dc electrification supply system design

Roger D White - 1 - 26/04/2011 Atkins Rail

DC ELECTRIFICATION SUPPLY SYSTEM DESIGN

Dr Roger D White Professional Head of Electrification

ATKINS United Kingdom

[email protected]

INTRODUCTION

Railway electrification has in the past been dominated by overhead contact wire and DC third/ fourth conductor rail electrification systems. The historical reasons for this have been the success of the DC traction motor and the necessity of a DC supply. Mercury arc rectifiers were originally used to provide rectification at substations with the DC power being transmitted to the traction equipment by the conductor rail or overhead wire. Success in producing mercury arc rectifiers capable of being operated on board the railway vehicle, enabled railway AC electrification systems to become a reality in the 1950/60's.

It should be noted that DC is still the most common form of railway electrification system in the world.

HIGH VOLTAGE SUPPLY FEEDING ARRANGEMENT

The Local Grid Network

The HV AC incoming supplies from a National Grid or Regional Electricity Companies (REC), or Railway Generators provide the feed for the DC traction power substations. The local supplies for stations, tunnel-cooling fans, auxiliary circuits including batteries, chargers and uninterruptible power supplies (UPS) for computers and signalling circuits are also provided by the REC.

The electrical supply is fed to the railway at typically 132kV, 66kV or 33kV and the electrical power is then (on larger systems) distributed through a separate AC network at a medium voltage of 33kV, 22kV or 11kV by the railway/metro. This supply is used to provide traction power at regular intervals around the railway network. With light rail/super tram applications it is usually only necessary to provide the supply at a couple of points and is therefore obtained directly from the electricity supply utility.

Positioning of HV Supply Points

The number of feed points to the railway network will depend upon the size of the railway system and the capacity required at any particular point. On urban mass transit systems EHV supplies may not be available and space for substations and feeders are difficult to obtain and expensive. It is sometimes necessary therefore to provide the supply from local 33/66kV supplies. This is convenient from the supply authority viewpoint and provides a high degree of supply integrity; however it does raise two problems, control of the traction voltage regulation and HV voltage harmonics.

Incoming Feeding Arrangement

The AC switchboard will have in addition to the incoming breakers, local supply breakers and feeder breakers to other substations. The local supply breakers provide the supply to traction transformer-rectifier units, and auxiliary step down transformers, which are required in the immediate vicinity.

The incoming supplies from the electrical utility will have its own protection provided by the Supply Company. The protection of supply cables require the co-operate with the Supply Company to agree on the relay settings necessary to achieve stability and discrimination.

Power is supplied into the railway network at several points, and it is necessary to ensure that the incoming feeds are not paralleled. An interlocking scheme enables the system to be fed from all power sources but ensures that an electrical break prevents paralleling taking place. This arrangement gives the most reliable delivery of power to the railway even if one supply source fails completely.

39


Railway LV Feeding Arrangement and Switchboard

A typical LV supply arrangement is given in Fig 1 where one circuit breaker is connected to each side of the Bus Coupler Breaker which feeds step down transformers (33kV/11kV) and the 11 kV switchboard, these circuit breakers are equipped with the protection relays to protect the step down transformer.

A typical 11kV distribution network is provided throughout this system where all the passenger stations have 11kV switchboards. At each passenger station the 11kV would be transformed to 415V 3 phase for domestic supplies. To give a high level of security of these supplies, duplication is provided at each location.

Providing the supply capacity for mass transit railways is more complex as there can be up to 10 trains on a

particular substation, some of which are motoring, coasting, braking or regenerating. The worst scenario occurs when all the trains are accelerating or regenerating simultaneously. The design of the electrical supply system must be such that it can cope with worst case scenario. The limitation of the number of trains in any particular section is the responsibility of the Operation Control Room.

On DC intercity lines, or freight routes, the positioning of incoming supply feeders and substations do not pose such a problem. Land is easier to come by and space is not at a premium. Since each substation only supplies one or two trains and the acceleration/deceleration is slower, the positioning of supply points, substations, and the specification of their capacity is relatively easy.

Figure 1 Typical feeding Arrangement for DC Electrification Systems

40


DISTRIBUTION OF THE DC TRACTION SUPPLY

One of the main drawbacks when using DC traction systems is the fact that electrical energy is universally generated by supply authorities in the form of alternating current. This means that for a DC traction system the railway authority has to provide its own converting plant. Due to the high levels of current that are drawn in the conductor rail or overhead catenary, the system voltage can experience severe regulations. To overcome this, substations are spaced at regular intervals. This normally results in the operation of a high voltage AC distribution network linking the lineside substations. Obviously this increases the capital expenditure when the various systems are compared.

Conductor Rail and Overhead Line

The conductor rail or overhead power supply is designed to operate within specified voltage limits and it is necessary that the traction unit is capable of handling the voltage provided. The traction motors and control gear are required to be adequately insulated to the maximum operating voltage of the supply network. Traction motors for DC systems are normally wound for 600/750 V and connected permanently in parallel for 600/750 V DC supplies, or connected in series parallel pairs for 1500 V DC operation. On 3000 V DC supplies the traction motors are normally wound for 1500V operation and connected two in series on full voltage and as a result they are larger in diameter and more difficult to install under the low floors of multiple unit stock. The establishment of DC supply voltages have traditionally been chosen to meet the needs of the DC traction motor control.

In the past the main advantage of the DC supply system compared to the AC supply system is that a less complex traction control system is required, however with the advent of high power GTO (gate turn off thyristor), IGBT (insulated gate bipolar transistors) and the microprocessor, 3 phase drives are becoming more common on both AC and DC electrification systems. With the advent of 3 phase drives the DC voltage is not a design requirement for the traction engineer due to the ability of the traction input converter to set the DC link voltage to the inverter drive, it is likely therefore with new schemes using three phase drives that other system voltages could be used.

Positions of the Lineside Traction Sub-Stations

A detailed analysis is needed to establish the correct positions of sub-stations on the railway system, a simple mathematical approach has been included. (Calculation of voltage regulation and short circuit situation). Having

established the working voltage 600V, 750V, 1500V or 3000V the exact position of the substation has to be decided. This decision is made on the technical performance of the power system. It is also necessary to take into account other factors which will determine the final choice; availability of land, position of junctions and crossovers, provision of road access up to the main door of the substation building in order to facilitate the transport of spare items of plant and any necessary maintenance test equipment.

The most economic distance between substations is for 600V DC 3-4km; 750 V DC 5-6km; 1500V DC 8 - 13km and 3000V DC 20 - 30km.

The distribution voltage for heavy metro and freight is 1500V, 3000V DC overhead, and therefore requires less isolation and clearance than for AC electrification. The mechanical strength of the overhead line conductor becomes the main factor in overhead design, making the conductor sizes not dissimilar between 1500 V DC and 25 kV AC Where the power requirements exceed the capability of the overhead catenary it is necessary to include parallel feeds along the overhead masts. Connections are made at regular intervals to the catenary to ensure good current sharing.

The 3000 V system is applied almost entirely to the main line in order to maximise substation spacing, with 750 V for tram and 1500 V DC supplies predominantly chosen for urban mass transit or light rail systems. It should be noted that London Underground use a fourth rail system with -210V and +420V conductor rails.

Factors Influencing Substation Spacing

The substation spacing is determined by the traction loads and the maximum permissible voltage drop in the conductor system, i.e. including both outgoing and return conductors that can be reasonably tolerated without too seriously affecting either the system efficiency or the train operation. With DC systems a voltage drop in the order of 15 to 30% has usually been allowed, although the lower percentage value is to be advocated.

