POWER SWING: DETECTION AND PREVENTION

: Power swing is a phenomenon that is basically caused by large disturbances in the power system. If not blocked, it can cause incorrect operation of distance protection and result in incorrect or unwanted disconnection of transmission line switches. Furthermore, if not prevented, it can lead to serious damage to the generator. This paper describes the basics of power system stability during power swings, lists the effects on the distance or other relay during power swing, and presents methods for detecting power swing, both traditional and advanced. Additionally, it provides a description of the settings of the Power Swing Blocking (PSB) scheme to meet these requirements.


INTRODUCTION
Power system have changes, faults, disconnection line, disconnection of a large load or production plant, lead to adaptation of the remaining generators to new stable conditions.Power system works with all generators in synchronized under normal conditions and has a balance between load and production.When the disturbance "removes" a significant amount of load or output, the rotor of the synchronous generator is given a new angle relative to the stator to restore balance.Due to the greater inertia of rotary machines, changes occur slowly and oscillate.This adjustment of rotor angles to disturbances to achieve a new stable operating point is known as a power swing [1].During power swings, the angle between different areas of the power system fluctuates [2]. Figure 1 shows the amplitudes of a) current and b) voltage during power swings.The appearance of the maximum voltage and current depends on the speed of change of the power angle between the two areas and is designated as the slip frequency [4].The sliding frequency can be 1-3 Hz (slow oscillation) and 4-7 Hz (fast oscillation) ( [5], [6]).This paper covers the basics of the appearance of power swings and out-of-step and different schemes for detecting these conditions, and influence on the distance relay, which should work correctly and selectively.The first section describe power system stability and the power swing phenomenon.Influence of the power swing on the distance relay is given in the second section.The third section lists power swing detection schemes and the fourth section describes power swing scheme setup.In the fifth section, the conclusions are given, and at the end, the literature and biography of the author are listed.

