The Effect of Unbalanced Impedance Loads on the Short-Circuit Current

Abstract

Conventional short-circuit studies often neglect the load current because the short-circuit current (SCC) flowing from generators is much greater than the SCC that is affected by various loading conditions. As distributed or clustered loads that are unbalanced in phases are connected to the grid, they can also change the magnitude and phase angle of the SCC, despite their small capacities. Thus, the objective of this study is to present algorithms that are able to analyze such an impedance unbalanced load. For this purpose, this study initially derives an SCC model of the unbalanced impedance load in phases. Since the proposed SCC model requires the pre-fault voltage, it uses a power-flow analysis algorithm that iteratively calculates the current that is to be injected and the pre-fault voltage, using the bus impedance matrix. Then, the proposed SCC calculation algorithm transforms the unbalanced loads into equivalent impedances, using the pre-fault voltage, and adds them to sequence networks as input data, using the proposed SCC model. The proposed algorithms are verified in various case studies. As a result, the proposed SCC calculation algorithms are more accurate, because they do not neglect unbalanced loads.

Steady‐state short‐circuit current calculation for internally limited inverter‐based distributed generation sources connected as current sources using the sequence method

“…In addition to the zero SCC of phase b, SCC in Table 1 increases as load capacity increases because the loads are converted to equivalent impedance and added to the original network, ie, buses 2 and 4. 28 Thus, SCC also slightly increases as the load increases, as shown in Table 1. Table 2 shows that SCCs when the 100-MVA IBDG was connected as a current source are higher than when connected as a voltage source.…”

“…We examined SCC for load capacity from 0 to 25 pu with IBDG source capacity = 1.0 pu. Figure shows that equivalent impedance increases to infinity when load = 0 because the squared nominal voltage is divided by 0, from the equations in Kim . In this case, SCC is 1.6357∠ − 56.45° pu, corresponding to the case where loads were ignored (Table ).…”

“…The SCCs for phases a , b , and, c flowing from bus 1 to bus 2, ie, currents flowing on the delta side, are 0.6709∠ − 75.01°, 0, and 0.6709∠104.99° pu, respectively, which provide the first validation. In addition to the zero SCC of phase b , SCC in Table increases as load capacity increases because the loads are converted to equivalent impedance and added to the original network, ie, buses 2 and 4 . Thus, SCC also slightly increases as the load increases, as shown in Table .…”

“…Figure 9 shows that equivalent impedance increases to infinity when load = 0 because the squared nominal voltage is divided by 0, from the equations in Kim. 28 In this case, SCC is 1.6357∠ − 56.45°pu, corresponding to the case where loads were ignored ( Table 2). In contrast, as the load approaches infinity, equivalent impedance is treated as a short circuit because the squared nominal voltage is divided by infinity.…”

The modeling of tap‐changing transformers and P‐V buses using the sensitivity impedance matrix

“…However, the detailed modeling of the power-flow calculation algorithm was not presented in the study. 17 Therefore, this article as an extended work of the study proposes an algorithm able to model tap-changing transformers for the iterative power-flow calculation method using the BIM. For this purpose, this study proposes a unified steadystate T shaped transformer model with a tap changer on the primary or secondary side, modified from the conventional model, [18][19][20][21] and decomposes the steady-state model into series and shunt components using a π-equivalent model.…”