CHAPTER 1: INTRODUCTION
3. CHAPTER 3: PROTECTION SCHEME FOR VSC-HVDC
3.3. The proposed protection method
3.3.2. Back-up protection
57 a) Wave velocity
Wave velocity (v) is mainly relies on the transmission cable property. It is calculated by equation (3-22) [67]: -
LC
v c (3-22)
b) Arrival time(s)
Arrival times are found using the current derivative calculated from the measured DC currents. The arrival of the wave front considered during location are only those above the derivative threshold. This value can be read from the corresponding plots. A MATLAB code is however developed for precise location.
58 Figure 3-19: Overcurrent protection method.
3.3.2.2. Differential protection
One of the most reliable techniques in power system protection is differential protection. The operating principle of this technique is to calculate the difference between the current entering and leaving the protected zone. This information is relayed to the converter stations via a telecommunications structure.
Differential protection is admired for its ability to protect a system selectively. This kind of selectivity exploits the principles of Kirchhoff first law at the node, where the sum of the currents in a node must be equal to zero, if the summation of the currents is different from zero it means there is a fault [91].
In a VSC-HVDC system, although Kirchhoff’s principle is still applicable, it should be noted that when a fault occurs on the DC cable, the DC current flowing through one end of the cable differs from the current flowing at the other end. This is a consequence of the resulting wave attenuation found in long distance DC cables and the communication delay [67]. VSC terminal arrangements or topology could also be a possible cause of unequal fault currents at faulty cable ends.
59 The algebraic sum (or difference calculated) will sometimes have an offset value not equal to zero. The value is used to detect DC faults. For this technique, the resulting difference is compared to a threshold for selective DC fault identification. If a part of the system exceeds the set threshold for a predefined time the protection is set to be enabled to isolate the faulty equipment from the rest of the system.
Figure 3-20: Differential protection method.
A flow diagram applying the principle of this method is shown in Figure 3-20. During a DC fault, the cable differential current value increases quickly and reaches a large positive value. This can also be used as an indication of the faulty cable since all healthy cable will measure a negative differential current after the fault.
60 Differential currents measured at all cable breakers are then compared with a positive threshold to detect the fault location. The differential current equation is represented by equation (3-23) where i_diff is equal to the-real-time local measurement IDCrectand IDCinvrepresent the DC current from the rectifier and inverter VSC terminals respectively and t0 represents the initial time of the system [70].
) 1 ( )
(
_ I t0 I t0 ms
i diff DCrect DCinv (3-23)
Unlike most mentioned detection techniques, the differential protection is highly selective. However, information from each converter must usually be transferred between two ends of the DC cable. For very long cables, the sensitivity of the technique is lowered. The response time increases due to the increased communication time. Errors in the communication system also contribute to the schemes vulnerability.
The lack of selectivity and prolonged time delay are also undesirable characteristics for a MTDC systems and so the protection strategy has not been selected for implementation in the developed MTDC VSC- HVDC system.
3.3.2.3. Derivative protection.
Current or voltage derivatives have proven to be very useful in HVDC protection, particularly for the location of faults in a system. To detect a fault through derivative protection, a calculated threshold magnitude is compared to the sum of the measured current and voltage derivative. The process is shown in the flow diagram in Figure 3-21 [184]. A trip signal is triggered if the resulting derivative magnitude exceeds the set threshold. The current derivative is usually the only input used for determining faulted DC cable. Since voltage is usually measured at each DC bus for each VSC station, its results are no good indication of locating the DC cable that is faulty. DC-link capacitors closer to the fault are the station components that first get discharged during a DC fault. They contribute the most to the fault current.
Because of the large change in the current of the faulty line, the current derivative is higher for the faulty line than for the rest of the network [70].
A DC fault very close to the converter yields higher derivative magnitudes. The detection of faults using the technology is also quite fast. This complies with the requirements of the protection system that state that VSC stations closest to DC faults are the most vulnerable and have a short reaction time. The biggest advantage of the derivative protection technique over the mentioned possible back-up protection strategies is its ability to be selective. In this method, the sign of the current derivative points to the fault location. A derivative magnitude with positive polarity means the fault is in the designated protection area. A negative derivative magnitude on the other hand indicates a fault is located outside its assigned zone of protection [83].
61 The main advantage of derivative protection is its detection speed and selectivity. The location method is however very sensitive to noise [11]. To determine the relevant threshold settings, detailed network studies are to be carried out to ensure that the protection scheme implemented is stable for all other disturbances except those that affect the DC line. For the implementation of derivative protection, the line currents and their derivatives during a fault are simulated beforehand. The current derivative thresholds must be evaluated based on the system parameters and are different for different topologies. These threshold values depend greatly on the cables and the DC-link capacitor size.
Figure 3-21: Operational process of voltage derivative protection.
62 A weighted sum of the derivatives is calculated using equation (3-24). The weight of current derivative is represented byK1and voltage derivatives byK2. K1is typically rated at 1 and K2 at 0 [83]. This implies that only the current derivative contributes to the weighted sum when determining a faulted cable.
dt K dV dt
K )dI ( )
( 1 2
(3-24)