Article citation information:
Olczykowski,
Z., Wojciechowski, J., Młyńczak, J. Reliability and
quality of the power supply for rail transport. Scientific
Journal of Silesian University of Technology. Series Transport. 2017, 96, 139-149. ISSN: 0209-3324. DOI: https://doi.org/10.20858/sjsutst.2017.96.13.
Zbigniew
OLCZYKOWSKI[1],
Jerzy WOJCIECHOWSKI[2],
Jakub MŁYŃCZAK[3]
RELIABILITY AND
QUALITY OF THE POWER SUPPLY FOR RAIL TRANSPORT
Summary. The
subject of this paper deals with the reliability of the power supply and
quality of electricity on the railway networks. The paper discusses the impact
of changes in the area of traction substation load on parameters characterizing
the quality of electricity. It analyses the quality of electricity in supply
networks for the traction substation, as well as on auxiliary lines and in
trackside power supply. Particular attention is given to the propagation of
interference caused by traction DC appliances for the trackside power supply.
This is important due to the fact that these networks provide the power supply
for devices that are responsible for the safety of train traffic. The paper
also discusses the impact of major failures (blackouts) in power systems on
transport, especially rail transport.
Keywords:
rail transport; power supply reliability; blackout power quality
1.
INTRODUCTION
Electric traction, being one of the
largest consumers of electricity, requires power to be supplied from two
independent sources. Once converted to DC, a significant part of the energy
supplied from the power system to a traction substation is used for driving the
traction units. The remainder is used for the substation’s auxiliaries and for
operating trains. Surpluses are sold to other customers; the trackside power
supply is used for this purpose.
Due to the nature of the substation,
which includes non-linear elements (rectifiers) and considerable loads,
electric traction impacts on the power system from which it is powered. By
pulling deformed currents, it introduces higher harmonics in the network. Rapid
power changes, resulting from starting up a train set, for example, cause
further fluctuations and voltage dips. These disturbances are felt both in the
substations’ supplying systems and in the auxiliary lines and trackside power
supply. The propagation of disturbances in the trackside power supply affects
the equipment supplied from this supply. This is particularly important, since
the trackside power supply supplies train traffic control equipment. Railway
interlocking devices have a direct impact on rail transport safety [9,17].
A very important issue concerning
all transport areas, especially rail transport, is power supply
reliability. The lack of electricity supply completely immobilizes trains that
use electric traction. The longer the power outage, the more severe impact it
has on passengers. When power systems covering large areas fail for a period of
several days (blackout), a massive transport chaos ensues. From rail, air,
public to individual forms of transportation, everyone depends on electricity
supply. This is particularly detrimental in large metropolitan areas, where a
large number of passengers must be carried at one time. In the case of a
failure lasting several days, rail transport based on diesel locomotives can be
one of the main measures to ensure delivery of food, medicines and fuel for
cities deprived of electricity. In such a case, it is very important to provide
power for railway interlocking.
2. ELECTRIC
POWER QUALITY ASSESSMENT CRITERION
The norm EN-50160 [5] was assumed to
be an assessment criterion of electric power quality. This norm concerns the
parameters of supplying voltage occurring in public distribution networks. This
norm also determines the voltage parameters of low (up to 1 kV) and medium
(from 1 to 35 kV) voltage networks. The norm specifies voltages measured in
supplying points (connectors) and conditions of nominal operation of supplying
networks. Presented in the norm recommendations are the following allowable
values of voltage parameters of low- and medium-voltage networks: frequency of
voltage, value of voltage, the change of voltage, fast changes in voltage,
subsidence of voltage, short-term interruptions in supplying voltage, long-term
interruptions in supplying voltage, instantaneous overvoltage between wires of
networks and the earth, transient overvoltage between wires of the networks and
the earth, asymmetry of supplying voltage, the influence of the harmonics of
voltage and the voltage signal on supplying voltage, and their use in transmitting
information.
On the basis of the EN 50160 norm,
Polish regulations determining the basis parameters of electric power quality
have been elaborated [11,13].
3. ANALYSERS
AND PLACES FOR THE MEASUREMENT OF ELECTRIC POWER QUALITY
Measurements of electricity quality
indicators, analysed in the article, were carried out on the power supply
networks of various traction substations. According to the EN 50160 norm,
devices having special certificates and meeting the norm of network parameters
should be used to record parameters. The records should be performed over at
least a period of one week. Power quality indicators were also recorded during
short measurement intervals. This allowed us to assess the impact of dynamic
substation load changes on energy quality. Fig. 1 shows the part of the
substation supplying system along with the places of meter connection.
