Beatrice Lang
Inverters are a major source of electromagnetic interference (EMI) and radio frequency interference (RFI). This is due to the nature of their function, which involves converting input AC power to DC voltage and then to pulse voltages. The pulse-shaped load voltage and current drawn from the power grid contain high-frequency components that cause electromagnetic wave radiation and subsequent interference. This interference can affect electronic devices and circuits, causing issues with their operation. While it is challenging to eliminate inverter interference entirely, there are several techniques to minimise its impact. These include soft-switching, grounding, filtering, and shielding methods.
Techniques to prevent electrical interference from inverters
| Characteristics | Values |
|---|---|
| Soft-switching | A method that can reduce the EMI generated in inverters |
| Guard rings in a PCB | Can assist in EMI reduction |
| Power integrity and signal integrity design choices | Only work if they keep noise within the CMOS noise margin |
| Grounding | The grounding wire should be designed with the right impedance and arrangement |
| Filtering | Prevents noise from propagating further along the wires |
| Shielding | Prevents electromagnetic interference from leaking out |
| Isolation | Disconnects the transmission of electromagnetic noise |
| Distance from other devices | Interference becomes more severe when other devices are close to the inverter |
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What You'll Learn
Soft-switching
Since inverters utilize switching devices, they are a major source of EMI. Soft-switching is a method that can be used to mitigate this interference. In soft-switching, the power switch in an inverter changes its state (from turn-on to turn-off or vice versa) when the voltage across it and/or the current through it is zero. This is in contrast to hard-switching, which continues to generate EMI even when EMI filters are used.
There are two types of soft-switching: zero voltage switching (ZVS) and zero current switching (ZCS). In ZVS, the switch is turned on or off when the voltage across it is zero. In ZCS, the switching state is changed only when the current through the switch is zero. Soft-switching enables the inverter to operate at higher frequencies, reducing the size of the inverter's dc-link capacitors and associated inductive components.
The use of soft-switching in inverters can also enhance efficiency by reducing switching losses. This is because soft-switching eliminates the overlap of current and voltage transitions during each switching cycle, reducing or eliminating voltage or current overlap. This can lead to a significant reduction in switching losses, improving the overall efficiency of the inverter.
In addition to reducing EMI and improving efficiency, soft-switching can also be advantageous in terms of the design of the inverter. Soft-switching allows for the use of smaller auxiliary switches and resonant inductors compared to the main inverter circuit components. This can result in a more compact and lightweight inverter design, which is beneficial in various applications, especially in electric vehicles where size and weight are important factors.
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Grounding
Firstly, it is important to understand that grounding the inverter chassis or metal cart is generally recommended. This is because, in the event of a fault, the current will flow directly to the ground, tripping the unit and preventing it from becoming a live metal box, which could be extremely dangerous.
When it comes to solar inverters, proper grounding is crucial to prevent lightning strikes and static charges from damaging the system. The negative battery post and metal chassis parts should be grounded to the "safety ground" to protect the equipment and people. Additionally, grounding is necessary to discharge static electricity and ensure the system's electromagnetic compatibility.
To achieve proper grounding, all electrical components should be grounded at the same location, known as single-point grounding. This is important to prevent voltage differences during a lightning strike, which could cause the inverter to fail.
Furthermore, it is recommended to ground the DC battery negative to earth ground and run a ground wire to the inverter chassis ground stud. This helps prevent shock hazards by ensuring that any metal parts do not become "hot" if they come into contact with an AC or hot/neutral wire.
In some cases, a Faraday shield may be constructed around the inverter to keep interference inside. This can be made of ferrous or non-ferrous metal and should be connected to its own RFI" earth ground. Additionally, a bond wire should be added from the RFI ground to the system's protective earth.
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Filtering
There are various types of EMI filters available, including common-mode filters and differential-mode filters. Common-mode filters are placed on the AC output of the inverter and work by suppressing differential-mode noise and passing common-mode noise through capacitors and inductors. Differential-mode filters, on the other hand, are placed on the DC input of the inverter to reduce noise between two lines, such as the positive and negative lines in a power supply.
In addition to common-mode and differential-mode filters, other filtering techniques can be employed. For example, ferrite beads can be placed on the DC and AC cables to absorb EMI and provide additional EMI suppression. Toroid cores can also be used, although they can be heavy and require careful installation away from the battery end to avoid creating an antenna effect.
When designing EMI filters, it is important to consider the specific voltages, currents, and frequencies involved. For example, the type of ferrite used should be selected based on the frequency range in which it will be effective. LC low-pass filters can be designed to filter out specific frequencies, but the load must be considered as it can impact the filtering response.
Overall, filtering is a critical technique to reduce EMI in inverters, and the appropriate type of filter and design considerations will depend on the specific application and requirements.
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Shielding
In addition to shielding the inverter itself, interference can be reduced by shielding the transmission of electromagnetic noise. This can be achieved through isolation methods, such as relays, isolation transformers, or optoelectronic isolators.
Another method of shielding is to construct a screen around the entire inverter, which is then connected to the ground. This creates a Faraday shield, which keeps interference inside. This technique is similar to how reinforcing steel bars in parking garages block radio signals from reaching a vehicle's antenna.
Nearly all equipment used in PV systems is digital, and almost any metal will offer some shielding from digital noise. Metal enclosures are common for inverters and other equipment, and metal conduit will also act as a shield.
Federal Communication Commission regulations require enclosures to shield the EMI made by inverters and other electronics to certain attenuation levels.
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Isolation
To achieve effective isolation, several methods can be employed:
- Grounding: It is recommended to ground the inverter power supply at a single point, preferably using a short and thick wire. Grounding helps discharge static electricity and can prevent interference from spreading to other equipment.
- Filtering: EMI filters are commonly used on both the input and output sides of inverters to minimise electromagnetic interference (EMI). These filters can include X and Y capacitors and common mode inductors, which help to disconnect the electromagnetic noise propagation path.
- Shielding: Shielding can be used to stop fields in the air and prevent noise propagation. For example, shielded wire can be used to prevent noise from HVAC systems.
- Soft-switching: This technique reduces the generation of EMI by controlling switching devices such as BJTs, MOSFETs, or IGBTs, resulting in a more sinusoidal waveform.
- Power Isolation: In some cases, power isolation may be necessary to prevent interference. For example, in the case of HAM radio operators, ensuring clean power can help mitigate interference issues.
While it is challenging to completely eliminate interference from inverters, these isolation techniques can significantly reduce their impact on nearby equipment and systems.
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Frequently asked questions
There are a few methods to prevent electrical interference from inverters, which are a major source of electromagnetic interference (EMI). Firstly, soft-switching is a method that can reduce the EMI generated in inverters. You can also use grounding, filtering, and shielding techniques to prevent interference.
Soft-switching is a method of controlling switching devices such as BJTs, MOSFETs, or IGBTs to obtain the desired output voltage. By using soft-switching, you can reduce the electromagnetic interference generated by the inverter.
Grounding involves connecting the inverter to the ground using a thick and short wire. This helps discharge static electricity and redirect electromagnetic interference away from sensitive equipment.
Shielding involves creating a barrier around the inverter to prevent electromagnetic interference from escaping or entering. This can be done by using conductive materials such as metal shells or housings to block the electromagnetic waves.
