Electronics Guide

Converter Topologies

Modern AC traction systems typically use a four-quadrant converter (4QC) on the line side, connected to a DC link, followed by a motor inverter driving AC traction motors. DC traction systems use a simpler configuration with a chopper controlling power to DC motors or an inverter driving AC motors.

The 4QC controls the power factor and harmonic content of current drawn from the catenary. Well-designed 4QC systems can achieve near-unity power factor with total harmonic distortion below 5%, dramatically reducing low-frequency harmonics compared to older phase-controlled systems. However, the switching transitions create high-frequency emissions that require filtering.

The motor inverter controls traction motor current to achieve desired torque and speed. Typical switching frequencies range from 500 Hz to 2 kHz for high-power systems, with higher frequencies possible in lower-power applications. The switching creates spectral content at the switching frequency and its harmonics, as well as broadband emissions from the fast switching edges.

Switching Transients

The switching transients in traction converters are the primary source of high-frequency emissions. When an IGBT or other switching device turns on or off, the voltage and current change rapidly, with transition times typically measured in hundreds of nanoseconds. These fast transitions contain spectral energy extending to frequencies well above 100 MHz.

The steepness of switching transients is a design trade-off. Faster switching reduces switching losses, improving efficiency and reducing cooling requirements. However, faster switching creates more high-frequency emission content. Gate drive design can control the switching speed to balance these requirements, slowing transitions just enough to meet EMC requirements while maintaining acceptable efficiency.

Parasitic inductances in the power circuit create voltage overshoot during switching, adding to emission levels and potentially damaging components. Low-inductance bus bar designs and careful component layout minimize these effects.

Common-Mode Emissions

Common-mode currents are a major source of radiated emissions from traction systems. The rapid voltage changes on motor cables couple through parasitic capacitances to the cable shield and vehicle structure, creating currents that flow through unintended paths and radiate from cables acting as antennas.

Motor cables in traction systems can be many meters long, providing effective antenna structures at frequencies where the cable length approaches a quarter wavelength. At 75 MHz, for example, a quarter wavelength is approximately 1 meter, well within the range of typical motor cable lengths.

Common-mode filtering at the converter output reduces the amplitude of common-mode currents. Common-mode chokes present high impedance to common-mode currents while allowing the differential motor currents to pass. The effectiveness of these filters depends on the common-mode impedance achieved, which is limited by parasitic capacitances in the choke winding.

Output Filter Design

Output filters between the traction converter and motors serve multiple purposes: smoothing the PWM voltage waveform to reduce motor heating from harmonic currents, limiting dV/dt to protect motor insulation, and reducing radiated emissions from the motor cables.

dV/dt filters, also called du/dt filters, are simple LC circuits that slow the voltage rise time at the motor terminals. These filters reduce both motor stress and high-frequency emissions, though they add weight, volume, and cost to the traction system.

Sinusoidal filters provide more complete filtering, producing a nearly sinusoidal voltage at the motor terminals. These filters are larger and more expensive but can significantly reduce emissions and motor losses.

Power Flow Reversal

During regenerative braking, the traction motors operate as generators, and the motor inverter operates as a rectifier, transferring power to the DC link. The line-side converter then inverts this power and returns it to the catenary.

The control dynamics during regeneration differ from motoring, potentially affecting the harmonic content of the catenary current. Converter control design must ensure low harmonic distortion in both operating modes. Testing during regeneration is essential to verify emission compliance.

DC System Effects

On DC traction systems, regenerative braking raises the catenary voltage as regenerated power enters the supply. If the voltage rises too high, the braking system must limit regeneration or switch to rheostatic braking to dissipate power as heat.

Voltage fluctuations from regenerative braking affect other equipment connected to the DC supply. Voltage peaks can stress insulation, while rapid fluctuations can affect the operation of converter controls on other trains. DC supply system design must accommodate these effects.

Receptivity and Energy Storage

The ability of the supply network to absorb regenerated power affects how much regeneration is possible. If no other trains are motoring to absorb the power, and the infrastructure cannot return power to the grid, regeneration is limited.

Energy storage systems at substations or on vehicles can improve receptivity by absorbing regenerated power for later use. These systems, typically using batteries or supercapacitors, have their own EMC considerations including converter emissions and battery management system EMC.

