Static Var Generator (SVG)
The principle of the SVG static var generator is very similar to that of Active Power Filter, as demonstrated in the picture below. When the load is generating inductive or capacitive current, it makes load current lagging or leading the voltage. SVG detects the phase angle difference and generates leading or lagging current into the grid, making the phase angle of current almost the same as that of voltage on the transformer side, which means fundamental power factor is unit.
Delta’s PQC series SVG is also capable of correcting load imbalance.
Delta PQC Series SVG static var generator is also modular in design, and the Delta SVG system consists of one or several SVG modules and an optional Liquid Crystal Monitor & Control Panel (LCM). Each SVG module is an independent reactive power compensation system, and users can change the SVG rating by adding or removing SVG modules.
SVG modules and LCM panel can be embedded in Delta’s standard SVG cabinet or in a customized cabinet. There are usually breakers, cable terminals and Surge Protection Device (SPD) in the SVG cabinet.
Active harmonic filters are more of a system solution. How do they work?
Harmonics come from the nonlinear load. Active harmonic filter, also called harmonic correction units, is parallel devices that act like a noise cancellation system and inject equal and opposite frequencies to mitigate harmonics. The filters can also provide additional current to correct the power factor. So, what’s left coming from the source flowing back to the utility is only a nice, clean current that is in phase.
For example, if we run four 6-pulse variable frequency drives at the same time, we have a harmonic spectrum of 5th and 7th and 11th and 13th harmonics. The waveform will show a significant amount of harmonic current in the total harmonic distortion. When the active harmonic filter is turned on, it will inject equal and opposite harmonics to cancel what’s there. The waveform now will be clean and in phase. If we go back and look at the harmonic spectrum, the current distortion is very low.
Adding active harmonic filters can be a good harmonic solution for power systems. Though more costly than other options, if you have multiple drives running all the time and multiple drives as backup, harmonic filters would be a reliable way to capture any and all of the harmonics coming from the loads. However, one important thing to know from a system design standpoint is that your drive should have either a DC choke or an AC line reactor to minimize the harmonics coming out of the drives.
Active harmonic filters typically come in 50, 60, 100, 200 and 300 ampere units that you can parallel. Another benefit to using active harmonic filters is that you cannot overload them, because once they put out the maximum harmonic current and power factor correction that they can, they stop producing at that level—whether it’s 100 or 120 amperes, whatever the case may be.
Using active harmonic filters on your power system provides a system-level solution for internal and external harmonic protection.
What are active harmonic filters?
Active harmonic filters are parallel filters (which means the current doesn’t go through the filter) that are used to reduce, or mitigate, harmonics to tolerable levels as defined by IEEE-519. Active filters use a set of transistors and capacitors to filter (or clean) the current wave by injecting inverse currents to cancel out the undesired harmonic components. Active filters are significantly more expensive than passive filters and take up more space. Size is an immense factor in system design today and should be accounted for when deciding on what type of harmonic filter is right for you.
Active filters can work with multiple drives; when the active filter reaches its limit, it won’t overload. In addition, if an active filter breaks, it won’t stop the motor (since current isn’t going through the filter); it just won’t filter the current wave.
Active harmonic filters can effectively cancel harmonic distortions from the network. This blog post will explain the key criteria that should be kept in mind when buying an active harmonic filter.
1.1. Inverter topology
Most modern AHFs are built on 3-level NPC inverter topology which brings several benefits compared to AHFs built on the conventional 2-level topology. In 3-level topology, the switching frequency and voltage stress are distributed among the IGBTs. Reduced stress extends the lifetime of the power electronics. Higher efficiency, lower losses and lower noise levels are also achieved. These make the overall cost of ownership much lower.
Depending on design and topology, AHFs can have higher or lower losses. Checking the losses is important as they will reduce the life cycle cost of the investment. Usually AHFs have about 2-3% losses (depending on rated power). AHFs built on 3-level NPC inverter topology have lower losses than 2-level ones. Depending on the user profile, reduced losses create a potential for considerable financial savings if the LCC is calculated over a period of few years.
1.3. Response time
Some power quality phenomena occur extremely fast, requiring the mitigation to be even faster. If the process is affected by fast voltage fluctuations or transients, it is very important to evaluate the AHF’s overall response time.
