For interconnection two power sub system, e.g. a utility system and a distributed generation unit some important subject shall be considered as following:
A- Insulation Coordination
Any equipment, including distributed generators, connected to the utility system (even for brief periods) must be able to withstand the normal and abnormal voltages that can be experienced on the utility system. With a properly designed distribution system, these normal and abnormal voltages should not cause damage to or reduce the reliability of any connected equipment. This statement applies to all connected equipment regardless of ownership including the utility, distributed generator, or other customer Utility equipment is designed with a certain basic impulse insulation level (BIL). BIL, in general terms, is a measure of the ability of a piece of equipment to withstand normal and abnormal voltages. Lightning and switching of equipment are two common sources of high abnormal voltage transients.
For example The US 13.8 kV system is designed for 95 kV BIL, the standard level for this voltage class. The US distribution system is in a geographical area subject to significant lightning. US line equipment is protected by utility-grade distribution surge arresters rated at 12 kV, which is the normal recommended rating for 13.8 kV multi-grounded neutral distribution systems. 12 Kv arresters have a rated discharge voltage of 39 kV for a standard 10 kA 8x20 sec lightning test waveform for which arresters are rated. Thus, a 12 kV arrester with this discharge voltage would achieve a 143% protection margin, PM, for 95 kV BIL equipment:
This has proven to be a reliable margin and is typical for utility distribution systems of this voltage class. This margin is necessary for several reasons, including:
- Equipment BIL declines as it ages. Having a large protective margin allows the equipment to remain in service longer.
- Many lightning stroke currents are larger than 10 kA. For example, a 40 kA 8x20 sec lightning test wave will yield a discharge voltage of 48 kV in a standard 12 kV arrester.
- Many lightning strokes have a much steeper rate-of-rise than the standard 8x20 sec test waveform and produce significantly higher discharge voltage across the arrester and the leads connecting the arresters to the system.
- Lightning surge voltages tend to double when they reach the end of a cable or an overhead line.
These factors shrink the protective margin considerably.
In contrast to standard utility line equipment, the protective margin for rotating machine generators proposed for DG applications to be directly connected to the US system is often very low in comparison. Depending on the current magnitude and the rate-of-rise of an incoming surge, the margin provided by US arresters will be insufficient to protect these machines for many of the voltage levels expected to be induced from common lightning strikes to the distribution system.
One might think that applying an arrester with a lower discharge voltage rating might protect such machines. However, this approach would result in these arresters attempting to protect the entire distribution system for some abnormal, but expected, low frequency surge phenomena that might occur. Examples of the latter condition include unfaulted phase voltage rise during single line-to-ground faults and resonant conditions that can arise on phase conductors. The neutral grounding reactance in the substations, while limiting the ground fault current, increases the unfaulted phase voltage rise. The voltage rise on unfaulted phases can subject arresters to excessive transient overvoltage (TOV) duty, causing the arrester to fail and isolate itself from the system. Unfortunately, this failure and resulting isolation would leave the machine vulnerable to the next lightning or switching transient overvoltage.
Using a standard utility service transformer for interconnecting the generator will generally resolve the insulation coordination issue. Such transformers are designed for this kind of service and provide the necessary separation of insulation levels from the high voltage winding to the low voltage winding. The reader is reminded that it is not possible to guarantee absolute protection from voltages induced by lightning surges. Even with a properly coordinated transformer, lightning may enter the electrical system from another path and induce voltages that cause damage to the machine insulation.
B- Underground Cable Express Feed
US may choose to provide a waiver to the interconnection transformer requirement if
1. The proposed DG site is served by underground cable directly from substation bus,
2. There are no other customers on the feeder (express feed),
3. It can be done safely without other technical issues identified during the feasibility study such as harmonics, overvoltages due to switching transients, and overvoltages during islanding.
Such an arrangement minimizes the risk of lightning strikes to the feeder because the substation end is well shielded as presumably would be the generator end of the distribution feeder. A mixed overhead/underground feeder, even if an express feed, can create significant risk of failureif the overhead portion is struck by lightning. The resulting surge will travel long distances down the cable where the voltage will tend to double when the surge reaches the transition in system construction at the generator.
