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پرسش نیروگاه شماره 42 - حفاظت جزیره ای
تفاوت جزیره با ریز شبکه از نقطه نظر حفاظتی چیست؟
نویسنده : محمد عباسی - از: ایران
 
#1
ساعت: 08:39 - تاریخ: 27 آبان 1394
با سلام
کارکرد جزیره ای نیروگاه به چه معناست؟ 
نویسنده : میلاد فرقانی - از: ایران
 
#2
ساعت: 11:34 - تاریخ: 5 آذر 1394
با گسترش شبکه و ضرورتهای جدید پیش آمده، موضوع تولید پراکنده در دستور کار متولیان صنعت برق قرار گرفته است. بدین معنی که از ظرفیت خالی شبکه های محلی کوچک صنعتی برای تأمین بار دیگر مصرف کنندگان شبکه از طریق شبکه سراسری اقدام گردد. اتصال شیکه های محلی کوچک به شبکه دسته ای از ملاحظات بهره برداری و حفاظتی را روی میز قرار داده که نیازمند بررسی است. شبکه کوچکی که قرار است خود را هم با ملاحظات و مشخصات فنی یک شبکه بزرگ تطبیق دهد و هم نیازمندیهای محلی خود را درصورت جدایی با قابلیت اطمینان و ذسترسی پذیری کافی پوشش دهد ، باید از طراحی مناسب برای این منظورها بهره مند باشد.
با جدا شدن یکباره بخش کوچکی از سیستم قدرت که Embedded generator خاص خود را دارد، مخاطرات حفاظتی به شرح زیر برای سیستم کوچک جدا مانده یا جزیره ای شده قابل ذکر است که باید در پیش بینی های حفاظتی مد نظر قرار گیرد.

Islanding
An Islanded situation is where the generator(s) and some local loads have become disconnected from the main distribution system. The usual situation is a section of distribution line (or the complete distribution line or lines), becomes disconnected from the main distribution / transmission system. The generator(s) output may balance with the load demand.

3.9 Hazards of Islanding

The hazards created during islanded operation are:
1. Unearthed operation of the distribution system
2. Lower fault levels
3. Out of synchronisation recloses
4. Voltage levels outside statutory limits
5. Reduction in quality of supply
6. Risk to maintenance personnel

3.9.1 Unearthed operation

Usually the distribution line is only earthed at the grid substation. At the generating site the high voltage (HV) side of the transformer is in delta configuration with no earth. When the distribution line, and its generator(s), are disconnected from the grid sub-station the line is not earthed. This is a potential hazard as line voltages may become excessive in a fault. This is detected by Neutral Voltage Displacement protection

3.9.2 Lower Fault Levels
In an islanded condition the fault level will be reduced. The fault level contribution from the grid substation has been lost. The reduction in fault level will affect the operation of protection on the distribution line. There may not be sufficient current generated in a fault condition to operate the protection.

3.9.3 Out of Synchronisation Recloses
During islanded operation the island will not maintain synchronisation with the grid system.
When the grid system attempts to reconnect the islanded distribution line to the grid system there will an out of synchronisation situation at the reconnection point. If out of synchronisation recloses occurs, there will be sudden and potentially large power flows and voltage disturbances. This may cause damage to distribution, generator and consumer equipment. The use of synch check or dead line check relays on auto reclose breakers will protect against out of synchronisation reclosure. If auto reclosers are to be used on distribution lines that have emebedded generators connected to them, the reclose time should be at least twice the maximum disconnection time of any generator protection systems.

3.9.4 Voltage Levels
The voltage levels of an islanded distribution line are controlled by the generator voltage regulators, the generated power and the load demand. Voltage levels may go outside statutory or normal operating limits.

3.9.5 Quality of supply
The quality of supply of an islanded distribution line will generally be reduced. It will be subject to voltage variations as there is limited generating capacity and may be little control on the output of this capacity.

3.9.6 Risk to Maintenance Personnel
There is a risk to the distribution system maintenance personnel from islanded operation. For example, a line may be disconnected for maintenance purposes and the maintenance personnel may assume the line will become, or remain, dead. Should a generator continue to be connected, or become connected later, this will be a hazard for them.

3.9.7 Islanding summary
Due to the hazards listed the operation of an islanded situation is generally not allowed. There may be an advantage to consumers, and generating companies, to allow islanded operation.
This is the maintenance of supply when a line has been disconnected. This advantage is small and the hazards outweigh the advantages. It may be allowed in special situations with appropriate additional protection
 
نویسنده : حمید - از: فیروزآباد فارس
 
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