In considering maximum voltage drop that can be allowed in the system, the following factors should be borne in mind so far as running rail return systems are used.

The greater the feeding length, the greater will be the rail potential above earth. This should be limited on account of the danger of risk to passengers and railway personnel. On 660 Vdc third rail systems, rail potentials are typically 30V with respect to earth, while on 1500 Vdc overhead systems rail potentials are typically 60-120V. Voltages in excess of this will be obtained under

41


more severe operational and feeding modes. The greater the rail potential the greater the risk of electrolysis.

Factors affecting the Length of the DC Feeding

i. System Loading The system loading can be obtained from analysis of train operating schedules or from the output of a multi train simulator. This will enable the optimum number of rectifier substations to be selected; also the substation plant capacity.

ii. Conductor Section The greater the cross section area of the conductor system the less the electrical resistance and hence the greater the distance for a given voltage drop with a specified load. The capital cost of the conductor system however increases directly as the conductor cross section area; furthermore the greater the cross section area, the greater the weight and resultant load on the supporting structures in overhead systems. With ground collection systems the conductor rail cross section is usually approximately equal to that of a running rail, though in certain busy sections of the line, at junctions, a larger cross section is employed. With DC overhead systems the limiting factor for the cross section area is the load on each supporting insulator, hence the conductor cross section is usually determined by mechanical considerations though its electrical conductivity in DC systems is the governing characteristic. Where the line current exceeds the current rating of the overhead catenary separate feeder cables have to be provided on the supporting structures.

iii. Circuit Breaker Tripping Current The length of section fed by one substation must be such that its electrical resistance, including both outgoing and return conductors, does not exceed the minimum system voltage divided by the circuit breaker tripping current. With a 1500 V DC overhead system, the loop resistance is in the order of 0.0375 ohm per km. Assuming a 10% volt drop at the far end and a breaker setting of 4000 amps, then loop resistance must not exceed:

90% x 1500 = 0.3375 ohms 4000

(which is equivalent to approximately 9 km of track.) Normally the substations are situated at approximately 8 km intervals so that the loop resistance for a fault at the end of the section with one substation out of service would be 0.3 ohms; this gives a margin of 0.3375 - 0.3 = .0375 ohms,

which could be included in the fault resistance and breaker operation would still take place.

iv. Inter- tripping With both substations feeding into a track section, then inter-tripping of feeder breakers is commonly used. If a fault occurs within a section of overhead line then the breaker nearest to the fault would trip first and by means of pilot wires the corresponding section breaker in the substation at the end of the section remote from the fault could be made to trip. This would have the benefit of allowing longer feeding sections.

DC FEEDING ARRANGEMENTS

The normal feeding arrangement is with the substations all connected in parallel. There is a DC circuit breaker at each end of the feeding section to provide protection under fault conditions. Each substation feeds from a common DC busbar through DC circuit breakers in both directions. The feed is separated by a bypass isolator; this is normally open and closed only when it is necessary to bypass the substation.

Normal Feeding Arrangement

This is where all of the feeder DC circuit breakers are closed, providing double end feeding of the section of traction supply. The isolator at each substation is normally open, providing an isolated feeding section of traction supply, and ease of protection of the feeding section. Where regeneration is applied, the isolator at each S/S is normally closed.

Tee Feeding Arrangement

This is implemented at a substation when a DC feeder circuit breaker at one end of the feeding section is open. The tee feeding arrangement is achieved by closing the bypass isolator, allowing the remaining DC circuit breaker at the substation to feed the traction supply in both directions.

Single End Feeding Arrangement

Single end feeding arrangement on double end fed sections is a temporary feeding arrangement following the loss of the feed from a track feeder DC circuit breaker. This mode of feeding is normally only temporary and if the feed cannot be restored within a reasonable period of time, the traction supply would normally revert to tee feeding.

Bypass Feeding Arrangement

42


This occurs when there is a loss of supply at the substation or failure of the feeder DC circuit breakers in both directions. Under this condition the associated isolator is closed. The traction supply is provided from the feeder DC circuit breakers at the adjacent substations. In the case of single end fed sections, it is necessary to close only the appropriate by pass isolator.

Loss of Supply from Transformer Rectifier Units:

The supply system is designed with more than one transformer rectifier unit at a location. The traction supply is normally designed to operate with one transformer rectifier unit out of service at any location. If the complete substation is lost, the substation should operate as a track paralleling hut.

TRACKSIDE DC SUBSTATION

Most silicon rectifiers on traction systems use a three phase 50 Hz national or railway supply intake. Three phase rectification arrangement is used to reduce harmonic distortion at the point of common coupling and to reduce harmonic content in the DC supply. Where the mercury arc rectifier have been replaced by a silicon rectifier the double star transformer with inter phase transformer is employed. The advance of the silicon rectifier makes more simple arrangements of design.

The advent of high reverse repetitive peak voltage withstand of the diodes in excess of 4500V makes the series connection of diodes in rectifier design unnecessary, for three phase bridges and for traction line voltage of up to 1800V. Where higher system voltage of 3000V DC is required, two three phase bridges can be connected in series. A low ripple on the DC system voltage can be achieved with the connection of one bridge in star and the other in delta.

Rectifier Design

The function of the rectifier is to convert the three-phase current into direct current. In the past, mercury arc rectifiers have been used, however it is now normal to install naturally ventilated silicon rectifiers. This has become possible with the increase in area of the silicon wafer and has created what is practically a short circuit proof rectifier.

Natural ventilation of the rectifier means that there are no moving parts and therefore an increase in reliability, economic benefits and minimum maintenance. Silicon diode rectifiers are very robust, efficient (low on state losses) and able to sustain large fluctuations in temperature, high over current and over voltage rating (reverse).

The use of capsule/disc construction permits a wide range of the mean forward current and allows a minimum number of devices to be connected in parallel in each arm.

The voltage level of the supply system and the transformer rectifier arrangement will give the characteristic DC voltage. The DC system voltage should be such that it complies with the train operating requirements whether 650V/1500V or 3000V DC

Rectifier Protection

Short Circuit Protection The rectifiers are protected against short circuit and overloading by a thermal relay and over current time relay. These relays will trip the rectifier AC supply, removing the system driving force.

Internal Short Circuits Internal short circuit of the rectifier will occur due to the failure of one of the rectifier arms. This will produce a two-phase transformer fault current and reverse current flowing in the faulted rectifier arm. The fault is interrupted by the diode fuse on the line side which isolates the faulted diode, a micro switch on the diode fuse indicates that the fuse has ruptured.

Where the diode is of sufficient rating it is sometimes considered suitable not to include a separate fuse.

Over voltages Over voltages originating from the AC supply or the traction DC supply are normally attributed to switching or interruption of the power system. These over voltage are attenuated with a resistor capacitor network provided on the DC side of the rectifier.

Commutation Protection Commutation hole storage protection circuits are applied to each diode. These protect the devices from high voltages that are generated during the commutation of the diode.

The use of these over voltage devices can be dispensed with as the reverse voltage ratings of the devices is increased.

Mechanical Construction Rectifier

The semiconductor diodes are mounted on aluminum extruded heat sinks. These are required to ensure that the device does not operate outside its normal junction operating temperature. The diodes are normally connected in parallel in a bridge and are hermetically sealed against the ingression of dust and moisture etc. The diode fuses and micro switches are mounted adjacent to the diodes, the diode units are then mounted

43


so that the heat sink fins can circulate air freely. The AC and DC busbars are arranged at either the top or bottom of the cubicle. The rectifier arm can then be connected to provide any arrangement that is required.