Low-Frequency Harmonics

Low-frequency harmonics in the catenary current result from the rectification and conversion processes in the traction system. For AC systems, these harmonics are related to the supply frequency; for DC systems, they represent ripple currents from the DC-to-AC conversion for the motors.

The 4QC topology used in modern AC traction systems can achieve very low harmonic distortion through active control. The converter switches at a frequency much higher than the supply frequency, using pulse-width modulation to synthesize a current waveform closely matching the desired sinusoid. Total harmonic distortion of 3-5% is readily achievable.

Older traction systems using phase-controlled thyristor converters produce much higher harmonic content. The phase control action creates current waveforms with significant low-order harmonics. Where these systems remain in service, their harmonic emissions must be considered in signaling system compatibility assessments.

High-Frequency Harmonics

High-frequency emissions from traction converters extend from the switching frequency through hundreds of megahertz. At frequencies above the switching frequency, emission levels generally decrease but can still be significant, particularly at frequencies corresponding to resonances in the circuit or antenna-like structures.

Input filters between the catenary and the 4QC attenuate high-frequency emissions before they can propagate along the overhead line. These filters typically include inductors to limit high-frequency current and capacitors to provide a low-impedance return path for high-frequency currents.

The main transformer in AC traction systems provides some inherent filtering due to its leakage inductance. This inductance, combined with capacitors, forms part of the overall input filter structure.

Track Circuit Interference

Harmonics at track circuit operating frequencies can interfere with train detection if they propagate through the traction return path. Standards specify maximum permissible current levels at specific track circuit frequencies, with very low limits at frequencies commonly used for track circuits.

Meeting these requirements typically requires specific filtering targeted at track circuit frequencies. Notch filters or trap circuits provide high attenuation at specific frequencies, reducing emissions at track circuit frequencies without significantly affecting converter performance.

The return current path through the rails is critical for track circuit compatibility. Impedance bonds at track circuit boundaries allow traction return current to flow while providing impedance at track circuit frequencies. The design of these bonds affects both traction return current distribution and track circuit operation.

Voltage Fluctuations

The varying load from accelerating and braking trains causes voltage fluctuations on the supply network. These fluctuations must be limited to prevent effects on other connected loads, including signaling and station equipment.

Supply network design includes sufficient conductor capacity and voltage regulation to maintain voltage within acceptable limits under varying load conditions. Substation spacing is determined in part by acceptable voltage drop at maximum load.

Power Factor

The power factor of the traction load affects the efficiency of power delivery and the voltage regulation on the supply network. Modern 4QC converters achieve near-unity power factor, minimizing reactive power demand on the supply.

On DC systems, power factor at the supply substation depends on the rectifier design and loading. Phase-controlled rectifiers have load-dependent power factor, while uncontrolled rectifiers with passive filters typically achieve power factor above 0.9.

Unbalance in Three-Phase Systems

Single-phase AC traction systems draw power from one phase of the three-phase grid, creating load unbalance. This unbalance causes negative-sequence currents that can overheat generators and motors connected to the grid.

Substation transformer connections can reduce the unbalance seen by the grid. Scott connections, Le Blanc connections, and other arrangements distribute the single-phase traction load across the three-phase supply in a way that minimizes unbalance.

Power Transformer Effects

Traction substations include power transformers that step down the utility voltage to the traction supply voltage. These transformers generate power-frequency magnetic fields that can affect nearby equipment and must be considered in substation siting.

Transformer saturation during energization or under fault conditions creates harmonic currents that can propagate into both the utility supply and the traction network. Controlled switching and pre-insertion resistors can reduce inrush currents and associated harmonics.

Rectifier Emissions

DC traction substations use rectifiers to convert AC supply to DC for the traction system. These rectifiers generate harmonics on the AC supply side and ripple on the DC output.

Twelve-pulse rectification, using two six-pulse bridges with a 30-degree phase shift, cancels the 5th and 7th harmonics, reducing total harmonic distortion. Further pulse multiplication can reduce harmonics further but adds complexity and cost.

DC output filtering reduces ripple and high-frequency emissions on the traction supply. Inductors limit the ripple current, while capacitors provide a low-impedance path for high-frequency currents.

Equipment Room Protection

Substation control and protection equipment must operate reliably despite the electromagnetic environment created by the power equipment. Equipment rooms are typically located away from the main power equipment and may include shielding.

