overload currents.
Overcurrent protection is a broader concept that encompasses overload protection as a subset.
While overcurrent relays can provide overload (thermal) protection for resistive loads, they are generally unsuitable for motor overload protection. Overload relays typically employ longer time delays compared to overcurrent relays.
The following constitute classes of overcurrent relays:
5.1. Instantaneous Overcurrent Relay (Define Current)
A definite-time overcurrent relay operates instantaneously when the current exceeds a pre-defined threshold. Upon reaching or surpassing this predetermined value, the relay trips without intentional time delay, as depicted in its characteristic curve. The relay's configuration is adapted based on its location within the network, with the threshold current setting decreasing as the relay's location moves further downstream from the source.
For instance, an overcurrent relay located at the downstream end of a distribution feeder will be set to operate at a lower current level compared to a relay at the source end, especially in cases where the feeder impedance is substantial.
When the feeder impedance is substantially lower than the impedance of the upstream network, achieving selective coordination between overcurrent protection devices at the feeder's source and downstream ends becomes difficult. This lack of discrimination is particularly pronounced at higher short-circuit current levels.
The limitations of definite-time overcurrent protection include poor coordination and a restricted range of settings. Conversely, in applications where the protected element or feeder impedance is relatively high, instantaneous protection offers the advantages of minimizing relay operating time for critical faults and maintaining selectivity within systems utilizing relays with diverse operating characteristics.
The calibration of instantaneous overcurrent relay settings depends on both the relay's location within the electrical network and the characteristics of the protected element. For instance, when applying instantaneous overcurrent relays to protect inter-substation lines, the setting is typically established at a minimum of 1.25 times the root-mean-square (RMS) current associated with the maximum symmetrical fault level at the downstream substation. The setting procedure initiates at the most remote substation and proceeds progressively towards the source.
For distribution lines terminating at MV/LV transformers, the instantaneous overcurrent relay setting is typically set at 50% of the maximum short-circuit current at the current transformer (CT) location or within a range of 6 to 10 times the maximum circuit rating. When the relay is installed on the transformer's primary side, its setting is typically between 1.25 and 1.50 times the short-circuit current contribution from the low-voltage (LV) bus, reflected to the high-voltage (HV) side..
This value is sufficient to ensure coordination with the high transient current associated with transformer magnetizing inrush during energization.
Figure 2 – Instantaneous Overcurrent Relay with Definite-Time Characteristic
Definite-Time Operation: Trips after a fixed time delay when the current exceeds the relay's pickup value.
Current Magnitude Sensing: Responds solely to the magnitude of the current, independent of time.
Constant Operating Time: Exhibits a fixed operating time characteristic.
Absence of Intentional Time Delay: No intentional time delay is introduced beyond the inherent relay operating time.
Coordination by Impedance: Coordination between definite-time relays relies on the variation of fault current magnitude with fault location due to differing source-to-fault impedances.
Graded Current Settings: Relays are set with progressively increasing pickup current values as their location moves closer to the source. The relay furthest from the source has the lowest pickup current.
Rapid Operation: Typically operates within 0.1 seconds or less.
Application: These relays are typically applied to outgoing feeders
5.2. Definite Time Overcurrent Relay
For a definite-time overcurrent relay to initiate a trip, two criteria must be met: the current must exceed the relay's pickup setting, and this overcurrent condition must persist for a duration equal to or greater than the relay's time setting.
Definite-time overcurrent protection is characterized by a fixed operating time, independent of fault current magnitude, once a pre-defined current threshold is exceeded.
Overcurrent relay settings at various locations within the network can be coordinated to ensure selective tripping. This coordination aims to trip the breaker closest to the fault first, with minimal delay, followed by subsequent tripping of upstream breakers with progressively longer time delays.
This relay type incorporates a current setting, also known as the pickup, plug, or tap setting, which defines the current threshold for relay activation. Additionally, a time dial setting is provided to specify the precise relay operating time.
A limitation of this protection scheme is that a high-magnitude short-circuit fault near the source may result in a relatively long clearing time.
Figure 3 – Definite-Time Overcurrent Relay
Modern relays may incorporate multiple protection stages, each with independent current and time settings.
Constant Operating Time: The operating time remains constant.
Independence from Current Magnitude: Operation is independent of the fault current magnitude above the pickup value.
Adjustable Settings: Includes pickup (or tap) and time dial settings, enabling precise control of the operating time delay.
Simplified Coordination: Facilitates easier coordination with other protective devices.
Consistent Tripping Time: The tripping time is independent of variations in the source impedance and the fault location.
Limitations of the Relay
Loss of Supply Continuity: Downstream supply continuity is interrupted during faults.
Undesirable Time Delay: The inherent time delay, while adjustable, can be undesirable for rapidly clearing short circuits.
Coordination Challenges: Coordination can be complex and may require adjustments with changes in system loading.
