Signalling Power Supply Part -2

Calculate MCCB Circuit Breaker rating ,Wire and Cable sizing(Down Stream -Power Distribution Cabinet)

Excel spread sheet is prepared with load requirement for each group ,including AC voltages and DC voltages (with converters) .

Adding all the load will give the total power for the power supply system .

As an Example :- If Point Machine and Signals requires 110V AC ,which can be be clubbed into one group or as separate group to feed from different isolation transformers with Static Automatic Transfer Switch for redundancy purpose .Interlocking cabinet ,AutoMatic Train Super Vision /Control Cabinet ,Communication Cabinet ,Secondary Train Detection Cabinet (Track Circuit /Axle Counter ) might need 230V with two independent supply (One from UPS supply Source and One from Rail Supply Source ) to feed the Redundant hardware of the respective cabinets .Similar feeder requirement can be clubbed together as per operator practice .In order to read the dry contact (for Point Machine Detection,other field status ) into I/O card file or to Pick a relay we might need 24V DC or 48V DC .All DC requirement of same voltage can be grouped together .These DC voltages are generated with the help of AC/DC converters (N+1 OR N+N *arrangement as per end operator requirement) which requires a fail proof 230V AC source which we might use a static Auto Transfer Switch (ATSw) ,Refer the below sketch.

Note* : Assume Total DC load requirement is 500 Watts ,If one of the AC/DC converter capacity is 500 Watts .One number of AC/DC converter is required (ie . we need 1 No of AC/DC converter rated for 500Watts to feed the load or 2 Nos of 250Watts which makes N ) .To make an N+N arrangement we use 2 nos of AC/DC converter(or 4 ) and the output is paralleled to same bus bar .There by if one converter/group is failed other will be continuously feeding the bus bar .There by making 100% redundancy .By using independent ATSw ,to feed both N will make the availability even better. In nutshell by adding all the reactive or active load will get us total load requirement of Power System .

Sample Calculation : we get total reactive power requirement from the excel table =60KVA (while adding all the total static and dynamic load for one single feed at any instance of the operation) and end operator specification requires an additional 20% capacity with feeders to be fully wired for future expansion and down stream isolation transformers shall not be loaded not more than 75% rated load .

So the total load requirement = Actual Load X 20% Spare Capacity /75% rated load

=60KVA x 1.2 /0.75

=96 KVA

In order to achieve 100% redundancy we need two independent MCCB and Transformer with 96KVA load for the Power Distribution System .Both sources can be UPS or One UPS and One Normal Supply .There by achieving redundancy from single point failure(Refer the figure) .Both these sources are fed into two independent MCCB which is feeding 100KVA transformer (Next higher size of total load 96KVA) .Let the down stream cabinet transformer be 3 phase 4 Wire transformer and we can segregate and balance the load on each phases at secondary of the transformer for each group (230VAC , 230V/110VAC ,48/24V DC (AC/DC converter with input supply 230V AC ) making use of different phases by balancing the load on 1 :1 Isolation Transformer (400V ,3 Phase ,4 Wire Transformer)

Down Stream Power Distribution Cabinet MCCB Rating Calculation

Total Load is =96KVA (100KVA Transformer)

Total Current on each MCCB =Reactive Power /1.73 (Root 3 ) x Voltage

=96 /1.73 x 400V

=138.72 Amps

We shall consider the inrush current of the transformer ,which is 1.5 times of total current (worst case)

So total current will be =138.72 x 1.5 =208 Amps

We can select the next higher available rating for MCCB =250 Amps

This means we need two sources to feed these two MCCB in the Power Distribution Cabinet(Down Stream Incomer)

Cable and wire size shall be selected based on this load or voltage drop from feeder or Short Circuit Current which ever is the highest .

Upstream UPS and Battery to feed these sources shall be designed for Online operation .Battery banks are designed based on the back up hours needed as per end operator requirement (say 8 hours or 24 hours or 48 Hours ).As UPS design itself is a long topic will include UPS design in another article .

Detail Design the sub system .

We have selected the down stream transformer ,MCCB rating which are fed from Two UPS or 1 UPS and 1 Non UPS Main source Supply .

Upstream sources (UPS or Normal ) shall be connected to MCCB which is fed to Primary of Transformer .Out put of the Transformer is fed into a 3 phase bus bar (4 wire ) .Power shall be distributed from this bus bar into each drives /cabinet protected with MCB's

From the excel spread sheet, for each group MCB has to be designed as per Reactive Power value .

Say for Example if Cabinet 1 require two sources of supply and each load is 1000VA .MCB rating is calculated as below

MCB 1 =1000/230=4.34 Amps and the next available size is 6Amps selected for MCB 1 .Redundant supply also have same 6Amps protection(MCB 2)

As per the grouping in the excel ,other distribution to be made to feed all the drives and cabinets.

A sample single line diagram is shown below for better understanding .(Not a perfect schematic !)

Detailed drawings to be produced further from block diagram /Single Line diagram for each feed with correct MCB rating and wire sizes are selected based on the current carrying capacity.

Procurement and Manufacturing.

Equipemnts and components shall be procured as per the designed parameters and built into cabinets (earth metallic enclosures) with correct IP requirement defined by the end operator .An IP 42 cabinet shall have a roof with fan .Filters to be designed and installed ,if EMC test is failed as per relevant standards requirements (EMC standards and requirements will be covered in another topic) and further tested to clear EMC test

Factory Acceptance Test.

A detailed procedure to be prepared to perform factory acceptance test ,to test Insulation monitoring , Earth Leakage ,No load test ,Full load test etc and the readings shall be recorded in the corresponding recording templates and duly signed by relevant parties.

Installation of the System

Factory tested cabinet shall be transported to project site for installation and completion of UPS and other sub system integration wiring .

Power On and Integrated Test .

A professional Engineer shall witness the power on after his inspection and Testing and commisioning team will further perform the integration test to all end drives

Part 3 will cover ,Earthing ,Earth Leake Detection ,Insulation Montitoring ,Local and Remote Indication.