THE BASIC STABILITY OF THE POWER SYSTEM AND THE POWER SWING PHENOMENON
Power systems, in normal conditions, operate with nominal frequency, with very small deviations of the order of 0.02 Hz for larger systems and up to 0.05 Hz for much smaller systems.In normal conditions, electricity production and load are balanced.Considering the classical model of a system with two generators as shown in Figure 2, the transmitted power can be represented by the following equation: (1) sin Where: -E S and E R are the phase voltages of generators S and R, respectively; δ is the angle by which E S leads E R , given by δ = φ Sφ R ; where φ S and φ R are rotor phase angles of generators S and R, respectively; -Z T is the total impedance between the two generators, consisting of Z S , Z R , and Z L , and is calculated as Z T = Z S + Z R + Z L ; -Z S and Z R are the phase impedances of source S and R, respectively; -Z L is the transmission line impedance.
Figure 3: Power-angle curve [7] Angular stability refers to the property of all synchronous generators in the system to remain in synchronism after a disturbance.It relies on the ability of each individual generator to maintain a balance between the rotating magnetic field and the rotation of the rotor.If one generator falls out of synchronism, it jeopardizes the stability of the entire system.An out-of-synchronism condition can usually be observed within 2 to 3 seconds after the disturbance that caused it.
Angular stability can be divided into two subtypes: 1. Static Stability: Static stability refers to the power system's ability to remain in synchronism with small disturbances.These disturbances are typically shortterm and impulsive in nature.The system's response to these disturbances is observed, and if it is stable, it returns to its initial equilibrium state.For the system to have angular stability with small disturbances, it must remain in synchronism despite changes in production and consumption.2. Transient Stability: Transient stability, on the other hand, is the ability of a system to remain in synchronism after a major disturbance in the network.It deals with the system's ability to maintain stability and avoid falling out of synchronism following significant disruptions.
For determining transient stability, the most important are the Pδ curves, which illustrate the relationship between the working power and the rotor angle.These curves provide valuable insights into the system's stability during power swings and help in assessing the power transfer capability under various operating conditions.
In Figure 4, two curves are shown.The first curve, marked with the number 1, illustrates the relationship between P (power) and δ (angle) when the system is in a normal steady state.The system operates at the operating point "a", which corresponds to the rotor angle δ a , with the input mechanical power Pm.If one of the lines is switched off, the reactance will increase, and the maximum power that the system can transmit will decrease.This reduction in maximum power is shown by curve 2 in Figure 4.
Figure 4: Pδ system curves [8] In the event of such a failure, the working power that the generator delivers to the system will be significantly less than in normal operation.Figure 5 shows Pδ curves for three cases: the first case represents normal operation before (2) Figure 2: Model of a system with two generators [7] Figure 3 graphically shows the angle-power curve, which is the relationship between the transmitted power and the angle (δ) between the two ends.By increasing the angle δ from 0° to 90°, the power transfer increases and then decreases when the angle δ increases above 90°.Typically, power systems operate below the maximum power transmission angle of 90°, where the power transfer corresponds to a certain power value (P 0 ) at the angle (δ 0 ).The maximum power transfer occurs when: the occurrence of a short circuit, the second curve depicts the system during the short circuit event, and the last case displays the system after the short circuit disturbance has been cleared.The short circuit is removed by turning off the switch, and the removal time depends on the speed of the relay and the switch.
In case a) of Figure 5, the failure deflection time is δ c1 , and in case b), that time is δ c2 , with both cases having constant mechanical power P m .If the stator losses are ignored, the powers P m and P e are equal before the failure occurs.When a short circ uit occurs, the system moves from operating point "a" to point "b."Due to inertia, the rotor angle cannot change immediately.Since the power P m is now greater than the power P e , the rotor accelerates until the protection system activates, bringing the system to the working point "c."After the line on which the short circuit appeared is turned off, the system moves to the operating point "d," where the mechanical power is less than the electrical power, causing the rotor to slow down.
As the rotor speed remains higher than the synchronous speed, the rotor angle continues to increase until the energy obtained during acceleration is consumed, shown by surface A 1 in Figure 5. On the other hand, area A 2 represents b) Figure 5: Pδ curves in case of three-phase short circuit [8] a) the consumed energy.Due to energy consumption, the system moves to the operating point "e," where A 1 and A 2 are equal.Despite the electrical power still being greater than the mechanical power, the rotor continues to slow down, causing its speed to drop below the synchronous speed, which further decreases the angle δ.The system returns to the operating point "d" and continues to move along curve 2. In this example, we see that the system is stable because, due to the occurrence of a three-phase short circuit, the generator independently returns to synchronism.
However, in case b), when the system reaches the operating point "e," it is observed that the area A 1 is larger than the area A 2 due to the longer time of fault elimination.At this point, the rotor speed is greater than the synchronous speed, causing the rotor angle to continue increasing.As the angle δ further increases, the system moves into the region where the mechanical power becomes greater than the electrical power.As a result, the rotor accelerates again, leading to the system falling out of synchronism.This scenario demonstrates that the system is unstable.

