Three-Phase Faults - TRANSFORMER SIMPLE STEP-UP

Electrical Fundamentals

TRANSFORMER SIMPLE STEP-UP

A) Three-Phase Faults

Three-phase (3Ø or 3P) Faults occur when all three phases are connected together with low impedance. We typically create balanced 3Ø Faults when creating simulations which are the equivalent of leaving safety grounds on a system and then energizing that system.

Figure 1-46: Three-Phase Fault 3-Line Drawing

A three-phase fault causes all three currents to increase simultaneously with equal magnitudes. The magnitude of fault current will depend on the impedance and location of the fault, as well as the strength of the electrical system. The safety grounds connected to the grid in our example will cause much higher fault currents than the same safety grounds connected to an isolated generator. Similarly, a fault closer to the source (Figure 1-46 – Left Side) will produce more fault current than a fault at the end of the line (Figure 1-46 – Right Side) because there will be more impedance between the source and the fault in the second situation. The current will lag the voltage by some value usually determined by the voltage class of the system.

All three voltages will decrease simultaneously with equal magnitudes. The magnitude of fault voltage will depend on the impedance and location of the fault, as well as the strength

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of the electrical system. The voltage drop will not be as significant if the safety grounds are connected to the grid compared to the huge voltage drop that will be observed in an isolated system which could cause the voltage to drop to zero. Similarly, a fault closer to the source will create a larger voltage drop than a fault at the end of the line because there will be more impedance between the source and the fault.

The following phasor diagrams demonstrate the difference between a “normal” system and the same system with a three-phase fault. Notice that all phasors are still 120° apart, the voltages stayed at the same at the same phase angles, and the current magnitudes increased with a greater lag from the voltages.

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Figure 1-47: Three-Phase Fault Phasor Diagram

It is always a good idea to simulate conditions as close to a true fault when performing tests and a 3Ø fault simulation should have the following characteristics:

• Prefault: nominal: 3Ø voltage magnitudes with 120° between phases in the normal phase rotation.

• Fault Voltage: Reduced, but identical, voltage magnitudes on all three phases with no change in phase angles from the prefault condition.

• Fault Current: Larger than nominal current on all three phases with identical magnitudes and 120° between phases with the normal phase rotation. The current should lag the voltage by 60 to 90°.

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www.valenceonline.com B) Phase-to-Phase Faults

Phase-to-phase (P-P or Ø-Ø) faults occur when two phases are connected together with low impedance. A Ø-Ø fault can occur when a bird flies between two transmission conductors and its wing-tips touch both conductors simultaneously. Phase-to-phase faults are described by the phases that are affected by the low-impedance connection. The B-C fault in Figure 1-48 is a Ø-Ø fault that connects B-phase and C-phase.

Figure 1-48: Three-Phase Fault 3-Line Drawing

The magnitude of fault current will depend on the impedance and location of the fault, as well as the strength of the electrical system. A fault closer to the source will produce more fault current than a fault at the end of the line because there will be more impedance between the source and the fault in the second situation. The fault current will lag the fault voltage by some value usually determined by the voltage class of the system.

If you follow the flow of current in Figure 1-48, you should notice that the current flows from the B-phase source into the fault, and then returns to the source via C-phase. Basic electrical theory states that the current flowing in a circuit must be equal, so the B-phase and C-phase currents must have the same magnitudes. However, relays monitor current leaving the source so the relay will see this fault as two equal currents with opposite polarity.

Therefore, when we simulate a P-P fault, the currents injected into a relay must have the same magnitudes and be 180° apart from each other. The current flowing through the actual fault will be equal to 2x the injected currents.

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Injectected Current Ib Ic

Fault Current Ib Ic Fault Current Ib@ 150 Ic@30

Fault Current 2 Ib@ 150

Figure 1-49: Fault Current vs. Injected Current

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The effect of a P-P fault on the faulted voltages is even more complex. The faulted voltages will have equal magnitudes because the impedance between the source and the fault on each faulted phase should be equal. A fault closer to the source will create a larger voltage drop than a fault at the end of the line because there will be more impedance between the source and the fault.

The faulted voltage angles are also affected because the ratio of reactance and resistance in the circuit changes when the fault is introduced. There is a lot of information to consider when creating the correct voltage magnitudes and angles and we discuss the calculations in detail in Chapter 15: Line Distance (21) Element Testing. For our purposes in this chapter, it is important that you be able to recognize a Ø-Ø fault as the following phasor diagrams demonstrate. Notice that the faulted voltages have collapsed and come together to change the voltage triangle from an equiangular/equilateral triangle (three equal magnitudes and angles) in prefault to an acute, isosceles triangle (two equal magnitudes, two equal angles, and all angles are less than 90°).

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Figure 1-50: Phase-to-Phase Fault

It is always a good idea to simulate conditions as close to a true fault when performing tests and a Ø-Ø fault simulation should have the following characteristics:

1. Prefault: nominal, 3Ø voltage magnitudes with 120° between phases in the normal phase rotation.

2. Fault Voltage: Reduced, but identical, voltage magnitudes on both phases affected by the fault. The phase angle between the faulted phases should be less than 120°.

3. The unaffected phase voltage should be identical to the prefault condition.

4. Fault Current: Increased, but identical, current magnitudes on both phases affected by the fault. The phase angle between the faulted phases should be 180°.

5. The unaffected phase current does not change between prefault and fault.

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6. Sequence Components

The vector addition examples covered in this book are very simple when compared to the variables that can occur during a real fault. Trying to use simple vector addition to determine the cause of a fault or even understand and predict fault characteristics can become a nightmare, especially when you realize that most of the electrical principles we use today were developed before computers or even calculators. Sequence components were introduced in 1918 by Dr.

C. L. Fortescue to help simplify the analysis of unbalanced systems to better understand fault characteristics so that protective relays could better protect the electrical system.

Sequence components are a mathematical model of an electrical system to help understand electrical networks during faults. These theories were designed to interpret faults, so it makes sense that more advanced relays use sequence components to detect faults and many do. Sequence components cannot be measured directly. They must be calculated using measurements of individual phase magnitudes and angles.

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