Fig.
1. Places of measurement of electric power quality
Two kinds of electric power quality
meters (Memobox 800 and QWave), as well as well as the Panensa-MEF
flickermeter, were used for measurement purposes. The analysers were connected
by current and voltage transformers to the following circuits: supply lines for
the substation; line of own needs; and non-traction networks.
4. ANALYSIS OF MEASUREMENT DATA
The traction system is closely
linked to the supply system of power lines. No voltage in the power grid
automatically affects the traction line, which makes power redundancy a must.
However, there is an inverse relationship present. A system of rectifier units
or substation load changes on the DC side affect(s) the quality of electricity
in substation feeders, and may cause interference in electric appliances on the
low-voltage side, powered from auxiliary transformers and trackside transformers.
Below is an analysis of changes in the basic electricity quality indicators.
4.1. Power lines for the traction substation
The power system is connected to the
DC traction in the traction substation supply grids. Most of the substations
are powered by medium-voltage lines. For a single-stage transformation of
energy, the traction’s impact on the power system is smaller. This is due to
the higher short-circuit capacity at the interconnection point.
An analysis of measurement results
recorded in the substation feeders with six-pulse
rectifier systems is given below.
Fig. 2 shows a summary of the
electricity quality indicators recorded during a week-long measurement cycle.
Fig. 2. Summary of the electricity
quality indicators recorded during a week-long measurement cycle
During measurements, the higher
harmonics of the supply voltage were recorded (see Fig. 3). This is due to the
influence of rectifiers installed at the substation. On the voltage spectrum,
there are harmonics (5,7,11,13,17,19,23,25) that are characteristic for
six-pulse rectifiers.
Fig. 3. The higher harmonics of the
voltage in the power supply line of the traction substation
The measurements also registered
higher flicker indicators (Fig. 4), which is indicative of fluctuations or
voltage dips (see Table 1).
Tab. 1
Voltage dips measurements in the
power supply line of the traction substation
The measurements also registered
higher flicker indicators (Fig. 4), which is indicative of fluctuations or
voltage dips (see Table 1). During one of the events, the substation’s supply
voltage decreased by more than 40%, down to 17,906 V.
Fig. 4. Voltage changes and
short-term light flicker Pst
4.2. Line of traction substation
needs
The main task of a traction
substation is to convert AC voltage to DC voltage, as well as deliver it to the
railway traction network. An additional task of the traction substation is
powering different non-traction objects with, for example, AC 230/400 V
electricity for powering its auxiliaries, which is provided by two auxiliary
transformers. These transformers supply auxiliary circuits, which are essential
for the operation of the traction substation (e.g., safeguards, control and
signalling circuits), most of which are powered by DC voltage. The operation of
these devices (especially older ones) is slightly affected by the quality of
power that supplies the substation, alongside disturbances associated with the
substation’s variable load. However, the introduction of modern safeguard
equipment, control devices and data transmission devices increases power
quality requirements.
Fig. 5. Voltage fluctuations on
power lines of the traction substation and its line of own needs
Voltage fluctuations in the
own-demand circuits are closely related to voltage changes in the supply line
of the substations (see Fig. 5). The correlation coefficient between the
voltages is 0.989 (Fig. 5 in decreasing order).
4.3. Trackside power supply
Unlike the devices connected to the
auxiliary circuits, the trackside power supply supplies various appliances
(with variable power input, pulling deformed currents, sensitive to
disturbances). For this reason, appropriate supply voltage parameters must be
guaranteed to a greater extent than in auxiliary circuits. As shown later in
this paper, the trackside power supply circuits are also affected by
disturbances resulting from the rectifier units (limitation of the fifth and
seventh harmonics for 12-pulse sets) and changing substation load. In addition,
we have disturbances caused by appliances connected to the trackside power
supply circuits. This is particularly important for small short-circuit
capacities at the interconnection point.
PKP Energetyka holds licences issued
by the Energy Regulatory Office for the distribution, transmission and trading
of electricity. Using trackside power supplies, it sells electricity to other
consumers in the energy market. For this, an appropriate quality of electricity
must be ensured [11].
Fig. 6 shows the voltage waveform in
one phase of the trackside power supply. There were three voltage dips, as
confirmed by the UNIPEDE table and the waveform of one of the dips in reduced
size (see Fig. 6). The voltage dips were caused by rapid changes in the load of
the traction substation.
