215 kV
Most of the industrial plants would purchase transformers with OffLTCs. In this case, based on the discussions with the utility and due to the expected significant variations in the day/night voltage profile, we would prefer transformers with automatic OnLTCs to make sure we have a stable voltage in the plant at all times.
The plant voltage profile is not determined solely by the utility but also by the plant motor load. Plant reactive MVAR load will likely have to be partly drawn from the utility, as explained in Chapter 13.
The smaller plant transformers, which distribute power to lower voltages, will generally have (±5%) OffLTC tap changers. Taps for each transformer will have to be set to obtain the most comfortable voltage profile throughout the plant during the normal plant operation and for large motor starting. This can be determined by a computer load flow study and confirmed during the plant operation. With the choice of OnLTC on our main transformers, we can consider that our plant distribution voltage will be relatively constant at 13.8 kV at all times, irrespective on what the utility throws at us.
The typical voltage drop criteria to be considered in the design of the plant distribution system is <15% for large motor starting, and <3% for large motor while running.
2.4.1 Source Impedance
This is the system subtransient impedance Z″ representing the generating capacity of the utility at the POI. It also includes the impedance of the interconnecting transmission line. The source impedance is derived from the short‐circuit level at the plant as advised by the utility. The figure given will likely be based on a present and future generation planned by the utility. This value will be used as the base for determining the interrupting ratings of the plant circuit breakers that connect to the transmission line and the voltage regulation and capability of the plant large motors to start properly.
We have to determine the source impedance for two different extreme cases, the maximum and the minimum values, as follows:
The maximum source impedance (minimum fault level) when the utility is operating on light load with a minimum generating capacity connected to the grid. This source impedance will be used for voltage regulation calculations and large motor starting duty. If the supply network is weak (low short‐circuit level), soft, or variable frequency starting may be required for starting large motors in order to satisfy the utility flicker requirement and to minimize the impact on other nearby customers connected to the grid.
The minimum source impedance (maximum fault level) is when the utility is operating on high load with maximum generating capacity. This impedance will be used to determine the short‐circuit interrupting duty of the plant circuit breakers.
For our system studies and calculations, we will use MVAb = 30 MVA figure as our per unit MVA base. This is the base rating of our main incoming transformers: 30/40 MVA, 230 to 13.8 kV.
2.4.2 Line Conductor
We received the conductor (name) information from the utility. It will be a single Hawk ACSR (aluminum conductor steel reinforced) conductor. The overhead conductors are symbolically called by the names of birds. The data for the Hawk conductor can be obtained from online sources. The best source of data for the conductors is the old T&D Westinghouse handbook, from which we find the following data:
HawkType: ACSR, 477 kcmilStranding: 26/7Ampacity: 660 AResistance: 0.135 Ω/kmInductive reactance: 0.24 Ω/kmCapacitive reactance: 0.188 Ω/km
The Hawk line carrying capacity is well in excess of our plant requirements. Utilities like to build lines with sufficient capacity for future expansions. The maximum expected current from the plant at 230 kV is 100 A at 40 MVA. The line length from the plant to the local utility source of generation is estimated at 120 km.
Calculate the line parameters in pu for the system studies:Now, calculate line characteristics in pu(2.1)
Convert line characteristics (pu) from one MVA base to another:(2.2) (2.3)
Convert line characteristics (Ω) from one system voltage kV base to another:(2.4) (2.5)
The utility “informed” us that fault level at our plant bus, projected for the future with possible expansion is 10 kA at 230 kV. This is calculated approximately as: MVAsc = 4000 MVA.
Therefore, we can now calculate the source impedance at our 230 kV bus as follows:
Or pu source impedance if kA fault interrupting current from the source (power utility) is given:
This source impedance will be used in our computer studies to represent the utility at our plant point of interface. It is a conservative value that will provide plenty of margin in our calculation.
However, for our quick hand calculations and for our interrupting ratings of the switchgear, we will rate our equipment on a conservative basis of an infinite fault level from the utility (zero source impedance). Therefore, the utility can now be called an Infinite bus at the point of interface with our plant.
If we now calculate the fault MVAsc from the Utility on our selected MVAb base, it is:
2.4.3 HV Circuit Breaker Fault Interrupting
Now we can determine the incoming 230 kV breaker as follows: It has to have interrupting capacity of at least 4000 A. We select 3 phase, 245 kV, 1200 A, 40 kA fault interrupting, basic impulse level BIL: 1050 kV peak (see Figure 2.5).
Why 40 kA? This may sound too excessive for our requirements, but these are the ratings at the low end for the 230 kV equipment.
The transformer differential protection scheme and metering would need current transformers (CTs) on