Substation Performance Ref. Figure 4 on Page 9

The detailed electrical performance of the substation has to be designed into the electrification system at the outset The performance is dependent upon the traction load specified by the operating business, and the level of supply redundancy required. Provision for future extension of the system, the increase in train loads, and the change of type of vehicles to be operated must also be considered.

In the design of a traction substation it is necessary to take into consideration the following aspects:-

i. Traction sub station rating ii. Traction supply and converter arrangements iii. DC Traction supply voltage iv. The regulation of the DC traction voltage v. The characteristic DC short circuit fault current vi. The power factor of the traction rectifier unit vii. The production of harmonics in the input AC

supply current, and the distortion to the input supply voltage.

1 Typical Traction Substation Rating: Design specification for Greenwich Traction Substation (Jubilee Line Extension)

i Rated capacity 2400kW i Rated DC traction voltage 600VDC i Rated DC traction current 4000A i Short circuit protection 40 kA 0.25 sec i Transformer 22kV/415V i Rated Capacity 2 MVA i Short circuit protection 40 kA 0.25 Sec

A typical design specification for a 750V supply system:-

i Rated capacity 3000 kW i Rated DC traction voltage 750 V DC i Rated DC traction current 4000 A i Overload capabilities 150% 300% 450% of the

rated load current for permitted overload periods.

150% 6000 A for 1 hr 300% 12000 A for 1 min 450% 18000 A for 10 secs

Typically the voltage regulation will vary by up to 6% at full load current over the linear portion of the

characteristic, with a specified maximum and minimum values of supply voltage over the operating range

2 Transformer and Rectifier Circuit Arrangement

Ref. Figure 2 and 3 The pulse characteristic of the supply system is primarily defined by the transformer winding and converter arrangement. A number of simple arrangements of the transformer windings may be chosen with a 3 phase AC supply system to provide 6, 12, 18, and 24 pulse DC output voltage. Other ripple frequencies may be achieved using two converters and windings, which are phase displaced or wound in an alternative star/delta configuration.

A 12 pulse rectifier therefore can be obtained by connecting two separately fed phase displaced, 6 pulse systems in series or parallel. The arrangement will provide the necessary 30o displacement of the supply to provide a twelve pulse ripple when the respective bridges are connected in series or parallel.

The arrangement of the windings during the design and construction of the transformer determine the short circuit reactance, commutating reactance and the operating load loss due to winding resistance. These design parameters are responsible for dominating the DC short circuit fault current level, the operating DC voltage regulation level, transformer efficiency, transformer and converter power factor and the level of harmonics produced in the supply side.

44


Figure 2 12-Pulse Parallel Bridge Converter

Figure 3 12-Pulse Series Bridge Converter

45


3 DC Traction Supply Voltage European regulation EN 50163 (Supply Voltage of Traction Systems)

Table 1 DC Traction System Voltages

Definition of operating DC System Voltages

600*

750 1500

3000

lowest non permanent voltage duration 10min,

lowest permanent voltage duration indefinitely,

400 500 1000

2000

nominal voltage designed system value,

600 750 1500

3000

highest permanent voltage duration indefinitely

720 900 1800

3600

highest non permanent voltage duration 5 min.

770+

950^

1950

3900

*Future DC traction systems for tramways and local railways should conform with system nominal voltage of 750, 1500, 3000V

+In case of regenerative braking a voltage of 800 may be admissible

^In case of regenerative braking a voltage of 1000 may be admissible

4 The Regulation of DC Traction Voltage Regulation of the substation is a vital characteristic of the DC electrification system. If the regulation is too steep then the train will not have sufficient volts to maintain train timetables. Raising the voltage at the substation will provide a higher voltage on the load but may produce excessive voltages under no load conditions. A lower regulation is achieved by reducing the impedance of the supply transformer. This however will increase the short circuit fault current which will require a higher rating for the circuit breakers and converters. The optimum design is therefore a compromise between regulation of the substation and the level of short circuit fault current.

Advanced transformer designs using coupled secondary allow low regulation over the normal load range and yet limit the short circuit current. This is achieved by using secondary windings which are coupled to achieve the correct short circuit to regulation relationship.

Calculation of Voltage Drop in the Feeding Network For accurate calculation of remote DC faults computer modelling is necessary utilising the following data:

i substation is modelled as a transformer of known impedance,

i a rectifier whose characteristics include overlap; i power rail and return conductor being

represented by a resistance and inductance both of which may vary with the frequencies of the load current.

The following calculations are simplified to show the principles involved. The supply is assumed to be a constant voltage supply with zero source impedance and the power rail and return conductor purely resistive.

One train in a double track section without track-sectioning equipment (Ref Figure 5 on page 10)Let the distance between the substations be L metres, the resistance of the conductor be Rc :/m and the resistance of each rail be Rt :/m. If both substations have the same voltage at their busbars then the maximum voltage drop will occur when the train is at the mid section.

The maximum voltage drop Vdmax = Rc L I + RtL I (V)

or Vdmax = I L(Rc + Rt) (V)

assumptions:- i where L is the distance in metres between

substations; i where I is the maximum current taken per train; i one running rail is available for traction return

circuit; i IR - current per rail; i IR -I (if one rail available per track). i IR -I/8 (if two rails available per track).

One train in a double track section with track-sectioning equipment (Ref Figure 6 on page 10)Again the maximum volt drop occurs when the train is at the midsection. However, the track section cabin provides a means of paralleling the conductors, hence

Voltage Drop = Rc L I + Rt L I (V)

= (IRc + IRt)L /8 (V)

Assume one running rail is available for traction return current..

46

Rog

er D

Whi

te

- 9

-

26

/04/

2011

A

tkin

s Rai

l

Figu

re 4

Trac

tion

Subs

tatio

n T

rans

form

er a

nd R

ectif

ier

Perf

orm

ance

Cha

ract

erist

ic

47


.

Figure 5 Voltage Regulation of Train in Double Track Section without Track Sectioning Equipment

Figure 6 Voltage Regulation of Train in Double Track Section with Track Sectioning Equipment

48


5 Characteristic DC Short Circuit fault CurrentThe DC short circuit level is an important design characteristic of the power supply system. The fault level must be such that it does not interfere with normal train operation. The level must be also be such that the short circuit rating of the rectifier devices is not exceeded. The short circuit rating of the rectifier devices is specified by the surge rating of the device under a fault condition. It is also necessary to ensure that the transformer windings are adequately braced against stress brought about by short circuit forces. Finally, the DC track circuit breakers must be capable of successfully interrupting any short circuit.

A short circuit at the output of the rectifier terminals applies a balanced three phase short circuit to the AC system. Normal circuit theory can be applied to establish the fault current which flows. The secondary circuit of the transformer and rectifier can be represented as a source e.m.f., supply reactance and supply resistance per phase. This produces a DC fault current that rises to 40-80kA in 10-20mS. The di/dt of the fault current is dependant on the inductance of the supply system and the inductance of the overhead line the return current system.

(t) = Vm sin (Zt + ) source emf

the AC current which flows can be calculated as

ShortCircuitacZ L E e

r t

L

R2 Z2

L 2

E

R2 Z2

L 2.

sin Z t atanZ L

R

)

ac

L = supply and line inductance R = supply and line resistance Z= angular velocity of the supply rad/sec

The analysis can be used to establish the instantaneous fault current obtained from faults remote from the substation itself, provided that a value of line inductance and resistance can be established.