Inapplicability to Long Transmission Lines: Not suitable for long-distance transmission lines where rapid fault clearing is crucial for system stability.
Poor Discrimination: Difficulty in distinguishing between fault locations when the impedance between those locations is small, resulting in inadequate selectivity.
Application:
Definite-time overcurrent relays find application in:
Backup protection for distance relays on transmission lines, employing a time delay.
Backup protection for differential relays on power transformers, also with a time delay.
Primary protection for outgoing feeders and bus couplers, utilizing adjustable time delay settings.
5.3. Inverse Time Overcurrent Relay (IDMT Relay), Encompassing
Inverse-time overcurrent relays are characterized by an operating time that is inversely proportional to the fault current magnitude. Higher fault currents result in faster relay operation, while lower currents lead to longer operating times. Several inverse time characteristics exist, including standard inverse, very inverse, and extremely inverse. Discrimination is achieved through a combination of current and time settings. The relay's operating time is inversely related to the magnitude of the fault current.
Inverse-time relays are also commonly known as Inverse Definite Minimum Time (IDMT) relays.
Figure 4 – Inverse Definite Minimum Time (IDMT)
Inverse-time overcurrent relays exhibit faster operating times with increasing current magnitudes, as illustrated in the provided characteristic curves. These relays are available with various inverse-time characteristics, such as inverse, very inverse, and extremely inverse, to accommodate specific application requirements.
Refer to Figure 5.
Both definite-time and inverse-time overcurrent relays require coordinated time settings to ensure selective tripping, where the relay closest to the fault trips first. This coordination process is known as time grading. The time difference between the operating times of two adjacent relays at the same fault current level is defined as the discrimination margin. Typical discrimination margins range from 0.25 to 0.4 seconds for electromechanical and static relays and approximately 0.2 seconds for digital relays.
Figure 5 – Representative Time-Current Characteristics of Overcurrent Relays
Where: (A) Inverse characteristic; (B) Very inverse characteristic; (C) Extremely inverse characteristic; (D) Instantaneous characteristic. TD = Time Dial Setting.
Calibration of both definite-time and inverse-time overcurrent relays involves determining two key parameters: the time dial setting (TDS) and the pickup current setting. The TDS, also referred to as the time multiplier, is assigned to each relay within the system, establishing its operating time delay while accounting for time grading and the required discrimination margin. The pickup setting defines the current threshold at which the relay initiates operation.
The operating time of an overcurrent relay can be adjusted by modifying the time dial setting (TDS). Increasing the TDS results in a longer operating time (slower response), while decreasing the TDS leads to a faster response. Typically, the TDS ranges from a minimum of 0.5 (fastest) to a maximum of 10 (slowest).
Operation Above Pickup: The relay operates when the current exceeds its pickup value.
Current-Dependent Operating Time: The operating time is inversely proportional to the magnitude of the fault current.
Dual Characteristic: Exhibits inverse-time characteristics for lower fault current values and approaches definite-time characteristics at higher current levels.
Characteristic Transition: Inverse characteristics are typically observed for plug setting multiplier values below 10. Between 10 and 20, the characteristics transition towards a definite-time response.
Typical Application: Widely employed for the protection of distribution lines.
Inverse-time overcurrent relays are categorized into three types based on their degree of Inverness:
Figure 6 – Inverse types
5.3.1. Moderately Inverse Time Overcurrent Relay
The operating time of inverse-time overcurrent relays exhibits a tolerance of 5% to 7.5% of the nominal value, as defined by applicable standards. This inherent uncertainty, along with the desired operating time, necessitates a coordination margin (grading margin) of 0.4 to 0.5 seconds.
This type of relay is applied when the fault current magnitude is primarily dependent on the generation source and not significantly influenced by the fault location.
A normal inverse-time overcurrent relay exhibits a relatively small change in operating time for a given change in current.
Application:
Normal inverse-time overcurrent relays are commonly applied in utility and industrial power systems, particularly in situations where the fault current magnitude is primarily determined by the system's generating capacity at the time of the fault.
5.3.2. Very Inverse Time Overcurrent Relay
Enhanced Inverse Characteristic: Exhibits a more pronounced inverse relationship between current and time compared to standard IDMT relays.
Application in Systems with Fault Current Variation: Suitable for applications where the fault current decreases significantly with increasing distance from the source.
Effective for Ground Faults: Particularly effective for ground fault protection due to their steep time-current characteristic.
Fault Location Dependence: Applied when the fault current magnitude is primarily dependent on the fault location.
Reduced Grading Margin: The steeper characteristic allows for smaller grading margins, typically in the range of 0.3 to 0.4 seconds.
Independence from Generation Changes: Appropriate when the fault current is relatively independent of normal variations in system generating capacity.
