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Abtract: Electrical Distribution Systems

Electricity cannot be stored or stockpiled in large quantities, so generating  capacities  must  meet  peak load requirements instantly. The move from early DC systems to high-voltage AC transformers solved voltage drops during long-distance transmission and varying voltage requirements, leading to huge growth in the electric industry, interconnecting generation sites, and higher voltages to support larger electric networks with increased power-carrying capacities. Electric distribution equipment and systems  are  conceptually  simple  now but use sophisticated technologies that are still  developing rapidly and require a mathematical approach (beyond the scope of this manual) for a full   understanding of design and operation.

Common voltages are noted for transmission lines (345,000 to 765,000 V), distribution systems (12,000 to 138,000 V), and medium to large customers (2,400 to 12,000 V). Higher transmission voltages are unproven in commercial applications; the highest operating transmission voltage still is 700 to 800 kV. High-voltage DC (HVDC) is cheaper for distances over 500 miles.

Doubling HVDC cable voltage increases power-carrying capability fourfold, but design, installation, and maintenance costs rise. This chapter describes components of university systems where electricity (generated on campus, bought, or both) is received and distributed to points on campus (not supplied in electricity utilization voltages to individual buildings).

Switching Stations

These stations switch electric power from a larger transmitting system to the university system and switch electric power generated on campus into the system. A switching station usually has more than one feed (or more than one station serves the university system, each with a separate feed). A two-feed system has manual or automatic switching and greater service reliability. A triple-feed system is advisable for life- sustaining applications (e.g., medical facilities) if economically feasible. Switching station transformers reduce high-voltage electricity from the incoming system to the voltage required by the university distribution system and operate at lower voltages because of safety and economics. Switching stations house fault detection and circuit interruption facilities to protect against exterior faults and overcurrents.

Switching Substations

Substations in the distribution system perform functions similar to those of switching stations. Substation elements include (1) switches, fuses, and circuit breakers; (2) switchgear; and (3) transformers.

Switches, Fuses,  and  Circuit  Breakers.  Switches  are used only during normal operations; fuses operate only under abnormal conditions; and circuit breakers perform in either. All are rated based on system voltage, continuous current capacity, and short-circuit current interrupting capacity.

Metal-Clad and Metal-Enclosed Switchgear. In a specific type of electrical equipment, circuit breakers, disconnecting devices, relays, metering, potential transformers, and current transformers are in separate metal compartments. A removable switch assembly is metal-clad; if not removable, it is metal-enclosed.

Transformers. Different generation, transmission, and utilization requirements dictate different voltage and current combinations, so voltage and current at high power levels must be changed (transformed). Power transformers enable energy generation at any needed voltage (much higher for transmission over long distances) and delivery at still another voltage.

Transformers have two basic elements, two or more windings insulated from each other (and the core) and a usually thin, insulated, laminated-sheet steel core.

Power transformers that step down distribution system voltages are classified by core construction type (core and shell) and cooling type. (1) Ratings indicate permissible winding temperatures at full load and 40ºC (function of kilovolt-amperes). (2) Each of the four transformer insulation classes (A, B, F, H) has NEMA specification and temperature limits. (3) Transformers in parallel can be used to meet power load demand, but they must be a matched set (primary and secondary voltages, line frequency, transformer connection, turns ratios, impedances that can share the load based on relative kilovolt-ampere ratings). (4) This section describes and compares the three common transformer types: dry transformers using clean filtered air (explosion-free, self-extinguishing, less expensive, larger than equivalent oil or PCB transformers, used for small loads); oil transformers, workhorses of power distribution systems (indoors or outdoors, self-cooled or air-cooled); and PCB transformers (fire resistant, regulated since 1976, mostly replaced or retrofitted).

High-Voltage    Conductors    and    Circuits Substations use two basic circuit  systems  to  feed building systems. In the radial system, separate feeders radiate from the substation with subfeeders (branches) that split off to transformers serving individual buildings  or building clusters.  In the network system, secondaries  of two or more transformers are linked, with adjacent transformers supplied from the same or different feeders, so distribution is at utilization voltages. Three main factors determine distribution voltages: transmission energy loss versus equipment cost, conductor strength, and overhead versus underground installation. A mathematical trade-off exists between cost of transmission energy loss and cost of transmitting equipment. Conductor strength is considered for overhead transmission lines (with aesthetic and routing issues) versus underground transmission lines (more expensive, most common in new construction).

Physical cable strength helps define electrical conductive capacity. Power cables serve as the arteries of high-voltage networks, so electrical network reliability can be improved (and costly downtime decreased) by improving cabling.

Cable Insulation. Paper-impregnated lead cable and varnished cloth were the initial insulation workhorses. Since the 1970s, various polyethylene compounds have been used as insulation, with good insulating characteristics (e.g., high moisture resistance, low temperature characteristics, high ozone resistance, greater abrasion resistance), lighter weights, and easier terminations and splicing.

Termination and Splicing. Different splicing kits and techniques are available, but one must use proper splice size and type, follow manufacturer recommendations, and be performed by skilled staff. Splices and terminations are usually cable system weak points, requiring careful installation and maintenance.

