2. Technical Aspects of Grid Interconnection

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2. Technical Aspects of Grid Interconnection 2.1. Introduction 2.1.1. The Evolution of Interconnected Systems Electricity grid interconnections have played a…
2. Technical Aspects of Grid Interconnection 2.1. Introduction 2.1.1. The Evolution of Interconnected Systems Electricity grid interconnections have played a key role in the history of electric power systems. Most national and regional power systems that exist today began many decades ago as isolated systems, often as a single generator in a large city. As power systems expanded out from their urban cores, interconnections among neighboring systems became increasingly common. Groups of utilities began to form power pools, allowing them to trade electricity and share capacity reserves. The first power pool in the United States was formed in the Connecticut Valley in 1925. As transmission technologies improved, long dis- tance interconnections developed, sometimes crossing national borders. The first international intercon- nections in Europe came in 1906, when Switzerland built transmission links to France and Italy. One of the great engineering achievements of the last century has been the evolution of large syn- chronous alternating current (AC) power grids, in which all the interconnected systems maintain the same precise electrical frequency. Today, the North American power system is composed of four giant synchronous systems, namely the Eastern, Western, Texas, and Quebec interconnections. The Eastern interconnection by itself has been called the largest machine in the world, consisting of thousands of generators, millions of kilometers of transmission and distribution lines, and more than a billion differ- ent electrical loads. Despite this complexity, the network operates in synchronism as a single system. So does the Western European interconnection, which reaches from the UK and Scandinavia to Italy and Greece, embracing along the way much of Eastern Europe (for example, Poland, Hungary, Slovakia, and the Czech Republic). Synchronous interconnections among countries are expanding in Central and South America, North and Sub-Saharan Africa, and the Middle East10. At the same time that synchronous AC networks have reached the continental scale, the use of high voltage direct current (HVDC) interconnections is also rapidly expanding as a result of technical progress over the last two decades. HVDC permits the asynchronous interconnection of networks that operate at different frequencies, or are otherwise incompatible, allowing them to exchange power with- out requiring the tight coordination of a synchronous network. HVDC has other advantages as well, especially for transmitting large amounts of power over very long distances. Fundamentals of both AC and DC interconnections are discussed below in Sections 2.2, 2.3, and 2.4 of this Chapter.  T. Hughes (1983), Networks of Power: Electrification in Western Society, 1880-1930, Johns Hopkins University, Baltimore, MD.  Rincliffe, R.G. (1967), “Planning and Operation of a Large Power Pool.”. IEEE Spectrum: 91-96. January 1967. The PJM (Pennsylvania/New Jersey/Maryland) grid was next to be developed, in 1927. 10 F. Meslier (1999), “Historical Background and Lessons for the Future,” in J. Casazza and G. Loehr, The Evolution of Electric Power Transmission Under Deregulation, IEEE, Piscataway, NJ; pp. 28-31. 16 Multi Dimensional Issues in International Electric Power Grid Interconnections 2.1.2. General potential benefits of grid interconnections There are number of technical rationales for grid interconnections, many of which have economic compo- nents as well (as described in Chapter 3 of this Report). Technical rationales for grid interconnection include: ã Improving reliability and pooling reserves: The amount of reserve capacity that must be built by individual networks to ensure reliable operation when supplies are short can be reduced by sharing reserves within an interconnected network. ã Reduced investment in generating capacity: Individual systems can reduce their generating capac- ity requirement, or postpone the need to add new capacity, if they are able to share the generating resources of an interconnected system. ã Improving load factor and increasing load diversity: Systems operate most economically when the level of power demand is steady over time, as opposed to having high peaks. Poor load factors (the ratio of average to peak power demand) mean that utilities must construct generation capacity to meet peak requirements, but that this capacity sits idle much of the time. Systems can improve poor load factors by interconnecting to other systems with different types of loads, or loads with different daily or seasonal patterns that complement their own. ã Economies of scale in new construction: Unit costs of new generation and transmission capacity generally decline with increasing scale, up to a point. Sharing resources in an interconnected system can allow the