Fault Analysis in Power Systems Part 2a

We are going to go over the high-level overview of the seven steps to better understand the process holistically. In the remaining parts, we will review each step in much greater detail to gain a good intuitive understanding of the involved process.

Step 1: Convert the system into per unit values. The reason for this conversion is to eliminate the voltage levels that occur in power systems due to voltage transformation. This transformation adds complexity when calculating fault currents. By converting the system into per unit values, we bring the entire system into a single per unit level. An example of this is covered in Part 2b.

Step 2: Identify the type of fault being analyzed. There are symmetrical faults that do not give rise to zero sequence or negative sequence components because they are perfectly balanced. They only have positive sequence values. On the other hand, there are unsymmetrical faults that require the calculation of positive, negative, and zero sequence components separately. Due to these different fault types and their characteristics, it is crucial to identify the type of fault being analyzed as the remaining steps depend on it.

Step 3: Draw the sequence networks for the unfaulted system. The unfaulted system refers to drawing individual sequence network diagrams for the system. In later steps, we will connect these diagrams in a specific way.

Step 4: Take the unfaulted sequence networks and modify/interconnect them according to the type of fault to create a faulted sequence network diagram. This diagram will be used to calculate, by hand, the current and voltage quantities during a faulted condition. This step is explained in greater detail in supplementary videos Part 2c and Part 2d.

Step 5: Hand-calculate the sequence current and voltage quantities during a faulted condition. Calculate the positive sequence current, negative sequence current, and zero sequence current, depending on the type of fault involved. Keep in mind that the current and voltage quantities in Step 5 are per unit values.

Step 6: Convert the per unit values calculated in Step 5 into three-phase current and voltage quantities that can be intuitively understood and used. This conversion requires a special sequence transformation based on the principles of symmetrical components. We recommend watching the symmetrical components video series Part 3a and Part 4b for a better understanding of this transformation.

Step 7: This step can be skipped when calculating a fault that occurs on the low voltage side of the transformer. All current and voltage quantities calculated are based on the low voltage side. If you want to analyze a fault occurring on the high voltage side of the transformer, there are special adjustments and calculations needed. Supplementary videos Part 2e and Part 4a provide more information on this step, but you may skip it for now if it seems complex.

All these seven steps are complex, and it is necessary to review them one by one in greater detail, which will be covered