lightning days, are normally included in the tender document for the vendors to design the equipment accordingly.
2.3.3 Reliability Criteria
Before we start putting things together, let us clarify the design reliability criteria, discussed in Chapter 21.
In order to ensure adequate availability and continuous plant operation, the power distribution system must be designed to tolerate and override certain failures of the equipment. Generally, the system will be designed for “a single contingency failure” of the principal distribution equipment. This is sometimes called “a single outage contingency.” In other words, the design will fully cover for a single failure of one major piece of equipment, such as transformer, pump, motor, but not for a simultaneous failure of two pieces of similar major equipment.
Therefore, each pump system, which is considered a critical primary part of the operating plant shall include 2 × 100% units. In some cases, for larger pumps, 3 × 50% pumps could be used. For sump pumps, roof fans, heating, ventilating air conditioning (HVAC), etc., which are considered the plant auxiliary services, there shall be no immediate substitutes. The switchgear busbars from the plant distribution transformers will be bussed together through bus tie breakers to allow for feeding the plant loads from a single transformer, in case of an outage of one transformer. Failure of some smaller distribution transformer may be tolerated by reconnecting the load to alternate sources of power supply.
Recovery from power outages will be either by having spares cable connected or piped, or by switching capability to feed power from alternate sources such as closing the bus tie circuit breaker.
No contingency consideration will be given to the failures of power cables, lines, or pipes, which can be replaced or fixed relatively quickly, except for the high voltage (HV) single conductor cables used in power plants for 138 kV and higher, where one additional spare phase is added and laid out next to the operating cables. Therefore, during the plant design, one may ask: “What if…?” But not: “What if…, and if…?”
2.4 Connection to Power Utility
The investors will likely not be interested in building this industrial facility unless they have spoken to the utility and were assured that there is available generating capacity to power the project.
Let us talk to the Utility to acquire the information we need to build our plant power distribution system. Here are some of the issues to be clarified by the Utility engineers:
Power agreement: firm or interruptible
Tariffs, for power demand and time of use
Line voltage and its daily and weekly profile
Frequency off‐limits
Power factor tariffs and penalties
Source impedance, inclusive of the transmission line impedance (conductors)
Double‐ or single‐circuit incoming transmission line
Generating capacity, firm power, how many units are available, and their ratings
Method of line protection
Lightning level (number of lightning days/yr) in the area
Let us review each of the aforementioned issues:
Power agreement: The plant owner will sign a power agreement with the Utility. If the plant needs power 24 hours a day, every day, the owner will look at signing a power agreement for uninterruptible firm power supply, if available. An interruptible power supply allows the utility to occasionally cut the power supply in a specific amount or in total. Naturally, this contract comes with a lower tariff. The plant owner will likely insist on an uninterruptible power supply (UPS).
Tariffs: In addition to the nominal charge for kilowatt‐hour consumed, the utility will likely have additional tariffs as demand charge, peak load, and reactive power hour consumption (MVARh consumption). The demand is the load averaged over a specified time (15 minutes, 30 minutes, or 1 hour) in kW or KVAR. The peak load may be the maximum instantaneous load or a maximum average load over a designated period of time. The reactive energy charge may be applicable for the load operating at <95% power factor at the point of interconnection (POI).
Voltage operating range: For this plant, 69, 138, and 230 kV voltages can be used. In this case, 230 kV is available and preferred. We have to determine the percentage range of voltage oscillations received from the utility and how stable it is. The next thing is to decide if our plant will need an automatic on‐load tap changers (OnLTCs) on the main incoming power transformers, or simple off‐load tap changers (OffLTCs). See a typical transformer in Figure 2.4.
Figure 2.4 Large oil filled transformer.
Assume a 20% adder to the cost of a transformer with an automatic tap changer. On the other hand, an automatic LTC can, in addition to keeping the plant voltage constant, better regulate the flow of power and save us some money in penalties charged by the utility for the low plant power factor.
If the voltage swings are large, OffLTC changers may not be able to provide a manageable operating solution. They can regulate the plant voltage manually to a preselected percentage tap, while the OnLTCs manage the plant voltage automatically and linearly to the full tap range of ±10% on the primary winding.
If OffLTCs are employed, the operator will have to shut down the plant in order to change the taps, if desired. Naturally, manual tap changes cannot be performed on a regular daily basis. Voltage at night may be higher than during the day. So once you set the taps, that is it. You may be forced to change the tap settings again if the operating conditions alter.
Suppose, you have decided that your incoming transformers are to be rated 230 to 13.8 kV. Also, you were informed that the incoming voltage from the utility varies from 215 to 245 kV, but most likely toward the lower range (see Chapter 24 for more details on transformers).
Let us examine in Table 2.1 the voltage range at the secondary side of the transformers with OffLTCs for the various taps and voltage swings:
Transformer voltage: 230 to 13.8 kV
Taps on primary side range: ±10% in 2.5% steps.
Based on the aforementioned, a choice would be to operate the transformer with OffLTC at −2.5% taps for the primary voltage range of 215–245 kV. Negative taps on the transformer primary winding are the taps of choice used for boosting the plant 13.8 kV voltage. On the other hand, the OnLTC, if used, will maintain voltage relatively steady in smaller tap movements within the full tap range.
Table 2.1 Secondary voltage for primary grid voltage.
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Grid voltage
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