Low Voltage Active Filter as a Medium Voltage STATCOM for Windpower

Fortum Kville Power station

Kville power station compenstated by Comsys STATCOM design.

When the 32 MW Kville wind power station was being built in Sweden, the local grid owner Fortum was looking for alternatives for inductive compensation.  The long underground cable length cause a dynamic capacitive reactive power problem that normally is compensated using a large MV inductor. The inductor is very large and costly device at these sizes and Fortum wished to investigate other alternatives.

Comsys used its extensive knowledge from MV applications in applying its liquid cooled low voltage Active Filter with a step up transformer to create a 2,5 MVA STATCOM solution to solve the problem. If applied correctly, an active technology is very compact and flexible enabling high availability. Comsys liquid cooled modular design offers a high degree of redundancy and availability as the modules can be operated individually.

A further complication was the requirement to measure at the PCC on the 130 kV level so the Low Voltage ADF STATCOM worked through two step-up transformers. After extensive simulations by Comsys, the system was designed and supplied through the turn key integrator Siemens.

Comsys modular design AHF

Comsys P700 modular STATCOM solution

The active filters where installed in an existing building and the step-up transformer was installed outside, saving valuable indoor space and requiring no additional transformer cooling.

The solution dynamically compensates the capacitive reactive power and keeps it in line with the utility’s requirement. Due to the STATCOM following the load dynamically and observing both voltage and current, optimal grid conditions are ensured during all operating conditions.

The investment cost was reported to be lower than using the customized inductor solution proving the competitiveness of small active STATCOM versus passive options.

The ADF P700 STATCOM is a perfect solution in a dynamic environment such as wind farms. It is as cost effective and compact as a passive solution but with superior performance.

AHF Compensates Thruster and Refrigeration Compressor

Danfoss used an Active Harmonic Filter to compensate the THD of their installed 960 kW of VFDs for thruster and refrigeration compressor on the fishing vessel Gitte Henning #8. The AHF ensured to keep the installation within class requirements. Read more at:

Danfoss VFDs on Gitte Henning #8

Active Harmonic Filter as a Tool to Save Money

Power quality mitigation products are not only used to fulfil regulations such as IEEE-519 and G5/4. They have actual effects saving both power and increasing productivity. This presentation by ABB exemplifies some of their early case studies indicating savings of up to 10%. Note that these savings compare to having no harmonic mitigation.

The cases below clearly show the great business value of implementing a high power quality standard within your facility.  The secondary effects of complying with IEEE 519 or similar standards enables the local grid to be dimensioned for less reactive power and harmonic current thus saving money through thinner cabling, smaller transformers etc. Power quality mitigation is not only a cost in the general investment calculation but a tool to save money.

ABB presentation: PQF-Energy_Savings

IEEE 519 and Active Harmonic Filters

Electric Grid

Electric Grid (Photo credit: Wikipedia)

Standards governing distiortion parameters in the electric grid such as IEEE 519, G5/4, EN 61000, EN 50160 and D-A-CH-CZ among others most often require voltage harmonic distortion to be below 5-8%. These are all recommended, not obligatory practices.

Although adherence to standards such as IEEE 519 is not obligatory, more and more utilities and other parties of interest are using these standards as a benchmark to place demands on their customers. This is a way for them to be able to guarantee disturbance free delivery on their end. It is also used as a part of an active environmental agenda to show a decreased energy usage and reduced energy costs for many energy intensive processes.

One way to meet the new requirements is to simply reduce the harmonics to an acceptable level. Many modern active harmonic filters can pinpoint the harmonic orders that are contributing, and the compensation power can be optimized to meet the requirements in the most cost efficient way.

The modern Active Harmonic Filter is one of the most efficient harmonic solutions the market today. Filters are commonly available in a 208 – 480V version and a 480 – 690V. The Active Harmonic Filter can be combined with 6 pulse drives and will be placed in parallel with the load, minimizing the need of compensation power to 20 – 30 % of the load. The parallel placement will also ensure the redundancy in the design, which is a major advantage in a critical applications. Modular solutions, which are now more commonly available gives a dynamic and agile solution to work for future improvements to existing machinery. This is all in keeping with the spirit of standards such as the IEEE 519 toward a sustainable energy future.