INFLUENCE OF POWER SWING ON THE DISTANCE RELAY
Power swings pose a significant challenge to the operation of protective relays, as the impedance measured during power swings can enter the relay's operating range [1].This can lead to unnecessary tripping in an already unstable system, further exacerbating the instability [1].The stability of power swings depends on the power transmission capability of the line and the rotor angle.A power swing that is stable will eventually lead to a steady state, while a power swing that is unstable may result in a loss of synchronization, which is known as loss of synchronism [9].To address these issues, protective relays employ two important schemes: the Out-of-Step Blocking (OSB) scheme and the Out-of- Step Tripping (OST) scheme.The OSB scheme prevents protective relays from operating during stable power swings, allowing the power system to return to a stable operating state.On the other hand, the OST scheme is activated if the system loses synchronization, leading to a controlled separation of unsynchronized parts to prevent further damage.
Distance protections or other relays should not be activated during unstable or stable power swings.Instead, they should allow the power system to return to a stable operating state.Distance protection elements that tend to operate during steady or transient power swings should be temporarily disabled to prevent disconnection of the system at random or in locations not preselected.
In modern relays PSB function is available to differentiate between power swings and faults, blocking distance protection or other relay from operating during power swings.However, in the event of faults occurring during power swings, they need to be accurately detected and resolved with high selectivity and reliability.Serious disturbances in the power system can lead to a significant separation of rotor angles between groups of generators, potentially resulting in the loss of synchronism between these groups or neighbouring power systems [9].
To prevent equipment damage and system breakdown, the separated areas must be automatically and promptly isolated from each other.Ideally, the separation should occur at predetermined locations to maintain a balance of production and load in each area.However, achieving the desired balance may not always be possible, requiring some form of load shedding in the isolated area to avoid a complete breakdown.Uncontrolled circuit breaker switching during Out-of-Step (OOS) events can cause equipment damage, pose safety risks to personnel, and contribute to cascading shutdowns and breakdowns in larger areas of the power system.
Overall, the correct functioning of OSB, OST, and PSB schemes is crucial in ensuring the stability and reliability of the power system, avoiding widespread outages and mitigating potential damages.
For this reason, it is necessary to turn off certain elements of the power system in a controlled manner to prevent equipment damage, massive power outages, and to minimize the effects of disturbances.The OST function achieves this separation.Its main purpose is to distinguish stable from unstable power fluctuations and initiate the separation of a part of the power system at predetermined locations and the appropriate phase-angle difference between the source voltage and the system voltage.This is done to maintain the stability of the power systems.
In the electric power system, the generator frequencies are not constant, the magnitude of the voltage is also affected by other factors.After a disturbance, there will be a change in power in the power system.Most of these swings are small, but some can be large enough, and others might lead to an unstable power swing.Figure 6 shows the paths as seen by the relay located on bus S that can occur in the power system when power swings occur.Additionally, two distance protection zones are also shown [7].
As shown in Figure 6, the distance zones can detect some of the power swings and potentially trigger the switch to trip.While some actions may be desirable, many could lead to unwanted tripping, especially during steady swings.To prevent unnecessary tripping, PSB schemes, also known as OSB schemes, are used in trip-blocking protection schemes at selected remote zones [7].
The system model depicted in Figure 2, consisting of two machines, is used to analyse the performance of distance protection during power swings ( [10], [11], [12]).By examining the apparent impedance seen by the relay V R is the assumed reference phasor, and δ represents the angle of the transmission line.The relationship between the transmitted power S R to the receiver and δ is described as follows: Figure 6: The impedance paths that the distance protection on busbar S records during power swings [7] (3.a) or Since for transmission lines, the reactance component is more dominant compared to the resistance component, Z T ≈ jX T .Thus, equation (3) becomes: Similarly, the transmitted power at the sending point can be calculated: Thus, the differential power that can be absorbed by the transmission line using equations ( 4) and ( 5) can be calculated as follows: It can be noted that the absorbed power per line depends on the angle and reactance of the line X T .
( ) We can calculate other quantities used in power swing detection functions and observe their dependence on system impedances.Assuming that V S = V R = V, we can examine the quantities used to detect power swings: Impedance: Impedance change rate Z: It can be observed that most of them depend on the impedance X T .
Failures are typically permanent compared to power swings, which are slow events and disappear after a short period.This information is used in distance relays to distinguish between short-circuit faults and power swings [13].

The fundamental problem of power swings detection
Power swings can lead to changes in the load impedance, potentially causing the impedance to drop below steady state levels.This can trigger unintended operation of relays at various points in the network.Such undesirable relay actions can exacerbate disturbances in the power system, leading to significant power outages or even a complete collapse of the system.It is crucial that distance protections do not activate unexpectedly during dynamic changes in the system, such as unstable or stable power swings, and allow the power system to return to a stable state.
To prevent unwanted relay operations during power swings, modern relays incorporate the PSB function [14].The primary purpose of the PSB function is to distinguish between supply faults and power swings, effectively blocking distance protection or other relay elements during power swings.
Special attention must be given to the effects of power fluctuations, particularly in cases of OOS conditions, which are synonymous with unstable power swings [15].Uncontrolled disconnection of circuit breakers during OOS states can result in equipment damage, pose safety risks to operating personnel, and contribute to cascading interruptions and disconnections of significant portions of the power system.Therefore, the main goal of the OST function is to differentiate unstable from stable power swings and separate the system into areas at predetermined network locations, considering the corresponding phase angle difference voltage between the source and the system.This approach is essential for maintaining power system stability and ensuring continuous power supply.

