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By Suman Pathak
Posted 3 years ago

POWER QUALITY ISSUES IN RAILWAY ELECTRIFICATION

Rail Electrification

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POWER QUALITY ISSUES IN RAILWAY ELECTRIFICATION

The power quality issues through electric railway development are overviewed as follows : 

 

Voltage Unbalance
Voltage imbalance (also called voltage unbalance) is defined as the maximum deviation from the average of the three-phase voltages or currents, divided by the average of the three-phase voltages or currents, expressed in percent. The most frequent problems of voltages are associated with their magnitudes. The major problem is unbalanced currents produce unbalanced voltages. Traction motors and other related loads in trains are designed to function properly with reduced voltage amplitude by 24% or increased amplitudes by 10% than the nominal voltage of electric railroad drives.

System imbalance

System imbalance is the most serious problem in electric railway power quality because most trains are single phase, and a single-phase load produces a current NSC (Negative Sequence Current) as much as a PSC (Positive Sequence Current). A new traction power supply system adopting a single-phase traction transformer and active power flow controller (PFC) is proposed. In the new system, the power quality problems caused by single-phase traction load are solved on the grid side and the continuous power can be

Arcing

The interaction between the pantograph/catenary of overhead systems or between brushes and the third or fourth rail causes arcs because of dynamic latitudinal tolerance between the wheels and rail. Arcs will occur, which can distort voltages and currents and produce a transient dc component in the ac systems causing a breakdown of dielectrics.

Flicker
As the train passes between two adjacent substations voltage sag may happen and affect other customers electrical light
performance so-called flicker.

EMI/EMC

The movement of rolling stock along an electrified track produces Electromagnetic interference in the system. EMC covers a wide range of phenomena, including inductive noise in parallel communication lines, impulse noise from lightning and traction transients, the production of hazardous voltage under step and touch conditions, and the appearance of stray currents.

EMI and EMC are very complicated for high-speed railway systems. Nowadays, the investigation in EMI/EMC high-speed railway is highly relying on simulation and measurement.

Waveform Distortion
Waveform distortion is defined as a steady-state deviation from an ideal sine wave of power
frequency.


There are five primary types of waveform distortion:
 1. DC offset

    The presence of a dc voltage or current in an ac power system is termed dc offset

 2. Notching

   Notching is a periodic voltage disturbance caused by the normal operation of power electronic
  devices when current is commutated from one phase to another. 

 3. Noise 

  Noise is the unwanted electrical signal with broadband spectral content lower than 200 kHz
 superimposed upon the power system voltage or current in phase conductors, or found on neutral
 conductors or signal lines. 

 4. Interharmonics

 Voltages or currents having frequency components that are not integer multiples of the frequency
 at which the supply system is designed to operate (e.g., 50 or 60 Hz) are called inter harmonics.
 

5. Harmonics

Harmonics can be best described as the shape or characteristics of a voltage or current waveform relative to its fundamental frequency. The ideal power source for all power systems is smooth sinusoidal waves. These perfect sinewaves do not contain harmonics. When waveforms deviate from a sinewave shape, they contain harmonics. These current harmonics distort the voltage waveform and create distortion in the power system which can cause many problems.

In short (Harmonics are sinusoidal voltages or currents having frequencies that are integer multiples of the frequency at which the supply system is designed to operate.)

Types of Harmonics:

Odd and Even Order Harmonics:
As their names suggest, odd harmonics have odd numbers (e.g., 3, 5, 7, 9, 11), and even harmonics have even numbers (e.g., 2, 4, 6, 8, 10). Harmonic number 1 is assigned to the fundamental frequency component of the periodic wave. Harmonic number 0 represents the constant or DC component of the waveform. The DC component is the net difference between the positive and negative halves of one complete waveform cycle.


Total Harmonic Distortion (THD) is defined as the measurement of the harmonic distortion present in a waveform. The power quality of a power system is inversely proportional to THD. More harmonic distortion in the system, lower will be the power quality and vice versa. THD is equal to the ratio of the RMS harmonic content to the fundamental:

 


Where Vn-rms is the RMS voltage of nth harmonic in the signal and Vfund-rms is the RMS voltage of the fundamental frequency.


The Destructive Effects of Harmonic Distortion


A power system’s ability to perform at optimal levels is compromised when harmonic distortion enters the system. It creates inefficiencies in equipment operations due to the increased need for power consumption. The increase of overall current required creates higher installation and utility costs, heating, and decreasing profitability.


Harmonics in Electrified Railways


It is well known that the rapid spread of power electronics brought along not only great advantages but also some drawbacks as they are the main sources of harmonics and voltage waveform distortion.
Harmonic has emerged as a matter of great interest for electrical power system engineering. The electrified railway is one of the main harmonic sources in utility. Because electrified railway is supplied by High Voltage (HV) power system directly, lots of harmonic (mainly including 3rd, 5th, and 7th) produced by electric locomotive penetrate in the whole utility from HV. Compared with normal load, the most characteristics of traction are random time-varying and non-symmetry. So, the harmonic of traction load is very different from the normal load of utility.

