CHAPTER 1: INTRODUCTION
2. CHAPTER 2: LITERATURE REVIEW
2.4. Current protection techniques
2.4.1. Fault identification
During steady state operations, a power system carries normal voltages and currents which results in the safe operation of the system [80], [81]. An electrical fault would therefore be the deviation of voltages and currents from the nominal values or states.
15 As mentioned previously, a VSC-HVDC system may experience disruptions in the form of AC-side faults, inner converter faults and DC cable or overhead line faults. Internal faults occur inside the converters and include IGBT-shoot through and short-circuits across DC rails. These types of faults are usually managed by the control systems whose task is not only to govern the basic functioning of each converter in a VSC-HVDC system but also to take care of the various disturbances that can affect the system [61]. They are less frequent as compared to external AC/DC faults or faults that occur in cables or overhead transmission lines. AC faults in a VSC system on the other hand include combinations like the single line-to-ground fault, the line-to-line fault, line-to-line-to-line (3 phase fault) and a line-to-line-to- line-to-ground fault [62].
Since they are backed by a mature protective scheme, AC faults can be prevented from influencing the grid side converters by using switch-gear like fuses, mechanical disconnectors and AC circuit breakers [13], [78]. VSC topologies are therefore mostly threatened by DC-side faults. External faults or DC-side faults can be further classified as either, short-circuit DC faults, ground DC faults, overcurrent and over- voltage [52]. Amongst these, the DC short-circuits are the most severe while the ground faults are the most frequent to look out for. During a DC short-circuit fault, the IGBTs can be blocked for self- protection leaving reverse diodes exposed to overcurrent. Analysis of the fault response is thus essential in determining the challenges involved in DC grid protection.
2.4.1.1. Line-to-line faults
Short-circuits shown in Figure 2-2 are otherwise known as line-to-line faults (where Trrect is the rectifier side transistor, Lrect is the rectifier-side reactor, Iconv1 is the converter current and CDC1 is the converters DC-link capacitance). Short-circuits usually occur when an object falls across the positive and negative line in the case of overhead lines (OHL) or when insulation fails in cables [82]. They may also occur in the event of a failure of a switching device causing the line to short [62].
Cables are almost immune to these type of faults, in fact the mentioned facts on cables when it comes to faults almost makes them an obvious choice when designing VSC-HVDC systems [71]. OHLs are however used in some systems because of certain reasons that put them at a better advantage than cables.
The fault currents caused by short-circuits may lead to the melting of transmission lines as the overhead lines, cables and windings will experience excessive heating. This may even cause a fire or an explosion [83], [84]. Stability of the power system may be adversely affected and can even lead to cascade tripping or complete shutdown of the power system.
16 Figure 2-2: Rectifier side of VSC-HVDC illustrating line-to-line faults.
2.4.1.2. Line-to-ground faults
DC-link and DC cable ground faults as shown in Figure 2-3 are usually known as line-to-ground faults or just ground faults (the components are as described in Section 2.4.1.1). They occur when the positive or negative line is shorted to the ground. In overhead lines, this may be due to lightning strikes that may cause the line to break or fall to the ground creating a fault.
In a cable connected system, ground faults are almost always caused by insulation deterioration and breakdown due to physical damage, environmental stress and electrical stresses [11]. Although ground faults are less frequent in cable systems, they are often permanent and require removal of the affected network until the disruption is cleared. A ground fault results in the rapid discharge of the faulted poles capacitance [85]. The negative and positive pole will therefore experience an imbalance of DC-link voltage. As the voltage of the affected line begins to fall, high fault currents flow from the capacitor as well as the AC grid [86]. The high fault currents are known to possibly damage the convertor or capacitors. Ground faults behaviour are highly dependent on the systems earthing configuration. At present, there are two main grounding schemes [62]:
1. Through the neutral point of AC-side transformer.
2. In the DC-link capacitors.
Figure 2-3: Line-to-ground faults.
17 A fault’s current magnitude and severity depends on a variety of factors like the location of the fault and the damage caused due to the fault [52]. While analysing a given fault’s severity, it is usual practice to refer to a standard fault condition for given voltage level. If DC-side faults are not taken care of accordingly, they could irreversibly damage the valves, diodes and other sensitive equipment of the system [86], [87]. This is one of the main reasons that raises the importance of VSC-HVDC protection.
Transmission line protection in particular, is based on the protection of system transients that occur during a fault condition. The DC faults mentioned previously are most likely to occur in overhead lines or transmission cables. The main characteristics of the mentioned faults is that if permanent, they cannot be extinguished until the current is brought down to zero. This will result in the collapse of the system’s line voltage while the current sharply rises. Protection should dependably discriminate between these different fault cases and take appropriate action quickly. Quick reparation of permanent faults in VSC-HVDC is essential for minimising down time and outage costs [78]. Normal converter control is usually not adequate to extinguish DC fault current. It is imperative that the fault be identified quickly, accurately and selectively to continue the operation of the system.