Power swing detection methods
The methods used to detect power swings and implement schemes for PSB, OSB, and OST functions can be categorized into two groups: traditional methods based on the rate of change of impedance and advanced methods implemented in microprocessor relays.

Traditional methods
These methods focus on detecting differences in the rate of change of the positive sequence impedance vector, enabling the distinction between a power swing and an OOS condition.The underlying principle is that due to the inertia of the system, it takes some time for the rotor angle to increase during power swings.As a result, the rate of change of the impedance phasor is relatively slow during power changes, whereas in the event of a fault, the impedance changes rapidly with high speed [14].
Practical implementation of measuring the rate of change of impedance often involves two impedance measuring elements along with a timing device.If the measured impedance remains within the set limits of the two impedance measuring elements during the specified time, the relay recognizes it as a power swing and activates the signal to block the distance protection action.After the designated time, the relay will trip the protected transmission line breaker if the power swing condition is not reset.The timer is initiated when the apparent impedance enters the outer characteristic, as shown in Figure 7.When apparent impedance remains between the inner and outer zones for a predetermined time, the PSB element activates, and the set distance protection zones are blocked during that time.
In the case of the OST scheme, when a loss of synchronism occurs, it can use the same or different measuring elements.When the criteria for power swing detection are not met, and the OST scheme is selected, all relay zones are temporarily blocked to prevent premature tripping [15].The impedance vector "Z" is continuously monitored, and when it leaves the power swing region, its "R" component is checked.If the R component retains the same sign as at the point of entry, it indicates that the power swing is in the process of stabilizing.However, if the vector passes through the Mho characteristic (trace "C" in Figure 8), it suggests a loss of synchronism.
Traditional methods are [14]: a. Scheme with two constraints for PSB and OST functions, b.Single-constraint scheme for the OST function, c.Concentric characteristic schemes.

Advanced methods
New digital technologies provide engineers with the ability to develop new protective relay functions and implement new methods to detect power swings.The literature ( [14], [16] ) describes numerous innovative techniques that eliminate the need for user-input settings and significantly streamline the application of power swing detection and protection.These advanced methods leverage the advantages of digital signal processing and automation, making power system protection more efficient and reliable.
Part of the methods determines power swings based on a continuous impedance calculation.The impedance calculation is performed for each 5 ms step and then compared with the result from the previous 5 ms.As a result, two continuous deviations of impedance change due to power swing can be predicted.
Other methods involve calculating the centre of the electrical system, which is the point or points in the system where the voltage becomes zero during an unstable power swing.By detecting the rate of change of zero voltage, these methods can determine whether a power swing is occurring.This approach provides a robust and accurate means of identifying power swings without the need for user-defined settings.
In addition to the mentioned methods, a synchrophasorbased out-of-step relay measurement is also proposed for detecting and taking measures for power swings.Many customer services are currently evaluating the use and application of synchronized measured system phasors.As this technology continues to evolve, new and innovative methods of detecting power swings are sure to be developed.

Limitations for advanced methods
The three non-conventional (advanced) methods mentioned above can effectively detect very fast power swings.However, for extremely slow power swings with slip frequencies of 0.1 Hz or less, they may benefit from supplementation with the conventional impedance rate-ofchange method.Generally, such slow power swings are less likely to enter the distance protection zones.During very slow power swings, conventional schemes can be configured with much smaller ΔZ between the inner and outer blinders.This adjustment is necessary because the impedance will take a considerable amount of time to pass between the two blinders.Additionally, a complementary conventional internal blinder can be placed as close as possible to the distant zones that need measurement, further enhancing the detection capabilities.

Other methods
Other power swing detection methods have been used, are currently under research, or are being implemented.This includes the R-Dot scheme described in the literature [14], the use of synchrophasors and waves.

