Power Swing Detection and Testing Methods

 

What is a Power Swing? 

Power swings are typically caused by large disturbances such as a fault on a transmission line, sudden changes in power demand, or the sudden loss of a generator or transmission line. When these disturbances occur, the power system undergoes a rapid change in the power flow, and the balance between the generation and the load in the system is disrupted. As a result, the voltage and current in the system oscillate back and forth, causing a power swing.

Related Article: Fundamentals of Generator Protection

What Causes Power Swing? 

Power swings in power systems can be caused by a variety of factors, such as but not limited to: 

1. Faults on transmission lines - A fault on a transmission line can cause a power swing in the power system. When a fault occurs, the power flow in the system changes, and the generator must adjust its output to maintain balance. The adjustment of the generator output can cause a change in the rotor angle, leading to a power swing. 
2. Load changes - A sudden change in the load demand in the power system can cause a power swing. When the load changes, the power flow in the system changes, and the generator must adjust its output to maintain balance. The adjustment of the generator output can cause a change in the rotor angle, leading to a power swing. 
3. Generator trips - A sudden trip of a generator in the power system can cause a power swing. When a generator trips, the power flow in the system changes, and the remaining generators must adjust their output to maintain balance. The adjustment of the generator output can cause a change in the rotor angle, leading to a power swing. 
4. Switching operations - Switching operations, such as the opening or closing of a transmission line or a transformer, can cause a power swing in the power system. The switching operation can cause a sudden change in the power flow, leading to a power swing. 
5. Control system malfunction - A malfunction in the control system of the power system, such as a failure in the automatic generation control system, can cause a power swing. The malfunction can cause the generator output to change rapidly, leading to a power swing.

During a power swing, the generator experiences changes in its output and rotor angle, which can affect its performance and stability. The generator must adjust its output to maintain balance when the power flow in the system changes, causing fluctuations in the power output of the generator. A power swing can also cause changes in the rotor angle of the generator, affecting the synchronization between the generator and the power system and leading to instability. Overloading of the generator may occur during a power swing, causing damage to the generator and the power system equipment. Voltage fluctuations can also occur, affecting the performance of the generator and connected loads. The loss of synchronization is another effect of a power swing, which can lead to instability and damage to the generator and power system equipment. Understanding the effects of power swings on generators is essential in developing effective power swing detection and mitigation methods to ensure the stability and reliability of the power system.

Therefore, power swing detection is an important function in power systems protection, as it helps to prevent system instability and power outages. There are several methods that can be used for power swing detection, and they can be broadly classified into two categories: traditional methods and advanced methods. 

Figure 1. Load angle illustration

When a generator is connected to a power system, it supplies electrical power to the load and maintains the balance between the generation and the load. The load angle is an important parameter that determines the power flow in the system. Figure 1 illustrates that when the load angle becomes too large, the system stability can be lost. If the load angle becomes too large, it means that the power flow in the system is unbalanced, and the generator is not supplying enough power to the load. This can lead to a loss of system stability, which means that the power system becomes unstable and cannot maintain a steady state.

To prevent the loss of system stability due to a large load angle, various methods are used, such as power swing detection and mitigation methods, which are designed to detect and correct the oscillations in the power system caused by disturbances or faults. The use of these methods ensures that the power system remains stable and can maintain a steady state under different operating conditions.

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Power Swing Detection Methods

The rate of change of impedance is a power system protection technique used to detect power swings and prevent the instability of the power system. It is a frequency-based method that measures the rate of change of the impedance seen by the relay during a power swing. During a power swing, the impedance seen by the relay changes rapidly due to the oscillations in the voltage and current. The RoCoF method measures the rate of change of this impedance and compares it to a predetermined threshold. If the RoCoF exceeds the threshold, the relay operates and initiates a corrective action to stabilize the power system.

Concentric Characteristic Scheme

The concentric characteristic scheme is a type of distance protection scheme used in power systems to detect and isolate faults on transmission lines. The scheme consists of a set of concentric circles on the impedance-distance plane, where the center of the circles represents the location of the relay and the radius represents the reach of the relay.

When a fault occurs on a transmission line, the impedance seen by the relay changes, and the change in impedance is reflected on the impedance-distance plane as a point. The location of the point on the plane corresponds to the distance and impedance of the fault from the relay. If the point falls within the reach of the relay, the relay will operate and initiate a corrective action to isolate the fault.

