Homeland Security

Electrical Distribution System Overview

Modern power grids are extremely complex and widespread. Surges in power lines can cause massive network failures and permanent damage to multimillion-dollar equipment in power generation plants. After electricity is produced at power plants it has to get to the customers that use the electricity. As generators spin, they produce electricity with a voltage of about 25,000 volts [a volt is a measurement of electromotive force in electricity, the electric force that pushes electrons around a circuit]. The transmission and distribution system delivers electricity from the generating site (electric power plant) to residential, commercial, and industrial facilities.

The electricity first goes to a transformer at the power plant that boosts the voltage up to 400,000 volts for distribution through extra-high voltage (EHV) transmission lines. When electricity travels long distances it is better to have it at higher voltages since the electricity can be transferred more efficiently at high voltages. High voltage transmission lines carry electricity long distances to a substation. At transmission substations a reduction in voltage occurs for distribution to other points in the system through high voltage (HV) transmission lines. Further voltage reductions for commercial and residential customers take place at distribution substations, which connect to the primary distribution network.

Utility transmission and distribution systems [T&D] systems link electric generators with end users through a network of power lines and associated components. In the United States typically the transmission portion of the system is designated as operating at 69 kilovolts (kV) and above, while the distribution portion operates between 110 volts and 35 kV. A further distinction is often made between primary distribution (voltages between 2.4 and 35 kV) and secondary distribution (110 to 600 volt) systems. Industrial and commercial customers with large power demands often receive service directly from the primary distribution system.

Transformers are a crucial link in the electric power distribution system. Utility transformers are high-voltage distribution transformers typically used by utilities to step down the voltage of electricity going into their customers' buildings. Distribution transformers are one of the most widely used elements in the electric distribution system. They convert electricity from the high voltage levels in utility transmission systems to voltages that can safely be used in businesses and homes. Distribution transformers are either mounted on an overhead pole or on a concrete pad. Most commercial and industrial buildings require several low-voltage transformers to decrease the voltage of electricity received from the utility to the levels used to power lights, computers, and other electric-operated equipment.

Transformers consist of two primary components: a core made of magnetically permeable material; and a conductor, or winding, typically made of a low resistance material such as copper or aluminum. The conductors are wound around a magnetic core to transform current from one voltage to another. Liquid insulation material or air surrounds the transformer core and conductors to cool and electrically insulate the transformer. Many different distribution transformer designs are available to utilities, depending on the loading patterns and needs of the end-user. Transformer engineers modify transformer design and vary material depending upon the needs of a particular utility (cost of energy, capacity, etc.).

A blackout is a condition where a major portion or all of an electrical network is de-energized with much of the system tied together through closed breakers. Any area whose tie-lines to the high voltage grid cannot support reasonable contingencies is a candidate for a blackout. System separations are possible at all loading levels and all times in the year. Changing generation patterns, scheduled transmission outages, and rapid weather changes among other reasons can all lead to blackouts. Separations due to dynamic instability are typically initiated by multiple contingencies such as loss of corridors, several transmission circuits, several generating units, or delayed fault clearing.

The system just prior to a blackout may not be dynamically unstable but in an overloaded condition. At such loadings, the collapse may come about due to damage to thermally overloaded facilities, or circuits contacting underlying facilities or vegetation. When an overloaded facility trips, other facilities will increase their loadings and may approach their thermal capabilities or relay trip settings.

Voltage collapse is the process by which voltage instability leads to the loss of voltage in a significant part of the system. This condition results from reactive losses significantly exceeding the reactive resources available to supply them. Circuits loaded above surge impedance loadings and reduced output of shunt capacitors as voltages decline can lead to accelerating voltage drops. Voltage collapse can look like both a steady-state problem with time to react and a problem where no effective operator intervention is possible. It is very hard to predict the area that will be affected or electrically isolated from the grid.

