CH14 | Power Supplies
Signalling
SIGNALLING BOOK | CHAPTER 14
1. Introduction
As Signalling system and its elements seeks 100% availability in almost all units and subsystems. Seamless power supplies are required for all electrical products to drive and achieve highest level of availability .It is possible to achieve various levels of redundancy based on end operators requirement and capacity to invest to make it 100% available at all time !
We can categorize Signalling power supply into centralized and distributed .Centralized are the one which Signalling Equipment Room has its own feeders with alternate sources of upstream supply with UPS back up (Number of Hours for back up needed are decided by the operator) and feeding into wayside location cases along the line ,where as distributed option has individual Main and alternate feeders available at each location cases /huts along the line with relevant UPS back up.
Both has its own pros and cons .For a centralized power supply ,it requires higher capacity feeders and bigger dia conductors to reticulate into trackside location cases. Alternatively can step up the source at SER and step down at location cases to reduce cable cost.Where as distributed system requires main source feeder and alternate feeder at each location .Selection shall be based on budget and trade off between pros & cons for both system.
2. Steps to design the power supply
There are many practices which could be implemented .Below steps are the one I apply for designing a power supply system if something specific is not asked for .It depends on individual designers /employers/operators practice
- Familiarize end operators requirements from contract or detailed specification.
- Understand country based standards required to be applied for the system.
- Gather power supply requirements of each drives.
- Assess the supply requirement for grouping based on voltages.
- Identify Static and Dynamic Load.
- Prepare the load requirement excel with possible grouping.
- Calculate Reactive Power for Transformers
- Calculate feed Circuit Breaker rating ,wire sizing
- Detail Design of the sub system .
- Procurement and Manufacturing of one Power Distribution Cabinet (PDC)
- Type approval to comply EMC requirements ,IP requirements and other needs to be approved for the railways.
- Mass production of power distribution cabinets
- Factory Acceptance Test.
- Installation of the System
- Power On and Integrated Test .
2.1 Familiarize end operators requirements from contract or detailed specification
Contract or Technical Specification or System Requirement Specification define the requirements to be followed for the power supply design .It could include the local rules ,standards and practices to be applied ,redundancy requirements (Eg:-N+1 ,N+ N ) ,Spare Capacity Requirement ,Rated load for transformer (Eg: Transformer shall not be loaded not more than 75 % of rated load ),local and remote monitoring features ,UPS back up requirements ,Battery Types ,Hours of back up needed ,pre-emptive warning of components needed (advance warning for the product before its going to be faulty ) ,Insulation Monitoring Requirements ,Earth Leakage Protection Requirements ,Lightning Protection ,Type Test (For EMC /IP) ,How system shall behave when earth leakage happens (Eg:IT Earth Systems per IEC 60364-Clause 4.3.3.1),require to warn for maintainer to rectify the fault at secondary side of a transformer ) ,detailed calculation and other requirements (Eg:- detailed calculation for each feeder ,each fuse/wire size ,calculation for identification of maximum short circuit current ,Voltage drop ,maximum load to validate the wire are considered for protector and wire/cable selection.)
2.2 Understand country based standards required to be applied for the system
Each country have either its own standard or follow reputed standards ,which shall be applied while designing a signaling power system .Again this is based on technical spec ,some time some requirement may be over ruled in technical specification.As an Example ,Australia seeks MEN (Multiple Earth Neutral) ,where as Signalling system require IT earthing system per IEC 60364.Direction shall be sought from customer unless otherwise specified.Country standard/practice includes earthing systems requirements ,wire colour coding ,EHS requirements (RCD fitted MCB) and code of practices.
2.3 Gather power supply requirements of each drives.
Designer shall gather the approved system architecture with power needs ,product data sheets (Detailing reactive Power /Active Power ,Frequency /Voltage Tolerances,start up current ),any tools for validation of the calculation comes under this step.
