The smart grid is projected to rely heavily on the penetration of renewable energy sources (RESs) such as solar, wind, photovoltaic, fuel cell, micro-turbine to facilitate distributed generation which will later become the cornerstone of the smart grid system. These RESs compel the increased use of power conversion devices which incidentally depend on the application of power electronic interfaces notable for inherent electromagnetic disturbances. The application of power electronic interfaces in the smart grid generally results in the flow of electromagnetic interference (EMI) currents with attendant issues ranging from interference propagation to interference aggregation and then to system performance degradation . The EMI issues within smart grid automated substations have been reported to depend on the magnitude of fault currents, their flowing paths and harmonic contents.
EMC and Power Converters
Undesired voltage or current is called interference and their cause is called interference source. The high-speed converters are the source of interference. Interference propagated by radiation in area around of an interference source or by conduction through common cabling or wiring connections. In this study power electronics conducted emission is considered only. Equipment such as computers, receivers, amplifiers, industrial controllers, etc that are exposed to interference corruption are called victims. The common connections of elements, source lines and cabling provide paths for conducted noise or interference. Electromagnetic conducted interference has two components as differential mode and common mode
The power converters are used to connect the renewable energy resources, storage devices and the grid together electrically. In other words, the power electronic interface receives power from the distributed energy source and converts it to power at the required voltage and frequency. The AC-DC converters are used to convert AC output to DC, the DC-DC converters convert DC-DC (change voltage level) and DC-AC converters convert DC source to grid-compatible AC power. Broadband EMI with operating frequency up to several MHz are generated by electronic converters and are coupled between circuit elements by magnetic or electric field. Rectifiers and inverters are some of the types of electronic converters with high frequency generated input current harmonics and output voltage related interference which may affect the normal operation of communication and control systems near inverters. When circuit space becomes important, the size of converters and EMI filters can be reduced by simply increasing there switching frequency. However, this high switching frequency of converters promote the generation of electromagnetic interference. Modeling and analysis of EMI problems relating to DC-DC converters and a design example of EMI filters used to mitigate conducted EMI.
High frequency converters incorporating power electronics interface is a verifiable source of EMI. An example is a 4-quadrant frequency converter consisting of two insulated gate bipolar transistor (IGBT) based three-phase bridges and an intermediate two-way circuit using four-quadrant operation. The conducted electromagnetic emissions at the frequency range 9kH – 30MHz result from the switching of IGBT devices with characteristic high dv/dt causing significant EMI. There is also EMI penetration into Low Voltage and Medium Voltage electric grid. The radiated EMI is another significant type of the EMI disturbances and it is often modeled as an antenna problem where the ac-dc bus and metal cabinet in the converters (for instance) present different impedance characteristics under different operating conditions.
The power converters are constructed with power electronic devices with EMI issues that create harsh electromagnetic environment that devices must endure. The generated EMIs are conducted through coupling paths over other circuitry to accumulate in some cases in certain areas of circuits. Power switching events cause conduction/switching losses, controller losses, charging and discharging stray losses. Switching losses depend on the switching frequency and may contribute significantly to EMI noise. The EMI noise increases with the switching frequency. So for higher frequency applications, the EMI is particularly significant. The EMC enables equipment to perform satisfactorily in its electromagnetic environment without introduction of new electromagnetic disturbance above the required limits in accordance with International Electrotechnical Commission standards (IEC). Thus, a high EMC requirement is key to the success of the smart grid.
EMC and Pulse Width Modulation
The random pulse width modulation (RPWM) technique and its hybrids have been applied in to spread the EMI noise spectrum produced at switching frequency and its harmonics and consequently reducing the EMI generated by active pulse width modulation control filter used in switch mode converters. In case of randomised pulse-position modulation (RPPM), RPWM, and randomised carrier-frequency modulation with fixed and variable duty cycles are focusing on analytical derivations for discrete and spectra switching signal of dc-dc/two-level converter waveform modulated using different probability density functions. A comparative analysis of periodic carrier-frequency modulation (PCFM) and random carrier-frequency modulation (RCFM) techniques, the analysis, the RCFM scheme gives higher high-frequency suppression than the PCFM scheme at the same frequency but introduces higher low-frequency output harmonics into the converter.
EMC and Heat Sink
In power electronics, wide band gap materials enable faster transients and higher switching frequencies. Therefore, more efficient power electronic systems can be realised, but also greater EMC problems in magnitude and frequency arise. Heat sinks, which are often necessary for cooling power semiconductors, are a source of EMC issues. In power electronic applications a cooling concept is often required for the semiconductors due to the occurring losses. Typically, heat sinks are made of aluminium and their conductive behaviour can cause EMC problems even if an electrical isolation is used. Voltage transients are transferred from power semiconductor to the heat sink via capacitive coupling. For safety reasons, heat sinks are typically grounded so that common mode current arises due to the current flowing through the ground connection. Furthermore, radiated interferences appear because the aluminium heat sink acts as an antenna.
EMC and FACTS devices
Another potential source of electromagnetic concerns in the smart grid is the envisaged increased deployment of flexible ac transmissions system (FACTS) used for AC transmission systems performance and efficiency enhancement. FACTS technology is generally based on power electronics used in the past in remote places for transmission network control. Currently, it is envisioned that smarter, smaller sized and cheaper FACTS controllers will find increased deployment in distribution networks in the near future. However, the operation of FACTS switches has been reported to inherently lead to the generation of electromagnetic interference noise with far-reaching effects on nearby electronic devices which are required to be well immune from EMI disturbances for reliable performance.
FACTS devices generate a high frequency steady state electromagnetic noise due to continuous transient switching of power electronic components such as thyristor valve. FACTS devices consist of components such as DC-AC inverter, interfacing inductor banks, harmonic blocking transformer, step-up transformer, associated control hardware and software. These are the culprit FACTS devices that are responsible for high electromagnetic disturbances that make the operation of FACTS devices worrisome despite their important roles in renewable energy integration and control. Furthermore, FACTS devices may be used for reactive power compensation in solar power plant to facilitate voltage stability and the flow of active power in the power system.
Fundamentally, FACTS controllers are based on controlled switching power-electronic devices used for power conversion and compensation. For high flexibility and wide substation load variation tolerance, new FACTS-based equipment is finding increasing applications in bi-directional transmission system’s reactive power generation and absorption control. Coupled with electromagnetic disturbances caused by the switching frequency of FACTS devices, the massive deployment of power electronic devices makes the EMI more disturbing. In general, FACTS equipment-based EMI depends on the equipment layout parameters, thyristor valve switching characteristics and stray parameters associated with the thyristor-controlled reactor (TCR), thyristor-switched capacitor (TSC), and the static synchronous compensator(SSC).