Voltage collapse is an event that occurs when an electric system does not have adequate reactive support to maintain voltage stability in which the sustained voltage level is controllable and within predetermined limits. Voltage Collapse may result in outage of system elements and may include interruption in service to customers. Apparent Power, the product of the volts and amperes, comprises both real and reactive power, usually expressed in kilovoltamperes (kVA) or megavoltamperes (MVA). Real Power is the rate of producing, transferring, or using electrical energy, usually expressed in kilowatts (kW) or megawatts (MW). Reactive power is the portion of electricity that establishes and sustains the electric and magnetic fields of alternating-current equipment. Reactive power must be supplied to most types of magnetic equipment, such as motors and transformers. It also must supply the reactive losses on transmission facilities. Reactive power is provided by generators, synchronous condensers, or electrostatic equipment such as capacitors and directly influences electric system voltage. It is usually expressed in kilovars (kvar) or megavars (Mvar).

The system restoration sequence and timing will be directly impacted by the various sizes, types, and state of operation of the system generating units prior to the blackout. After a system has blacked out, the system operators perform a survey of the system status. Circuit breaker positions will not provide a reliable indication of faulted versus non-faulted equipment. Breakers can be found in the closed position, but the associated transmission facility is faulted. If the system blackout is storm-initiated, this condition is quite possible. The storm can continue to damage equipment after the system is de-energized. Also, equipment with neutral connections, such as reactors, transformers, and capacitors, may be locked out from the neutral overcurrent conditions during system shutdown. These facilities may be in perfectly serviceable condition. Most relay systems will remain reliable and secure during restoration, provided there is adequate fault current available to activate the relaying. The most questionable relay reliability issues come from reclosing relays.

A power generating unit separated from the may have islanded and continue to generate power for its station auxiliary load. With no system load on the generators, the station auxiliary demand will be quite small, and the steam generators output may be difficult to control. Immediate load addition may be required to keep the steam generator from tripping or having the steam turbine trip out on overspeed. Other units may be able to operate indefinitely on their auxiliary load.

An electrical utility which experiences an operating capacity emergency seeks to balance its generation to its load to avoid prolonged outages of service. The emergency reserve inherent in frequency deviation may be used as a temporary source of emergency energy. A utility unable to balance its generation to its load removes sufficient load to permit correction of the outage. In the event of a capacity deficiency, generation and transmission facilities are used to the fullest extent practicable to promptly restore normal system frequency and voltage. If all other steps prove inadequate to relieve the capacity emergency, the system may take immediate action which includes but is not limited to manual load shedding. Unilateral adjustment of generation to return frequency to normal may jeopardize overloaded transmission facilities. Voltage reduction for load relief is made on the distribution system. Voltage reduction on the subtransmission or transmission system may effective in reducing load; however, voltage reduction would not be made on the transmission system unless the system has been isolated from other interconnected systems. If the overload on a transmission facility or abnormal voltage/reactive condition persists and equipment is endangered, the affected system or pool may disconnect the affected facility. shutdown. If abnormal levels of frequency or voltage resulting from an area disturbance make it unsafe to operate the generators or their support equipment in parallel with the system, their separation or shutdown would be accomplished in a manner to minimize the time required to re-parallel and restore the system to normal.

After a system collapse restoration begins when it can proceed in an orderly and secure manner. Restoration priority is normally given to the station supply of power plants and the transmission system. Even though restoration is intended to be expeditious, system operators seek to avoid premature action to prevent a re-collapse of the system. Customer load is normally restored as generation and transmission equipment becomes available, since load and generation must remain in balance at normal frequency as the system is restored. When voltage, frequency and phase angle permit, the system operator may resynchronize the isolated area with the surrounding area. In order to systematically restore loads without overloading the remaining system, opening circuit breakers may isolate loads in blacked-out areas. Reenergizing oil-filled pipe-type cables must be given special consideration, especially if loss of oil pumps could cause gas pockets to form in pipes or potheads.

After determining the extent of the blackout and assessing the status of system equipment, the switching operations necessary for system reintegration represent a significant portion of the restoration process. Depending on the specific utility's requirements, there are two general switching strategies which may be used to sectionalize the transmission system for restoration. The first is the "all open" approach where all circuit breakers at affected (blacked out) substations are opened. The second strategy is the "controlled operation" where only those breakers necessary to allow system restoration to proceed are opened.