مهمترین موضوع مرتبط با نوع اتصال ترانس واسطه مابین شبکه و ژنراتور، وجود هارمونیک های جریانی در سیستم است. همانطور که می دانید ژنراتورها بسته به گام سیم پیچی اعمالی روی آنها مولد هارمنیکهای جریانی متنوعی هستند. در گامهای سیم پیچی 5/4 و 7/6 وجود هارمونیک 3 و مضارب آن می تواند دردسر ساز باشد. استفاده از ترانسفورماتور حائز اتصال مثلث می تواند سبب حذف این گروه از هارمونیکهای جریانی باشد. از کارکردهای مهم دیگر ترانسفورماتور واسطه امکان هماهنگی عایقی دو سیستم با الزامات متفاوت است. با وجود یک انتخاب صحیح همچنین می توان سیستم زمین مناسب با هر شرایطی را در دو طرف ترانس ایجاد نمود. در زیر مزایا و معایب اتصالات مختلف ترانسفورماتور میانه تشریح شده است.
a- Grounded-Wye (utility) –Grounded-Wye (generator) Interconnection TransformerThis connection is perhaps the most common three-phase transformer connection on some utility systems, although not necessarily on the UI system. Advantages and disadvantages related to the use of this transformer connection with respect to DG application include:
More economical than other connections in some applications, particularly at 25- and
35 kV class voltages. Less concern for ferroresonance in cable-fed installations; some core designs may be more prone than others. DG ‘sees’ same imbalance that utility system ‘sees’
2. Harmonic Distortion
- Will directly pass zero-sequence harmonic currents (such as the 3rd harmonic)
3. System Protection
- No phase shift in system voltages (relaying); can detect primary side voltages with low-voltage relays
- DG may feed into any type of fault that is on the utility system.
- Utility will supply fault current for internal generator ground faults, increasing fault damage.
- Does not necessarily provide a ground reference when islanded despite the fact that both windings are solidly grounded. Ground reference will be provided by the generator and/or load.
4. Insulation Coordination
The insulation level of the transformer provides proper insulation coordination between system and load-side equipment.
b- Delta (utility) – Grounded-Wye (generator) Interconnection Transformer
This is the most common connection for serving three-phase loads on the medium system.
Advantages and disadvantages related to the use of this transformer connection with respect to DG application include:
1. Harmonic Distortion
- Triplen harmonics from the DG do not reach the utility system.
- Depending on generator neutral grounding, 3rd harmonics in the DG may cause excessive current in the DG-side neutral.
2. System Protection
- Provides some isolation from voltage sags due to utility-side single lineground
(SLG) faults, allowing the DG to better ride through voltage sags.
- Does not feed directly into utility-side SLG faults (it can contribute through other ground sources on the utility system).
- Cannot provide an effectively grounded system during islanding or open conductor conditions
- Difficult to detect some utility-side SLG faults from the generator side by voltage relaying alone. The delta winding tends to hold the generator side voltage magnitudes up and in the proper phase relationship. Primary-side relaying is often required to ensure rapid detection.
- Very prone to ferroresonance in cable-fed installations, especially during open conductor fault conditions. Instantaneous overvoltage relaying (59I) is often required to ensure prompt detection of this condition.
3. Insulation Coordination
Provides proper insulation coordination between medium voltage system and load-side equipment.
c- Grounded-wye (utility) –Delta (generator) Interconnection Transformer
This connection is rarely used on utility power distribution systems to serve loads, but is considered by many as the best way to interconnect large three-phase DG. This transformer connection is often referred to as a “grounding bank” and has a number of special characteristics that must be considered in its application on utility power systems like UI’s distribution system:
1. This connection acts as a source of ground current and will supply ground fault current into the utility system as long as the transformer is connected to the utility system even without the DG connected to the transformer. This additional fault current must be considered when designing a DG application using this connection. A single UI substation may have several DG units connected to it and the ground fault current contribution must be limited by using a neutral grounding reactor (see Neutral Grounding Reactor Sizing below).
2. The normal voltage imbalances on the distribution system will cause current to circulate in the transformer winding continuously. This will result in transformer heating and consume a portion of the transformer capacity. This circulating current can also be limited by selecting an appropriate neutral grounding reactor size.
3. The grounding bank action serves to limit ground overvoltages that can occur during momentary islanding periods when the DG system is separated from the utility system. Neutral grounding reactors selected to limit fault current contribution and circulating currents must have a sufficiently low impedance to maintain an“effectively grounded” system during periods when the connection to the main utility grid is lost.
Advantages and disadvantages related to use of the grounding bank interconnection transformer configuration for DG applications include:
1. Harmonic Distortion
- Triplen harmonic currents that might be produced by the generator are blocked by the delta winding and cannot flow on the generator side and, therefore, are not passed on to the utility power system.
- Triplen harmonic currents already present in the distribution system from other sources will tend to flow into transformers with this winding connection, contributing to transformer heating.
- The flow pattern for triplen harmonic currents on the distribution system is altered, which could be either beneficial or detrimental to telecommunications interference and neutral-to-ground voltages depending on the path taken by the currents. This alteration in flow pattern for triplen harmonics is difficult to predict prior to construction and can require mitigation after commissioning should complaints arise from other customers.
2. System protection
- Utility-side faults are generally more readily detected by the DG system protection because the transformer itself actually participates in all ground faults. This characteristic generally allows the DG to disconnect more quickly.
- Should the DG become isolated from the utility source (islanded), this connection helps the DG system present an effectively grounded source to the utility distribution system and avoid the resonance and overvoltage issues of other connections.
- Contributes strongly to all ground faults, which can increase damage due to high fault currents. A neutral impedance can minimize the impact.
- The presence of multiple DG installations supplied by the same UI substation using this transformer connection increases the complexity of coordinating transformer and neutral reactor sizing.
3. Insulation Coordination
Provides proper insulation coordination between distribution system and load-side equipment.