POWER SYSTEMS - Design of an Urban HV Distribution Network

Introduction

The objective of this assignment is to do the basic planning and protection design of the sub-transmission and high voltage distribution network of a typical suburban "zone" substation. It has been designed to tie in with system studies to be done on the Power System Simulator in the Sir William Tyree (SWT) laboratory (the lab work will enable you to confirm a number of the relay settings determined as part of this assignment).

A zone substation steps "sub-transmission" voltages of 33 kV down to "high voltage distribution" voltages of 11 kV. Distribution substations on the 11 kV feeders in turn step voltages down to 415 volts for domestic and commercial use. The sub-transmission system is fed by a grid supply substation, modelled in this exercise by a single 132kV/33kV star-star transformer (but in reality would be several transformers in parallel). From this a sub-mesh of 33kV lines feed the sub-transmission network, modelled in this exercise by a single equivalent line. All impedances are shown on the diagram.

The zone substation will be constructed in two stages: initially two 25 MVA 33/11kV transformers supplying 8 outgoing 11kV feeders and ultimately three 25 MVA transformers and 15 outgoing 11kV feeders. For sake of analysis, the supply area can be assumed to be roughly circular and of uniform load (refer fig 1). Initial diversified load density (i.e. the average electrical load on the network) will be 1.0 MVA/km2 and the ultimate load density 2 MVA/km2.

On the basis of reliability, the substation always runs one of its transformers as a spare, the design capacity of a 2 transformer substation is only the rating of one and that of a 3-transformer substation is the capacity of 2 transformers. No spare capacity is provided at local distribution substations. The 11kV network is run as a "ring-main" system, where any 2 feeders can carry the load of 3 (i.e. the design capacity of the cables is based on an average load equal to 2/3 of the cable maximum rating). However, all the feeders in practice do not hit peak load at the same time (not uniformly loaded) and a "diversity factor" (DF) of 0.7 should be applied (definition is: DF = Total Substation Load/Sum of individual feeder loads, i.e. zone substation load = 0.7 * individual feeder peak loads; conversely an individual feeder peak load = zone sub load/no of feeders/0.7).

The distribution substations vary in size from "kiosk" types of 750 kVA to commercial designs of 1500 kVA capacities. All the zone and distribution transformers are 3-phase, delta-star types; delta on the HV, earthed star on the LV; the zone transformers may be earthed via an earthing reactor. The substations and network are all three-phase, 50 Hz.

The Design Task

The design task is as follows. Analyse the system and recommend:-

- Confirm the suitability of the zone transformer impedance, and zone transformer neutral earthing impedance (if used),
- The distribution transformer impedances (for both 1500 and 750 kVA sizes),
- The 11kV cable size,
- The zone substation transformer 11 kV CT ratio, to suit the transformer's differential protection scheme (allow for star-delta transformation),
- The zone substation transformer overcurrent and earth fault relay settings on HV (33kV) and LV (11kV) sides (relays ‘A' and ‘B'),
- The sub-transmission feeder overcurrent and earth fault relay settings,
- The 11 kV feeder overcurrent and earth fault relay setting (relay ‘C'),
- The HV overcurrent relay setting on both the 1500 and 750 kVA distribution transformers (relays ‘D1' and ‘D2'). Refer to figure 2 for the A, B, C and D relays locations.

There are both differential and overcurrent relays on each side of zone substation transformer, and overcurrent relays on each outgoing 11 kV feeder, and on the HV side of each distribution substation. The zone substation transformers have 500:5 Amp CTs on the 33kV side, and a choice of 1200:5, 1500:5 or 1800:5 on the 11kV side. The 11 kV feeders have 400:5 Amp CTs and for the distribution transformers, there is a choice of 60:5, 80:5, 100:5 or 150:5 Amp CTs. The primary rating CTs on the 11kV side of the zone substation transformers will be chosen to suit the differential protection and the overcurrent relays will have the same CT ratio. All CT secondaries (and hence also all overcurrent relays) will be 5 amps. You will have to specify the secondary current rating on the 11 kV sides of the differential relays on each zone substation transformer.

Attachment:- Assign.pdf