Detuned Power Factor Correction Case Study – Plastics Factory

We were asked to attend site to carry out a power factor correction survey on a plastics factory who, due to their impending increase in load, had a requirement for more power factor correction. As a normal part of our power factor correction survey our survey engineer measured the voltage harmonics to determine whether any power factor correction recommend would have to incorporate detuning reactors to avoid harmonic overloading of the capacitors. During this survey the voltage harmonics were recorded as being in excess of 8%. Such a level of harmonic voltage distortion is beyond even the IEC compatibility limits, meaning that any equipment connected to such a distorted supply cannot be guaranteed to function correctly.

BY INSTALLING OUR DETUNED POWER FACTOR CORRECTION SYSTEM WE MANAGED TO NOT ONLY PREVENT A DAMAGING HARMONIC RESONANCE, BUT ACTUALLY REDUCE THE LOAD HARMONICS, POTENTIALLY INCREASING THE LIFE OF THE CLIENTS PLANT.

The installation at the time of the survey included a 400 kVAr power factor correction bank arranged in four steps of 100kVAr, all of which was in circuit at the time of the test. The immediate suspicion was that the power factor correction capacitors were forming a resonant or at least partial resonant circuit with the supply transformer leading to a magnification of the loads natural harmonic levels. In order to test this theory the capacitor banks were all switched out of circuit and the harmonic voltage measured once again. Upon doing this the voltage distortion was found to be in the region of 3.5% to 4.0%, clearly supporting the theory that the capacitors were exacerbating a previously moderate harmonic issue. Upon closer analysis of the specific harmonics contributing to the overall harmonic distortion it was discovered that the 5th harmonic was the dominant harmonic.

In order to determine the true extent of the problem a period of harmonic logging was commissioned with the original power factor correction operating. Figure 1 shows the profile of the voltage total harmonic voltage distortion captured with the original power factor correction running. The first couple of days of the analysis were during the weekend and consequently no plant was operating. Once the plant started operating on the Sunday night there was an immediate jump in the voltage THD, with the distortion level peaking at more than 8%. Figure 2 shows the profile of the 5th harmonic voltage taken during the same period. It can be seen that this profile is almost identical to the THD profile, indicating that the 5th harmonic is by far the dominant harmonic.

From the predicted increase in loading it was calculated that the total future power factor correction requirement was 600 kVAr. Clearly this amount of power factor could not be connected without detuning reactors as it would result in harmonics levels even higher than the elevated values existing at the time. In cases such as this the most common solution would be to install capacitors with 189 Hz detuning reactors, which prevent the magnification of harmonics for all of the harmonics from the 5th order upwards. Normally the total harmonic distortion of a supply consists of contributions from a number of harmonics whose magnitude decreases as the harmonic order increases. As has already been mentioned, the harmonic distortion at this site is overwhelmingly dominated by the 5th harmonic and as such any reduction in this harmonic would have a profound impact on the total harmonic distortion. With this in mind it was recommended that a 210 Hz detuned power factor correction system was installed at this site. The effect of tuning the power factor correction to 210 Hz is that as well as preventing magnification of all of the major harmonics it actually provides filtration of the 5th harmonic.

The larger the capacitor bank, the greater the filtration effect is and bearing in mind the recommended capacitor bank size for this site was 600 kVAr the potential filtration was significant.

Based upon the commercial and technical arguments presented for this project, the installation of a 600 kVAr, 210 Hz detuned power factor correction system was commissioned. Figure 3 and Figure 4 show, respectively, a profile of the voltage total harmonic distortion and 5th harmonic distortion with all of the replacement detuned power factor correction in circuit. It can be seen that as opposed to the original level of 8%, the voltage THD with the detuned capacitors in circuit was barely 2%. In addition to this the 5th harmonic voltage no longer dominates the voltage THD to the same degree as before, this time only constituting approximately 1.6% of the 2.2% distortion.

Conclusion

From the preceding results it is apparent that the old power factor correction was producing a serious harmonic resonant condition. The additional harmonics imposed upon the supply transformer would have produced significant additional stress upon the supply transformer, potentially reducing the lifespan of the transformer and compromising the integrity of the supply.

By installing 210 Hz detuned power factor correction, a solution to the poor load power factor has been implemented that provides both an economical and technical advantage, resulting in a supply that operates efficiently with little potential for mal-operation of connected equipment due to excessive voltage harmonic distortion.

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Packaging Company Network operating company WPD East Midlands

Reactive power charge at time of survey – the bill for this site was not specific about unit charges for kVArh, but instead just gave the total reactive power charge. This charge would have been based upon not achieving an average power factor of 0.95 and averaged £250.00 per month.