The value of the DC fault current can be achieved by adding each of the secondary line currents point by point during the DC short circuit. With a 6-pulse system this will include the inverse of the line currents shown. The DC fault current obtained will be the typical fault current. Alternatively the current can be approximated to

Idc = Im (1- e -(R/L)t )

Calculations of Short Circuit Currents The circuit breaker setting must be low enough to enable the circuit breaker to trip when a short circuit occurs, yet the setting must be sufficiently high to cover the maximum current likely to result from normal

operation of the schedules. When calculating fault currents precise details of the track bonding and conductor supply arrangements are necessary.

(a) Single unit substation without track-sectioning equipment (figure 7)

The circuit breaker, shown open, immediately trips, but current can still flow to the fault from the next substation as shown. The short circuit current Isc is approximated to the current flow from substation B, this is to a first approximation only:

Isc = V (A) L (Rc + Rt)

where V is the voltage at the substation busbars and only one running rail is available for traction return current.

If there are two tracks and all the running rails are bonded together at each substation for connection to the negative busbar, then the resistance of the return conductors is halved increasing the current to: Isc = V (A) L (Rc + Rt)

(b) Single-unit substation with track-sectioning equipment (figure 8)

This is most easily solved by applying Kirchhoff's second law, i.e. in a closed loop the sum of the applied e.m.f.s is equal to the sum of the products of current and resistance. Hence :

VA = IaRc L + (Ia + Ib)Rc L (V)

VB = Ib Rc L + (Ia + Ib)Rc L + I Rt (V)

Assuming the voltage at the substations remains constant, these two equations can be solved for IA and IB, and one running rail is available for traction return current.If there are four running rails able to return fault current is as shown:

VA = IARc L + (IA + IB)Rc L (V) VB = IB RcL + (IA + IB)Rc L + IB Rt(V)

49


.Figure 7 Short Circuit Fault Current Double Track Section without Track Sectioning Equipment

Figure 8 Short Circuit Fault Current Double Track Section with Track Sectioning Equipment

50


6 Power Factor of the Transformer Rectifier Unit The overall power factor of the rectifier unit and the traction supply transformer are normally specified to be better than 0.9 pu (including distortion factor).

Power Factor = Irms (fundamental) x cos Irms (total)

cos (phase angle of fundamental)

The distortion factor needs to be included within the power factor equation to take into consideration the effect of the line current harmonics.

Distortion factor = Irms (fundamental) Irms (total)

7 The production of harmonics in the DC and input AC supply current. Any complex wave can be resolved into its Fourier Series; that is a series of sinusoidal waves of specific amplitude, frequency, and phase. Complex waveforms are therefore, the summation of a specific set of even and odd harmonics as indicated below.

(t) = 0.50+ 1sin(1t-1) + 2sin(2t+2) + .

The rectifier circuit is a major source of harmonics in the AC supply and the DC traction supply. This effect is due to a number of system parameters including:

Rectifier pulse number Balance of the firing circuits Supply voltage symmetry Transformer tolerances

The rectifier switches the load current from one phase to the next; this is necessary to keep the polarity of the DC voltage positive. This process is called 'commutation'.

The line current cannot switch instantaneously due to the effect of the leakage reactance; this process is called 'overlap'. During the overlap period all devices are conducting in the outgoing and incoming arms of the rectifier. This produces a short circuit on the input and the output to the rectifier.

The input supply voltage waveform and the output DC waveform will therefore be interrupted, producing a notch and oscillation due to the RLC characteristic of the voltage waveform supply system. This has the effect

of distorting the AC supply waveform at the point of common coupling.

The input current to the rectifier produces line current with a stepped waveform due to the switching strategy of the rectifier converter.

Harmonic distortion levels are specified by the Electricity Council recommendation G5/4. This lays down the limits of harmonic current which may be generated back in the supply network. As a result of design predictions it is possible to decide whether 6, 12 or 24 pulse will meet the supply authorities regulations. In urban areas with systems supplied at 33/66/132 kV it is normal to use 12 or 24 pulse rectifiers to ensure compliance with G5/4 AC Side Harmonics.

Characteristic harmonics: the DC output voltage waveform produces a ripple that is related to the pulse number of the converter. This will produce harmonics in the load current waveform that are typically related to 300Hz, 600Hz, 900Hz etc. depending on the rectifier pulse number. The characteristic DC side harmonics are therefore related to the pulse number.

AC side harmonics can be characterised by the Fourier Analysis of the quasi square waveform. This produces harmonics that are related to Vhn = n pulse 1.

PulseNumber

DC side AC Side

p np np 1 6 0,6,12,18,2

41,5,7,11,13,17,19,23,25

12 0, 12, 24

1, 11,13, 23,25

The harmonics are responsible for undesirable effects including Overheating capacitors Overheating generators and induction motors Instability in converter control systems Interference with control systems Noise on telephone lines Interference with signaling systems

The main mechanism of reducing harmonics on DC electrification systems Increase of the converter pulse number Installation of filters

Uncharacteristic harmonicsThe uncharacteristic harmonics are produced by : unbalance in three phase systems positive,

negative and zero phase sequence

51


voltage waveform distortion unbalance in phase impedances inaccuracy in the converter delay angles supply frequency variation

The harmonics of low uncharacteristic orders are normally much smaller than those of adjacent characteristic harmonics in the converter itself, however on the ac side the uncharacteristic harmonic may be of about the same magnitude as those of the characteristic harmonics that are produced.

Under normal conditions the frequency of the National Grid transmission system is required to be within 1% of the nominal frequency i.e. from 49.5- 50.5 Hz. The design of a users plant and apparatus must enable operation within this range within this range also complying with.

47.5 - 52.5 Hz continuous 47 - 47.5 Hz operation for a period of at least

20s is required whenever the frequency falls below 47.5 Hz.

Permitted Harmonics and the Engineering recommendation G5/4 Stage 3 assessment is applicable to connection of non linear equipment with supply systems having a Point of Common Coupling (PCC) at 33kV and higher voltages.

Requirements of the Railway Company i. The railway system is required to provide the

characteristics of the load to be installed, when this is a non linear equipment,

ii. Prediction of the Total Harmonic Distortion (THD) is required to assess all harmonics up to and including the 50th harmonic.

Requirements of the NOC (Network Operating Company)

iii.NOC is required to provide the system harmonic impedance values at the PCC which will enable the customer to evaluate his system harmonic performance.

iv. The existing distortion that already exists on the system is required to be measured.

v. The prediction of the total harmonic levels by the addition of the results of existing and new harmonics.

Calculations i. For unbalanced harmonic conditions, the phase

with the highest THD should be used.

ii. For individual harmonics which have the summated magnitude and hence the greatest THD, the measured and calculated values of distortion are assumed to peak at the same time and to be in phase.

Vhp = Vhm + Vhc (Total = Measured + Calculated)

For other harmonics an average phase difference of 90o is assumed at the time of maximum THD _______ Vhp = Vhm +Vhc

Total = Measured + Calculated

The THD is then given by

THD

2

50

h

Vhp 2

Planning and Compatibility Levels Planning Levels are the levels for public supply system harmonics and are specified in the IEC Basic Standards IEC 61000-3-3, these levels are used in the design study to ensure that any increase in load on the system does not cause adverse reduction of performance.

Compatibility Levels for public supply system harmonics are specified in the IEC Basic Standards 61000-2-2 and 61000-2-12. The immunity test levels for equipment are higher levels based on the specified compatibility levels. If the network distortion exceeds the relevant compatibility level, experience has shown that there will be a sudden increase in equipment failure and customer complaints.

System Planning Levels (IEC 61000-3-6)

Compatibility Levels

400V 5% 8% (IEC 61000-2-2)

6.6, 11, 22 kV

4% 8% (IEC 61000-2-12)

>20kVand


the individual and the THD harmonic voltage planning levels for the relevant supply network.