2.4 Assess the supply requirement for grouping.
In this step ,we identify the possibility of power grouping .Systems which require two separate sources of supply (Two UPS supply or Two Main Supply or One UPS and One Main Supply) ,subsystem which require one source ,but from a fail proof Auto Transfer Switch (ATSw),Types of Voltages (AC/DC with rating) ,Reticulated Supply to other destinations are the factors to be considered.
Note :While preparing a load calculation excel ,these grouping will prove useful
2.5 Identify Static and Dynamic Load.
As you know some of the loads to the power system are permanent (static) ,where as some loads are required while the drive operates(dynamic) .As an example ,Point Machine draws current when system calls the machine to operate ,similarly only one signal operate out of a two or three aspect signal at any time, and all relay don't operate at same time to mention some examples (Some relays are normally up but some are picked as per control logic .Designer shall identify these factors to save cost and to avoid over designing . There could be numerous point machines in the layout which don't operate at same time .Designer shall either identify himself /Herself or get identified from Interlocking expert how many points can be operated at same time to decide the total load for the system .Its waste of money to provide feeder capacity for 60 point machines when maximum 20 can be operated at any time ) .Designer shall also identify any point sequencing are considered in the route table .This means if there are 10 point machines to be called for a particular route setting and if all are lying in unfavorable position from previous train move or have self normalizing feature in the interlocking and route requires opposite move ,interlocking can call five of them first and next five with a time delay .This will be defined in signaling interlocking control table
Another factor is electro mechanical components .High current is only required in few seconds at start up to gain initial torque for a point motor and reduce gradually which will be defined in the product manual.This factor shall also be considered .I like to consider starting current as worst case instead of operating current when other factors are optimized to the maximum.
2.6 Prepare the load requirement excel with grouping.
Now its time to record all these in an excel sheet as per the grouping ,some employers /operators might have predefined tools ,if so please use the validated tool.
Creating new tool /programmed excel require authentic review ,verification to make it error free.This table can group as per the reactive power(Apparent Power) or Active Power to identify the rating of feeder and transformers.
If the data sheet provide active power ,designer can consider to convert to reactive power based on power factor of the product .Electronic products have close to unity power factor theoretically ,where as heating elements might have power factor of 0.8 or less .This tool contains all the different voltage requirements with its group to arrive at total load to the Power System .
2.7 Calculate MCCB Circuit Breaker rating ,Wire and Cable sizing(Downstream -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 clubbed into one group or as separate group to feed from separate isolation transformers with Automatic Transfer Switch.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)
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 which is considered as ‘N’ unit ) .To make an N+N arrangement we use 2 nos of AC/DC converter and the output is paralleled to same bus bar .There by if one converter(N) 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.
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 downstream 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 downstream 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)
2.8 Downstream 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(Downstream 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 .
2.9 Detail Design the sub system .
We have selected the downstream 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(Figure 1) is shown below for better understanding
Detailed drawings to be produced further from block diagram /Single Line diagram for each feed with correct MCB rating and wire sizes. This shall be selected based on the current carrying capacity.
Figure 1. Sample Single Line Diagram
2.10 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
2.11 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 to cover all the functionality and the readings shall be recorded in the corresponding recording templates and duly signed by relevant parties.
2.12 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.
2.13 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
3. Earthing Systems
Functional Earthing and Protective earthing(PE) are carried out at the electrical installations .A functional earth connection serve the purpose other than electrical safety and might carry electrical current as part of normal operation .For the functional earthing cases a special terminal is provided for the installer to connect external earth usually for the purpose of noise reduction .Screen earthing of a cable is a functional earth .
A protective earth is used to protect the operator by means of reliable ground connection to make sure the touch current wont exceed certain values .In nutshell Protective earth is intended to protect personals from electric shock during an earth fault .This earthing is performed for any metallic exposed part to the Main earth terminal in the Signalling Room.
Fault current will flow through this conductor to earth which in turn on to the protective devices such as RCD (Eg residual Current Device Fitted MCB) to safely open the circuit within 0.4 seconds,where as functional earth is used to reduce radio frequency noise .Functional Earthing and Protective Earthing must be connected to separate earthing system and can be tied together through Potential Equalisation Clamp.(PEC) .