In an electrified railroad, the traction power is delivered to the catenary by substations, which in turn receive their supply from the utility network. For the electric utility transmission systems, the alternating current catenary is an unfavorable consumer. 


Two major reasons for this are:
i)    the catenary is a single-phase load, which power consumption unbalances the main supply three-phase system,
ii)   the use of power electronics converters to drive traction motors, generates harmonic currents that perturb interconnection busbar voltage.


Basically, the power quality issues in railway electrification systems include the studies of the influence of traction loads on three-phase utility systems. Most of the high-speed trains are single-phase loads. Due to a large amount of power electronics application to the motor driving circuits of trains, they contribute to the high harmonic currents flowing to the railway catenary system.  Traction load is varying dynamically, and arcs may occur because of pantograph/catenary and switching actions. Modern drive trains rely on power electronic converters combined with transformers, which inject low amounts of current harmonics into the supply system.


Therefore, power quality must be considered in all aspects of the design for every system dealing with electric power systems.
Some especially connected transformers are widely used, such as V-V, Scott, Le Blanc, and Modified Woodbridge connection schemes have been utilized in traction substations to compensate for negative sequence current (NSC) of the grid-side. Due to the nature of time-varying traction loads, it is almost impossible to compensate the whole NSC in all loading conditions. 
Passive filters have been adapted to suppress harmonics in electrical railway systems. Among derivations of filters, a C-type filter (CTF) introduces no power loss at the fundamental. frequency and performs as a first-order high-pass filter at tuned resonance frequency.

Accordingly, the CTF is generally used to mitigate high-order harmonics caused by the PWM converters of the traction trains and prevent harmonic resonance. Although passive devices are affordable with a simple configuration, their performance is not satisfactory when operational conditions are varying. Therefore, active devices in AC electric railways have been proposed to resolve this issue. Static VAR compensators (SVC) and static synchronous compensators (STATCOM) were proposed to compensate the load reactive power of trains dynamically. Since electric locomotives introduce harmonic contents, there is no chance to compensate harmonics by these devices, concurrently. 
Many other strategies have been also proposed for power quality improvement in electric railways, investigated in a comprehensive historical perspective. Nowadays, power quality improvement strategies have developed to a mature degree for new electric railway systems, among which Railway static Power Conditioners (RPC) and its alternatives (e. g. APQC, HBRPC, HPQC) have the main place. 
These compensation schemes are connected to the TSS secondary, as shown in Fig. , and theoretically operate based on instantaneous active/reactive power theory, in which the three-phase currents at the TSS primary side are supposed to be: 
(i)    three-phase symmetrical,
(ii)    fully sinusoidal with no relevant harmonic content
(iii)    aligned with the three-phase voltage featuring negligible reactive power. 

Thereafter, the difference between the load currents and the ideal currents must be generated by the compensator, called compensation currents. The compensator operates as an independent three-phase current source, generating the desired compensation currents.

The RPC consists of two single-phase back-to-back converters sharing the same DC-link capacitor through which active and reactive power are applied compensates voltage, NSC, total harmonic distortions (THD), and PF simultaneously and each AC side of inverters are connected to the two phases of the secondary side of feeding transformer, main phase, and teaser, respectively. These inverters work as effective power balancers and reactive power compensators. For example, if a load of the main phase is larger than that of the teaser, the RPC transfers effective power from the teaser bus to the main phase bus. This system works to balance the effective power of different phases and compensate for reactive power to reduce voltage unbalance and fluctuation.

The various structures of the RPC such as active power quality conditioner (APQC), half-bridge RPC (HBRPC) and Hybrid power quality conditioner (HPQC) were presented. These devices can perform at a full compensating method which results in grid-side power factor unity, zero current unbalance, and harmonics.

 

DIFFERENT COMPENSATORS USED IN SCOTT CO-PHASE SYSTEM 


Hazards of Poor Power Quality Problems in Railways

Impacts on Signaling and Communications:

Track circuits are designed to work with a special frequency that must not have any interference with the power frequency. But in presence of harmonics, communication signals may be affected by harmonic frequencies, resulting in erroneous signals and faulty train positioning, which lead to a disaster. Also, high-order harmonics may cause an interference problem between communication and power systems.

Malfunction of the protective system:

Protection relays may operate incorrectly in the presence of harmonics and NSCs of currents and voltages. Traction load injects many harmonics and NSCs resulting in the malfunction of the protective system.


Decreased Utilization Factor: Since the traction load is a large single-phase load, it results in high current NSCs, which will flow in only two phases, and it decreases the utilization factor of the transmission line.

Incorrect Operation of Transmission Line Control Systems:

Voltages and currents sampling is based on fundamental components of either voltage or current. Every control system in the transmission line would work not appropriately because traction loads inject large amounts of harmonics and NSC current into the transmission lines.

 

 


References: " IEEE Paper 10.1109/TIE.2014.2386794”, IEEE Paper 10.1109/TVT.2017.2661820, IEEE Std 519-1992
 

 

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Suman Pathak - Posted 3 years ago

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Deepu Dharmarajan - Posted 2 years ago

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