Generally if several machines are operated in parallel, and only one machine grounded, this effect is accentuated in the grounded machine. For example, consider four generators operating in parallel as shown in Figure below. The reactances shown are in percent based on the individual machine ratings.
The phase-sequence-reactance diagram is shown by Fig.(b) from which Z0 = 2.2; Z1= 2.2 and Z2, = 2.2. The sequence components for a line-to-ground fault are
The three-phase fault current of an individual generatol is Xg/X1= 100/8.8 = 11.4 times full load. Consequently, in the case illuskated, grounding only one generator will cause that generator to carry 22.8/11.4 or twice the current it would have on a three-phase fault. Since mechanical stresses are proportional to the square of the current, they would be equal to four times the three-phase short-circuit stresses. It will be observed that the line-togroundfault current is always greater than the threephase fault current, and that the situation becomes partitularly serious as the number of paralleled machines increases. In an installation of this kind, some form of neutral impedance device is a necessity.also when two generators are operated in parallel at generated voltage, there is the possibility of circulating harmonic currents. This is true whether the neutrals are interconnected or not. The two conditions necessary for the: flow of harmonic current are: the prcsence of a resultant harmonic voltage, and a path for the flow of current. It is important to note the term “resultant.”
If the two machines are duplicates and are being operated under identical conditions, they will probably generate the same harmonics of about the same magnitude and phase position. If the harmonics are thus equal and opposite, there will be no resultant voltage available to circulate harmonic current. If, hovever, the machines are dissimilar, one may generate harmonic voltages that the other does not. There will then be a circulating harmonic current between them whose magnitude is equal to the resultant harmonic voltage divided by the impedance at the harmonic frequency.
When faced with a requirement to parallel dissimilar generators, a system designer has several options to avoid problems associated with generator incompatibility:
- If possible, require that new or replacement alternator equipment be identical to existing equipment. This may or may not be practical depending on the voltage harmonics produced by the alternators in the existing system, especially if the machines are of significantly different kVA ratings. In machines other than 2/3 pitch arrangements, the fact that the machines are the same pitch may not be enough to eliminate problems, because differences in third-order harmonics could still cause significant neutral current flow. Where this is practical, it is probably the best solution. While this may sound like an extreme suggestion, it should be recognized that the alternator on a generator set represents only about 10% of the total factory cost of the machine, and that alternators do age over time regardless of their limited use in standby applications. It is prudent to replace an alternator when it is more than 25 years old as part of a paralleling upgrade in a system.
- Use a three-wire distribution system. By avoiding a solid neutral connection between the genset bus and the loads, the designer is free to let the neutral of the dissimilar machines in a system float,* so that the most common cause of harmonic problems is minimized by removing the path on which the most disruptive current can flow. (The harmonic currents will still cause heating in the machines, but the disruptive effect of current flow in the neutral is eliminated.) In these systems, loads that require a neutral connection will be required to be served by a delta/wye transformer to develop the required neutral connection. The designer should carefully specify the neutral grounding design and monitor the installation in these systems, because an errant neutral-to-ground bonding will result in neutral current flowing through the grounding (earthing) conductors in the system, which represents a potential hazard for electric shock and for fire due to overheating of conductors. Downstream transformers can be used to provide four-wire service to loads that require it. Connect neutrals of like-pitch machines only. Note that line voltage systems (those operating at less than 1000VAC) are usually required to have a neutralto- ground connection. In a parallel application the ideal location for this bonding point is in the system switchgear, so that there is only one neutral bond for the system. Consideration must be given to the magnitude of loads requiring the neutral connection versus loads that can operate only on the three phases. System loads will naturally balance out as long as there is sufficient line-to-neutral capacity in the system.
- Add neutral contactors in the link between the gensets and switchgear neutral bus to connect the neutral only on the first unit to close to the bus. This has a similar impact to the previous recommendation, but allows any machine to be the first connected to the system. In this design it is particularly critical for the failure modes of the neutral contactors to be considered. Alarms should be raised by failure of a neutral closure to operate correctly in either opening or closing mode. Dual neutral contactor position indicating contacts (one “a” and one “b” from different switches) should be used to be more certain of the state of the neutral contactor.
- Install reactors in the neutral leg of each generator to limit current flow at third- and higher-order frequencies. Reactors can be tuned to specific frequencies that are the biggest problems, but typically they are designed for 150/180 Hz, as this is the most problematic harmonic. The major issue in the use of reactors is their cost, and the custom nature of their design, making them problematic to acquire and install quickly. Also, the failure of the reactor may go undetected for a long time, resulting in a change in the effecting bonding arrangement of the system and potential unexpected hazards. Compensate for the incompatibility by oversizing theneutral conductor and derating the alternators.
-Technical information from Cummins Power Generation
-Westinghouse Transmission and Distribution Reference Book