Example :- Cabinet case earthing is a type of Protection Earthing ,Shield /screen earthing of a cable is a type of functional earthing in a signalling installations.
4. Type of Protective Earthing System.
There are few types of earthing system (Protective Earthing ) implemented as per country practices. These can be broadly classified as TNS Earthing System ,TNC Earthing System ,TNC-S Earthing System ,TT Earthing System and IT Earthing System. This can be applied on to the primary side of the isolation transformer of a downstream Power Distribution cabinet. The supply directly feeding the signalling equipment shall be isolated from earth(floating) as per IEC 60364 IT system (e.g. 600 V a.c and 110 V a.c supply). Refer to Clause 4.3.3.1.All the supply fed to the signalling element will have floating neutral (IT earthing system ) with neutral cut through Insulation Monitoring Devise or Earth Leakage Device .
The ELD/IMD device shall be used for first fault condition monitoring only – the ELD device shall not be used to trip circuit protection during either a first or second fault condition. The first fault in the IT systems should be identified and fixed immediately to avoid a second fault from occurring.Refer Figure 1 to identify IMD connected to the secondary side of the Isolation Transformers and Earth Leakage Relay connected at the Primary side of the transformer in which the relay contact is used to trip the MCB and there by isolating the group .This is to avoid nuisance tripping of upstream supply when earth fault occurs in a power group in the downstream.
T(Terre or earth) Denotes that the Power Distribution System at SER is solidly earthed independently of the source earthing method.
N(Neutral) Denotes that a low impedence conductor is taken from earth connection at the source and directly routed to the Power Distribution System at SER (Signalling Equipment Room Signalling Power Supply) for the specific purpose of earthing of the PDC system
S (Separate) Denotes that the neutral conductor routed from the source is separate from the protective earthing conductor ,which is also routed from the source (Upstream council/Rail supply )
C(Common ) Denotes that the neutral conductor and the protective earthing conductor are one and the same conductor used
4.1 TNS (Terre ,Neutral, Separate ) Earthing System.
Here Terre stands for Earth .In this type of earthing system neutral conductor routed from the source is separate from protective earthing conductor ,which is also routed from the source .Upstream supply normally tapped from Council (Government Electricity Authority ) or Railway exclusive supply which is the source for power distribution system .This is used as supply source in the Power Distribution Cabinet or UPS depends on the power supply distribution requirement at the downstream on a signalling power distribution cabinet .For a three phase source ,will have five wires (Phase 1 ,Phase 2 ,Phase 3 , Neutral and Exclusive earth ) and a single phase source have three wires (Phase ,Neutral and Earth ) leading into your Signalling Power Distribution Cabinet or UPS .Here Neutral and Earth are separate conductors and earthed at source.No separate dedicated earth used at Power Distribution Cabinet End in the Signalling Equipment Room. Refer Figure 2 for TNS earthing system details
Figure 2 TNS Earthing System
4.2 TNC (Terre Neutral Common ) Earthing System .
Here C denotes that the neutral conductor and the protective earthing conductor are ONE and same conductor is used .For a three phase source will have four wires (Phase 1 ,Phase 2 ,Phase 3 ,Combined Neutral and Earth ) and two wires for a single phase system (Phase and combined Neutral and Earth) In this type of system joined Neutral and Earth are earthed at source(upstream) end and destination end (downstream ) on separate earth electrodes.There could be additional electrodes at source end as shown in the Figure 3
Figure 3 TNC Earthing System
4.3 TNC-S System
This earthing system is an enhanced version of TNC system .In this type of system a three phase source(Upstream) will have four wires (Phase 1 ,Phase 2 ,Phase 3 ,Combined Neutral and Earth ) and two wires for a single phase system (Phase and combined Neutral and Earth) .That is,Joined Neutral and Earth at source(Upstream) end but separate Neutral and Earth conductor at downstream(PDC) end .In nutshell the difference for TNC-S from TNC system is that there are five wires used at downstream end(PDC ) joined into four wires towards source(Joined Neutal and Earth toward Source) for a three phase system and three wires (Phase ,Neutral and Earth ) joined earth and Neutral towards source and joined Neutral and Earth in a single phase system .Refer the Figure 4 below for TNCS-S earthing system details .
