CHAPTER 1: INTRODUCTION
5. CHAPTER 5: RESULTS AND DISCUSSIONS
5.3. VSC-HVDC protection strategies
5.3.3. Isolation technique
To avoid any damage to the VSC-HVDC network, in this section, the hybrid DC circuit breaker was inserted into the developed model for isolation. The breakers were to be mainly triggered in cases where excessive currents that threaten the networks reliability were detected. The requirements of the DC breaker depend highly on the systems configuration and on the current limiters that might be included. It was also very important to ensure that the stations voltage ratings were not exceeded when switching high DC currents caused by DC faults. The literature of the hybrid DC breaker was thoroughly highlighted in Chapter 2, therefore in this section, only simulation results and discussion of the hybrid breaker are presented.
It is important to note that in this study, DC CBs were modelled as ideal switches with the corresponding overall interruption time which includes both the detection and isolation time. This indicates that other breaker factors including on-state losses, maximum withstanding current, etc. were not considered. The design of a more practical type of DC CB is however still worth more attention and research in the future.
For optimal results, the isolation method must work within a set interruption time to avoid reaching the high peak currents that will damage the system’s components. It should also work synchronously with the detection techniques implemented. To clarify DC line fault clearing characteristics, the main parameters that are considered for discussion include: -
• Circuit breakers co-ordination with the detection technique.
• Fault current interruption of hybrid breaker
For these tests, the DC fault was applied in cable DC12P, 50 km away from VSC1. For reasons stated in previous sections, only results of positive pole are shown. In addition, since the fault was injected between the VSC stations, results of the breaker located in VSC1 (i.e. DCbreaker12P) are presented.
5.3.3.1. Confirmation of hybrid DC breakers co-ordination with detection techniques
To validate the operation of the CB in the network, various fault cases were investigated. In this study only three of cases have been discussed. This includes a scenario considered as normal operation of a DC CB during a DC fault (Case A), a scenario where the detection technique failed to respond accordingly (Case B) and a scenario where the DC breaker had an error and was unable to isolate the threat in the network. Zero in the results represents the OFF state, while a one represents the ON state. In addition,
‘Fault’ indicates the DC short-circuit fault signal while W12P and W12N were the output signals for each pole in the station. SW12 is the activation switch for the back-up detection algorithm, DWTdc12P is the signal from the DWT primary detection scheme, derdc12P is the derivative back-up signal, breakerCtrl is the DC breaker controlling signal and ACBRK1 represents the AC breakers control signal.
118 a) Case A- During a short-circuit DC fault
When a fault was injected into the VSC-HVDC system, the DC current rose towards very high peak magnitudes. The systems protection scheme should detect and isolate the fault within the set interruption time. Figure 5-28 shows the results obtained during a DC short-circuit fault. The results depict the signals sent to switches in an event of a DC fault. When a fault was injected at t=5s, the ‘Fault’ signal rose to one to indicate that a fault was present in the system. Since the fault was line-to-line, signal W12P and W12N were triggered, indicating that they have both been affected. When all the systems components perform their function as designed, switch SW12 should remain at zero, indicating that the back-up protection has been currently disabled to send signals to the breaker. As expected, the signal from the DWT detection technique and the DC breaker control were both triggered to one, sending an instruction that the breaker must immediately isolate the affected cables from the rest of the system. This means that the main IGBT switches found within the DC CB were opened and so current was then commutated to the arrester path (i.e. path designed to absorb energy from the system). The back-up switchgear control signals were both not triggered confirming that the main protection techniques were fully operational.
Figure 5-28: Breaker signals for breaker located terminal VSC1 for case A.
b) Case B- Error in the main detection technique
Although a system may be equipped with a strong and reliable protection scheme, errors constraining its normal operation are never ruled out. Failure to configure the networks components correctly for example, may cause the main protection algorithm to fail. When this occurs, back-up protection must be readily available to perform the function of isolating the fault to ensure the robustness of the protection scheme. To investigate the networks response for such scenarios a fault was forced on the DWT control algorithm causing it to crash. Results of this test are presented in Figure 5-29.