Interharmonics are usually caused by synchronization issues. If the installation includes interharmonic sources, the manufacturer should be consulted as not all AHFs can deal with this. It is a common issue with cycloconverters or some types of older wind turbine generators.
1.5. Harmonic compensation capacity
Harmonics can be seen in the odd and even orders. Common capacity for AHFs is 25th or 50th harmonic order. Sometimes there is a claim of being able to mitigate the 51st harmonic, which has little value as harmonic orders of 51st and above do not appear in electric power systems. An important issue is that the AHF can offer the possibility of selecting which harmonic order to compensate. For some devices, it is possible to select the whole harmonic spectrum (1st to 50th, odd and even), but for some others only few harmonic orders can be selected. Depending on the application, the capacity to compensate a certain harmonic order is a critical issue affecting the performance of the whole system.
1.6. Electromagnetic compatibility (EMC)
In some countries there are strict guidelines regarding the EMC. To be sure that the AHF is not causing interference it must be fitted with a properly designed EMC filter.
1.7. Derating according to harmonic order
The rating of an AHF is usually defined at nominal load (at 50/60Hz). As the AHF works further up the harmonics, its capacity compared to nominal starts to derate. For example, a derating of 50% at the 13th harmonic order means that a 100A AHF has only the capability to compensate 50A at the 13th harmonic order.
Derating is a matter of how robustly the AHF is designed. This capability is more dependent on the change rate of the current than just the frequency and magnitude of the current (all different frequencies, their magnitude and phase have an effect). Because of this, a derating curve cannot show the capability of the AHF. The only way to verify the real compensating capability of a device is to check its di/dt capacity. This compensating capability is clearly better in 3-level NPC inverter topology AHFs compared to 2-level topology devices.
1.8. Physical footprint and modularity
Most suppliers offer several installation alternatives: Cubicle type, wall mount or loose modules that can be locally installed inside new or existing cubicles. A modular AHF design allows customers to adapt to potential changes in future harmonic filtering and reactive power compensation needs. Modular design means that it is possible to add easily to the AHF’s capacity within the existing configuration, saving both costs and space.
AHFs are offered in a range of voltages, most common is 200V up to 690V. Some manufacturers can produce AHFs for higher voltages, up to 1000V, without step-up transformer, reducing costs and footprint. It is possible to connect AHFs to high voltage (over 1kV) systems using a suitable step-up transformer. Step-up (or step-down) transformers can reduce compensation performance due to increased impedance between the AHF and network.
There are different HMI setups for AHFs. Some have a very simple front HMI while others include graphs showing the current and voltage waveforms and many further functions in different languages. A great added value is to have at least a web-based interface allowing in-depth monitoring and control functionality.
1.11. Commissioning software
Commissioning and service of AHFs without proper tools can be time consuming. Some suppliers provide software for this. Minimum required functionality should be that the system performs a self-check of voltage and CT phase order, CT polarity check, self-diagnosis and self-calibration. Such features will find installation errors before they can cause problems and will shorten the commissioning time. If the AHF does not have this type of software the commissioning becomes more complex and might require external support adding to the system costs.
1.12. Smart Grid functionality
Some AHFs have a built-in power quality analyser to calculate the required compensation. Some suppliers enable the user to connect all AHFs on site through a web-based architecture. An operator can then have an overview of the status of all connected AHFs and log them. This enables the possibility to log events that could have caused production disturbances, monitor individual AHFs, and remote monitoring and analysis capability.
1.13. Control of detuned capacitor banks
Very often AHFs are installed at sites together with existing or new contactor or thyristor switched detuned capacitor banks. Some suppliers offer the possibility to control the steps of these banks directly from the AHF’s control system through dedicated digital outputs in the AHF. By doing this it is possible to use the comprehensive power quality monitoring and reporting features of AHFs to accurately monitor all the parameters of the installation and control the total power quality improvement needs. This brings an optimal system integration, efficient operation, cost savings on the control system, and the possibility to build hybrid var compensators (HVC).
1.14. Cancellation of harmonics in neutral (4W)
Typically, active harmonic filters are installed to cancel harmonics distortions created by AC and DC drives or UPS systems. However, use of LED lighting as well as other single-phase loads in the buildings also produces tripplen harmonics which accumulate in the neutral wire and should also be mitigated. Therefore, active harmonic filter should be easily configurable to 3W or 4W applications.
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