Practical Power Plant Engineering. Zark Bedalov

Practical Power Plant Engineering - Zark Bedalov


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2.12). In tropics, use concrete poles and steel cross‐arms (see Chapter 9). The distribution lines will use Hawk ACSR 466 kcmil (660 A), or Partridge ACSR 266 kcmil bare conductor having more than 470 A capacity in free air and high pulling strength. High capacity conductors are used not only to maintain low voltage drops but also for their tensile strength for pulling and to be able to implement longer spans between poles.

      The lines will be protected from lightning by an overhead shield wire on the top. The shield wire, popularly called overhead power ground wire (OPGW), will also contain fiber optic single mode, multicore cable inside it. All the metallic pole hardware will be grounded to a pole ground wire, which will then be led to the pole grounding rods. Depending on the soil conditions, the grounding rods will be installed at every pole in the dry areas and every several poles in the moist, low soil resistive areas.

Photo of the MV distribution lines using wood poles and wood cross-arms.

Transmission level, 69 kV and higher Solid (zero resistance) → No fault limitation
MV 3.3–33 kV Resistance → Limitation to 100 A
LV 400–600 V Solid (zero resistance) → No Limitation
Generators, 5 kV and higher High resistance → Limitation to 5–10 A

      The zero resistance (solid) grounding is now officially called “effective grounding.”

      In some parts of the world, the LV systems for manufacturing plants are left ungrounded or resistance grounded to allow the plant operation to continue, but to clear the fault at some convenient time. A phase to phase fault occurs if the plant receives another fault on a different phase. In that case, the affected part of the plant is tripped.

      Brief reasoning for the above methods of grounding and standardization are given below. For further discussion on the above standardization (see Chapter 5).

      The HV and EHV equipment at this voltage level is expensive. The transformer γ/Δ winding configuration with solid (effective) grounding with no resistance on the primary winding allows the manufacturers to apply lower BIL levels for the HV windings and to lower the cost of transformers and other equipment. The line tower footing ground resistance must also be kept low to reduce the BIL level of the line, thus cutting the cost of towers while offering the use of lower clearances and shorter insulator strings.

      The grounding/station service transformers are connected to the 13.8 kV buses with fused links.

      The MV 13.8 to 4.16 kV transformers are Δ/γ. The LV neutral is resistance grounded to limit the fault current to about 100 A. The selected fault current is low enough to limit the damage caused by ground faults to the MV equipment and high enough to be used for selective relay protection and most importantly to limit the insulation overstressing due to spiking arcing ground faults caused by the system capacitances.

      The LV systems will use 13.8 kV to 480 V, Δ/γ transformers. The LV (star) side will be solidly grounded with no intentional resistance. This will cause the ground fault currents to be as high as phase fault currents and can be operated by the phase current protection. In the mining industry, LV neutrals are resistance grounded as shown in Figure 2.10.

      This is addressed in more details in Chapter 5.

      The standby DG may be grounded as the MV system to limit the fault current to 100 A.

      Based on the foregoing discussion, the plant distribution transformer selection will be as follows:

       Grid transformers (2): 230 to 13.8 kV, YNd1, 30/40 MVA, ONAN/ONAF, oil Immersed, solidly grounded.

       Process plant transformers (2): 13.2 to 4.16 kV, Dyn11, 12/15 MVA, ONAN/ONAF, oil immersed, 100 A grounding resistor.Figure 2.13 Plant phasing diagram.

       Plant transformers (2): 4.16 kV to 480 V, Dyn11, 2/3 MVA, ONAN/ ONAF, dry type, solidly grounded.

       Plant transformers (7): 13.2 kV to 480 V, Dyn11, 1/1.5 MVA,

      ONAN/ONAF, dry type, solidly grounded.

      Furthermore, note that all the 480 V buses are not in phase. Those fed from the 13.8 kV buses are 30° apart from those fed from the 4.16 kV buses: for instance T41 and T31, respectively.

      It is important to note that the 480 V buses from those transformers cannot be interconnected together. On our project in this book, this type of phasing may not be important, as the plant power distribution system is of radial nature without a need for synchronizing.

      In other larger plants, where there is a requirement for synchronizing, it is essential to establish proper transformer winding configurations to suit. Engineers must learn how to assign correct transformer winding configuration and phasing orientation for the transformers on the electrical diagrams.


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