construction of larger facilities with lower unit costs. ã Diversity of generation mix and supply security: Interconnections between systems that use different technologies and/or fuels to generate electricity provide greater security in the event that one kind of generation becomes limited (e.g., hydroelectricity in a year with little rainfall). Historically, this complementarity has been a strong incentive for interconnection between hydro-dominated systems and thermal-dominated systems. A larger and more diverse generation mix also implies more diver- sity in the types of forced outages that occur, improving reliability. ã Economic exchange: Interconnection allows the dispatch of the least costly generating units within the interconnected area, providing an overall cost savings that can be divided among the component systems. Alternatively, it allows inexpensive power from one system to be sold to systems with more expensive power. ã Environmental dispatch and new plant siting: Interconnections can allow generating units with lower environmental impacts to be used more, and units with higher impacts to be used less. In areas where environmental and land use constraints limit the siting of power plants, interconnections can allow new plant construction in less sensitive areas. ã Coordination of maintenance schedules: Interconnections permit planned outages of generating and transmission facilities for maintenance to be coordinated so that overall cost and reliability for the interconnected network is optimized. Some costs and benefits of interconnections are difficult to quantify, but as a rough figure of merit it has been estimated that interconnections in North America have resulted in an overall annual cost sav- Technical Aspects of Grid Interconnection17 ings of $20 billion in the 1990s, and that the Western European interconnection has resulted in reduced capacity requirements of 7-10 percent. 2.1.1. Technical complexities and risks of grid interconnections The fact that interconnections between power systems are increasingly common does not imply that they are as simple as connecting a few wires. Interconnections obviously entail the expense of constructing and operating transmission lines and substations, or in the case of HVDC, converter stations. Intercon- nections also entail other costs, technical complexities, and risks. For AC interconnections especially, a power system interconnection is a kind of marriage, because two systems become one in an important way when they operate in synchronism. To do this requires a high degree of technical compatibility and operational coordination, which grows in cost and complexity with the scale and inherent differences of the systems involved. To give just one example, when systems are interconnected, even if they are oth- erwise fully compatible, fault currents (the current that flows during a short circuit) generally increase, requiring the installation of higher capacity circuit breakers to maintain safety and reliability. To properly specify these and many other technical changes required by interconnection requires extensive planning studies, computer modeling, and exchange of data between the interconnected systems. The difficulties of joint planning and operation of interconnected systems vary widely. As with mar- riages, from the institutional and administrative standpoint, coupled systems may become a single entity, or they may keep entirely separate accounts. Within the North American interconnections, for example, there are hundreds of electric utility companies that are entirely separate commercial entities. Customers receive power from, and pay bills to, the utility that serves their area, for example Consolidated Edison. They may do so without even knowing of the existence of the Eastern interconnection. Yet all the utilities in the Eastern interconnection are in a technical marriage that dictates or constrains key aspects of their technology choices and operating procedures. Within countries, there are typically common technical standards for all utilities, which reduces the complexity of interconnecting separate systems. In different countries, on the other hand, power systems may have evolved quite separately, with very different standards and technologies, which adds an extra layer of technical complexity to interconnections. Institutional and administrative features of power systems in different countries are also likely to differ in many ways, and these differences invariably affect the technical and operational dimensions of an interconnection. Issues ranging from power trading agreements to reliability standards, while expressed in technical terms, often must be resolved within the realm of policy and political economy. As one expert on international interconnections has remarked, “many technical, organizational, commercial and political problems have had to be solved to get large networks linked by international interconnections to operate”11. The greatest benefits of interconnection are usually derived from synchronous AC operation, but this can also entail greater reliability risks. In any synchronous network, disturbances in one location are quickly felt in other locations. After interconnecting, a system that used to be isolated from disturbances in a neighboring system is now vulnerable to those disturbances. As major blackouts in North America 11 F. Meslier (1999), “Historical Background and Lessons for the Future,” in J. Casazza and G. Loehr, The Evolution of Electric Power Transmission Under Deregulation, IEEE, Piscataway, NJ; p. 32. 