A new method for preventing the operation of distance protection due to power swings
This method uses a time-invariant (d-q) frame to distinguish between swing and error, Mechraoui and Thomas [17] were the first to use this method.They monitored the load angle between the relay and the end of the protected zone in order to identify the fault and unblock the relay.
The operation is similar to PSB, but the expected behaviour is for the apparent impedance to pass through both the inner and outer zones (see Figure 8).
ISSN:2566-3143, eISSN:2566-3151, DOI: 10.2478/bhee-2023-0010 It prevents the distance relay from operating under power swing conditions and allows it to operate if a fault occurs.In this method, the distance relay is not blocked during power swings but instead transfers voltage and current to the d-q frame.Therefore, once the power swing is detected, the voltage and current are transformed into a time-invariant d-q frame with the rotating frequency as the swing frequency.Subsequently, they are used in the calculation of the distance protection impedance.
In studies of the transient and dynamic stability of large power systems, the variables of all power systems components, except for synchronous machines, are represented in a reference frame that rotates at synchronous speed.All known real transformations for these components are also encompassed in the transformation of the arbitrary reference frame.We could formulate one transformation (Park's Transformation) of an arbitrary frame of reference that could be applied to all variables [4].The change of variables that formulates the transformation of the 3-phase variables of the stationary circuit elements into an arbitrary reference frame can be expressed as: where ( ) ( ) ∫ ξ is the dummy variable of integration, and ω is the angular velocity of the arbitrary frame.Certain large system disturbances cause severe oscillations in machine rotor angles and significant swings in power flows.The rotor oscillation can be separated from the nominal speed as an arbitrary frame, and stationary variables such as voltage and current can be transformed into this (d-q) frame.In this frame, the frequency of the rotating frame is equal to the sliding frequency.If the voltage and current are transformed into a (d-q) frame with a sliding frequency, the swings are completely removed from their waveforms.

The proposed anti-blocking algorithm during power swings
The basic method for preventing distance relay operation during power swings is blocking.A power swing block is a device that sends a continuous block signal to a distance relay upon identifying power swings, preventing it from taking any action until a fixed block time has elapsed.
A new method is proposed that does not entirely block the distance relay when power swings occur.Instead, it utilizes a function to eliminate the swing components from the variables used in the distance relay.
The basic difference between the conventional and the proposed method is illustrated in Figure 9.In this state, the distance relay is unable to detect the power swing, thereby preventing false tripping during swing events.However, the proposed method ensures that the distance relay functions correctly and responds appropriately when a fault occurs.In this technique, after detecting the power swing, the voltage and current signals are transformed to a (d-q) frame with the slip frequency before being used in the distance relay.Since the slip frequency is a time-varying variable, a relay cannot directly determine its exact value due to the complexity of power systems.However, an estimation of the average slip frequency can be made.
In the system, during power swings, the amplitudes of signals oscillate, and they are generally equal to their amplitudes in a stationary state or exhibit only a small change.Under such conditions, the signal obtained from the (d-q) transformation remains nearly equal to the amplitude before the transformation.However, if the amplitude is further increased, it does not significantly affect the operation of the distance relay.This is because the amplitude change resulting from the (d-q) transformation occurs in approximately the same ratio for both the voltage and current signals.
In addition to fault detection during power swings, the new method offers another advantage, which is the monitoring of voltage and current after their transformation into the (d-q) frame.This monitoring enables the observation of changes in slip frequency, which can be utilized to optimize the swing removal process.Unlike the conventional PSB approach, where the relay is blocked for a fixed time, the proposed method's continuous monitoring of signals allows for more dynamic decision-making.With the proposed method, it becomes possible to make informed decisions regarding whether to continue the transformation in the (d-q) frame or stop it.

