IEEE Std 1250 pdf free download
IEEE Std 1250 pdf free download.Identifying and Improving Voltage Quality in Power Systems.
Interconnection of the generation, transmission, and distribution systems takes place in an electrical substation. Substations may include transformers that raise or lower the voltage depending on the need. A substation that has a step-up transformer increases the voltage while decreasing the current, whereas a step- down transformer decreases the voltage while increasing the current for distribution. Electric power may flow through several substations between generating plants and consumers, and it may be changed in voltage several times.
The generation and transmission components are typically connected in an interconnected grid fashion. Within the “grid,” the transmission lines transport bulk power for long distances that typically cross multiple service territories and multiple utilities. Figure 2 shows a simple transmission system, referred to as a transmission network, illustrating how most of the substation buses have more than one source. In most circumstances, the loss of a single line or generator should not cause overloads within the remaining network. This offers a high degree of reliability because power can be maintained to most buses even with the loss of a line or source.
Distribution lines (commonly called primaries) are usually not interconnected but are designed in a radial fashion except in some cities that use a mesh or network distribution scheme. Traditional radial distribution systems consist of a source originating at a substation in which the system voltage is stepped down (see Figure 3). The distribution bus has breakers that feed lines (feeders) that carry the power to many customers in an area. There are usually line protective components (reclosers and fuses) downstream of the substation breaker on distribution lines. These components create situations in which only a portion of the distribution line may need to be de-energized to clear a fault (short circuit), thereby saving many customers on the line from experiencing interruptions unnecessarily. This protection scheme is commonly called line sectionalization. With smart grid becoming more common, another distribution protection scheme called segmentation is being utilized. Segmentation involves electronic line reclosers communicating with one another and isolating the smallest amount of customers closest to the fault location. It also involves switching portions of one circuit over to other circuits in some cases. DER sources can connect to distribution feeders near the substation or out on the feeder. Typically, there is a line protective component placed at the DER metering point.
3.2 Basic types of power quality variations
It is useful to divide power quality characteristics into the following two basic categories:
a) Steady-state (continuous) power quality characteristics. This refers to the quality of the normal voltage supplied to a facility. How much can the voltage magnitude vary from the nominal value? How distorted is the voltage or current? What is the imbalance among the three-phase voltages or current? What is the magnitude, frequency, and angle of each phase? All of these characteristics can be quantified, and limits for the variations can be developed.
b) Disturbances. This refers to power quality variations that occur at random intervals and are not associated with the continuous characteristics of the voltage. The variations include sustained interruptions, momentary interruptions, voltage sags (and swells), and transients. All of these disturbances can impact a facility, depending on the equipment susceptibility and investments that have been made in power conditioning.
Each of these two basic categories has indices associated with it. Indices provide the foundation for characterizing the supply system power quality levels in a consistent manner. Indices can be used to establish baselines of performance as a function of system characteristics. The following subclauses describe indices that can be used to describe power quality levels in both of these major categories along with example benchmarking results that can provide the basis for establishing targets and limits.IEEE Std 1250 pdf download.
