The main power quality issues affected by Distributed Generation are: 1. Voltage Regulation 2. Sustained Interruptions 3. Harmonics 4. Voltage Sags.
Issue # 1. Voltage Regulation:
This is often the most limiting factor for how much Distributed Generation can be accommodated on a distribution feeder without making changes.
It may initially seem that Distributed Generation should be able to improve the voltage regulation on a feeder. Generator controls are much faster and smoother than conventional tap-changing transformers and switched capacitor banks. With careful engineering, this can be accomplished with sufficiently large Distributed Generation.
However, there are many problems associated with voltage regulation. In cases where the Distributed Generation is located relatively far from the substation for the size of Distributed Generation, voltage regulation issues are often the most limiting for being able to accommodate the Distributed Generation without changes to the utility system.
Issue # 2. Sustained Interruptions:
This is the traditional reliability area. Many generators are designed to provide backup power to the load in case of power interruption. However, Distributed Generation has the potential to increase the number of interruptions in some cases.
Much of the Distributed Generation that is already in place was installed as backup generation. The most common technology used for backup generation is diesel gensets. The bulk of the capacity of this form of DG can be realized simply by transferring the load to the backup system. However, there will be additional power that can be extracted by paralleling with the power system. Many Distributed Generation installations will operate with better power quality while paralleled with the utility system because of its large capacity.
However, not all backup Distributed Generation can be paralleled without great expense. Not all Distributed Generation technologies are capable of significant improvements in reliability. To achieve improvement, the Distributed Generation must be capable of serving the load when the utility system cannot.
Issue # 3. Harmonics:
There are harmonics concerns with both rotating machines and inverters, although concern with inverters is less with modern technologies.
There are many who still associate Distributed Generation with bad experiences with harmonics from electronic power converters. If thyristor-based, line-commutated inverters were still the norm, this would be a large problem.
Fortunately, the technologies requiring inverters have adopted the switching inverters. This has eliminated the bulk of the harmonics problems from these technologies.
One problem that occurs infrequently arises when a switching inverter is installed in a system that is resonant at frequencies produced by the switching process. The symptom is usually high-frequency hash appearing on the voltage waveform. The usual power quality complaint, if any, is that clocks supplied by this voltage run fast at times. This problem is generally solved by adding a capacitor to the bus that is of sufficient size to shut off the high-frequency components without causing additional resonances.
Harmonics from rotating machines are not always negligible, particularly in grid parallel operation. The utility power system acts as a short circuit to zero-sequence triplen harmonics in the voltage, which can result in surprisingly high currents. For grounded wye-wye or delta-wye service transformers, only synchronous machines with 2/3 pitch can be paralleled without special provisions to limit neutral current.
For service transformer connections with a delta-connected winding on the Distributed Generation side, nearly any type of three-phase alternator can be paralleled without this harmonic problem.
Issue # 4. Voltage Sags:
This a special case because Distributed Generation may or may not help.
The most common power quality problem is a voltage sag, but the ability of Distributed Generation to help alleviate sags is very dependent on the type of generation technology and the interconnection location. Figure 6.6 illustrates a case in which Distributed Generation is interconnected on the load side of the service transformer. During a voltage sag, DG might act to counter the sag. Large rotating machines can help support the voltage magnitudes and phase relationships. Although not a normal feature, it is conceivable to control an inverter to counteract voltage excursions.
The Distributed Generation influence on sags at its own load bus is aided by the impedance of the service transformer, which provides some isolation from the source of the sag on the utility system. However, this impedance hinders the ability of the Distributed Generation to provide any relief to other loads on the same feeder.
Distributed Generation larger than 1 M W will often be required to have its own service transformer. The point of common coupling with any load is the primary distribution system. Therefore, it is not likely that Distributed Generation connected in this manner will have any impact on the voltage sag characteristic seen by other loads served from the feeder.