EL DOCENTE EMOCIONALMENTE INTELIGENTE

Simulation modeling is a valuable tool that can be used to explore the potential impacts of DG on electric systems. For example, a Virtual Test Bed simulation platform suite was constructed in one detailed study to examine both power quality and reliability issues associated with DG installations (GE Corporate Research and Development, 2003). The Virtual Test Bed models the utility’s power delivery system, the loads, and the DG. In this study, parametric analysis is used to examine the influence of the amount of DG on a feeder, the location of the DG relative to the loads, (lumped at the beginning, middle, or end of the feeder, or uniformly distributed along the feeder), inverter-based and rotating DG technologies, DG local voltage regulation strategies (either operation at a power factor of 1.0 or the DG provides voltage regulation based on local conditions), two radial feeder lengths, and the presence or absence of capacitor banks on the feeder.

The analysis of protection and reliability in this study included: transient response and fault behaviors (capacitor switching and fault behaviors); reclosing; anti-islanding scenarios; and power systems

dynamics and stability. Some of the conclusions from this analysis, which focused on the behavior of DG units with power electronics, were that:

“A fault analysis found that the fault current contribution of a standard induction motor is usually much larger than that of current controlled inverter-DG. … the DG, in this example, provides some damping to high-frequency oscillations. Other findings include:

• Local distribution system dynamics are most affected by DG trips.

• Distributed generation controls do not have a major impact on local dynamics when the connection to the host utility is maintained.

• Anti-islanding schemes (of the type tested here) appear to be effective at destabilizing islands containing multiple DG units and loads with relatively complex dynamics.

• Voltage and power regulation tend to act contrary to the anti-islanding schemes.

• Widespread penetration of DG units at the load appears to be benign with respect to system response to bulk system disturbances.

• Anti-islanding schemes (of the type tested here) appear to have little impact on system response to bulk system disturbances.

• Aggressive tripping of DG units in response to under voltages appears to present a substantial hazard to the bulk system, and was shown to bring down the entire U.S. western system in one extreme case (GE Corporate Research and Development, 2003).”

Another analyst used a probabilistic reliability model to compare the options of adding DG or adding another feeder to a local distribution network. Using the Expected Energy Not Served as the reliability index, this model is able to optimize both the size and location of alternative DG units. The input for this model includes values for the annual failure rate of each system component, the repair time, and

switching times. For example, for the network studied, substations were given failure rates of

0.02 occurrences per year, line sections of 0.04 to 0.12 occurrences per year, and DG of 5 occurrences per year, with repair times of 4 hours for the network resources and 50 hours for the DG resources. For this network, an additional feeder was able to reduce the Energy Not Served from over 17 MWh per year to less than 5 MWh per year. Three possible DG configurations were identified that provided that same level of reliability (Chowdhury et al. 2003).This study is enlightening because it recognizes that DG can improve system reliability even if it is not 100% reliable itself, that is, that physical assurance

requirements are no more appropriate for DG resources than for any other network resource used to provide reliable service.

In 2003, Oak Ridge National Laboratory (ORNL) performed a study entitled “Quantitative Assessment of Distributed Generation Resource Benefits.” In this study, ORNL quantified the benefits of system

reliability in terms of a reduction in the LOLP of DG (Hadley et al. 2003). Reliability of the Pennsylvania/New Jersey/Maryland Interconnection (PJM) system was simulated across multiple scenarios of differing generation unit sizes. The study shows that improvement in the LOLP is achieved when generation expansion needs are met with ten small plants compared to a single large plant of the same size. For example, in one scenario, generation expansion was designed to be met by a new 100 MW single unit and in the alternative scenario as ten 10 MW units. Many other paired scenarios of single or multiple units of generation capacity were also analyzed.

The study results indicate that the LOLP for each pair of scenarios was always lower in the scenario with the higher number of units. This suggests that a system in which capacity expansion is comprised of many DG units, rather than one central station power plant, can provide more reliable service to customers. The study draws the following conclusions:

“Based on the … analysis there is a small but positive value to having capacity added at the unit size of DG as opposed to typical central station size. The main beneficiary may be society. If reserve margins are fixed by PJM at a certain percentage of demand, or by the largest single contingency, then society will benefit by increased reliability at the same amount of capacity. This can also lead to lower electricity prices since high cost plants will not be called upon as

often. If, however, the ISO chooses to lower the required reserve margins, then utilities may benefit by not having to have as much reserve capacity on hand, through either ownership or the capacity market (Hadley et al. 2003).”

The study also indicates that DG units can be used to improve system reliability even though each

individual unit is less than 100% reliable. That is because the same rules of redundancy and diversity that applies to central station plants, or any other component of the power system, also applies to DG.