Available Supply Capacity (ASC) charge – £1.59 / kVA / month.

During our site survey the following load vectors were recorded:

kVA – 637
kW – 452
PF – 0.71
kVAr – 448

From the metering information the estimated maximum demand load vectors were as follows:

kVA – 867
kW – 650
PF – 0.75
kVAr – 573

In order to correct the maximum demand condition to a more economical level of 0.97, 400 kVAr of power factor correction was offered.

With the above capacitors connected the maximum demand condition was predicted to be as follows:

kVA – 673
kW – 650
PF – 0.97
kVAr – 173

The reduction in maximum demand predicted = 867 – 673 = 194 kVA

Potential annual savings
From the above information, the following savings were calculated:

Excess reactive power – on average the client was receiving £250 / month in reactive power charges. By installing the equipment recommended this charge was virtually eliminated as an average power factor greater than 0.95 would be maintained. Achieving annual savings of £250 x 12 = £3,000.

Available supply capacity – the reduction in maximum demand of 505 kVA represents an annual available supply capacity saving of 194 x 1.59 x 12 = £3701.52

Total potential annual savings =

£6701.52

Installed cost of equipment recommended
The total installed cost of the equipment recommended was £8499.00

Payback period of equipment
From the cost of the equipment and the potential annual savings, the payback period of the equipment was calculated as approximately 15 months.

Result of survey
The equipment recommended was installed. The reactive power charges were eliminated and the client is presently evaluating their reduced maximum demand before re- negotiating their available supply capacity.

THIS INSTALLATION OF POWER FACTOR CORRECTION AT THIS PACKAGING COMPANY ACHIEVED SAVINGS OF ALMOST £7000 PER ANNUM, ACHIEVING A PAYBACK PERIOD OF ONLY 15 MONTHS.

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Rice Distributor Network operating company – UKPN – London

Reactive power charge at time of survey – 0.329p / kVArh in excess of 33% of the kWh (i.e. a charge is imposed only if an average power factor of less than 0.95 is achieved)

Available Supply Capacity (ASC) charge – £0.93 / kVA / month

During our site survey the following load vectors were recorded:

kVA – 2209
kW – 1684
PF – 0.76
kVAr – 1430

From the billing information the estimated maximum demand load vectors were as follows:

kVA – 2497
kW – 1923
PF – 0.77
kVAr – 1593

In order to correct the maximum demand condition to a more economical level of 0.96, 1000 kVAr of power factor correction was offered.

With the above capacitors connected the maximum demand condition was predicted to be as follows:

kVA – 1992
kW – 1923
PF – 0.96
kVAr – 593

The reduction in maximum demand predicted = 2497 – 1992 = 505 kVA

Potential annual savings

From the above information, the following savings were calculated:

Excess reactive power – on average the client was receiving £1500 / month in reactive power charges. By installing the equipment recommended this charge would be eliminated as an average power factor greater than 0.95 would be maintained. The potential annual saving is thus £1500 x 12 = £18,000.

Available supply capacity – the reduction in maximum demand of 505 kVA represents an annual available supply capacity saving of 505 x 0.93 x 12 = £5635.80

Total potential annual savings =

£23,635.80

Installed cost of equipment recommended
The total installed cost of the equipment recommended was £20,612.00

Payback period of equipment
From the cost of the equipment and the potential annual savings, the payback period of the equipment was calculated as approximately 10 months.

Result of survey
The equipment recommended was installed. The reactive power charges were eliminated and the client is in the process of re- negotiating their available supply capacity.

BY INSTALLING A BESPOKE POWER FACTOR CORRECTION SCHEME PFC ENGINEERING MANAGED TO ACHIEVE SAVINGS OF £23,000 PER ANNUM FOR A MAJOR UK RICE DISTRIBUTOR, ACHIEVING A PAYBACK PERIOD OF LESS THAN 12 MONTHS.

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Power Quality Case Study – Electronic Component Distributor

We received a request from a major electronic component supplier to investigate a possible power quality issue resulting in the repeated failure of the communication system that controls the operation of the various conveyor systems within the warehouse. Every time this failure occurred a major re-boot of the warehouse control system had to be carried out with the result that a significant amount of time was lost.

BY IDENTIFYING AND RECTIFYING A CAPACITOR SWITCHING TRANSIENT PFC ENGINEERING MANAGED TO PREVENT THE SHUTDOWN OF

THE PRODUCT PICKING PROCESS, EACH OCCURRENCE OF WHICH COST THE COMPANY £50,000 PER HOUR IN LOST PRODUCT THROUGHPUT.

