breaker.
2.4.4 Double or Single Incomer Connection
The 30/40 MVA plant can be connected to the incoming utility transmission line with one or two transformers. This will depend on the plant reliability requirements. Earlier we determined that the plant must meet the “single contingency criteria.” Therefore, we conclude that the plant will be connected to the grid with two transformers (two incomers). Each transformer must be capable of carrying the full load of the plant. Figure 2.6 shows the substations diagrams with two and single incomers for the same plant. A switchyard with a single HV breaker would not meet our reliability criteria of full redundancy for the plant operation. This is mainly because a major transformer failure may cause a total plant shutdown for an extended period of time.
Figure 2.5 245 kV circuit breaker.
Figure 2.6 Double and single incomer diagram.
The transformers in our 30/40 MVA plant will be required to share the plant load, but each will also be capable of carrying the full load of the plant on their ONAF cooling in case one transformer fails.
Double power entry will be more reliable, though considerably more costly. A substation with two transformers, in addition to more HV switches will need three incoming HV circuit breakers to feed the two transformers. This substation will also require considerably more space.
For a smaller plant of up to 10 MVA, a single HV incomer may be considered acceptable as the cost of the HV breakers and two transformers might be excessive in comparison with the cost of the plant. In fact, a single incomer switching yard may include nothing more than a single H frame pole structure.
To support a smaller plant, several diesel generators (DGs) can be brought to site in trailers to temporarily replace a faulty transformer.
2.4.5 Utility Generating Capacity
This information we require to be able to determine if our plant will need some supplementary firm generation at site and also if we would be able to expand the plant in the future in case our ore body miraculously doubles up. We will look into a possibility of having a solar plant or a wind farm to augment our power sources. The solar and wind resources cannot be counted in as firm capacity, but simply as a source of power to displace the fuel consumption or import of power (see Chapters 25 and 26).
2.4.6 Firm Capacity
This term is used in generation to determine the overall available MW capacity not considering one unit (single outage failure); therefore, a power plant with two units of 50 MW each has a firm capacity of one unit (50 MW). That is one generator unit out of two units, or two out of three units. Therefore, a firm capacity of a generating plant is the available generation MW capacity not counting one unit which is held as spare. Firm capacity of a transmission line is defined as one out of two circuits. A single circuit line has no firm capacity.
2.4.7 Line Protection
Utilities use specific protective relays and have definite strategies for the line protection: two to three zone distance, negative sequence, etc. The utility will likely ask us to match the protection relaying at our end to that of their end and to coordinate the settings between the two ends. The line protection will likely be by pilot differential relaying with fiber optic communications.
2.4.8 Lightning
We can look at the historic data of the lightning days/year in the area. The plant may be in a desert environment with a low isokeraunic level or in some mountainous region of high lightning intensity. Hopefully, we can obtain this data from the utility and design the switchyard and the overhead lines accordingly with appropriate overhead shielding, grounding, and lightning arrester protection (see Chapter 10 for more details).
2.5 Main Plant Substation
Question? What is the difference between a switchyard and a substation? Is there a precise definition of one and the other? I have never heard one. In my circles, we called a facility with HV switches, circuit breakers, and transformers a substation. A switchyard was the same, but without transformers.
The main substation on this project will contain a number of major items of equipment: transformers, HV switches, HV circuit breakers, arresters, and protection panels as well as medium voltage (MV) switchgear connected to the low voltage side of the transformers. Specifications must be urgently written for the long lead items. In this substation, the transformers should be given a priority. Large transformers are long lead items, requiring 12—18 month delivery plus the procurement time. Transformers rated up to 10 MVA can be obtained within nine months. Often, large transformers may be purchased ahead of time with a provision of being cancelled if the project is not approved to proceed. To purchase the transformers, we need know: plant load, future load, voltages, and method of cooling.
Based on the projected load estimate let us assume the main transformers will be oil immersed/forced air cooled, as follows: two (2) 230 to 13.8 kV, 30/40 MVA, YNd1, ONAN/ONAF/ONAF(prov.), BIL 900 kV, 55 °C rise at 40 °C ambient. We will explain the details later.
For our plant, each transformer must carry 30/40 MVA on ONAN/ONAF/ONAF cooling. Each stage of fan cooling adds about 15% capacity to the base rating. In addition, we can specify the transformer to have a 55/65 °C temperature rise allowance. A transformer rated at 55 °C rise has about 10% spare MVA reserve over a transformer rated 65 °C rise of the same MVA rating. Obviously, a 55 °C transformer is built to a more efficient cooling design.
We choose a single ONAF cooling stage, including a provision (prov.) for adding additional stage of cooling fans if necessary in the future.
Remember, the transformer base rating is 30 MVA. This value will be used in the study calculations.
What are the designations for the transformer cooling?
ONAN: Oil Natural Air Natural → Without fans.
ONAF: Oil Natural Air Forced → With fans.
The transformers will be furnished with a conservator tank and all the standard auxiliaries. We noted the winding configuration as YNd1. Star Primary, HV Neutral solidly (effectively) grounded, Delta secondary, lagging 30° in counter clockwise (CCW) convention. The most popular winding configurations in the industry for high voltage (HV) transformers at 230 kV and above are Yd1 and Yd11.
Transformers up to 10 MVA can be ordered as “sealed tank design,” without a conservator.
The main and plant oil immersed transformers will be placed outside, next to the plant buildings in their independent vaults with oil containment basins. The walls