119 Figure 5-29: Breaker signals for breaker located terminal VSC1 for case B.
In the event of a fault in this case, the ‘Fault’ signal rose to one to once again indicate that a DC fault was present in the system. Signals W12P and W12N show that there was error in the detection technique as the both remain at zero giving off the idea that there was no fault in the system even though a fault signal has been triggered. After a short time, delay, signal DWTdc12P confirms that there was indeed an error with the main protection and so the back-up protection switch SW12 was immediately enabled allowing the current derivative detection technique to further assess the system for any possible faults. When the derivative protection detects the disruption in the system, the DC breaker signal breakerCtrl12P was again triggered to isolate the faulty cable. Even though by the help of the back-up scheme the network was still protected, after isolation it was recommended that the main detection be fixed and put back into operation as soon as possible to avoid further disruptions.
c) Case C- Error with the DC CBs
Due to natural variations such as switching transients and over-loading, the DC breaker can also face numerous problems that might prevent it from performing its functions properly. In this case a scenario was tested when the DC breaker was unable to isolate the affected cable despite instructions to do so. At this point the AC breaker control has to pick up the detection signal to identify the presence of a DC fault in the system. In complex power systems, this is usually done using sophisticated algorithms. In this study, however, a logic control algorithm will trigger the AC breaker to isolate the affected VSC terminals. Since this method has been designed to isolate VSC terminals and not DC cables only as with the case of DC CBs, the technique was only used as a last resort after all other protection strategies implemented have been exhausted.
120 In fact, the technique should only be enabled for very severe cases in order to maintain the reliability of the system. The technique will also ensure that the AC grids in which the VSC terminals are connected stay protected even in rare cases where the whole protection technique implemented fails.
Figure 5-30: Breaker signals for breaker located terminal VSC1 for case C.
In the event of a fault in this case, the ‘Fault’ signal rose to one to indicate that a DC fault was present in the system. Signals W12P and W12N also respond accordingly, showing that there was a fault present in the system. After a small time, delay, signal DWTdc12P resulting from the DWT detection technique confirms the presence of a DC fault and so the back-up detection protection switch SW12 was not enabled. Unlike the previous cases, during this test, the DC breaker signal breakerCtrl12P was unable to detect or read instructions from the detection signals. Fortunately, by the help of the back-up isolation strategy the network threat could still be isolated by the AC CB and so the network was still protected. As mentioned, the protection technique should be quickly fixed and put back into operation as soon as possible to resume the transmission of power to customers.
In summary, A DC CB was included in the MTDC VSC-HVDC system to isolate possible threats posed to the system. DC CBs installed in the network receive their trip order from the detection algorithms chosen for the MTDC network. This was verified in the cases discussed in this sub-section. AC CBs were included as part of the networks isolation criterion. They act as back-up protection for the DC CB.
5.3.3.2. Fault current interruption of hybrid DC breakers
The time that the DC breaker takes to react to the fault determines the interruption time of the breaker.
DC systems have no natural current zero crossing and so the interrupting device must be equipped with a mechanism to create current zero crossing.
121 This can be achieved in the general interruption processes that includes the creation of a counter-voltage that exceeds the systems voltage. Ideally, the fault should be isolated before the peak magnitude can be reached. Knowing the interruption, time plays a big role in determining the overall design of the protection scheme set-up. In this case current limiting reactors of 90mH were used at ends of each cable.
A current limiting reactor as the name suggests was included to limit the amount of current flowing into the DC breaker.
The correct sizing of these reactors is important as its size affects current flowing through the breaker, voltage across the breaker and the magnitude of energy absorbed by the DC breaker. Although the proposed protection scheme incudes detection and location using the wavelet and derivative techniques, in this section, overcurrent was used to analyse the fault current interruption time of the breaker. The simulation results for a fault occurring 50 km away from VSC1 on cable DC12p are presented. For this test, during a DC fault the line-to ground voltage (Vbreaker) measured right after the DC breaker drops to zero (as seen in Figure 5-31). The fault current (Ibreaker) reaches a peak of 5.56 kA at 5.0019s. At this point, the circuit breaker starts to build a counter- voltage to force the current to zero to allow for isolation. A maximum of 400 kV (Vbrkout) was reached by the circuit breaker during this process (shown in Figure 5-32). At 5.005s the current drops to zero meaning that the breaker has opened or isolated the cable from the rest of the system. During a fault current interruption by an HVDC CB, the system voltage restores before the fault current was completely cleared. The voltage recovery process starts the moment the CB begins to generate the counter voltage.
Figure 5-31: Current measured at breaker located at terminal VSC1.
122 Figure 5-32: Voltage measured at breaker located at terminal VSC1.
When given the breaking time and the maximum breaking current capability, the only adjustable parameter is the inductance of the HVDC reactor, which decides the rate of current rise. The HVDC reactor therefore needs to be selected in such a way, that within the breaking time, current does not reach a level higher than the maximum breaking current capability of the HVDC breaker. From these PSCAD simulation results, it can be concluded that the model shows good responses for opening and closing on the detection trip order. Breaking time has been governed by the response time of the protection, and the action time of the HVDC switch. A longer breaking time would require the HVDC switch to have an increased maximum current breaking capability. This increases the energy handled by the arrester and correspondingly leads to a higher cost for the HVDC breaker.