As the occurrences of conveyor shutdown were quite frequent we recommended that a power quality analysis be carried out for the period of one week during which time we asked the site engineers to log the time of any conveyor system faults so that any power quality events occurring at this time could be investigated further.

During the power quality logging period a failure of the conveyor system was logged at about 10am on the morning of June 6th 2005. Upon inspection of the data recorded it was apparent that a voltage transient occured at the time of the conveyor system failure. This voltage transient is shown in Figure 1.

The regular occurrence of such failures and the likely coincidence with transient voltage events lead us to suspect that the switching of an item of plant was responsible for the conveyor system failures. In order to determine whether this was the case the power consumed in cycles immediately before and after the transient event recorded.

Figure 2 shows the reactive power consumption immediately before and after the transient event associated with the conveyor system failure. It can be seen that there is a step change (reduction) in the consumed reactive power at this time. This fall in reactive could mean either an inductive load being turned off (possibly a motor) or a bank of power factor correction capacitors being energised. As there was no appreciable drop in kW during this time it was felt most likely that a capacitor bank was responsible for the transient event recorded.

Further power quality recording and logging of conveyor system failures yielded the same pattern, i.e. a step change in reactive power coincident with transients occurring at the same time as system failures. As a result of this we recommended that the old power factor correction incorporating 50 kVAr steps switched with non-soft switching contactors was replaced with 25 kVAr steps incorporating soft switching.

This recommendation was implemented and immediately the instances of conveyor system failure reduced dramatically, saving the client many tens if not hundreds of thousands of pounds.

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Voltage Optimisation Case Study Link Centre: Ice skating rink and leisure complex

The Link Centre is a large leisure complex, comprising an International sized ice skating rink, swimming pool, indoor courts, climbing walls, fitness studio etc and the local Library. The site operates seven days a week for fifty one weeks per year. The site would normally run anywhere between 400kVA and the maximum demand of approximately 750kVA, the over night base load was approximately 300kVA.

On site voltages were high, peaking at just under 250V (L-N). The whole site is fed by a single 1000kVA transformer. The Client has already implemented a number of energy saving measures typically fitting high frequency ballast lighting and variable speed drives to some of their larger motors.

VOLTAGE OPTIMISATION SAVED THIS LEISURE COMPLEX OVER £76,000 AND SIGNIFICANTLY REDUCED THEIR CARBON FOOTPRINT AS WELL.

Project

Installation was reasonably difficult with the main switch room located in the basement plant room of the centre and existing plant equipment restricting access. An additional plinth as flood protection was requested.

The existing ACB main breaker was reutilized reducing the overall cost of installation. Due to the limited access it was decided to install three single phase units instead of the normal three phase unit. The Dynamic Voltage Regulator (Voltmaster Plus), was specified at 1441 Amps per phase, 318kVA at the 221V (L-N) set output voltage, complete with an external bypass switch allowing the Client full control and independence over their supply and with enough excess capacity for future growth. All preparations were carried out beforehand with the final connections made within a four hour period on a Sunday evening, causing the least disruption to the Client.

Conclusions

The Voltmaster Plus outputs were set to 221V (L-N) as the optimised setting, giving just in excess of 220V (L-N) at the furthest point on site as requested by the Client.

The initial survey estimated a saving of a minimum 14% of the overall consumption. The Client required proof of these savings as confirmation of their own long term tests. It was decided to utilise the Voltmaster Plus’s ability to adjust phase voltages whilst on-line and load. A medium and a short term test would be performed, in each case returning the site voltages back to near maximum and then reducing again to the optimised setting. Results from these tests combined with other test data will allow the actual savings to be calculated.

The site voltages were increased to the maximum and allowed to run for a period. Then reduced to the Optimised setting of 221V (L-N) within less than a minute and allowed to run for the same time period. The step change in voltage (all three phase voltages are summed and averaged for clarity) and the consumption are clearly visible. The results were analysed and comparisons made with the short term tests.

Again, the site voltages were increased to maximum and allowed to run for a short period and then returned to the optimised value, the sequence was repeated on several occasions in short succession. The step changes in voltage (again all three phase voltages are summed and averaged for clarity) and consumption can be clearly defined. Analysed together with the previous results from the medium term test and other date, the calculated savings for this site will be approximately 15% of the usage, an increase over the previous estimate

Summary

The consumption of the Link Centre was 4,258,134kWh at an annual cost of approximately £510,310. The implementation of Voltage Optimisation utilising the Voltmaster Plus has now reduced this to 3,619,414kWh a saving of 638,720kWh, with a consequential reduction in emissions of 274,650kg of CO2 calculated at 0.43kg CO2/kWh. The cost savings of £76,547 pa will show a return on investment within less than 10 months.

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