1144 Power Quality

Distributed resources must provide the intended services to the users and electrical power networks. Fundamentally, these services are simply to provide high quality electric power in a safe manner when it is needed (i.e., reliability of supply) and in the quantity demanded. (Reliability — in the narrowest sense, the minimization of outages of any duration — is now considered by many to be a component of power quality.) New high technology equipment requires higher quality power. Furthermore, the rapid spread of computers and automated equipment has made customers aware of the effects of poor power quality. It is important to keep in mind that the user (possibly, but not necessarily, also the owner of the distributed resource) is buying services and not hardware, software, and operation and maintenance contracts. With the deregulation of the power industry, retail and commercial, electric power providers can and will differentiate their services through power quality. Therefore, power quality has moved to the forefront. At the same time, the advent of distributed generation has brought new problems that have the potential to degrade the power quality that existed before the installation of DG.

In more specific technical terms, power quality describes how closely the actual electrical signal at various points in the network (including at the terminals of the user's load device) follows the ideal stable, sinusoidal waveform that we associate with utility grade power. The term distortion is used to describe any deviation from this perfect sinusoid. Several important distortions or components of power quality are power interruption (complete loss of the waveform), voltage sag (decrease in amplitude of the waveform), voltage "flicker" (momentary voltage swings), and the presence of harmonics (higher frequency waves combined with the fundamental 60 Hz sinusoid). (See Figure 11.8.)

It is argued that distributed resource interconnection and operation will have both negative and positive impacts on the quality of power delivered to

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FIGURE 11.8

Effect of lagging power factor (courtesy of Nicoara Graphics).

the user. On the positive side, a decrease in service interruptions is ensured for the end users that can count on operating their on-site generation in the off-grid mode when required. On the negative side, it is possible for stability to be compromised when numerous distributed resources are operated on the same feeder. Because stability margins are specific to the configuration of the feeder and its loads, the distributed resources connected to the feeder, and the load being served at a given moment, general guidelines for determining whether there may be a stability problem cannot be defined.

Distributed resources have the potential to mitigate the effects of utility interruptions in service. Many end users would want a seamless transfer of their loads from the utility to the distributed resource should a utility power outage occur. Later, when utility service is restored, a seamless reconnection and synchronization of the two systems is desirable. In practice, a rather broad spectrum of configurations is available because system cost increases rapidly with decreasing transition time, and many users do not require virtually seamless transition. Some configurations are similar to conventional UPS systems, where, for example, an energy storage unit (typically a battery bank) immediately picks up the end user's load following the utility outage. The power is delivered to the load through an inverter that converts direct current (DC) to alternating current (AC) power. Seconds or minutes later, after the engine-generator set has reached rated speed, it is connected to the load and the battery/inverter is disconnected. Fuel cells are ideally suited for this application since they must already include energy storage to operate successfully when operating in the grid-independent mode. In addition, fuel cell power is delivered as DC power. Ideally, one power inverter would service both the fuel cell and the energy storage unit.

Simpler and less costly configurations use only the diesel generator unit, for example. Power will be interrupted for a few minutes while the diesel starts and reaches operating speed. In these simple systems, there is no transfer switch and no synchronizing gear, so when the utility power is restored, the engine-generator unit is disconnected from the load before the utility service is reconnected. The end user experiences a very short interruption in service in return for a much less expensive system. At present, the penetration of distributed resources into the electric distribution systems is very small; therefore, few reports exist on direct operating experience, particularly with respect to power quality.

Solar Stirling Engine Basics Explained

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