Fig. 6. Changes and dip voltage in
the network of non-traction needs
Voltage dips and short power cuts
are one of the most common disruptions in power grids. Being highly random,
they are virtually impossible to eliminate. Limiting the effects of these
disruptions requires large financial outlays. For computer equipment used to
supervise railroad traffic control systems, an uninterrupted power supply
becomes a must. In trackside power networks, the main cause of voltage dips is
the dynamically changing load of the traction substation.
As in the case of auxiliary
networks, the non-linear nature of the traction substation causes the
propagation of higher harmonics. Depending on the rectifier units used,
characteristically higher harmonics are generated. Fig. 7 shows the percentage
change in voltage higher harmonic content, recorded during one of the
measurement cycles. In the smaller box, the higher harmonics spectrum for the
measured voltage is shown.
Fig. 7. Higher harmonics and voltage
spectrum recorded on the line of non-traction needs
The voltage harmonics spectrum is
characteristic for six-pulse rectifier units. This spectrum is similar to the
spectrum of voltage measured on the traction substation’s auxiliary circuits.
Voltage waveform deformation directly affects the lifespan of electricity
appliances. It is assumed that a voltage deformation of THD=10% will reduce the
lifespan of single-phase motors by approximately 30%. Substantial voltage
distortions also cause many other risks.
5. RELIABILITY OF POWER SUPPLY FOR RAIL TRANSPORT
Electric traction is one the
appliances requiring increased supply reliability. This is secured by two
independent power lines fed into the traction substation (see Fig. 1). If the
primary power supply fails, the substation automatically switches to the backup
line. Feeding power into the substation in this way makes operating trains
possible if one of the power lines fails.
However, there are more and more
situations where consumers without power are in the order of thousands or even
millions. During a blackout in the US and Canada between 14 and 16 August 2003,
more than 55 million people had no power [7,2,1]; in Italy, on 23 September
2003, more than 57 million were affected [8,9]; and in Brazil, between 14 and
15 March 1997, more than 97 million were affected. During the biggest blackout
in India from 30 to 31 July 2012, more than 700 million people were affected
[14,15,16]. Moreover, these power cuts were so prolonged that they endangered
human life and caused substantial financial loss. One of the areas of the
economy that depends on electricity to a large extent is transportation. Once
we could distinguish between transport sectors that are directly dependent on
electricity, such as rail transport. Now, when there are system failures
(blackouts), we are dealing with a major transport chaos (see Fig. 8).
. 
Fig. 8. Looking like a scene from a
post-apocalyptic film, droves of people walked were forced to cross the
Brooklyn Bridge to get in and out of Manhattan [8]
Following the many blackouts around
the world, restoring the power was not be achieved swiftly. Thousands of
passengers had to be evacuated from subway cars and passenger trains. Many were
stranded at stations and airports (see Fig. 9).
Fig. 9. Passengers sit on a railroad
track in India waiting for power to return after
a massive blackout in late July 2012 [4]
Poland experienced a major power
system failure between 7 and 8 April 2008. Severe ice build-up damaged power
lines supplying the Szczecin agglomeration, leaving more than 650,000 people
without power. As in the case of other incidents, the lack of energy supply
caused traffic chaos in all transport sectors. The costs of the failure were
estimated at over PLN 100 million [12].
6. CONCLUSION
Rail transport is highly dependent
on electricity supplies from the power system. Disturbances and failures in
traction substation supply lines directly affect the operation of trains and
the entire railway infrastructure. IT equipment and railway control engineering
are particularly sensitive to these disturbances [10]. If a power failure
affects a large area of the national power system, resulting in a blackout,
railway transport is brought to a virtual halt. It is then that diesel engine
train sets must be used, with railroad traffic control being provided by backup
power, given that electricity generators provide power to traffic control
equipment.
Given its power and load, electric
traction brings into the system all kinds of disturbances. These disturbances are
also propagated within auxiliary networks and trackside power supplies. These
are mainly higher harmonics that result from using rectifiers in traction
substations, as well as voltage fluctuations and dips. They reduce the quality
of electricity. The right power quality must be guaranteed, especially in
trackside power supplies. These supplies provide power to traffic safety
equipment and other non-railway consumers. Improving the voltage parameters is
very expensive in many cases. Replacing old rectifier units with new ones or
using single-stage energy transformation will often limit the traction’s impact
on the power system. Increasing the cost towards improving the railway
infrastructure responsible for operating trains seems to be inevitable.
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Received 12.04.2017; accepted in revised form 19.07.2017
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