CHAPTER 1: INTRODUCTION
2. CHAPTER 2: LITERATURE REVIEW
2.5. Protection requirements
When developing VSC-HVDC systems, the design of a feasible control and protection scheme must be a priority. The proposed design fundamental principles for these systems are drawn from conventional HVDC systems and the mature HVAC networks [16], [34], [67]. These principles serve as a guideline that can be used for both protection schemes under development and those already installed. VSC-HVDC systems are not immune to disruptions, therefore it is always advisable that an electrical system is equipped with a very strong protection scheme. System protection is defined as the art and science of detecting and disconnecting faults and other abnormal conditions on a power system [147]. A protection scheme is tasked to monitor the system continuously, removing possible threats without interfering with or limiting the normal operation of the system. Important characteristics that are usually considered in the design of a protection scheme include: -
• Inclusion of features tasked to prevent failures and;
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• Features tasked with mitigating the effects of failure in cases where it does occur.
It is also just as important in a protection scheme that the following requirements are effectively met.
These are considered philosophies of protection [34] and include: -
• Speed: Is essential to avoid damage to equipment, where the faults are expected to be interrupted before they can damage the equipment.
• Selectivity: The protection system must have defined protection zones which would prevent the isolation of unaffected VSC-HVDC components. The system should also be able to distinguish the difference between normal operation, possible overload and a fault condition.
• Reliability: Includes dependability which implies that the protection scheme applied to prevent the network from being affected by faults should be readily available to offer its assistance only when it is required to, especially when considering large protection zones, in which large sections are isolated in case of a DC fault.
• Sensitivity: Is important to ensure that every faulty situation is to be accurately detected and cleared without exception.
• Robustness: The protection system must be able to operate even in degraded situations. When protection systems are duplicated this also provides redundancy and can aid in robustness.
• Seamlessness/stability: Where the system is expected to reach stable operation within an acceptable period after clearing the fault thereby allowing the system to continue operating securely.
It should be taken into consideration that even though existing methods for AC grids cannot be directly applied in DC grids since they are too slow to cope with the DC fault current phenomena, the development of DC-side protection system should be guided by the same highlighted protection requirements/ philosophies as they are well established in AC systems [16].
2.5.1. Protection requirements of a VSC based MTDC system
Protection measures, securing VSC-HVDC schemes from faults and disturbances are regarded as one of the main factors to have limited the growth of VSC-MTDC systems. Although vastly admired for their controllability and flexibility, due to their nature VSC-HVDC systems are defenceless against DC-side faults [148]. Proven protection methods in AC grids are not regarded as feasible for protecting VSC- MTDC systems against DC faults as they are too slow to cope with the DC fault current phenomena and require isolation of the entire MTDC system. Protection of MTDCs is regarded as even more challenging since they require even faster communication links. Much like point-to-point systems, the basic protection requirements for a MTDC transmission scheme are speed, selectivity, sensitivity and security.
31 For MTDC networks the emphasis is mainly on obtaining fast and selective DC line fault detection, location and isolation techniques [67]. To satisfy the high speed operational requirements, current fast fault detection and location algorithms include travelling waves measured at the MTDC terminals. These methods use the initial transient fault signatures for detecting and locating commonly occurring faults like pole-to-pole and pole-to-ground faults [149]. A selective protection scheme ensures dependability and certainty of correct operation once a fault occurs in a protection zone. As mentioned, currently favoured fault detection and location methods for HVDC transmission include travelling waves. However, due to multiple paths in the MTDC networks, this technique becomes more complex to implement and control.
Other commonly applied detection and location techniques include voltage and current differential as well as derivative methods [23]. As the technology progresses, there have been numerous other techniques presented to detect and locate DC faults in a VSC-MTDC system.
Some of these include the methods based on the electromagnetic time reversal (EMTR). This method on time reversal process is applied to travelling waves in transmission lines using measurements at one terminal only. Its details are further elaborated in [18]. Studies carried out in [150] also proposes that a DC fault is managed using what is referred to as the delayed-auto-re-configuration (DARC), for VSC MTDC networks. This protection scheme does not require additional components to be added to the existing system but instead the MTDC system is protected after the isolation of the faulted line section, where un-faulted terminals will be controlled and recovered to form a new MTDC configuration.
It is essential to note that besides focusing only on addressing the critical design requirements of selectivity and speed, the system’s grounding schemes play a huge role in influencing the fault detection and protection solutions [8]. Likewise, it is important to ensure that the designed scheme is equipped with back-up protection and the auto reclosing function for quick service restoration after temporary fault trips.
2.5.2. Future of VSC- HVDC transmission protection
The development of a robust protection scheme for a VSC-HVDC system will surely play an important role in the future of these systems. The current trend that is seen implemented for the protection of these networks, focuses a lot on improving fault location and or the development of a DC CB able to withstand the fast and high rising DC fault currents. The choice to focus on these properties is understandable, as it is a vital task to detect, locate and isolate DC faults before they can affect the rest of converter of the station. It is however also important to note that there have been commendable advances in other areas of VSC-HVDC protection. In addition to fault location and isolation, research has broadened into developing ways of limiting fault currents such as including FCLs to reduce the severity of DC faults.
32 From the available technologies, super-conductors emerge as a promising solution. More information on the super-conducting fault current limiters is presented in references [99] and [115]. The development of new valves is also proposed by Alstom and this technology is seen as an equally promising approach in protection. The innovation of this technology aims to combine existing VSC topologies to produce a prototype that is meant to deal directly with some of the constraints posed by VSC schemes. The Alstom hybrid valve combines concepts of the conventional converters (i.e. LCCs) with those of MMCs [3]. The technology results in relatively lower losses and low distortions presented by half-bridge MMC as well as DC fault blocking capabilities from full H-bridge MMC [3]. Unfortunately, this solution is mostly beneficial for stations still under development [151].
For VSC-HVDC networks that are already in existence, it is considered more feasible to develop DC isolators or breakers to perform the task of protecting existing VSC-HVDC networks from being affected by DC faults. For complete protection, an isolation device is always necessary in the system. These work by starving the DC fault for a long period of time so that that there are no possibility of system re-striking when the voltage re-appears [152]. DC switches although fast, are incapable of interrupting fault current [152]. A feasible solution is the ABB DC breaker. Although the breaker is praised for its numerous attractive traits, the prototype has yet to be implemented in real life .The costs associated with a device of its nature are not expected to be trivial [113]. Research therefore looks to find a similar technology that can perform these functions at a reasonable amount without affecting the networks reliability.
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