Low Harmonic Drive Optimization

Earlier on the blog, we have defined the Active Harmonic Filter (AHF) as the best low harmonic choice for variable speed drives. A common question often put is whether the drive has to be fitted with a line filter as well.

In theory a well designed Active Filter can compensate a drive without a choke but this may not be the optimal solution. This would create the need for a bigger Active Filter, and the drives rectifier would be stressed by the compensation power from the filter. This in turn would reduce the drive’s life span.

A very common solution is to install a 2% or 5% choke on the drive. Many high quality drives have such chokes fitted as standard. Fitting a choke reduces the harmonics from 85-100% to about 35-40% which is a very cost effective solution. The remaining, and significantly lower THD, means the size of the final and more expensive filtering is much smaller.

The choke will also dampen the compensation power affecting the drive. Leaving out the choke, the drive’s rectifier will degrade over time.

So the answer is yes, a Low Harmonic Drive system using Active Harmonic Filters will benefit from using a choke to complement. It gives the overall system a lower cost and higher availability.

Active Harmonic Filters – Buyer’s Guide

Active Harmonic Filters are growing in popularity as a method to mitigate power quality issues. There are several factors to consider when specifying an active harmonic filter. Typical applications for active filters are compensation of variable frequency drives and data-centers to reduce the load on UPS systems or compensating the effects of renewable energy sources on the grid.Active Harmonic Filter

What is an Active Harmonic Filter and what is its Application.

The general definition to describe this application is an analog or digital device that measures the power quality on the grid side. It then injects current to compensate any unwanted deviations from the standard 50 or 60 Hz supply. Deviations can be mitigated in full or partially.

What Factors to Consider when Specifying an Active Harmonic Filter

Losses

Depending on design, the filter has higher or lower losses. Check the losses as this will reduce the Life Cycle Cost on your investment. Some active filters have up to 1%-point lower losses, which depending on your user profile, means a potential for  considerable financial savings if calculated LCC over 5 years.

Harmonic Compensation Capacity

Harmonics are normally seen in the odd. Common capacity for active filters is 25th or 50th harmonic. Sometimes there is a claim of being able to mitigate the 51st harmonic, which has little value as harmonic order of 51 and above are normally not important.

Harmonics above the 50th are more difficult to measure as there are few PQ-meters that can handle such orders. There are however quite common sources such as Active Front End Drives that cause switching ripple above 3kHz, above the 60th harmonic (or above the 50th in 60 Hz systems).

There are a few Active Harmonic Filters capable of compensating such frequencies. Choose a filter according to the needs specified by your measurements.

A filter’s capacity to compensate a certain harmonic order is only part of the story. Another important factor is de-rating, discussed below.

Reponse Speed

Some power quality phenomena occur extremely fast requiring the mitigation to be even faster.  If your process is affected by fast flicker or transients, take special care to evaluate the response capacity of the filter. Flicker is a specific phenomena that normally requires special software to compensate flicker in a controlled environment.

Interharmonics

Interharmonics is commonly caused by syncronisation issues. If  your installation includes such interharmonic sources, the type of active filter changes and the vendor has to be consulted. This is a common issue on some types of older wind turbines.

EMC

In Europe there are strict guidelines regarding EMC. If you want to be sure that the active filter does not interfere, the filter must be fitted with a properly tuned EMC-filter.

De-rating

An Active Harmonic Filter’s rating is normally defined at nominal load, meaning at 50/60Hz. As the filter works further up the harmonics its capacity compared to nominal starts to de-rate. The de-rating curve is documented by all serious suppliers and should be available if you ask them.

A de-rating of 50%, at say the 13th harmonic, means that a 100A filter only has the capacity to compensate 50A at the 13th.  Naturally if you have harmonics of higher order it becomes more important to check the de-rating.

De-rating is a matter of how robustly the filter is designed. Some suppliers offer zero de-rating up to the 7th before capacity starts to fall.

Physical Footprint – How Much Cabinet Space is Required?

Most active filter suppliers offer several alternatives regarding installation. Wall mount, Cabinet and IP00 modules to install in cabinets. Efficient use of cabinet space translates to lower system cost. Some filters have a modular design and can be enhanced with further capacity without adding to the footprint.

Modularity

As mentioned, a modular design of your Active Harmonic Filter enables you to adapt the filter to potential changes in your future power compensation needs. The modular design means that you can easily add to the filter’s capacity within the existing cabinet, saving both cost and space.