5.3.3. Extremely Inverse Time Overcurrent Relay
Highest Degree of Inverseness: Possesses the most pronounced inverse time-current characteristic among inverse-time relays.
Overheating Protection: Suitable for protecting equipment, particularly machines, against thermal overload.
Current-Squared Relationship: The operating time is approximately inversely proportional to the square of the fault current.
Rapid Operation Despite Inrush: Enables the use of short time delays even with high inrush currents during switching operations.
Fault Location Dependence: Applicable when the fault current magnitude is primarily dependent on the fault location.
Independence from Generation Changes: Appropriate when the fault current is relatively independent of normal variations in system generating capacity.
Application:
Protection of distribution feeders with high inrush currents during switching (e.g., from refrigerators, pumps, and water heaters).
Coordination with fuses and reclosers due to their steep characteristic.
Protection of equipment such as alternators, transformers, and expensive cables.
5.3.4. Long Time Inverse Overcurrent Relay
Long-time overcurrent relays are primarily utilized as backup protection for earth faults. This application serves several critical purposes within a protection scheme:
Backup to Primary Earth Fault Protection: They act as a redundant layer of protection in case the primary earth fault protection (e.g., residual current device (RCD), ground fault relay) fails to operate correctly. This ensures continued protection against ground faults, even if the primary protection scheme is compromised.
Protection Against Persistent Earth Faults: Long-time overcurrent relays are designed to respond to sustained or slowly developing earth faults that might not be quickly detected by faster-acting primary protection. These persistent faults can lead to equipment damage or pose safety hazards if left unchecked.
Coordination with Downstream Devices: Their longer operating times allow for coordination with downstream protective devices. This ensures that the closest protective device to the fault trips first, minimizing the impact of the fault on the rest of the system. The long-time characteristic prevents the backup relay from tripping unnecessarily for faults cleared by downstream devices.
Protection Against High-Impedance Earth Faults: These relays can be more sensitive to high-impedance earth faults, which might not generate sufficient current to trigger instantaneous or fast-acting overcurrent protection. The longer operating time allows the relay to integrate the fault current over time, increasing its sensitivity to these types of faults.
Selective Coordination: By employing a long-time characteristic, these relays can be selectively coordinated with other overcurrent and earth fault protection devices in the network. This coordination ensures that the protective device closest to the fault operates first, limiting the extent of the outage.
Long-time overcurrent relays provide essential backup earth fault protection, offering redundancy, safeguarding against sustained faults, facilitating coordination with downstream devices, and exhibiting enhanced sensitivity to high-impedance ground faults. Their primary function is to improve the reliability and safety of the electrical system.
5.4. Directional Overcurrent Relay
In non-radial power systems (i.e., systems with sources at multiple points along a line), conventional overcurrent relays may offer insufficient protection. Directional overcurrent relays address this limitation by operating selectively based on the direction of current flow, tripping for currents in one direction and blocking for currents in the opposite direction. These relays require three criteria to be met for operation: the current magnitude must exceed the pickup value, a time delay must elapse, and the current must flow in the designated direction.
The directionality of current flow can be determined by using voltage as a directional reference. Specifically, the phase relationship between voltage and current is used to establish directionality. In a power system, current flow is considered positive or forward when it is in phase or lagging the voltage by an angle less than 90 degrees. Conversely, current flow is considered negative or reverse when it is leading the voltage, or lagging by an angle greater than 90 degrees. Directional relays utilize this phase relationship to discriminate between forward and reverse current flow. They are designed to trip only when the current flows in the designated (forward) direction relative to the reference voltage. This directional information is crucial for selective coordination and fault clearing in complex power networks.
In essence, voltage provides the reference for determining the direction of current flow.
Motor Protection:
Overcurrent relays protect against overloads and short circuits in motor stator windings.
Motor protection schemes typically utilize both inverse-time and instantaneous overcurrent elements for phase and ground fault detection.
These relays are generally applied to motors with ratings exceeding 1000 kW.
Transformer Protection:
In applications where the cost of more sophisticated overcurrent protection (e.g., inverse-time relays) is not economically justifiable.
Extensively as backup protection against external faults on power transformers.
Line Protection:
On certain sub-transmission lines where the expense of distance protection is prohibitive.
As primary ground fault protection on many transmission lines where distance relays are employed for phase fault protection.
Providing ground fault backup protection on most lines that utilize pilot relaying as the primary protection scheme.
Distribution Protection:
Overcurrent relays are well-suited for distribution system protection due to the following characteristics:
Simplicity and Cost-Effectiveness: Overcurrent relays are relatively simple in design and implementation, resulting in lower costs.
Non-Directional Applicability: In many cases, directional relays are not required, eliminating the need for potential transformer (PT) supply, further reducing cost and complexity.
Versatility: Phase-to-phase faults can be protected using a set of two overcurrent relays, while a separate overcurrent relay can be dedicated to ground fault protection.
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