Phasor-based algorithm

Block diagram of the proposed phasor-based reclosing algorithm is shown in Figure 4.31. Sim- ilar to the angle-based reclosing method, the phasor-based algorithm can also be initiated either by auxiliary contacts of the line circuit breakers or through analyzing CT secondary current as proposed in [19]. The faulted phase is determined and the process starts if single-phase-to- ground fault inception is confirmed. Present day line protection relays are equipped with phase selector function which reliably determines the faulted phases. In the proposed algorithm, as- suming that the faulted phase is known, VsnD andδD are obtained by derivating|Vsn|andδby

means of half-cycle LES algorithm, after anti-aliasing and CVT temporary filtering and pha- sor estimation [116]. Using these variables, first, the fault type is identified. For the permanent fault cases, three-phase tripping will be initiated immediately, while single-phase reclosing will be commanded once arc extinction is detected, for temporary faults.

Breaker interruption

detection

Determine Vh, Vk, Vs

Arc extinction detection (for transient faults) LES Derivation

Fault type recognition |Vsn| VsnD Anti-aliasing Filter + CVT Transient Filter + Phasor Estimation va vb vc δ

Final decision making

LES Derivation

δ _D

Figure 4.31: Block diagram of the proposed adaptive single-phase reclosing algorithm.

Fault type recognition

As mentioned in Subsection 4.1.2, after the line single-phase isolation, the ratio of VsnD to |V_snDN| is larger for temporary faults than permanent fault cases and the difference becomes

even larger after the positive peak, VsnDP. Hence, the voltage derivative ratio, V DR, as given in

(4.3) is defined for fault identification.

V DR= |VsnD

VsnDN

|(%) (4.3)

To calculate V DR, VsnD must be monitored after the line single-phase isolation to obtain VsnDN. Considering the fact that it takes around 3.25 cycles for V_snD to make a negative peak

and then recover to zero after the line single-phase isolation, as mentioned in Subsection 4.1.2,

VsnDN must be obtained within this time interval. For permanent fault cases, V_snDP appears

slightly after 3.25 cycles and then VsnDdrops to zero shortly, while it will remain positive as

the arc is being extinguished in cases of temporary fault. Therefore, by monitoring VsnDfor a

sufficient time interval after the line single-phase isolation, the fault type can be identified. In this research study, considering various delays such as local circuit breaker opening detection, delay between circuit breaker operation at both line ends, CVT temporary and relay filtering and estimations, it is proposed to monitor VsnD for 6 cycles or 100 ms after the line

single-phase isolation. Then, V DR is calculated at each protection pass for the 6th cycle, i.e. the time interval within 83ms to 100ms. In this case, 16 values for V DR will be obtained for the 6th_{cycle. The fault type will be recognized using V DR}t_{, the summation of the 16 obtained V DR}

values. In cases of permanent fault, V DRt _{is ideally zero although in practice it can have a very}

small value due to various sources of error. In cases of temporary fault, V DRt _{is considerably}

larger than the one obtained for permanent faults. In this research study, V DRt _{=5% has been}

chosen as a threshold for discrimination. For temporary fault detection, V DR must be larger than 5%, while, V DR<5% leads to permanent fault detection.

In the case of a close-in fault with very small resistance, voltage drops to zero immediately after the fault inception. Therefore, line single-phase isolation does not lead to further voltage drop. This means VsnDN will be detected as zero and this will be the case for all 16 values of

VsnDof the 6th monitored cycle. To cover this condition, permanent fault is detected in case the

faulted phase voltage magnitude stays less than 0.5% of the rated voltage for the entire 100ms monitoring time interval.

Arc extinction detection

In case of temporary faults, the main sign of arc extinction is some kind of settlement in system variables. In this case, both VsnD and δD settle down at/resonate around their final values as

the arc extinguishes and they are chosen for arc extinction detection purpose as discussed in Subsection 4.1.2.

Based on the comprehensive simulation study performed, it was observed that in most of the cases, the signs of the arc extinction appear in VsnDabout or before the real arc extinction,

whereas, these signs show up inδ_Dalways after the real arc extinction. Further, in case of fast arc extinction, it is hard to find a proper reference time to start monitoringδ_Dfor arc extinction detection as there are large transients on δ_D after the line single-phase isolation as shown in Figures 4.28 and 4.29. In addition, the arc extinction algorithm requires to analyze VsnDorδD

for several samples after real arc extinction to reliably detect the arc extinction. In this research study, it is proposed to use both quantities to increase the reliability and speed of detection.

In the proposed method, having recognized the temporary fault, initial arc extinction is de- tected if VsnDbecomes less that 10% of VsnDP for three consecutive protection passes. Then,

this initial arc extinction is confirmed by observingδ_D smaller than a very small value; 0.1◦_/s

is considered in this research study. This strategy increases the security of the reclosing algo- rithm and also the reclosing speed by eliminating the waiting time for arc extinction detection. The merit of involving VsnDP in the arc extinction detection algorithm is to provide an adaptive

threshold for VsnDsettlement detection as faster extinguishing arcs result in larger VsnDP. There-

fore, it is not needed to wait for VsnDsettlement detection for too long in such cases, while for

slower arcs, the algorithm will wait longer to ensure reliable detection as VsnDP is smaller.