Commissioning Software

Does the filter have built in commissioning software?  Commissioning and service of Active Filters can be quite time consuming. Ask for a review of the support software included in the machine. Some suppliers have an extra charge for the necessary software. 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 quickly find installation errors before they can cause problems and will also shorten the needed commissioning time.

If the filter does not have this type of support software the commissioning becomes much more complex and might even require external support adding to the system cost.

HMI

There are different HMI setups. Some have a very simple front HMI while others include graphs showing the current and voltage waveforms and many further functions. A great added value is to have at least a web-based interface allowing in-depth monitoring and control functionality. Then no extra PQ-meter is necessary.

Smart Grid Functionality

Active filters have a built in rudimentary power and power quality meter to calculate the required compensation. Some filter manufacturers make use of this fact and enable the user to connect all filters on site and company wide through a web based architecture.  An operator can then have an overview of the status of all connected cabinets and log them. This enables the possibility to log events that could or should have caused production disturbances, status monitoring of individual filters as well as remote control capability.  Email and text alerts to dedicated service personnel from the filter reduce response time dramatically.

IP/NEMA Class and Water Cooling

Water-cooled Active Filters enable very good cooling of the IGBTs, the most critical component in the Active Filter. Water-cooling reduces overheating immensely, which increases availability in the same way as for Variable Frequency Drives. The power density of the installation is also improved.

Voltage

Active Harmonic Filters are offered in a range of voltages.  Most common ranges are 380–415V, up to 480 V. Higher voltages up to 600 and 690V are also available without step-up transformer, reducing foot print. Some suppliers have the capacity to supply MV ratings as well, normally using a step-up transformer.

Power Losses and Low Harmonic Drives

Using the active filter in shunt applications has a lot of energy saving potential compared to using serial filters – either passive or active front end.

Here are some examples and what they mean to you as user. When seen as a system, the active filter in shunt mode offers a total system loss that is lower than that of the passive filter.

Passive Harmonic Filters

A passive filter has between 0.6-1.5% losses

Assuming a 6-pulse drive has 2% losses, the total system loss is the sum of the losses

Pdrive*PFilter = 2% + (1.5 <-> 0.6)%  =>  3.5% to 2.6% total system loss.

NOTE! This does not include an eventual voltage drop through the passive filter and its effect on the motor’s losses.

Serial Active Filter – Front End

An active front end drive essentially has twice the loss of a standard drive as the power has to pass through two IGBTs.

Pafe = 2% + 2% + 1% for the LCL-filter = 5% losses. Total system losses observed in documentation are 4.7-5%.

Shunt Active Filter

Unlike the serial solutions the shunt active filter only has to be sized according to the harmonic currents to be filtered out. Under normal conditions this means that in a IEEE-519 or G5/4 application the filter has to be sized 15-30% of the 6-pulse load.  This means the total system loss is also much lower even though the efficiency of the Active Filter is:

Pdrive + Padf = 0.02 + 0.02* (0,15 – 0,3) = 2.3 – 2.6 % in total system losses.

Summary from a System Owner’s View

Shunt Active Harmonic filters offer between 0 and 1.17% points lower power consumption compared to Passive Harmonic Filters.  This does not include any effect from voltage drop through the serial passive filter.

Shunt Active Harmonic Filters offer between 2.7 – 2.4 % lower power consumption compared to Active Front End drives.

Power Losses are a Significant Part of Your Life Cycle Cost Calculation

Minimising losses over time, especially in industrial process loads with more than 8000 hours of yearly operation, 1%-point saving in power consumption translates into significant value.

(Pdrive + Pcooling) (kW)* Yearly operation hours(h)*Net Losses(%) = Total cost saving potential

Energy Cost Estimate

Electrical power prices differ but the relation between cooling and electricity is roughly equivalent to

Pcooling = 0.3 * Pdrive

In the case of the AFE there are cases where the entire harmonic mitigation solution has been paid off in 2.5 years simply by choosing shunt Active Harmonic Filters instead of Active Front End thanks to lower power losses.

The Active Harmonic Filter is very competitive compared to both Passive filters and Active Front End. As the capital expenditure is very similar, a lower power consumption make the AHF a very good overall choice.

Furthermore the availability offered by a shunt installation where the drive can continue to operate even though the mitigation has failed offer a great upside through higher availability of the process.