Figure 2. Stable Swing

The concentric characteristic scheme is based on the principle that the impedance seen by the relay during a fault is proportional to the distance of the fault from the relay. By plotting the impedance-distance characteristic on the plane, the relay can determine the distance of the fault and operate accordingly. 

Figure 3. Unstable Swing

The concentric characteristic scheme is a simple and reliable protection scheme and is widely used in power systems. It can detect and isolate faults quickly, which helps to prevent damage to the power system equipment and maintain the reliability of the power system.

Pole Slip Detection Methods

During a power swing, the oscillations in the voltage and current can cause the rotor of the synchronous machine to lose synchronism with the rotating magnetic field of the stator, leading to a reduction in the power output and efficiency of the machine. The pole slip detection method can detect the loss of synchronism caused by the power swing and take corrective action to stabilize the power system.

The pole slip detection method for power swing detection uses the variation in the stator current and voltage to detect the loss of synchronism. When a power swing occurs, the stator current and voltage become out of phase, and the power factor decreases. The pole slip detection method measures the phase angle difference between the stator current and voltage and compares it to a predetermined threshold. If the phase angle difference exceeds the threshold, the pole slip is detected, and the corrective action is initiated.

Figure 4. Pole Slip Detection Method

The pole slip detection method for power swing detection is a reliable method for detecting power swings and preventing the instability of the power system. It is widely used in power system protection and is an essential part of the protection scheme for synchronous machines.

Blinder Scheme

The blinder scheme is a power swing detection method used in power systems to detect power swings and prevent the instability of the power system. It is an impedance-based method that uses a two-zone protection scheme to detect power swings.

Figure 5. Double blinder

The blinder scheme consists of two zones, Zone 1 and Zone 2. Zone 1 is designed to detect power swings with high accuracy and speed, while Zone 2 is designed to provide backup protection in case Zone 1 fails to detect the power swing.

1. Zone 1 of the blinder scheme uses an impedance-based criterion to detect power swings. It measures the apparent impedance seen by the relay and compares it to a predetermined threshold. If the apparent impedance exceeds the threshold, the relay operates and initiates a corrective action to stabilize the power system. 
2. Zone 2 of the blinder scheme uses a time delay and a less stringent impedance threshold to provide backup protection. If Zone 1 fails to detect the power swing, Zone 2 is activated after a time delay and detects the power swing using a less stringent impedance threshold.

The blinder scheme is a reliable and fast power swing detection method and is widely used in power system protection. It can detect power swings even in systems with high impedance, and it provides backup protection in case of a failure in Zone 1.

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Power Swing Testing Methods

A power swing is a phenomenon that can lead to instability in a power system. To prevent the instability and ensure system stability, power swing detection methods are used in power systems. However, these methods need to be verified and tested to ensure their effectiveness in detecting and mitigating power swings. Power swing tests are, therefore, essential to power system operation and protection. These tests involve simulating power swings under different conditions and verifying the accuracy and reliability of the power swing detection and mitigation methods. Power swing tests are necessary to optimize system performance, comply with regulatory requirements, minimize equipment damage caused by power swings, and maintain the reliability and safety of the power system. The importance of power swing tests cannot be overemphasized as they are necessary to ensure the uninterrupted power supply to customers and prevent power system outages.

Figure 7. Testing Methods | Source: Doble Engineering PVT. Limited

Power swing testing methods are used to verify the performance and reliability of power swing detection methods in power systems. The testing methods involve simulating power swings under different conditions and verifying that the power swing detection methods can detect and mitigate the power swings effectively.

1. System-level testing - this testing method involves simulating power swings in a laboratory or field setting using a power system simulator. The power system simulator can simulate different types of power swings and different operating conditions to verify the performance of the power swing detection methods. 
2. Hardware-in-the-loop testing - this testing method involves using a real-time digital simulator to simulate the power system and a physical relay to test the power swing detection methods. The relay is connected to the digital simulator, and the power swing is simulated to verify the performance of the relay and the power swing detection method. 
3. Field testing - this testing method involves testing the power swing detection methods in the field under actual operating conditions. A power swing is simulated by injecting a signal into the power system, and the performance of the power swing detection methods is verified by comparing the measured results to the expected results. 
4. Fault injection testing - this testing method involves injecting faults into the power system to simulate power swings and verify the performance of the power swing detection methods. The faults can be injected using various methods, such as software-based or hardware-based methods.

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• Title: Application Guidelines for Power Swing Detection on Transmission Systems
• Source: Joe Mooney and Normann Fischer | Schweitzer Engineering Laboratories, Inc.