18 Multi Dimensional Issues in International Electric Power Grid Interconnections and Europe in 2003 demonstrated, large-scale disturbances can propagate through interconnections and result in cascading outages, bringing down systems that had previously been functioning normally. In addition, long-distance interconnections with long transmission lines have potentially greater stability problems than is the case for shorter lines. Finally, many systems that have undergone electricity liber- alization in recent years have experienced large increases in transmission capacity utilization, reducing reserve margins. Minimizing the likelihood that an interconnection will lead to such problems as voltage collapse, dynamic and transient instability, or cascading outages due to propagated disturbances requires careful planning and well-coordinated operation. 2.2. Technical parameters of interconnection 2.2.1. Basic Electrical Parameters This section describes the basic electrical parameters and units of measurement used in electric power systems. It is meant to provide the non-technical reader with the concepts needed for a general under- standing of the technical issues discussed in subsequent sections. AC & DC Electric power comes in two forms: alternating current (AC) and direct current (DC). These forms are characterized by the behavior of their waveforms: AC alternates between positive and negative polarity with respect to ground, while DC does not. In power systems, AC is generally a sine wave, while DC is a constant value. Early electricity systems, such as Thomas Edison’s Pearl Street Station in New York City, which provided the world’s first public electric service in 1882, were DC. However, by the beginning of the 20th century AC systems had become standard worldwide. The main reason for the adoption of AC was that it is relatively simple to change AC voltage levels by using transformers, while it is difficult to change DC voltages. The development of solid-state power electronics in recent years has allowed an increased use of DC in the form of HVDC interconnections, but otherwise power systems remain AC. Frequency Frequency is the rate at which an alternating current changes from positive to negative polarity, measured in cycles per second, or hertz (Hz). There are currently two widespread world standards for power system frequency: 50 Hz in most of Europe and Asia, and 60 Hz in North America and in other places strongly influenced by the U.S. power industry, such as South Korea. The choice of 50 and 60 Hz systems in different locations is a consequence of historical legacies rather than the inherent technical superiority of one or the other. However, the range of possible frequencies for power systems is constrained by practical concerns. For example, a century ago many electric railroads operated at a frequency of 25 Hz, but 25 Hz was never adopted for general use in power systems because frequencies at that level cause electric lights to flicker. At the other end of the scale, frequencies well above 60 Hz result in higher impedances, leading to unacceptably high transmission and distribution losses. Voltage Voltage is the difference in electric potential between two points in an electric circuit. A difference in potential causes electric charges to flow from one place to another, just as a difference in heights causes Technical Aspects of Grid Interconnection19 water to flow from one level to another. Voltage is measured in volts (V), and sometimes in thousands of volts or kilovolts (kV). In power systems, two important measures are the maximum voltage and average voltage at any par- ticular point. Maximum voltage is important because insulation and safety equipment must be designed to protect against the highest voltage encountered. Average voltage is important because the amount of energy supplied to an end user or lost in transmission lines is a function of the average voltage and cur- rent. For DC systems, maximum and average voltages are the same, because DC voltage doesn’t oscillate. For example, the output of a 120 V DC power supply is a continuous 120 V relative to ground, and this is both the maximum and average voltage. For AC systems, different measures are required. In a 120 V AC system, the voltage actually oscillates in a sine wave between + 170 V and – 170 V relative to ground. The maximum voltage, also called amplitude or peak voltage, is thus 170 V. The simple arithmetic average of this waveform is actually 0 V, since the positive