An underground express feed does not guarantee immunity from voltages induced by lightning because lightning may strike a nearby structure and induce voltages that enter the generator through other conductors in the electrical system. Lightning has also been shown to occasionally strike buried cables through the soil. Therefore, careful attention to grounding, bonding, and ground referencing must still be given even if the entire system is underground.
The express feed arrangement also minimizes the exposure to the impacts of disturbances related to serving other customers.
A common complicating factor is the need for an alternate source in case of underground cable failure. In many cases on the US system, this alternate source will be provided by an open-wire overhead system. If the DS is to be operated while connected to such an alternate feed, a waiver cannot be granted.
If it is a critical that the DG operate when the main feed has failed, a second dedicated underground cable feed could be considered in lieu of an interconnection transformer.
C- Neutral Stability during Islanding
The UI distribution system is designed assuming that the source of power provides an
“effectively grounded” system. That is, there is little neutral shift during both normal and abnormal operation (usually to less than 125% of nominal voltage). An ungrounded system would allow the phase-to-ground voltages to shift up to 173%. This level of voltage shift cannot be tolerated in the multi-grounded neutral system, especially with distribution transformers connected line-to-ground.
For a significant percentage of the faults that occur on the US system, the US fault interrupting device will operate before the DG breaker is able to disconnect from the system. This sequence of tripping will leave the generator isolated on the system in an island, if only for a few cycles.
Some direct-connect generators have been proposed as wye-connected with a neutral grounding resistance. This arrangement is common for a cogenerator connected behind a transformer with a delta winding. However, a directly-connected generator with a high neutral impedance will result in severe overvoltages during islanding conditions, especially if the ground fault remains and/or there are significant wye-connected (line-to-ground connected) loads. Depending on the nature of the loads on the system and their overvoltage protection, US may have to require an interconnection transformer of the appropriate winding configuration even if the DG site is supplied by an express feed.
Rotating machines are not commonly thought of as harmonic sources. However, they can be significant sources of harmonic currents depending on how the machine is designed and how it is connected to the system.
US has had experience with directly connected wye-connected alternators injecting excessive amounts of third harmonics onto the system. These “triplen” harmonic currents on utility power systems are the source of many power quality problems. Triplen harmonics include the 3rd harmonic and odd multiples of the 3rd. Multi-grounded neutral systems generally cannot tolerate the large amounts that can come from certain machine designs.
If the proposed design is determined to be a significant source of triplen harmonics, an interconnection transformer with an appropriate winding configuration will be required to block these harmonics. Utility central station generation is also faced with this problem. It is usually solved by interfacing the generator to the transmission grid through a delta-wye step up transformer. The delta winding blocks the triplen harmonic currents.
E- Interconnection Transformer and Generator Harmonics
The presence or absence of an interconnection transformer can have an impact on the harmonic voltages and currents on the utility system and in the generator.
The absence of an interconnection transformer generally means that the generator, the utility system, and the utility system’s loads have a greater impact on one another. The following table summarizes the basic harmonic interaction concerns for the most common generator pitches for a solidly grounded-wye utility system.
The presence of an interconnection transformer, on the other hand, can provide some isolation between the distributed generator and the utility system and loads, particularly if one of the windings is delta-connected. The presence of a delta-connected winding does not provide a path for zero-sequence, and therefore might be of interest should triplen harmonic interactions be a concern.
F- System Neutral
The presence or absence of a system neutral and its grounding practices also plays an important part in the interaction of the harmonic voltages produced by fractional pitch generators. The absence of a system neutral, indicates that there is no path for zero-sequence currents to flow, and thus, the interconnection of a fractional pitch generator that does not eliminate 3rd harmonics is less of a concern for systems with this configuration.
The presence of a system neutral that is solidly grounded provides a relatively low-impedance path for the flow of 3rd harmonics. US’s distribution systems are mainly of this type. Therefore, generators which do not eliminate 3rd harmonics will add to the 3rd harmonic voltages on the overall system. It is possible that they may also increase the neutral-to-ground voltages in the vicinity of the generator.