Where existing harmonic voltage levels are more than the planning levels, the risk of disturbance to other customers is increased.

Compatibility levels for 66,132,275 and 400kV systems have not been specified internationally. The margins for the THD in the UK are 4% for 6.6, 11 kV, 5% 22,33kV, 2% 66 and 132 kV and 0.5% for 275 and 400kV.

Note The planning and compatibility levels quoted in G5/4 are the required limits at the point of common coupling.

Supplies that are derived from traction supplies cannot be expected to comply with these

53


Figure 9 Transformer Primary and Secondary Voltage

54


TRACTION RETURN CURRENT SYSTEMS

There are typically four earthing strategies for DC railways and these are detailed below:

Fourth Rail Systems: problem relating to leakage currents is

overcome by the insulation of the fourth rail (London Underground UK)

Running Rails as the return conductor: Rail Nominally Insulated (Network Rail UK) Diode Earthed (Hong Kong MTRC, Sheffield

Super Tram) Floating Negative Earth with Rail Potential

Control Devices ( Singapore MRT, Hong Kong LRT)

Rail Insulation

The railway normally uses the train wheels and running rails as the traction current return path to the DC substation, (except fourth rail systems). Where the rails are not insulated from the sleepers, the earth return is considerably more complex since the current may flow out of the rails and return at some other point (stray current). Most modern rail systems use high insulated rails to prevent the current from leaving the rails and corroding metallic structures

A very important aim when designing a DC electrified railway is to control return DC traction current and avoid passing these currents through the reinforcement of concrete structures, and in particular highly stressed concrete structures to avoid causing damage to the reinforcement through electrolytic corrosion.

The Running Rail is normally insulated from the sleeper / concrete pad by the use of insulation pads placed under the rails with an insulation value in line with the European Standard EN 50122-2. (Table 1 Page 6; No added rail insulation 0.5 S/km for open formation and 2.5 S/km for closed formation). This level can be improved with the application of track insulation mounting pads or polymeric insulation. This value of insulation however will reduce over time, due to degradation of the track insulation. For closed formation, improved levels of insulation can be achieved where the rail is to be embedded in the road.

The level of insulation is determined not only by the installation but the maintenance of the track bed. Every effort should be made to minimise the risk of ballast coming into contact with the rails, since ballast, especially when wet will reduce the insulation value of the rails to earth.

Hazards Associated with Rail Potentials The track is insulated from ground to minimise leakage. Introducing insulation means that voltages between the

rails and ground will occur. These voltages are a potential hazard to passengers and railway staff when coming into contact with rail or anything connected to it and the ground. Reduction of rail touch voltages can be achieved by clamping the rail to ground if a dangerous voltage is reached and resetting as soon as possible afterwards. It is necessary to have quick detection of the high voltage and then activate a "clamp", by a thyristor device, GTO device or contactor, to short out the voltage.

Diode Earthed System (Figure 10)

This arrangement includes a diode at the traction substation which is connected to the substation local earth. The inclusion of the diode will cause the rail potentials to be either at zero (diode is conducting) or above zero if the diode is reverse biased.

Characteristics of the diode earth system: Stray current is minimal where there is no rail

leakage Where there is rail leakage the diode returns

this to the substation The diode acts as a unidirectional connection

to remote earth Diode conducts with negative rail potentials

( typically at substations and regenerating trains)

There are particular difficulties in operating a diode earthed system in conjunction with regenerative trains, which may produce negative rail potentials and hence encourage diode conduction.

Floating Negative Return Current System (figure 11)

In this arrangement there is no intentional connection between the traction substation negative and the substation earth. The effect is to produce a system that floats about the remote earth potentials.

55


Figure 10 Typical Rail Potentials in Diode Earthing System

Substation Rectifier

Vrail

Diode Earth

Traction Current

Traction Return Current

DC System Voltage

Catenary

Main Earth bar VrailVrail Vrail

Figure 11 Typical Earth Currents and Rail Potentials in DC Negative Floating Return System

56


System configuration and generation of High Rail Potentials High traction load can cause high rail potentials which subsequently cause spurious trips of the Over Voltage Detection Circuits.

High Rail potentials may occur under the following:

Multiple train loads Substation outage Single end feeding Long feeding section; i.e. during substation

outage Remote Regenerative Train

The likelihood of all these circumstances occurring simultaneously is rare; however the concurrence of some circumstances will happen on a regular basis.

Rail Potentials under Fault Conditions

The magnitude of the fault impedance is dependant on a number of factors Fault Impedance Substation impedance Impedance of the OLE rails and bonds Feeding length i.e. emergency feeding

Earth Fault Path dependant on: Traction return rails Earth wire or fault current return wire Remote earth

The magnitude of the rail potentials dependant on: Magnitude of the fault current Feeding length i.e. emergency feeding Path of the fault back to the substation

Earthing Management Plan

Statutory documents ISBN 0105437743 Health and Safety at Work

Act 1974 SI 1989 No. 635 Electricity at Work

Regulations1989.

Relevant Standards UK and European European Standard EN 50122 Part 1 electrical

safety and earthing European Standard EN 50122 Part 2 provision

against stray currents BS 7430 Code of Practice for Earthing BS7671 17th Edition of the IEE Wiring

Regulations

G59/1 Electricity Association Engineering Recommendation

LU Standards MR-S-PO-0009-Part 1Earthing Code of

Practice MR-S-PO-0008 Earthing management 1-106 Earthing and Bonding of LU Electrical

Networks E7720 A3 Engineering Standard 25kV 50Hz

immunisation SSL-SE-0858-A1 Earthing Practice 1985-

Signal Engineering.

Network Rail NR/SP/ELP/21085 Network Rail Earthing and

bonding Standard RT/E/21032 Network Rail Earthing Systems

57


Figure 12 Earthing and Bonding on DC Electrified Railways

NGCSubstation

Earth Wire

DCVoltage

R D White 2004

Signall ingPSU

Track circuit

Telecoms.Cable

TunnelStructure

Telecoms PSU

Rebars

CopperEarth Mat

RECSupply

Track StructuresLocalRECS

StationApparatus

StationMetallic

Structures

Figure 13 Earthing and Bonding on DC Electrified Railways (LUL)

58


14 Typical Cross Section of Cables found in a DC Mass Transit Railway

59


REGENERATIVE TRACTION UNITS AND SUPPLY POINTS

3- Phase Traction Drives

The development of 3 phase induction drives has introduced higher power drives and the ability of regenerative braking energy back into the supply system. Three phase induction motors are attractive due to the elimination of the DC commutator. This reduces the chance of mechanical breakdown and eliminates the need for maintenance. The induction motor is able to develop more torque due to the control system, the motor design and the fact that there is increased space on the rotor. Size for size the induction motor is more powerful than the equivalent DC motor; consequently it has a higher power to weight and power to volume ratio. The capital cost of introducing inverter drives on traction equipment is close to DC drives, therefore AC drives are financially viable and also able to produce a superior performance. Careful analysis must therefore be undertaken if 3 phase drives are to be introduced to ensure that the supply network will operate satisfactorily.

The development of traction inverters and choppers has made the recovering of kinetic energy of the train and returning it to the supply common place. The introduction of microprocessor control for traction drive enables reliable control of the traction drive and reduces the likelihood of any interference with the low frequency signalling circuits.

Other traction equipment can use energy that is regenerated back into the DC electrification network. If there is no traction unit available to use the regenerative energy, the system voltage will rise. It is vital the supply voltage does not exceed that specified for the DC electrification network (reference section 3.4 Table 1). It is necessary at times to cease regeneration or dissipate the energy in dynamic brake resistors. The inclusion of inverters at the substation along with the normal diode rectifier equipment, enables the power to be returned back to the supply system when other trains in the feeding section cannot absorb it.

