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Fault Current Analysis for Selection of Switchgear and Protection Scheme

Key Takeaways

  • Fault current is the abnormal amount of current flowing in a circuit to the location of a short-circuit. The impedances of the network components decide the magnitude of the fault current.

  • The symmetrical components method of fault current calculation is only useful during a three-phase fault events. In the occurrence of unsymmetrical faults—such as single-phase to ground faults, or, line-line faults—the fault current calculation methods used should consider each phase of the power bus separately. 

  • The approximate estimation of the fault current ensures that the selection and setting of the switchgear and protective equipment are adequate enough to interrupt the fault current. 

The approximate calculation of the fault current in different parts of the power network helps design efficient switchgear and protection schemes.

The magnitude of fault current varies with the type of fault. 

Power system protection relies mostly on switchgears to interrupt fault occurrences in a system. In any system design, a detailed analysis conducted to calculate the maximum fault current is the basis of switchgear selection. If a mechanically-sound switchgear fails to interrupt a fault, then it's time to revisit the fault current calculation.

Fault current analysis is the fundamental numerical technique used by power system engineers to find out the fault current in any electrical system, prior to its real occurrence. Fault current analysis is a crucial step in the selection of switchgear and other protective equipment in the power system. It is the tool used for fault current estimation prior to real fault events. 

When a power system network gets short-circuited, a heavy current starts flowing from the circuit to the fault location. The short-circuit current—or fault current—is capable of damaging the electrical components present in the fault current path. The detrimental impact of fault current makes it necessary to determine the maximum possible fault current that can flow in any part of the power system. 

Significance of Fault Current Analysis

When an electric power system undergoes fault, all the other equipment in the path of the fault gets damaged due to heavy current flow. It produces severe dips in the bus voltage,  destroys the loads, and ruins the power system balance. The large ground voltage at the fault location is a serious hazard to human life.

If you have a slight idea of the fault current value in a power system, you can de-energize the system when those currents start flowing. Early fault identification is only possible if you have an approximate estimate of the fault current in the system. Due to its high magnitude, it is easy to distinguish the fault current from other system currents. The manual de-energization is not a practical prevention method, therefore, switchgear and protection engineering is an important department in any power system planning project.

Fault current analysis is also crucial for:

  1. Selection of switchgear-In order to protect the rest of the power system from fault currents, switchgear is connected in electrical systems to operate under the fault events. The suitable selection of the switchgear in any application requires the calculation of possible fault current in the system. 

  2. Determining the setting, type, and location of the protective equipment- For example, in a three-phase induction motor, the single-phasing fault current is different from that of earth fault current. So, sensing the magnitude of different fault currents is very important in fixing the type, location, and setting of circuit breakers, relays, and other protective equipment.

  3. Rating of the protective reactors- Even though circuit breakers make or break the circuits, the short-circuit current in a power system is limited by installing protective reactors. The rating of the reactors depends on the maximum fault current in that power system network.

  4. Selection of bus-bars, current transformers- Under fault events, the electromagnetic forces produced are so large they ruin the busbars, current transformers, and associated equipment in the power system. The calculation of fault current is essential for selecting the rating of this equipment.

  5. Network planning- The expansion and extension of power systems can be planned only by considering the various voltages and currents in the network. This includes fault current calculation as well.

As we have seen, the significance of fault current analysis is great, and it is important to understand the methods involved. Short-circuit occurs in the power system due to symmetrical or unsymmetrical faults. Depending on the type and location of the fault, methods of fault analysis will differ.

Symmetrical Fault Current Calculation

Symmetrical faults are three-phase faults. In symmetrical faults, the short-circuit currents in the three phases are identical and displaced 120° from each other. Due to the symmetry, the fault current analysis is limited to only one phase. The steps involved in symmetrical fault current analysis are:

  1. Draw a one-line diagram of the power system chosen for fault current calculation. Include power, voltage, and percentage reactance marked for each component in the network.

  2. Each component—such as motor, generator, or transformer—in the power network will be of different kVA ratings. Choose a large kVA rating as the base value, and convert all the percentage reactances to the chosen base kVA.

  3. Replace all the components in the one-line diagram, with reactances calculated as per step 2. It is enough to draw the reactance diagram of only one phase, as all phases are symmetrical to each other. 

  4. Mark the fault location and calculate the percentage reactance (% X) of the network until the fault point.

  5. Find out the full-load current (I)  under the normal system voltage and selected base kVA.

  6. The short-circuit formula is listed in equation 1 and 2 below. 

    Short-circuit formula.

Short-circuit formula.  

Scope of Thevenin’s Theorem in Fault current analysis

The Thevenin equivalent model of the power bus at fault condition.

Figure.2: The Thevenin equivalent model of the power bus at fault condition.

Now, you may wonder why you couldn’t apply Thevenin’s theorem, if the fault is a short-circuit and fault current is short-circuit current. Here are the steps involved in Thevenin’s theorem:

  1. Take the network or power bus where the fault current is to be calculated. Calculate the Thevenin voltage (VTh)that appears across the fault in the bus. This voltage is equal to the pre-fault bus voltage.

  2. Determine the Thevenin impedance (ZTh) of the power bus across the point of fault location. For Thevenin impedance calculation, all the components in the circuit path until the fault location are replaced by their internal impedances.

  3. The faulty power bus can be drawn as shown in Figure.2 above. The fault current can be given as:

Ohm’s law equation.

Power system expansion plays a crucial part in the socio-economic and industrial growth of a nation. The supply of uninterrupted energy is the main aim of these expansions, and it showcases the overall development of a nation. The fault current analysis in network planning guarantees a power system that is well protected by an efficient switchgear and protection scheme. Power system engineers ensure the switchgear interrupts the fault on-time, by estimating the possible fault current. The fault current calculation helps to optimally select the circuit breakers and relays that will keep the power grid continuously energized.