Failures in electrical power systems

 

1- Faults in electrical power systems:

Modern power systems are designed with a high level of reliability and flexibility, allowing one or more elements to be taken out of service with minimal disruption. However, breakdowns in electrical insulation or equipment may still occur. These failures may result from electrical, mechanical, or thermal breakdowns—or a combination of these.

Table 1 summarizes the main types and causes of failures in electrical systems.

 

Table (1): Types and General Causes of Collapse

 

Type	Causes
Insulation	Design defects, manufacturing errors, installation errors, aging insulation, pollution
Electrical	Lightning, transient waves from switching operations, sudden voltage spikes
Thermal	failure of cooling medium, current surges, voltage increases, high ambient temperature
Mechanical	High current forces, earthquakes, foreign object collisions, ice or snow loads

 

Consequences of Faults

Malfunctions in power systems can lead to several consequences, including:

1. Abnormally high currents flowing through system components, causing excessive heating.
2. System voltages deviating from normal and acceptable ranges, risking equipment damage.
3. Unbalanced operation of three-phase systems, leading to improper equipment performance.     

2. Types of Faults

Electrical faults are generally classified into two main categories:

1. Open-circuit faults – caused by the opening of one or more lines.
• These faults do not generate high fault currents.
• Single-phase loads may continue to operate, but three-phase loads lose supply.
• Protection relays are needed to detect such faults.
2. Short-circuit faults – caused by contact between one or more phases and ground, or between two or more phases without ground.
• These faults typically result in very high currents.
• They create thermal and mechanical stress on network components.
• They may also cause disturbance in communication and control systems.

Common Types of Short-Circuit Faults

1. Three-phase fault (with or without ground)
2. Phase-to-phase fault
3. Double-phase-to-ground fault
4. Single-phase-to-ground fault

Simultaneous faults in two parts of a system are difficult to detect accurately, but they are rare and not usually a major cause of incorrect operation.                       

3-  Characteristics of faults

3.1 Fault Angles

The power factor (angle) of the fault current depends on the electrical source and circuit parameters up to the fault location.

• For ground faults, the power factor is influenced by the grounding method of the system.
• Typical fault angles are:

Voltage Level	Typical Fault Angle (Lagging)
7.2 – 23 kV	52° – 54.5°
23 – 69 kV	54.5° – 57.5°
69 – 230 kV	60° – 80°
230 kV and above	75° – 85°

Note: If transformer and generator impedances dominate, the fault angle will be higher. If cables contribute significantly to total impedance, the angle will be smaller.

3.2 Grounding of the electrical system

Grounding the electrical system affects both the value and angles of ground faults, and there are three ways of grounding:

1- Ungrounded (Neutral Point Isolated)

• Equivalent to grounding through phase-to-ground capacitances.
• During a single line-to-ground fault, voltages of the unfaulted phases increase.

2- Grounding is done during resistance (resistance or reactance)

• Limits fault current by inserting resistance/reactance between neutral and ground.

 3- Effective grounding system (the neutral point is directly connected to the ground)

• Neutral point is directly connected to ground.
• Single line-to-ground faults cause large currents, but voltage rise in unfaulted phases remains small.

The ungrounded system can be considered to be grounded by equal ground capacitors (Figure 1), and in identical electrical systems where the three capacitors to the ground are equal (N-E), and when a face grounding occurs (A),  the displacement triangle will appear as in Figure (1). Consequently, Vbg and Vcg will be approximately =

 of their natural original value.

 

 

Figure (1): Voltage Diagram for Shortening the Face of the Earth in an Ungrounded Electrical System

 

Conversely, in the case of a short circuit of one of the aspects of the radiological system with direct grounding, this will lead to a large short current in the case of ground failures, with a very small increase in the voltage of the non-short circuit faces.

 

Figure (2): Voltage vector to shorten the face of A to the ground

 
3-3 Fault resistance 

• Short circuits are often accompanied by arcs.
• Arc resistance varies with arc length and affects the fault current magnitude.
• For phase-to-phase faults, arc resistance usually has little effect (except at low voltages).
• For ground faults, arc resistance is more significant because arcs may be long and tower footing resistance adds to total impedance.

 

3-4 Deformation of faceted efforts during failures:

During faults, system voltages and currents become unbalanced.

• The largest voltage drop occurs at the fault location.
• Voltages between the source and fault point vary depending on the measurement location.
• Assumption: Z1 = Z2 = Z0 (positive, negative, and zero sequence impedances are equal).

 

At all faults assume:   Z1 = Z2 = Z0     Figure (3)