Currently there are a number of applications where energy is being regenerated by traction units, with the energy (allowing for receptivity) being returned to the DC electrification network. This energy is then utilised by other trains on the same network.

Technical Merits of Regenerative System i. Reduction of heat produced in underground

metro systems. ii. Elimination of brake resistors. iii. Reduction of brake dust in tunnels.

iv. Reduction of maintenance in tunnels. v. Improved efficiency of the Railway Network and reduction in energy costs.

Disadvantages of Regenerative Systems i. More capital investment in traction and supply

equipment. ii. More harmonics in the medium voltage supply. iii. Intelligent protection system is required on the

supply and traction equipment enabling it to discriminate with regenerating units.

iv. Adverse effects on stray current management

Regenerative vehicles on a DC Electrified Railway.

The following are system performance issues that need to be addressed by the traction and the electrification engineers to achieve compatibility and are required where appropriate to be written into the software code for the train and the Supply Protection System.

Variation in System Voltage. The system voltage limits will determine the operation of the regenerate vehicle. As the system voltage drops the following process is necessary for regenerative load and should reduce commensurate with falling voltage.

i. Power is reduced ii. Dont initiate regeneration iii. Stop Regeneration iv. Open traction unit circuit breaker

A local short circuit of the system will cause the collapse of the voltage. Under a remote short circuit the system voltage will not collapse as easily, therefore there is a greater chance of the train operating into the short circuit.

Interference between the train and the electrification system i. A train runs into an electrification system which is

isolated and earthed? i Train System must detect that there is no voltage

and not initiate regeneration i Train System should detect step change and

convert to rheostatic brake or friction brake.

ii. A train runs into an electrification system which is isolated and not earthed? and with no load for the regenerative energy: i System must detect that there is no system

voltage and not initiate regeneration i Train System should detect step change and

convert to rheostatic brake or friction brake.

60


iii. The train protection system should be designed to trip with a short circuit and subsequently allow the main track feeder breakers to also clear the fault.

iv. The Electrification System opens the Supply Circuit Breakers for whatever reason with a regenerative train in section? The train may or may not continue to regenerate.

v. On the reapplication of the Electrification Feeder Circuit Breaker? i Auto-reclose may not be considered

appropriate in case a regenerative train is operating within the system.

i Closure of the circuit breaker onto a regenerating train will produce a transient effect on the train and the electrification system.

i Before the system can close the supply circuit breaker should detect that there is no existing voltage due to a regenerative vehicle.

i Alternatively the system could be locked out for a set period to ensure that all regenerative trains have come to a standstill.

MAGNETIC FIELDS ASSOCIATED WITH THE OVERHEAD LINE

DC Magnetic Field ( ref. Figure 14)

The dc magnetic field created by the dc traction current in the overhead line or third rail is significant within and beyond the environment of the railway. The DC field causes interference due to the change in load on the system or under large di/dt during a short circuit.

The magnetic field will vary in magnitude due to the passage of trains typically between 1-100A/m. The limits for interference into electrical equipment are not detailed for DC or power frequency harmonics in EN 50082-2 Generic Immunity Standard Industrial and BS EN 50121.

The DC magnetic field during a short circuit will typically be up to 600 A/m.

Harmonic Magnetic Fields

Harmonic fields attributed to the power frequency will be significant within the environment of the railway. There may be an effect on equipment within the environment at these frequencies. The induction into lineside cables of harmonic content of the power system should also be considered.

CABLES AND CONDUCTORS

Applications ( ref. Figure 14) Cabling and Conductors (copper, aluminum or steel) are used extensively in the rail electrification system and are used in:

i 33kV & 11kV Distribution network, substation feed

i D.C, traction return cables i DC feeder cables. i Track bonding i Lineside cables i Overhead line conductors

National Regulations Cable design is subject to stringent standards and requirements for safety, safe installation and serviceability. This is particularly so in the case of tunnels where cable must comply with low smoke no halogen specification. Also it is especially important where the power supplies are subject to high temperatures whether influenced by the environment or internally by the current carrying capacity and load demands.

Cables are manufactured to a range of specifications including: BS 6853, BM/RT2120 and LUL/RSE/STD.

Cable Specification

Track Application For the majority of track applications a combination of copper and aluminum, concentric solid core and stranded cables is used.

For solid core the insulation in many cases is PVC applied to the cores when hot, then cooled and shrunk onto the cores. Older cables use oil impregnated insulated paper tapes. The outer core of the cable may be protected by steel tape, or galvanized wire armouring. This provides protection against abrasion and gives strength and protection where the wire is required to be drawn through cable ducts and laid on the track.

HV Supply and Distribution

DC electrification uses solid, gas or oil-filled cables. In ratings in excess 33kV. 33kV fluid filled and XLPE are used for substation applications feeds. The most common application of insulation for power cable feeds now use XLPE (cross linked polyethylene) Polyethylene is a good insulator, the main benefits of this being greater stability at higher operating temperatures during normal operation, degraded mode and short circuit.

61


DISTURBANCE EFFECTS INTO TELECOMMUNICATIONS CIRCUITS

( ref. Figure 14)

Safety of Personnel, maintainers etc

The integrity for a railway network is dependent upon the correct operation of the low voltage signalling circuits. The circuits use the running rails as well as cables which run parallel to the track and to the overhead traction conductor, therefore it is possible for the traction current to interfere with the low power signalling circuits due to electromagnetic or conductive mechanisms. It is necessary that the magnitude of this interference must be reduced to levels which will not threaten the safe operation of trains, nor the safety of any personnel who are likely to come in contact with a part of the signalling system.

The levels adopted are those specified in the International Telegraph and Telephone Consultative Committee (CCITT) directives concerning the protection of telecommunication lines against harmful effects of power lines.

With regard to signalling circuits, the longitudinally induced emfs should not exceed the following levels:

i Under normal conditions: 60 volts rms. In situations where there is no exposure to other than technical staff to any direct contact with signalling lines circuits, the limitation of 60V is normally raised to 110V, as this voltage is a common supply voltage for signalling systems.

i Maximum induced voltage occurring on lineside cable conductors will be 430V, the fault duration not exceeding 200mS.

Safety for humans Telecommunications Equipment (Longitudinal Voltage)

This is a disturbance resulting in degradation of useful signals and merely hampers the exchange of communications. The rating of individual manufacturers equipment will vary but should nominally be about rated to about 1000V. (new equipment)

The limit defined in the European Standard states that equipment should be able to withstand as specified for a 50Hz Railway EN 50121 - 4: i 150V 50Hz for traction current i 650V rms 50Hz for short circuit i 100A/m Power frequency magnetic field

Existing equipment may be specified in the UK to 430 volts rms.

Interference into telecoms equipment Transverse Voltage

Psophometrically weighted traction line current: Psophometric current is defined as the r.m.s addition of all the harmonic currents in the traction units primary current wave-shape, each harmonic first being attenuated in accordance with the appropriate CCITT weighting curve. The psophometric current therefore is an indication of the level of interference that will be produced at that point in time for the traction unit. The interference mechanism is by electromagnetic induction from the traction high power into the low power telecommunications networks. Conventional telecommunications networks only are affected. Digitised and optical links are inherently immune from such interference.

I pso = (I2n p2n)

where n = harmonic number p = psophometric weighting factor of the

nth harmonic

Immunity Concerns

The immunity concerns are related to high-energy surge on the overhead line, due to connection to the high voltage power system. The relative slow response of the diodes will provide a good immunity to fast transient effects including the effects of lightning strikes to the overhead line.