and negative voltages cancel each other out. Hence, another type of average is used, called root-mean-square (RMS). RMS is obtained by squaring the values of the voltage over one complete sine-wave cycle, determining its average value, and then taking the square root of that average. The result (true for any sine wave) is that VRMS = VPEAK / √2 = 0.707 VPEAK. For a household system with a VPEAK = 170 V, VRMS = 0.707 (170 V) = 120 V. Thus the common designation of a household electric outlet as “120 V AC” refers to the RMS value of the voltage. The voltages of power system components, such as transformers and transmission lines, are also generally given in RMS terms. Current Current is the flow rate of electric charge. In an electric circuit, charge flows from a point of higher voltage to a point of lower voltage through a conductor, just as water flows from a higher spot to a lower one through a pipe. Current is measured in amperes (A) or kilo-amperes (kA), where one ampere is a certain number of charges (to be precise 6.25 x 1018 charges, called one coulomb) flowing per second. As is the case for voltage, AC currents are generally described in terms of their RMS values. Resistance and Conductance Conductance describes the ability of an object, such as an electric wire, to allow electric currents to flow. The reciprocal of conductance is resistance, which describes how much the object resists the flow of cur- rent. Resistance is measured in ohms (Ω). The resistance of wire is a product of its resistivity (an inherent property of the material from which it is made, such as copper or aluminum, for a given temperature) and the dimensions of the wire. For a given material, the longer the wire is, the greater the resistance, and the larger in diameter the wire is, the smaller the resistance. In the analogy of water flowing from a higher to a lower spot through a pipe, resistance is analogous to the friction of the pipe. A narrow pipe has a higher resistance; a wide pipe has a lower resistance. Ohm’s Law Ohm’s Law describes the relationship between voltage (V), current (I), and resistance (R) across any ele- ment of a DC electric circuit: V = I∗R. Thus, for a fixed value of resistance – say for an HVDC transmis- sion line of a certain length and diameter – if the voltage is made larger, the current will decrease, and 20 Multi Dimensional Issues in International Electric Power Grid Interconnections vice versa. For example, if the resistance of a line is 25 Ω, and the current through the line is 1 kA, then the voltage drop across the line is V = 1 kA * 25 Ω = 25 kV. If the voltage on the sending side was 500 kV, then the voltage on the receiving side must be 25 kV less, or 475 kV. Power and Energy Power is the rate of energy flow, measured in watts (W), and sometimes in thousands of watts or kilo- watts (kW), or in millions of watts or megawatts (MW). For a DC circuit, the power passing through any element of the circuit (e.g. a transmission line, a generator, an electrical appliance) is the product of the voltage across it and the current passing through it: P = I∗V. The energy delivered by a power system is measured in kilowatt-hours (kWh), and sometimes megawatt- hours (MWh). In general, energy is equal to power times time. For example, a light bulb that draws 100 W of power and is in use for 10 hours consumes a total amount of energy, E = 0.1 kW * 10 h = 1 kWh. Note that power and energy are quite different concepts. If an electric oven draws 1 kW of power and is in use for an hour, E = 1 kW * 1 h = 1 kWh. In these two examples, the power levels are different but the energy consumed is the same, the difference being the length of time that each device is operated. Note that the basic unit of energy is the joule (J), while the basic unit of power is the watt, where 1 W = 1 J/s. Thus 1 kWh = 1 kW * 1 h = 1000 J/s * 3600 s = 3.6 million J. Resistive Losses When current flows against a resistance, some of its energy is lost in the form of heating. For a DC circuit, the resistive losses can be calculated using Ohm’s Law: PLOSS = I∗V = I(I/R) = I2R. To continue with the example under “Ohm’s Law” above, consider a 500 kV HVDC transmission line with 25 Ω of resistance, with 1 kA of current passing through it, and which has a voltage on the sending end of 500 kV, and a voltage on the receiving end of 475 kV. The total power being transmitted at the sending end of the transmission line is P = 500 kV ∗ 1 kA = 500 MW. Out of this 500 MW, the amount being lost to heating is PLOSS = (1 kA)2 ∗ 25 Ω = 25 MW. This constitutes 25 MW/500 MW = 5 percent of the power being transmitted. Very high voltages are used in transmission in order to reduce resistive losses to a tolerable level. In the example above, if the same amount of power were being transmitted (500 MW) but the sending volt- age were 125 kV instead of 500 kV, the current through the line must be I = P/V = 500 MW/125 kV = 4 kA; the current is four times higher to yield the same amount of
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