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What Is Power System Protection, Why Is It Required and Some Basics?

1.1 What Is Power System Protection?

Our modern human civilization is dependent on the electric power system to enable all of its critical functions: food, health, sanitation, security, commerce, and progress. The electric power system is dependent on protections. By electric power system, we are referring to power generation and a network of wires that connect generation to the load locations where it is utilized to power the functions above. Protections consist of an assembly of electric components, and consequently, are better referred to as protection systems. Protection systems continuously monitor the equipment that the power system itself is comprised of for abnormal operating conditions. Protections are automatic systems that once an abnormal condition is detected, quickly as possible isolates the abnormal condition by the tripping of circuit breakers or the operation of fuses.

Power system protection systems are referred to as secondary equipment, as the primary equipment is transformers, lines, buses, generators, capacitors, breakers, disconnectors, etc. Primary equipment is directly involved with electric energy supply and delivery. Protection systems are designed and installed to oversee and "protect" primary equipment and the integrity of the power system.

In essence, power system protections "protect" power system primary equipment and, thereby, maintain system integrity and safety.

Protection systems are to a power system as a panel circuit breaker/fuse is to a household electrical circuit panel.

In addition to protecting power system primary equipment, power systems also employ remedial action schemes (RASs), previously known as special protection systems (SPSs), to protect the integrity of the power system. RAS/SPSs can monitor frequency, voltage, and operating contingencies that require immediate system correct actions, among others.

Power system protections are classified as "mission-critical" assets, as failure to operate or, if they do not operate as intended, have grave consequences to the continued operation of the power system.

A protection system itself is comprised of Individual devices, sub-systems, and numerous pieces of equipment as follows:

• Protection relays that monitor the power system for abnormal conditions.
• Communication systems that are used as part of the overall protection system functionality.
• Voltage and current sensing equipment that steps down high-power system values to much lower values capable of being input into the protection relays.
• Direct current () auxiliary supply including batteries and their chargers used to power protection relays, auxiliary devices, communication systems and trip circuit breakers.
• Control circuitry working with protections to trip circuit breakers or other interrupting devices such as circuit switchers.

Most reliability organizations that oversee the adequacy of protections include the above-listed components as part of an overall protection system. Batteries are not included just the battery circuits. Also, circuit breakers are not included just the breaker trip coils are. However, batteries and breakers are key components of protection systems but fall under the jurisdiction of station engineering. The consequence of such definitions only impacts compliance and organizational accountability.

A typical protection system consisting of these components is illustrated in Figure 1.1 showing that a protection system consists of many components, or sub-systems: CTs, PTs, protective relays, auxiliary relays, control wiring, equipment mounting panels, DC power supplies, telecommunications, and breaker trip coils. A protection system, in the general case, is not just one device, or subsystem, it consists of several sub-systems, each containing several devices that represent the whole. To function correctly, each of the components or sub-systems must themselves operate correctly . it is a serial operation. Each of these sub-systems and their functions will be discussed in more detail in Chapter 2, Section 2.1

Figure 1.1 Illustration of a protection system for a transmission line [1].

It is not possible to design an electric power system that is immune to equipment failures and abnormal operating conditions. Therefore, all power systems must deploy highly reliable protection systems that can quickly detect abnormal conditions and take appropriate actions to mitigate abnormalities.

In the normal state of a power system, there is a balance of electric energy sufficient to meet the needs of the connected load, in real-time, and the power system operating quantities such as voltages, currents, and frequency, are all within the design ratings of the primary equipment.

Abnormal conditions result when system faults occur that cause these operating quantities to deviate beyond equipment ratings. Protection systems are designed to monitor power system quantiles for such abnormalities and operate to isolate these fault events that cause abnormal quantities. One prominent operating quantity that is drastically impacted by such events is current. System faults also referred to as disturbances, can cause normal load current to increase from several hundred amps to 70,000 A which can cause major damage if not cleared in fractions of a second. Currents of such high quantities can cause thermal damage, mechanical damage, forces are so high that metal bus bars can bend, equipment failures, fires, safety issues, and a collapse of the power system if not cleared within the short-time ratings of primary equipment.

Some examples of system events that cause abnormal conditions are as follows: lightning strikes (Figure 1.2), wind, ice storms, animal contact, equipment failures (Figure 1.3), car accidents knocking down electrical poles/equipment, etc., that cause short circuits or broken connections. Such events are also referred in the industry, as faults. Faults and their types, causes, and how to calculate fault values will be further discussed in Chapter 6.

Figure 1.2 Environmental risk lightning strike - Dallas Tx.

Source: NOAA Photo Library / Flickr / CC BY 2.0.

Figure 1.3 Failed transformer on fire - Thessaloniki Greece.

Source: Konstantinos Stampoulis / Firefighters.gr / Wikimedia Commons / CC BY-SA 3.0 GR.

1.2 Why Is Power System Protections Required?

Power systems are designed, planned, and constructed to limit failure modes and equipment damage and thereby enhance overall system reliability.

The power system is designed to balance performance and minimize the cost of energy delivery. The planning, design, and implementation of a power system is a balance of initial capital costs and ongoing maintenance costs with the potential cost impact of power system equipment failure.

Power systems are exposed and subjected to environmental elements such as rain, snow, ice, lightning (Figure 1.2), storms, and other such environmental risks. These risks cause, power system's primary equipment components to make unwanted contact with other components, referred to as faults which result in fault currents in the order of 10-100 times normal load currents. Transmission lines have the highest risk of environmental elements due to their increased natural exposure to the environment.

It should be noted that protection systems cannot prevent faults or equipment failures. They detect an abnormality by monitoring quantities such as increased currents, depressed voltages resulting from failures. A limited number of protection devices can respond to failures without directly monitoring electrical quantities example are gas, temperature, and light-sensing devices.

1.2.1 Minimize Primary Equipment Damage

Power system equipment is designed and constructed to limit failure modes. However, power system's primary equipment can and does fail for the following reasons:

1. (1) Soon after installation, due to either a design or manufacturing flaw.
2. (2) Equipment failures due to prolonged operation beyond the equipment's rated design parameters.
3. (3) Equipment failures due to adverse environmental conditions such as salt pollution, animal contact, or high wind and lightning strikes during storms.
4. (4) Equipment operated beyond their normal expected life span.

Primary equipment is designed to withstand a certain level of fault exposure. Protection systems operating within that exposure period will minimize damage to the equipment and possibly prevent catastrophic failure thereby, also decreasing equipment outage time. It should be noted the cost and installation of some primary equipment such as generators and large power transformers are in the order of US$10's of millions of dollars each. More significant than the cost is the time needed to manufacture as it can take up to one to two years as this type of equipment is only made by custom order.

1.2.2 Provide Continuity of Service by Minimizing Outage Time and Service

1. Power systems are classified as critical infrastructure due to the 24/7 dependence on electric power for modern-day life. Automatic protection...