SETTING OF POWER SWING SCHEME
Tuning the power swing element is typically accomplished through extensive and long-term stability studies.
Employing stability studies to adjust the power swing element is considered the most effective method.However, power swing elements can also be adjusted using known system conditions and certain assumptions about power system performance.These methods can be suitable for PSB schemes and may yield satisfactory results.Nevertheless, these methods might not work as effectively for OSB schemes.
Compared to other tuning methods, using an impedancebased tuning method has demonstrated favourable performance in most applications [14], particularly in cases where there are no significant changes in the source and transfer impedances.
When calculating equivalent impedances, an additional piece of information is required, known as the power slip rate.The power slip rate is equivalent to the rate at which the system oscillates or the rate of change of impedance observed from the relay location.It represents the speed at which the oscillations occur during power swings.
The references ( [14], [16]) outlines several steps for setting up the PSB blind scheme, as shown in Figure 7.
1. Set the outer characteristic impedance blinders inside the maximum possible load with some safety margin.This ensures that the outer blinders are sensitive enough to detect power swings effectively, but also provides a safety margin to prevent false tripping during normal system variations.2. Set the inner impedance blinders outside the most overreaching protection zone that is to be blocked when a swing condition occurs.The inner blinders should be positioned to encompass the zone that needs to be blocked during power swings, while still ensuring that the distance relay can accurately detect faults outside of this zone.3. Based on the outer and inner blinders set previously, the predetermined PSB timer value, T 1 , can be calculated from equation (12).Additionally, assume a maximum From Figure 10, it becomes evident that obtaining the appropriate source impedances in a complex power system is challenging.These source impedances are essential for accurately determining the blinder and PSB timer settings in the PSB scheme.Where: Z1S: local source impedance; Z1L: line impedance; Z1R: remote source impedance; Ang2R and Ang1R: machine angles at the Outer and Inner Blinder of the zone, respectively.
Comprehensive system stability studies are typically required to consider all possible contingencies when constructing the equivalent source impedance for a conventional PSB function.Moreover, a modern relay system with the capability of detecting "load violation (entering the load zone)" becomes essential, especially for situations involving long transmission lines with heavy loads [14].In such cases, the load area is in close proximity to the distance element that requires blocking during power swings.

CONCLUSION
Power swing refers to the fluctuation in three-phase power flow that occurs when the generator angles of different synchronous machines advance or decelerate relative to each other in response to various factors.These factors can include changes in load magnitude and direction, line switching, loss of generation, faults, and other disturbances within the power system.In real-world applications, PSB elements can be strategically placed using an impedance-based method, which involves developing system equivalents.Setting the tripping parameters for power swings requires data obtained from extensive stability studies.However, this process can be challenging and complex due to the numerous system conditions that need to be calculated, the creation of boundary equivalents, and determining the optimal locations to apply the scheme and separate the system.
Furthermore, it is not advisable to solely rely on PSB for unstable power swings without also implementing some form of OST protection at strategically planned locations.Due to the complexity of power swing protection systems, engineers responsible for selecting protective devices must carefully consider the power swing protection method incorporated into the chosen device.

R.
Muminović, A. Muminović: POWER SWING: DETECTION AND PREVENTION located on busbar S and studying the difference in angle change, the influence of power swings can be assessed.In Figure2, V S and V R represent the voltages at the sending point and the receiving point, respectively, while Z T denotes the total impedance between the two generators, comprising Z S , Z R , and Z L (Z T = Z S + Z R + Z L ).However, since the source impedances Z S and Z R are small and negligible, Z T practically represents the impedance of the transmission line.

Figure 8 :
Figure 8: "Mho" type detection of loss of synchronism Where is: Shaded zone: failure zone; A: Z moves into the OOS zone and slowly leaves; B: Z moves into OOS & Trip zones and slowly leaves; C: Z moves slowly = network becomes asynchronous; D: Failure = Z moves rapidly into the exclusion zone.
Advanced methods include: a. Continuous calculation of impedance ΔZ, b.Swing -Centre Voltage ( |V| cos φ ) and its rate of change, c.Synchrophasor-Based Out-Of-Step Relaying.

Figure 9 :
Figure 9: Structure of conventional and proposed method [19] b) Proposed method

Muminović: POWER SWING: DETECTION AND PREVENTION Faculty
Rasim Muminović was born in 1960, Kalesija.Graduated from the Faculty of Electrical Engineering in Tuzla in 1983, attended postgraduate studies in Zagreb and Sarajevo and defended his master's thesis at the Faculty of Electrical Engineering in Sarajevo in 2009.He has been employed at JP EP BiH in Sarajevo since 1984, working on protection testing, coordination of protection settings and analysis of power system operation.Design, construction and testing of high-voltage and mediumvoltage plants.Armin Muminović was born in 1992 in Tuzla.He completed a B.Eng. degree in electrical engineering at the of Electrical Engineering in Sarajevo in 2016.He has been employed at Energoinvest joint-stock company Sarajevo since 2016, working on testing primary equipment, designing, and implementing electrical power facilities up to 400kV.