The 3rd harmonic current problem is quite important for utilities. Machine vendors will often recommend a 2/3 pitch machine when it is known that the machine will be used for utility parallel operation. This minimizes the amount of 3rd harmonic current that might be injected onto the utility distribution system neutral.
G- Load Characteristics
If the loads connected to the distributed generator are sensitive to certain harmonics produced by fractional pitch generators, then those loads may need to be de-sensitized, or filtered, or other measures such as the specification of different fractional pitch generators may need to be considered. For instance, generators which increase the 5th harmonic voltages in the system may result in additional heating of induction motors, particularly if the distortion is magnified by resonance resulting from the addition of power factor correction capacitors.
H- ISLANDING CONCERNS
One of the primary objectives when operating distributed generation in parallel with the distribution system is that the DG unit separate from the utility source during most abnormal utility-side conditions, such as faults. Separation is required to allow utility fault clearing action to proceed, to ensure safety to utility personnel and the general public, and to ensure that damage to utility and/or customer equipment, including the DG equipment, is avoided. Separating the DG from the utility system helps to ensure that the utility protection systems operate as intended to return the system to normal operation and to ensure that the DG does not unintentionally keep all or a portion of the distribution system energized.
The DG interconnection system protection equipment is responsible for sensing select abnormal utility-side conditions, and for acting accordingly, separating the DG from the utility system within an acceptable time period. IEEE Std. 1547™ specifies the required time for a DG to disconnect from the utility in response to utility side voltage and frequency deviations as shown in Tables below.
If islanding overvoltages are judged to be a likely problem an interconnection transformer that maintains an effectively grounded system will be required. Effective grounding may be
accomplished by using a reactance grounded wye-delta transformer (wye on the utility side). The design of such transformer must be carefully coordinated with the UI protection department because the ground fault contribution of the transformers can result in excessive ground fault current and can upset feeder relaying coordination.
The selection of a neutral grounding impedance requires balancing conflicting goals. On utility 4-wire multi-grounded neutral systems like UI’s system, one can expect a continual flow of current in the grounding impedance. Therefore, a grounding reactor is used instead of a resistor to avoid the losses incurred by a resistor. The reactor must be large enough to limit the fault current to acceptable values yet small enough to maintain an effectively grounded system should the generator and transformer become isolated from the utility supply.
Note that the grounded wye-wye transformer commonly encountered in many distribution systems does not necessarily provide an effectively grounded system even with both windings grounded. It will depend on the connection and grounding of the generator and any connected load. This connection simply transfers the characteristics of the system on either side. A three-legged core design will provide some ground referencing due to lower zero-sequence impedance, but this core configuration is likely in the minority of utility distribution transformers.
M- OPEN-PHASE CONDUCTOR CONCERNS
While short circuit faults are the focus of many DG interconnection issues, open-phase faults also can result in severe overvoltages. Open-phase faults are common on open wire overhead utility distribution systems, but can also occur on underground systems. Perhaps, the most common type of event is a blown fuse on only one phase. Thus, line fuses are not recommended between DG interconnection locations and the substation bus whenever possible. Sometimes this is unavoidable. Also, jumpers, splices, and other connectors can burn off as a result of a short circuit fault or simply from aging effects.
Severe over-voltages as well as unstable load voltages can occur on the open phase, similar to conditions that can occur with any distributed generator where the interconnection is not effectively grounded. Most such conditions can be detected with appropriate primary-side voltage relays sensing each phase-to-ground voltage. However, negative-sequence current relaying can generally sense this condition more quickly and reliably, disconnecting the generator as quickly as possible.
A grounded wye (utility)-delta (DG) transformer can maintain an effectively grounded system if careful consideration is given to neutral grounding impedance selection. An effectively grounded system from the DG will minimize the overvoltages after a phase conductor opens.
The open-conductor condition may be detected by negative-sequence currents on either the utility side of the transformer or the generator side. Depending on the interconnection transformer winding connection, the condition may also be detectable by neutral overcurrent protection as well. Open conductor fault detection must be addressed by the DG protective relaying.