CHAPTER 1: INTRODUCTION
3. CHAPTER 3: PROTECTION SCHEME FOR VSC-HVDC
3.2. DC fault response
3.2.5. System configuration fault response
Although considered as an influencing factor, the effect of a systems configuration on the fault response of VSC-HVDC system is a topic that has not been covered extensively in literature. When discussing fault response current literature usually makes the assumption that the configuration is monopolar [65], [69], [75], [87].
45 It is however worth noting that some present and mostly future HVDC grids are expected to develop into systems with a bi-pole configuration. This is mainly because bi-pole configuration offers increased flexibility and higher extensibility in developing HVDC grids [75]. In this sub-section, the impacts of a system configuration and the main contributing factors to the fault current during DC-side faults are evaluated. Similarly, the important factors to consider during this analysis include the magnitude and rate of rise of fault current.
3.2.5.1. Symmetric monopole
Monopolar configuration can be classified into two main categories; the symmetric monopole and asymmetric monopole. Amongst these the symmetric monopole is the configuration most used in VSC- HVDC systems and will thus be considered for this evaluation. A symmetric monopole is a single converter with mid-point ground between positive and negative voltage polarities as shown in Figure 3- 11. The analysis of this configuration is the same as that described in Section 3.2.1 for 2 level converters.
Figure 3-11: Symmetrical monopole configuration.
If a DC short-circuit fault has disrupted the normal operation of the network, IGBTs can be blocked for self-protection, leaving reverse diodes exposed to overcurrent [74]. During a short-circuit fault in a DC transmission line, the capacitor will be discharged rapidly. When fault occurs in DC-side, the IGBTs can be blocked for self-protection during faults, leaving reverse diodes exposed to overcurrent [82].
The fault demands that both converters should be blocked. It is evident in Figure 3-5 that the DC fault causes a very sharp rise in the DC current, putting the converters and other components of the system at the risk of failure. Figure 3-5 presented in the Section 3.2.1. for 2-level converters shows the graphical results of the fault current response of a monopole VSC system in case of line-to-line fault, resulting from previously developed dynamic models. Similarly, the main contributions are from the DC capacitors and energy storage elements in the transmission cable. Their size directly affecting both the magnitude and peak time of the fault current. In the case of a ground fault, the faulty pole is temporarily grounded. The contributions from the AC side are restrained through the freewheeling diodes therefore there is zero steady state fault current.
46 There are over-voltages at the healthy pole because of contributions from the DC capacitors that lead to a steeply increasing circuit breaker current [87]. If both cables and transformers are rated to operate under DC-link voltage stress, the link can be operated as asymmetrical monopolar. All monopolar configurations however lack redundancy as, in the case of a DC fault, all terminals are affected by the high fault currents. The main requirement for this type of configuration is therefore that the fault clearing techniques clear the fault before the end of discharging of DC capacitors to ensure that VSCs can continue their operation and resume power transmission.
3.2.5.2. Bi-pole
In this configuration, two asymmetric monopole systems may form a bi-pole system via series connections. Its structure is shown in Figure 3-12. It can achieve double power rating of monopolar topologies but unfortunately, it exposes the transformer to DC stresses. In a point-to-point bi-pole HVDC link a fault that is more likely to occur is between one converter pole and the ground. The healthy pole conductor can be used as the return path in case of the single pole outage. However, a low voltage dedicated conductor (metallic return) is required to operate as the return path in meshed HVDC grids [38].
Figure 3-12: Bi-pole configuration.
In theory, the bi-pole configuration, is considered a more reliable and redundant technique favourable for implementation for future developments. Again, assuming that the bipolar system is mostly affected by single pole-to-ground faults, the healthy pole is still capable of transmitting at least 50% of the total active power, provided the power command to healthy VSC terminals is not reduced to zero. Furthermore, provided that the AC networks are sufficiently strong to tolerate the impact of DC faults while accepting additional power [38], [66]. This means that before, during, and after the fault, the positive pole converters operate independently from the negative pole converters. The redundancy of this configuration is however only valid if the system is only affected on one converter pole.
47 Should there be a rare case where both the converter poles are affected at the same time the entire system will be affected by the system threatening overcurrents as in the case of symmetrical monopoles [12]. Bi- poles are expected to give rise to much higher fault current levels due to the additional infeed from the AC side. The transients are also associated with the discharge of stray capacitors of the affected DC cables. As mentioned previously, not much has been covered in literature on the fault response of the bi- pole configuration as most evaluations assume that the system is monopole. Studies that have taken the initiative to address this issue are mostly limited only to ground faults [68], [86], [167], [168].
Wang [165] takes a different approach analysing DC fault in bi-pole HVDC grids, in particular taking unbalances and grounding relocation into consideration. The analysis is still however carried for ground faults. A representation of short-circuit faults is seen in the study of over-voltages by Jardini in [87]. This study proves by means of simulation that although the configuration is redundant, it also has limitations like in the case of a short-circuit fault. Protection scheme techniques for these systems remain primarily to detect, locate and isolate any potential threats to the system. In theory, for both configurations, during DC-side faults, the protection methods presented in Chapter 2 will apply for both configurations. The travelling wave method is still highly recommended for its speed and accuracy. To isolate the faulted sections during a DC fault, isolation techniques include the use of a DC CB. Additionally, AC CB are included to aid in protection, isolating the DC system in its entirety.