Cable entries are protected with filters and surge protection to prevent disturbances from propagating into the equipment room. Power supplies for control equipment include isolation and filtering to provide clean power despite supply disturbances.

Switching Transients

Opening and closing circuit breakers and disconnectors at sectioning posts creates voltage and current transients that propagate along the traction supply. These transients can affect equipment on trains and at trackside installations.

The rate of change of voltage during switching depends on the circuit parameters and the switching device characteristics. Modern vacuum or SF6 circuit breakers can create very fast transients that require surge protection at sensitive equipment.

Auto-Reclosing

Automatic reclosing after fault clearance subjects the network to repeated switching events. The timing and sequence of reclosing must be coordinated with protection on connected equipment to ensure that equipment can withstand the resulting transients.

Passage Through Neutral Sections

As trains pass through neutral sections, pantographs must leave and rejoin the energized catenary without drawing current. If current flows during the transition, arcing occurs, generating broadband impulse emissions and potentially damaging the catenary and pantograph.

Automatic neutral section passage systems on modern trains open the main circuit breaker before entering the neutral section and close it after exiting. The timing must be precisely coordinated with train position to minimize any arcing during the transitions.

Phase Separation

On AC systems, neutral sections often separate phases fed from different points on the three-phase grid. Connecting across phases would create a short circuit, making reliable detection and breaker control essential.

The transient disturbances during neutral section passage include the current interruption when leaving the first energized section and the voltage transient when connecting to the next section. Equipment on the train must withstand these transients without malfunction.

DC System Stray Currents

DC traction systems are particularly susceptible to stray currents because the direct current can flow through any conductive path to return to the substation. Underground pipelines, cable sheaths, and reinforcing steel in structures can all carry stray currents.

Stray currents cause corrosion where they leave a metallic structure to enter the soil. The corrosion rate is proportional to the current flow, making stray current control essential for protecting underground infrastructure.

Control measures include maintaining low rail-to-earth voltage to minimize the driving voltage for stray currents, insulating the rails from earth where practical, and providing adequate return current capacity so that the rail return path has low resistance compared to stray current paths.

AC System Induced Currents

AC traction systems create induced currents in parallel conductors through magnetic coupling rather than direct leakage. These induced currents flow in nearby cables, pipelines, and fences parallel to the railway.

The induced voltage is proportional to the traction current, the length of parallelism, and the separation. Long parallel runs close to the track can develop significant induced voltages that require mitigation for personnel safety and equipment protection.

Mitigation measures include maintaining separation between the railway and parallel infrastructure, using earthing to limit touch voltages, and installing drain cables to carry induced currents safely.

Rail Return Systems

In conventional rail return systems, the running rails carry traction return current back to the substation. Cross-bonding cables ensure current can distribute between the two rails, reducing resistance and balancing current flow.

The magnetic field from current in the rails cancels partially due to the current flowing in opposite directions in the two rails and in the catenary. However, perfect cancellation does not occur because the current paths are not identical, resulting in net magnetic field that decreases with distance from the track.

At signaling frequencies, impedance bonds at track circuit boundaries affect the current distribution. The design of the bonding system must balance traction return requirements with signaling system needs.

Dedicated Return Conductors

Some systems use dedicated return conductors alongside the track, separate from the running rails. This approach reduces the current in the rails, improving track circuit performance and reducing magnetic fields from rail current.

The return conductor may be a cable, a rail-like conductor, or the messenger wire supporting the catenary. The choice affects the field distribution around the railway and the inductance of the traction circuit.

Booster Transformers

Booster transformer systems force the return current to flow in a return conductor close to the catenary rather than through the rails. This reduces the magnetic field at ground level and can reduce interference with trackside systems.

The booster transformer primary is in series with the catenary, and the secondary is in series with the return conductor. Load current flowing in the catenary induces an equal current in the return conductor, confined to the return path.

Autotransformer Systems

Autotransformer (AT) feeding systems use a feeder wire at twice the catenary voltage, with autotransformers at regular intervals providing the catenary voltage. This system reduces losses and allows longer substation spacing.

In AT systems, the current distribution between catenary and feeder depends on train position relative to the autotransformers. The changing current distribution creates a complex magnetic field pattern that varies as trains move along the route.