Commutation Failure in HVDC ⭐️ (High Voltage Direct Current) occurs when the current flowing through the converter fails to commutate or switch from one valve to another. This can result in a sudden increase in voltage, which can damage the equipment and cause disruptions in the power system.
👉Commutation failure can be defined as an adverse dynamic event that occurs when a converter valve that is supposed to turn off, continues to conduct without transferring its current to the next valve in the firing sequence. Its occurrence causes temporary interruption of transmitted power and stresses the converter equipment.
👉System faults on the AC networks connected to a line-commutated HVDC system with thyristor based technology result in voltage disturbances in the form of voltage magnitude reduction and/or phase shift. If the disturbance is severe enough, the power transmission is interrupted by a so called commutation failure.
👉Commutation failure can be caused by various factors, including improper design or sizing of the HVDC system components, inadequate insulation, inaccurate control of converter firing angle, improper grounding, and faults in the system. To prevent commutation failure in HVDC, it is important to ensure proper design and sizing of the system components, adequate insulation, accurate control of converter firing angle, proper grounding, regular maintenance and testing, and installation of proper protection systems.
👉Commutation failure (CF) is an adverse and frequent dynamic event in high-voltage direct current (HVDC) systems based on line-commutated converter technology. It is most often caused by AC system faults in the receiving-end grid. CF causes a drastic increase in DC currents at the DC side, which poses a serious threat to converter equipment, and causes temporary interruption of transmitted active power, leading to instability issues on the AC side. Therefore, it is crucial to establish a systematic assessment method for CF caused by AC system faults, which can assist CF prevention and contingency response.
👉Methods based on electromagnetic transient simulation are believed to be more accurate, considering that the commutation process of HVDC converter valves is modeled in detail and a CF can be identified by checking behaviors of valve switching. Nevertheless, this method inevitably leads to heavy workload and long computation time for large-scale AC/DC hybrid power systems, which cannot meet application requirements of power system planning and operation.
👉Considering disadvantages of the simulation-based assessment method, another idea is to develop analytical assessment methods which are convenient for sensitivity analysis, screening studies, and so on. Earlier approaches to assess the vulnerability of the converter to CF used the maximum permissible balanced voltage drop on the converter’s AC bus. If the voltage dropped by more than this amount, CF was presumed to occur.
PREVENTION OF COMMUTATION FAILURE:
Methods to prevent CFs can be divided into four categories in general.
(I) optimize protection and control characteristics;
(II) install auxiliary equipment;
(III) coordinate DC power; and
(IV) modify converter structure. However, due to the complex causes of CFs in HVDC transmission systems, especially in multi-infeed systems, the existing methods cannot prevent all CFs under diverse and complex disturbances.
👉Commutation failure can be avoided by using a Commutation Control System (CCS) another option. The CCS is used to control the DC current of each phase of the power system, to ensure that the phase angle of the current remains consistent with the phase angle of the applied voltage. This helps reduce the chances of a commutation failure, as well as improving the power quality of the system. Other methods of avoiding commutation failure include using pulse-width modulation techniques, as well as using smooth current regulators to limit voltage transients.
COMMUTATION FAILURE IN A BULK POWER SYSTEM CAN BE AVOIDED THROUGH VARIOUS MEASURES....., INCLUDING:
1. Proper design and sizing of the HVDC system components: The HVDC system components such as converters, filters, and transformers should be designed and sized appropriately to ensure their reliable operation.
2. Adequate insulation: Adequate insulation should be provided for the HVDC system components to prevent breakdowns and commutation failures.
3. Accurate control of converter firing angle: The converter firing angle should be accurately controlled to ensure proper commutation of the HVDC system.
4. Proper grounding: Proper grounding of the HVDC system is essential to prevent ground faults and commutation failures.
5. Regular maintenance and testing: Regular maintenance and testing of the HVDC system components can help identify potential issues before they lead to commutation failures.
6. Proper protection systems: Proper protection systems should be installed to detect and isolate faults in the HVDC system before they cause commutation failures.
7. Monitoring system performance: Continuous monitoring of HVDC system performance can help detect potential issues before they lead to commutation failure. This includes monitoring system voltage, current, and power flow, as well as analyzing system data to identify trends and anomalies.
8. Load resources: By utilizing available load resources loss.
9. Proper training: System operators must receive proper training on HVDC system operation and maintenance to ensure they are equipped to identify and address potential issues before they lead to commutation failure.
10. Ensure that adequate control strategies are adopted to eliminate interaction between the HVDC system and its AC counterpart.
11. Make sure the switching surges are filtered out of the HVDC system to prevent any damage to the system components.
12. Utilize an effective communication network between the HVDC system and the AC System to facilitate monitoring, control, and coordination.
13. Perform preventive maintenance on the equipment to ensure its proper operation and reliable performance.
14. Ensure proper selection of equipment such as valves and switches as per the individual HVDC project requirements.
15. Adhere to the transmission capacity and voltage levels while operating the HVDC system.
16. Monitor the harmonic levels and power flows to ensure their optimal performance in the system.
17. Use proper protective devices and insulation to protect the system components from damage.
By following above measures, the risk of commutation failure in a bulk power system can be significantly reduced & ensure reliable operation of HVDC.
EVENTS AROUND THE WORLD ON COMMUTATION FAILURE OF HVDC
Commutation failure events have been reported around the world, including in countries like India, France, the UK, Japan, and the United States. Here are some examples (Error & Emission possible):
1. In 2012, a commutation failure occurred in the HVDC link between Sweden and Denmark, causing a blackout in parts of Denmark and southern Sweden.
2. In 2015, a commutation failure occurred in the HVDC link between Tasmania and mainland Australia, leading to a blackout in Tasmania.
3. In Japan, a commutation failure occurred at the Rancho Seco HVDC connection in October 2015. The failure was triggered by an external fault, leading to harmonic oscillations in the system and causing a partial bracing of the power line.
4. In the United States, two commutation failures occurred in October 2017 at the Tres Amigas hub, an energy facility in New Mexico. The failures caused an overheating of the converter transformers, causing significant degradation in the power system's stability.
5. In 2018, a commutation failure occurred in the HVDC link between China and South Korea, causing a power outage in parts of South Korea.
6. In India, for example, two commutation failure events occurred in April and May 2019 in the northwest state of Rajasthan. The events were caused by external faults and were attributed to a partial arc produced in one of the converter bridges due to incorrect commutation.
7. In France, a commutation failure event occurred in February 2019 at the Chams-Laogny HVDC connection, which runs between the Ardèche and Pas-de-Calais Regions. The event caused an overload in the power link and affected power supply to the 11.7 km long transmission line.
8.In the UK, a commutation failure occurred at the UK-Sweden Interconnector in June 2020. The failure stemmed from a sudden surge in the system's reactive power when the two converters were switched in opposite directions.
Summary, commutation failure events seem to be an issue that affects all countries with HVDC systems. Thus, appropriate mitigation strategies are key to ensuring the stability and safety of HVDC systems around the world. These incidents highlight the importance of proper design, maintenance, and protection of HVDC systems to prevent commutation failure and ensure reliable operation.