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Essentials of Power and Control

1.1 Introduction

The electrical grid is one of the most amazing hidden parts of our modern technological society. Is it not amazing that the small wires can move enough electrons to power massive machines and light millions of square feet in a fraction of a second? Further, it is done with some semblance of control without worrying about its underlying factors. So, what are the physical, mathematical, and sociopolitical dimensions of electrical energy? Traditionally, we have been unaware of these dimensions because we were just the end-user, aka an "electron-taker" (like "price-taker" in economics). At most, we would worry about getting reliable and affordable power. For example, how many of us know that when a branch of a tree (literally) falls in a distribution line the distribution utility's (including in the most advanced countries like the United States) major source of information is the phone call that you make to the utility? Yes, the distribution utilities might still not have much automation or many sensors and so must rely on phone calls. Even in a place where there are wholesale markets, many of us are not aware that electricity is traded like shares on the stock exchange. We would not want to be involved in the power system operations. We would not care about power system operations or attributes such as environmental values in the past.

These days, consumers are becoming sophisticated consumers or even "prosumers" (a consumer who also produces energy). The prosumer (we) wants reliable, affordable, cleaner, and resilient power. We want to improve the efficiency of our use, and some even want to generate local energy and contribute to the grid. Our decisions, actions, and involvement affect the physics, mathematics, and sociopolitical aspects of traditional power systems. Therefore, we must have a basic understanding of electrical systems and electricity flow primarily because this prosumer "we" is much intertwined with the system compared to our historical counterpart.

This chapter is meant for today's "us." After reading this chapter, you (e.g. graduate student, engineer, non-engineer, decision-maker, policymaker, and members of the informed public) will be:

• Acquainted with the electric power system, including generation, transmission, distribution system operations, and control.
• Acquainted with the general principles that guide this operation.
• Use this knowledge to make informed decisions in their day-to-day lives and work.

1.2 Basic Principles of Power and Control

An electrical power system is a network of components that supplies, transfers, and uses electrical energy. The electrical grid of Texas in the USA, or microgrid at a supermarket in Finland, or an interconnected network of micro hydro units in Nepal are all examples of electrical power systems. The power system generally consists of generators that generate the electricity and the transmission and distribution system (lines, towers, insulators, transformers, etc.) that carry and feed the power to homes, industries, and other end-users. In most places worldwide, the generators generate electricity that fluctuates between highs and lows over time, called the alternating current (AC). In exceptional cases, the electricity generated is constant over time (e.g. electricity used by trains, ocean liners, and most electric automobiles) called direct current (DC). We look at some examples later in this chapter. Note that electricity generated by centralized power plants can be used directly in trains and other vehicles. Sometimes it can also be stored in batteries, or by pumped hydropower and even geothermally. Pumped hydro is an arrangement of multiple water reservoirs that are located at different heights. When water flows from top to bottom, it generates electricity. It also includes a mechanism that pumps water back to the upper reservoir, especially when the electricity prices are low, to store it and use it when the electricity generation prices are high. Similarly, geothermal storage uses earth's underground heat for energy generation and storage. Electricity can also be co-generated on board and used in trains, boats, buses, and automobiles. These are often referred to as diesel-electric or hybrid systems (see Chapter 7 on different forms of generation).

1.2.1 Energy and Power

Energy is the capacity to do work. Work is the act of applying force over a distance, e.g. pushing a desk over a distance or moving electrons over a distance. Watt-hours or British thermal units (BTUs) are the most used units of measurement in electrical engineering.

Several other units measure energy, including N-m (Newton meter), joules, and calories. Watt is the instantaneous unit of power, also called electrical demand. Electrical energy is generally measured in watt-hours and is also called electrical consumption. Power is defined as the rate of producing or consuming energy and the time factor is important. For example, Sandra must move from point A to B. She can use a tiny scooter engine or she could use a jet engine. Which one do you think will move her faster? Her weight is the same, the distance is the same, and everything else is the same. Though both will require the same amount of energy, the jet engine will move faster because it has more power to do the work quicker. Figure 1.1 compares different units of energy and power.

Let us take the example of electrical power. If Sandra's light emitting diode (LED) light consumes 1?W of electrical power for one hour, this equals one watt-hour of energy. A thousand of these units are called kilowatt-hours (kWh). So, if Sandra's fan consumes one thousand watts for one?hour, her fan consumes 1?kWh or one unit of electrical energy consumption. Power can also be calculated from voltage and current. Electrical power is the product of current and voltage. For example, 1?W = 1?V?×?1?A.

Section 1.2.2 considers voltages and currents.

Figure 1.1 Different units of energy and power comparison [1].

1.2.2 Voltage, Current, and Impedance

Voltage is the amount of potential energy (a form of energy) difference between two points. Such a potential difference is due to the difference in charge between these two points. Voltage is measured in volts. Voltage is the difference in electric potential between two points in a circuit. Current is the flow of charge and is measured in amperes. Impedance, also sometimes called resistance (e.g. in DC circuits), is the property of any material that impedes current flow. It is measured in ohms.

Let us take an example of two interconnected beer mugs, as shown in Figure 1.2. Since there is a difference in beer level between the two mugs, the beer will flow from mug A to mug B. You will have a generator in an electrical system instead of mug A and loads (i.e. equipment that use electricity like a light or a fan) instead of mug B. The pipe connecting mugs A and B is the transmission and distribution system consisting of wires and transformers. The potential energy difference between these two mugs can be compared to voltage. The flow of actual beer can be compared to the current.

Figure 1.2 The beer mugs.

Another widely used way to explain voltage, current, and impedance is the fluids analogy. In this analogy, voltage is like pressure, and current is like flow.

The relationship between voltage, current, and impedance was described by Georg Ohm in the late 1700s. He stated that the current flow through a conductor is directly proportional to the voltage and inversely proportional to the impedance. If we represent voltage as V, current as I, and impedance as Z, according to Ohm's law

1.2.3 Alternating Current vs. Direct Current

Most of us know about the band AC/DC and their rock and roll. However, we are on a slightly different topic (equally interesting, like the band). Energy.gov describes this as a "war of currents." Thomas Edison generated electricity as DC (electrical current that does not change over time). It quickly gained popularity in the United States. However, it was hard to transmit it over long distances at that time. Nicola Tesla worked on another version: AC (electrical current that fluctuates and reverses over time). It was easy to convert it into higher or lower voltages and could be transmitted over long distances. General Electric vouched for Edison's DC and Westinghouse for Tesla's AC. Over time, both methods gained popularity. Fast forward to today. We generally use AC at our homes and DC in small electronics or while transmitting power at high voltages over long distances, etc. AC still has the advantage of being easy to convert into higher or lower voltages...