Immunity to RF fields will be high where diode rectification is used, extra consideration will be required where thyristor or other controlled devices are used.

Factors for Consideration with Induction Calculations

Traction Power Disturbances i Change of traction load or fault current ( time

varying) i DC Ripple (rectifier supply) i Coupling increases with harmonics (j2..f.M) i Fast transients ( power supply) i Normal load Current Typically < 500A i Fault current typically < 10kA

Factors for DC System Design i Geometry of the line conductors i Inductance of the DC overhead line i Proximity of control system cables

62


Factors necessary for the Calculation of Induced Voltage i Traction unit psophometrically weighted current i Power system cables HV 50 Hz ac power cables,

DC traction power cables i Receptor cables telecoms cables Signalling

cablesi Parallelism of cables i mutual impedance as a function frequency i screening factors of the earthed conductors

Calculations required by Railway Company i induced touch potentials i equipment potentials i induced psophometric transverse voltage

Cable Management Systems

Extensive cabling for services for power, signalling, communications, fire systems etc need to be routed throughout the railway. Multi-compartment cable management systems (cms) is used which allow for ease of installation whilst providing mechanical protection for cabling. Cms can run for very long distances in underground stations and may place sensitive telecoms and communications cabling in close proximity to noisy power cabling. These power cables may feed a variety of switching loads with associated harmonics. The disturbance current induces a longitudinal voltage in parallel signal/communications cables which may present a touch voltage or accessible voltage hazard. In addition, the longitudinal voltage will result in a transverse voltage in the victim cable which may cause interference to the victim circuit in question. A typical cms will have many cables in each of its multi compartments.

LIGHTNING AND LIGHTNING PROTECTION

Lightning strikes on or near railway equipment can generate large voltage surges that can disturb or damage railway operations. The various ways in which a lightning strike can affect railways are:

i. Direct strike to the overhead lines ii. Direct strike to the aerial earth wire or gantries iii. Nearby strike to ground induced voltages iv. Strike to ground further away rise of local

ground potential)

A direct strike to the lines can generate an overvoltage surge of several million volts. This will cause a flashover across the support insulators to the gantry. The surge current will then find various routes to earth depending on their surge impedance values. This will also generate an overhead line earth fault.

A direct strike to the earth wire is unlikely to cause a flashover across the support insulator, unless there is a

train at that point, as the lower impedance path will be via the earth wire and support gantry. However, the surge current will then find a similar path to earth as before but with different current surge values.

A nearby strike to ground will cause induced surge voltages of up to several 100kV to appear on the earth wire and the lines. No flashovers are likely to occur as most overhead wires will reach similar voltage levels. However, the strike current will flow into the ground and cause a local rise in ground potential (known as earth potential rise EPR - or rise of earth potential - ROEP). This may cause disturbance to electrical equipment or signal cables on the railway.

A strike to ground from further away will not cause damagingly high induced surge voltages but may still cause problems due to ROEP.

Adequate surge protection is required to be included in the system to protect lightning surges reaching the DC switchboards. Surge protection must be fitted on the incoming circuit of each DC track feeder circuit breaker, in addition to surge protection of the OHL

Lightning Protection on Railway Lines

Specific designs for railways are not provided within EN 50122-1; therefore guidance has been obtained from BS 7354 Code of Practice for design of high- voltage open terminal stations.

BS 7354 : 1990 Code of Practice "Design of high-voltage open-terminal stations". Section 7.3 Earthing see clause 7.3.9 states:

"An earth electrode, which may be part of the grid, should be provided as near as practicable to each set of surge arresters. The connections thereto should be as direct as possible. Earth connections to surge arresters should not pass through iron pipes which would increase the surge impedance of the connections. The earth connections of the arresters should be interconnected with the main earthing system since, to be effective in protecting the station equipment, a definite connection of low impedance between the equipment and the arresters is essential".

In terms of the railway, there is no copper earth mat as exits in HV substations; there is however an earthed rail returns system.

A copper bond (ideally flat in cross section) of the shortest possible route shall be used. Ideally a segregated earth for the lightning arrestors should be used; Where the lightning arrestor is also bonded to rail earth, signalling, telecommunications and LV earth connections should not be bonded to the rail within close proximity of the connection from the surge arrestor.

63


DISTURBANCE EFFECTS OF DC ELECTRIFICATION SYSTEMS

Characteristic System Behaviour: DC Traction Systems produce significant amount of ripple, which is present within the DC supply, the ripple is related to the pulse number of the rectifier. within the transformer rectifier unit. (Figure 9)

Resonant frequency

Resonance is related to the system characteristics of the power supply, rectification, dc electrification distribution and traction loads. The frequencies can occur from the resonant frequency of the input traction filter to MHz. System parameters that are responsible include: capacitance of the overhead line; leakage inductance of the overhead line; rectifier switching; inductance of supply transformer; traction input filters;

AC LV and HV Systems i. AC side harmonics in 3 phase supply ii. AC voltage distortion iii. Power Factor iv. 50Hz Disturbance to users on the

Railway LV/HV system i. Power Harmonic to users on the railway LV/HV

system

DC and Power System Harmonics i. DC Side Harmonics ii. Resonant, overlap effect, system capacitance iii. Traction line filter resonance iv. Magnetic and Electric Fields

Inductive and Radiated Effects i. High frequency radiated emissions ii. Traction load traction to regeneration. iii. Power arcs on the ramp end of the rails iv. Disturbance changes supply & traction v. Switching of the DC power (di/dt) vi. Longitudinal Transverse voltages

Return Circuit and Stray Current i. Stray DC current ii. Magnetic field traction/electrificationiii. Harmonics in the return circuits

RFI from the Distribution System i. current collection mechanism. ii. operation with multiple contact wires. iii. HV switching of the power system iv. resonance of the power system at MHz

v. excessive voltage stress across insulators vi. disturbances on the overhead line

RFI from the Traction Unit i. transients due to raising the traction unit pantograph ii. interaction of power system contacts earthing,

wheel rail, rail to rail iii. operation of multiple pantographs iv. switching of thyristors and semiconductors.

RFI from the Track Circuits v. track circuit current producing arcing poor wheel to

rail,vi. high voltage impulse track circuit.

64


Relevant UK and European Standards

i. IEC 60479-1 2005 Effects of current on human beings and livestock.

ii. BS EN 50122-1, Railway applications - fixed installations, Part 1: Protective provisions relating to electrical safety and earthing,

iii. BS EN 50122-2, Railway applications - Fixed installations - Electrical safety, earthing and the return circuit - Part 2: Provisions against the effects of stray currents caused by d.c.

iv. traction systemsv. BS EN 50122-3 Railway applications - Fixed

installations - Electrical safety, earthing and the return circuit - Part 3: Mutual Interaction of a.c. and d.c. traction systems

vi. BS EN 62305, Protection against Lightning, 2006. vii. BS EN50388:2005 Railway applications Power

supply and rolling stock Technical criteria for the coordination between power supply(substation) and rolling stock to achieve interoperability

viii. BS EN50119 Railway applications -Fixed installations -Electric traction overhead contact lines.

ix. BS 7671, Requirements for Electrical Installations. IEE Wiring Regulations Seventeenth Edition, BSI, 2008.

x. BS 6651, Code of Practice for Protection of Structures against Lightning, 1992.

xi. Guidelines for the Design Installation Testing and Maintenance of Main Earthing Systems EATS 41-24 1992.

xii. BS 7430, Code of Practice for Earthing, BSI, 1998.

xiii. Electricity Council Engineering S 5/1. xiv. BS EN 32605 Protection against lightning. xv. Engineering Recommendation P24 AC Traction

Supplies to British Rail 1984. xvi. BS EN 50162:2004 Protection against corrosion

by stray current from direct current systems applications.

