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Examoogle • User • info@examoogle.com • EE-2024-8821
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The following sequence currents were recorded in a power system under a fault condition Iₚₒₛᵢₜᵢᵥₑ = j 1.753 pu, Iₙₑgₐₜᵢᵥₑ = – j 0.6 pu, Izₑᵣₒ = – j 1.153 pu · The fault is
Line to ground
Three-phase
Line to line to ground
Line to line
Line to line to ground
Given: Positive sequence current I₁ = j1.753 pu, Negative sequence current I₂ = -j0.6 pu, Zero sequence current I₀ = -j1.153 pu.
Positive sequence current I₁ = j1.753 pu, Negative sequence current I₂ = -j0.6 pu, Zero sequence current I₀ = -j1.153 pu.
I1=I2=I0⟹LG Fault,I1+I2+I0=0⟹LL Fault,I1+I2+I0=0⟹LLG Fault
Analyze the relationship between sequence currents
In an unbalanced fault analysis using symmetrical components, the relationship between sequence currents identifies the fault type · Here, we check the sum of sequence currents: Isum=Ipositive+Inegative+Izero.
Isum=j1.753−j0.6−j1.153
Calculate the sum
Adding the given values: Isum=j(1.753−0.6−1.153)=j(0)=0.
Isum=0
Determine fault type based on conditions
Since Ipositive+Inegative+Izero=0, this is a condition for a line-to-line fault · However, if any sequence current is non-zero and they are not equal, it indicates a double line-to-ground (LLG) fault · Specifically, in LLG faults, all three sequence networks are connected in parallel · Since all three are non-zero and their sum is zero, it represents a Line-to-Line-to-Ground fault.
I1+I2+I0=0 and I0=0
C is correct because the sum of the sequence currents is zero (I1+I2+I0=0) while I0 is non-zero, which confirms a line-to-line-to-ground fault.
This concept is fundamental for relay coordination and protection studies; remember that LLG faults involve the parallel combination of positive, negative, and zero sequence impedance networks.