Figure 4 TNC-S Earthing System
4.4 TT System
In this type of earthing system ,there are four wires (Phase 1 ,Phase 2,Phase 3 and Neutral ) for a three phase system and two wires for a single phase system .Both source (Upstream ) and load (Downstream ) have exclusive earth and neutral is tied to earth at source end not at load end (downstream /PDC). Refer Figure 5 below for TT Earthing System details
Figure 5 TT Earthing System
4.5 IT Earthing System
This is similar to TT earthing system .However neutral is floating compared to other types of earthing system .Neutral is not earthed at source (upstream ) or Load (Downstream ) .That is your Power Supply Cabinet . Both upstream and Downstream has exclusive earth ,however source earth is routed via Insulation Monitoring Device which can use for monitoring the insulation fault.Accepted earthing practice for Railway control systems are as defined in IEC 60364 -4-41 .This means the earthing system of the Low Voltage supply directly feeding the signalling equipment shall be IT earthed (Isolated from Earth) through an Insulation Monitoring device or an earth leakage detector .An earth leakage in the secondary side of the transformer feeding the signalling gears shall not trip ,but it shall be warned to the operator when first leakage detected and maintainer to be send to identify the cause .This will not cut the feed to the system at same time at the primary side of the isolation transformer ,eearth leakge realys are implemented to prevent nuisance tripping to the upstream UPS or the alternate normal supply.Refer Fig 6 for TT earthing system
Figure 6 IT Earthing System
5. Power Earth ,Signalling Earth ,and Communication Earth
It is always advisable to have separate dedicated earthing system for Power supply subsystem ,Signalling System and Communication System . Communication and Signalling system exclusively requires functional earthing for EMC purposes where as Power Distribution System requires Protective earthing .Signalling Earthing System can be used to earth all the exposed metal part for the protective earthing of the signalling system and communication earthing system can be used for functional earthing and protective earthing of the communication cabinets respectively .
All these earthing systems can be tied together with a potential equalization clamp which is an open circuit between earthing systems and which conducts when there is an earth fault .
This practice is adopted in Sydney Practice
6. Earth Leakage Detection
An earth Leakage System and Insulation Monitoring system shall be implemented in the signalling system at primary of the isolation transformer for the Power Distribution System and Secondary side of the isolation transformer .Earth Leakage Relay contact can be wired on to the trip coil arm of the transformer to isolate the downstream from source when leakage exceed a certain set value .This set value shall be based on the maximum leakage current possible from the end equipment and the current that can electrocute the person comes in contact .For an example .If leakage current from a signalling cabinet is 40mA and above this current can electrocute a person ,ELR shall be set for 40mA and if it exceed this value the incoming MCB will isolate the source by tripping the system .
7. Insulation Monitoring Device (IMD)
This is another protective mechanism ,especially in IT earthing system which is used to monitor the leakage on the insulation of the conductor and can be locally and remotely monitored
8. Lightning Protection
Its advisable to implement lightning protection for the conductors from the source for a distributed power system when conductors are exposed to lightning and thunder .There are protective devises available to connect across the conductors which are available in the market .It shall be implemented based on your operator practice
9. Local and Remote Monitoring
Its also vital to read the status of various group of supply, incoming power availability ,UPS availability ,UPS on battery , Earth Leakage /Insulation Monitoring Status ,Current ,Voltage ,Frequency ,Synchronization ,ATSw status to be locally monitored (through local indication on the cabinet ) and remotely monitored (for maintainer to rush ) through Automatic Train Supervision system .Telemetry could be used to transmit these status remotely.
10. Summary
This article cover the downstream power supply cabinet requirements and its out of scope for Upstream Feeder and UPS design which is considered as Electrical scope .However will cover in another article for those who are keen on it
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