The voltage, power, frequency, load factor, and reliability capabilities of the transmission system are designed to provide cost effective performance. It is uneconomical to connect all distribution substations to the high main transmission voltage, because that equipment is larger and more expensive. Subtransmission circuits are usually arranged in loops so that a single line failure does not stop service to many customers for more than a short time. Such a system could be less prone to failure if parts of it were suddenly shut down. Voltage is stepped down before the current is sent to smaller substations. As reactive current increases, reactive power increases and power factor decreases. For transmission systems with low power factor, losses are higher than for systems with high power factor. As power systems evolved, voltages formerly used for transmission were used for subtransmission, and subtransmission voltages became distribution voltages. Voltages of 69 kV, 115 kV, and 138 kV are often used for subtransmission in North America. Utilities add capacitor banks, reactors and other components (such as phase-shifters; static VAR compensators; and flexible AC transmission systems, FACTS) throughout the system help to compensate for the reactive power flow, reduce the losses in power transmission and stabilize system voltages.

This is an everyday occurrence in AC systems, but one that can become disrupted when AC system components fail and place unexpected loads on the grid. Loops can be normally closed, where loss of one circuit should result in no interruption, or normally open where substations can switch to a backup supply. Factors that affect resistance and thus loss include temperature, spiraling, and the skin effect. The effect of the material is designated by the Greek letter ρ (rho, pronounced ro), which represents the resistance of a piece of the material with specific dimensions. The skin effect causes the effective resistance to increase at higher AC frequencies. Where R1 is the known value of the resistance at t1 and R2 is the resistance at t2. In the US customary system, ρ is defined by the resistance of 1 foot of metal with a cross-section of 1 circular mil (CM). Series resistance and shunt conductance are considered to be distributed parameters, such that each differential length of the line has a corresponding differential series impedance and shunt admittance. Series capacitors or phase-shifting transformers are used on long lines to improve stability. For AC it was 4,000 kilometres (2,500 miles), though US transmission lines are substantially shorter.

For intermediate-length lines on the order of 100 kilometres (62 miles), the limit is set by the voltage drop in the line. As of 1980, the longest cost-effective distance for DC transmission was 7,000 kilometres (4,300 miles). One example of a long DC transmission line is the Pacific DC Intertie located in the Western United States. The New York Times reported that American hackers from the United States Cyber Command planted malware potentially capable of disrupting the Russian electrical grid. A 2024 report found the United States behind countries like Belgium and the Netherlands in adoption of this technique to accommodate electrification and renewable energy. Finally, at the point of use, the energy is transformed to end-user voltage (100 to 4160 volts). The product of line length and maximum load is approximately proportional to the square of the system voltage. The terminal characteristics of the transmission line are the voltage and current at the sending (S) and receiving (R) ends. The line is assumed to be a reciprocal, symmetrical network, meaning that the receiving and sending labels can be switched with no consequence. The lossless line approximation is the least accurate; it is typically used on short lines where the inductance is much greater than the resistance.

Reconductoring is the replacement-in-place of existing transmission lines with higher-capacity lines. Figure 6 Various transmission line supports. The power transmitted by an AC line increases as the phase angle between source end voltage and destination ends increases, but too large a phase angle allows the systems at either end to fall out of step. Deep-cycle lead-acid systems often use a low-charge warning light or a low-charge power cut-off switch to prevent the type of damage that will shorten the battery's life. Understanding the temperature distribution along the cable route became possible with the introduction of distributed temperature sensing (DTS) systems that measure temperatures all along the cable. However, when "jump starting" a car, the high current can cause the rapid release of large volumes of hydrogen, which can be ignited explosively by a nearby spark, e.g. when disconnecting a jumper cable. These reactive currents, however, cause extra heating losses. Otherwise, the buildup of heat can cause a problem. If this kind of battery is over-discharged, the reagents can emerge through the cardboard and plastic that form the remainder of the container. Battery Knowledge - AA Portable Power Corp.

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