LV Networks

xvii. BS EN 50310, Application of equipotential bonding and earthing in buildings with information technology equipment, BSI, 2006.

xviii.BS EN 50310, Application of equipotential bonding and earthing in buildings with information technology equipment, BSI, 2006.

xix. Technical Specification 4124. Guidelines for the design, Installation, Testing and Maintenance of Main Earthing Systems in Substations, Energy Networks Association, 1992.

xx. HD637, Power Installations Exceeding 1kV a.c., CENELEC, 1999.

xxi. Recommendation K-27, Bonding configuration and earthing inside a telecommunication building, ITU-T, 1996.

xxii. IEC 60364-1, Low Voltage electrical installations - Part 1: Fundamental principles, assessment of general characteristics, definition, IEC, 2005.

Group Standards

i. GE/RT8016 Verification of Electrification Systems and Interactions with Other Systems

ii. GE/RT8023 Compatibility between Electric Trains and Electrification Systems

iii. GL/RT1253 Mitigation of DC Stray Current Effects

iv. GE/RT8270 Assessment of Compatibility of Rolling Stock and Infrastructure

v. GL/RT1254 Electrified Lines Traction Bonding

Network Rail Standards

i. AC Traction Supplies to British Rail ER P24 [1984] Electricity Association recommendation.

ii. NR/GN/ELP/00015, Signalling Power Supply Design

iii. NR/L2/TEL/30034, Radio Mast Lightning Protection and Earthing System, Network Rail, issue 02.

iv. NR/SP/ELP/21106 25 kV a.c. System Protection Calculations

v. NR/SP/ELP/21085 Specification for the design of earthing and bonding systems for 25kV a.c. electrified lines

vi. NR/SP/ELP/21036 Specification for 25kV Booster Transformers for a.c. electrified lines

vii. NR/SP/ELP/21078 Specification of design of the Return Conductor Systems for AC Electrified Lines

viii. NR/SP/ELP/21074 Overhead Line Equipment Allocation design

London Underground

i. MR-S-PO-0009-Part 1Earthing Code of Practice ii. MR-S-PO-0008 Earthing management iii. 1-106 Earthing and Bonding of LU Electrical

Networks; iv. 1-222 London Underground Cat 1 Standard

Electromagnetic Compatibility v. G-222 London Underground Manual of EMC Best

Practicevi. 1-193 London Underground Electromagnetic

Compatibility (EMC) with LU Signalling Assets

International Standards

i. Technical Specification 4124. Guidelines for the design, Installation, Testing and Maintenance of

65


Main Earthing Systems in Substations, Energy Networks Association, 1992.

ii. HD637, Power Installations Exceeding 1kV a.c., CENELEC, 1999.

iii. Recommendation K-27, Bonding configuration and earthing inside a telecommunication building, ITU-T, 1996.

iv. IEC 60364-1, Low Voltage electrical installations - Part 1: Fundamental principles, assessment of general characteristics, definition, IEC, 2005.

Technical Specifications for Interoperability (TSIs)

i. High speed TSI ENERGY ii. Conventional TSI ENERGY (in drafting 2006) iii. High speed TSI RST iv. Conventional TSI RST (in drafting 2006) v. Rules of the Route: Rules of the Route (by area /

territory). Defines operational access, blockages, engineering access

vi. Rules of the Plan: Rules of the Plan (by area / territory). Defines operating rules and allowances for operational access

vii. Working Timetable: Working Timetable (by route). Defines existing and planned service patterns

viii. Rolling Stock Diagrams: Rolling Stock Diagrams. Defines existing and planned utilisation of paths and thereby traction demand

BIBLIOGRAPHY:

1. 11th ETS and 13th RSCS IET Professional Development Course Notes 2010.

2. Systems IRSE Seminar Railway Interfaces; IEE Savoy 18 November 2004.

3. Health and Safety at Work Act 1974 ISBN 0105437743.

4. Electricity at Work Regulations 1989 SI 1989 No. 635.

5. Construction Design and Management Regulations 1994.

6. The Electricity Council Chief Engineers Conference, Limits for Harmonics in the United Kingdom Electricity Supply Engineering Recommendation G5/4 2002.

7. Proceedings of EMC in Railways IET Seminar Austin Court Birmingham 28th September 2006.

8. Proceedings of ASPECT 2006 IRSE Quality of service through Signalling and Communications Queen Elizabeth II Conference Centre Westminster London 16-17th March 2006.

9. IRSE ASPECT 2003; Signs of the Times for Train Control; Queen Elizabeth Conference Centre London UK

10. Symposium on Research and Development in Railway Engineering Asia Pacific; IEE and IEEE Hong Kong, Hong Kong March 2005.

11. Seminar Proceedings Traction Power Supplies IMechE Birdcage Walk London January 2004.

12. IEE Railway Industry Group Seminar DC Traction Stray Current Control 21.10.99.

13. International conference on developments in Mass Transit Systems IEE PEP04530, 322pp., 53 papers, ISBN 0-85296-703-9 & 978-0-85296-703-4.

14. Colloquium on Systems Engineering on Large Railway Projects IEE Railway Industry Group May 1997.

15. Colloquium on EMC in Electric Traction and Signalling IEE Savoy Place, Nov 1995.

16. International conference on electric railways in a United Europe IEE PEP04050, 210pp., 39 papers, ISBN 0-85296-631-8 & 978-0-85296-631-0, 1995.

17. IEE Conference on 'Main Line Railway Electrification' Publication 312 York University 1989; ISBN 85296384. X.

18. IEE Conference on 'Electric Railway Systems for a New Century'. Publication 279; ISBN 085296351-3.

19. GEC Traction Symposium Main Line AC Electrification.

20. Protective Relays For Rail Transport System GEC ALSTHOM.

21. Kimbark E.W 1971. Direct Current Transmission J Wiley and son.

22. Arrillaga, J et al, Power Systems Harmonics John Wiley & Sons 1985.

23. J E Buttery, D N Ebenezer and B P McCormick. "Electromechanical and electronic falling voltage track impedance devices for fault detection on DC track systems" Whipp and Bourne Ltd, UK.

24. L. L. Denning Influence of Commutating Reactance on the Design of DC Power Supply Converters, GEC ALSTHOM Publication.

25. J.G.Yu The Effect of earthing strategies on rail potential and stray currents in DC transit railways CEGELEC Projects Ltd UK.

26. Dr R J Hill Electric Railway Traction Part 1-7 Power Engineering Journal 1994.

27. J G Yu and C J Goodman, University of Birmingham Computer Analysis of touch and step voltages for DC Railways. Proc. of the third International Conference on Computer Aided Design, Manufacture and Operation in the Railway and other Advanced Mass Transit System Washington 1992 .

28. Stray Current Design Parameters for DC Railways Proc of the ASME/IEEE Joint Railroad Conference pp 19-28 1992.

66


29. Protective Relays For Rail Transport System Protection and Control GEC ALSTHOM.

30. ORR Web Site: Design standards Stray current Management Nov 2008

http://www.rail-reg.gov.uk/upload/pdf/TTGN3.pdf 31. ORR Web Site: Guidance of Tramways Sept 2007 http://www.rail-reg.gov.uk/upload/pdf/rspg-2g-trmwys.pdf

File IEE REIS DC 2011 Author Dr R D White April 2011, Professional Head of Electrification Atkins (UK) [email protected]

67

dc electrification supply system design

Documents