Cable Maintenance. The insulation resistance test determines insulation resistance between the conductor and ground. The dielectric absorption test provides better information than the spot test, plotting insulation resistance against time, but takes longer than the insulation resistance test. The high potential (hypot) test applies stress beyond normal cable use; the AC hypot test is used almost exclusively for insulation breakdowns, and the nondestructive DC hypot test is commonly used for maintenance.

Surge Arrestors. These devices act as grounds to protect against overvoltages (e.g., lightning, switching surges) and resulting flashover and equipment damage. The three surge arrestor classes are station (best), intermediate (lower energy discharge), and distribution (lowest energy discharge, least protection).

Electric Power Generation

Generators. Generators are electromechanical devices to convert mechanical energy to electrical energy based on Faraday’s Law (voltage is induced on a conductor moving through a magnetic field). Induced voltage is directly proportional to number of turns in the conductor, magnetic field strength, conductor speed, and nearness to 90° field crossing. Generator types include AC synchronous (most common in industry), asynchronous, emergency, and DC. This section details operations of (1) AC synchronous generators, structurally identical to synchronous motors; (2) asynchronous generators, induction motors driven faster than synchronous speed by a prime mover (simpler in construction, less costly, and limited in size because of harmonics) are used for small cogeneration units and wind-powered generators; and (3) emergency generators, with normal power source and emergency generator connected to an automatic (sometimes also manual) transfer switch and UPSs for computers, PBXs, and life-sustaining equipment).

Cogeneration is the production of more than one form of energy simultaneously, usually electricity  and  heat energy. Cogeneration  interconnection  raises  (often utility) concerns over personnel safety, service quality during normal versus emergency operations, and interconnection requirements that depend on interconnection voltage, transformer configuration, protection scheme, and onsite load and generation capacity. Overcurrent protection  coordination  is  simple in a utility distribution network, but cogeneration is more difficult to coordinate. Autoreclosures are an issue because cogeneration  changes  available  fault  and system coordination for in-house systems and the utility grid.   Another problem is islanding, when the cogeneration site operates independent of the reference voltage and frequency of the utility power grid and is no longer in synch with it. Harmonics generation must be studied for impacts on computers  and  sensitive electronic equipment. Required electrical protection depends on cogeneration unit size, generator type, in- house load, and interconnection voltage; local utility requirements; and site conditions. Cogeneration operators and utilities must coordinate to ensure network reliability and safety.

Utility-Connected Systems. The most practical way for universities to use renewable power is to connect it to a building also served by the local utility (grid-connected or utility-interactive systems). With net metering and interconnection  agreements,  excess  generated electricity can be fed back to utilities.  Such  systems do  not require many additional parts, but an inverter (to translate DC into AC current) is needed.

Battery Storage Systems. These systems are used when utility-connected systems are not an option, but they require a charge controller to regulate quality of electricity flowing from the system to the battery.

System Protection

Short-Circuit Faults. Electric current always follows the path of least resistance but is confined in a conductor by the dielectric around it, creating a  short-circuit  current (as high as 10,000 times the rated current) with a magnitude that is a function of power  source  capacity and conductor size and length (not load). The four  sources of short-circuit currents are the utility system, in-house synchronous generators, induction motors, and synchronous motors. Available fault from the utility is directly related to utility transformer size. When the system is in a symmetrical or asymmetrical fault condition, it can disrupt the transmission network (e.g., large currents, electrical arcing, higher or lower system voltage, three-phase system imbalance, power flow interruption). The protection system for a high-voltage system (composed of fuses, relays, and circuit breakers) safeguards against such disruptions. Fuse selection criteria are voltage rating, ampacity, and interrupting rating. Protective relay types (electromagnetic attraction, electromagnetic induction, thermal induction, electronic) must be reliable, selective, sensitive, and speedy. Mechanical circuit breakers break and reclose a circuit in all conditions.

Selection of System Protective Devices. Institutional systems use circuit breakers for applications requiring complex relaying or high continuous currents, but most applications can use either circuit breakers or power fuses (in widespread use; simple, economical, fast response, no maintenance).

Types and Symptoms of Failures. Major electrical failures are caused by insulation breakdown due to dirt, high ambient temperature, oil leakage, internal failure, overload, high-voltage surges, ferroresonance, corona, flashover, absorption of moisture and dust in cores, excessive vibration, and aging.

Instruments

Test Instruments. Electrical test instrument are tools to perform maintenance on power systems. For low voltages, multimeters measure voltage and resistance, and amprobes measure current. Infrared detectors, meggers, hypot testers, phase meters, glow sticks, and dielectric testers are used for high-voltage systems.

Metering. The need to accurately measure electrical energy became critical  after  the  1970s  energy  crisis. The kilowatt hour (kWh) meter measures cumulative electrical energy use over a time period.  Kilowatt  demand (kWD) signifies  maximum  power  demand within a time period. The difference between peak demand and substation rating indicates available spare capacity. Electric meters are subject to drift, so they are tested and calibrated periodically (based on service magnitude and critical importance of data).

Use of Capacitors for Power Factor Correction. The power factor (quantity by which apparent power is multiplied to obtain active circuit power) is an important value in load consideration and measurement. This component of current cannot be eliminated but can be neutralized by adding capacitors at the loads, in the substations, or on the lines (generally less costly).