Power Factor Correction (PFC)
Very low power factors (PF) have been observed in Australian agribusiness, with the average PF sat at approximately 0.6 – 0.7 but can drop to as low as 0.4 at times. Further, a low PF means you are potentially paying significantly more than necessary, either in excessive $/kVA costs, or significantly overrated kVA gensets and fuel consumption, for a given kW peak load. The diagram below show the correlation between kW, kVAr and kVA.
The diagram below shows how, by increasing the PF towards unity (1.0) the apparent power (kVA) and reactive power (kVAr) is also reduced.
PFC uses reactive elements (capacitors and inductors) to bring the site PF closer to 1. PFC can also improve motor life.
An example 175 kVAr PFC system was sized to bring the average power factor from ~0.6 to 0.9 for a 150 kW average load, and priced at $9,400 supplied and installed (excluding GST). With no $/kVA charge as part of the electricity bill, there is minimal economic justification for PFC; for a distribution network charging $10.4 to $15 /kVA/month, the simple payback period for such a PFC device increasing PF from 0.6 to 0.9 was estimated at 0.6 to 0.9 years. Note that solar photovoltaic systems if installed will reduce the site PF, so PFC may need to be considered if following this route.
It is estimated that 90% of Australian businesses receive electricity at a higher voltage than required, thus impacting efficiency, increasing energy consumption, and prematurely wearing plant. Voltage Optimisation can reduce the active power consumed by an equivalent magnitude of voltage reduction and reduce reactive power by a ratio of 1:1.7 or more.
As a case study, voltages were measured within a site serviced via the Queensand Ergon network in Q4 2018 at 419.6 V on a non-VSD motor. Allowing a voltage loss buffer (throughout the site’s system) of 2%, the incoming site supply was assumed at 428 V which is within the +10% AS60038 requirement). Assuming a voltage loss buffer (throughout the site’s system) of 2% with a target voltage of 390 V, the optimized incoming site voltage is calculated at 397.8 V, which means that the overvoltage is 30.2 V (428-397.8) which equates to a 7.1% overvoltage. Power costs savings are achieved by reducing consumption (kWh) and reducing maximum demand charges (i.e. kW, kVA and/or kVAR charges, often based upon the maximum demand for a site each month). Power quality is also improved by stabilising and balancing phase voltage supply, hence reducing motor overheating, reduced malfunctions of sensitive equipment, and reduced wear on equipment and electrical infrastructure. Reduced equipment wear is difficult to quantify hence the cost-benefit analysis presented below only considers savings from consumption and demand charges.
Percentage voltage reductions do not correlate directly to the percentage of energy (kWh) and demand (kVA / kW) savings, as the overall energy savings depend upon the type of load / equipment, the target voltage, amount of overvoltage, equipment utilization, etc. The expected savings of voltage optimisation is a 0% saving for DC equipment (LED lighting, inverter air conditioning, office IT equipment etc); approximately 3-5% for VSD driven motors; to 9-15% for motors operating at partial loading most of the time, oversized motors, HVAC and refrigeration systems. Across an entire facility, energy usage reduction of 12 to 14.4% are quoted for VO case studies,,. A 430 kVA/600 A VO system can be purchased for $60,590 with estimated installation and commissioning costs of $25,000. Energy and maximum load reductions of 7%, 12% and 14% were modelled, saving $10,825, $18,558 and $21,651, and delivering paybacks of 7.9, 4.6, and 4.0 years respectively.
Further considerations for VO:
- If you are the one in ten business that receives voltage within the range of 220 – 230 V single phase (approximately 400 V three phase) then the economic viability of VO will be reduced for your business.
- Sites with consistently high loads will have shorter payback periods whilst sites with short spikes then long periods of low loads will have a longer payback (as VO equipment is recommended to be sized for the high demand periods).
- The physical size of the equipment is similar to a fridge; and is defined by the electrical rating (kVA) required.
- VO should be positioned as close as possible to the main switchboard (MSB) in order to minimise the cost of cable to and from the equipment. Typically, up to about 20m away from the main switchboard is viable, beyond that the cost of cable can have a significant impact on the project.
- VO is installed in series with the electricity supply (the power is routed out of the switchboard, through VO, then back to the switchboard). Additional costs can be incurred where MSBs are very old or in poor condition.
For some facilities and depending upon the power reticulation infrastructure, voltage reduction could be achieved at minimal to no cost by changing the manual tap setting on the transformer (e.g. if the transformer is under the operational control of the facility or the facility has a designated transformer; or if adjustments can be made at the zone substation transformers) – this has been referred to as Conservation Voltage Reduction (CVR). A VO device allows such reduction to be achieved more accurately, because it has more tapings, enabling it to provide finer ranges of voltage output plus can be installed on-site.
Low levels of automation and reliance on manually operated plant results in slower response times, un-optimised operations, inefficient energy usage and higher power demand. An example of this is switching on all large plant at once at the beginning of a shift, leading to a very large load spike from starter current and sub-optimal matching of the plant load with the electricity costs associated with different sources of power. A more efficient method would be to automate the staging of equipment coming on- and off-line. The general function of an EMS is to monitor all power sinks and sources in a plant, logging generation and usage characteristics and aggregating this into a detailed model for automated decision making. Critical data can then be extracted to a side-wide dashboard to enable staff to make informed and meaningful decisions. When trends are defined, most processes may then be automated to bring further efficiency gains.
A quote for the supply and integration of an energy management system for an edge of grid power supply, existing 380 kVA diesel generator, and planned 98 kW of solar at a site with average load 150 kW at $114,300. An example of how an EMS would work is during the sunlight hours when solar is providing power at ~4 to 8 c/kWh (depending upon scale, installation costs, and solar radiation), the EMS throttles the genset (generating power at approximately 32 c/kWh) higher during the peak period where power is charged at over 50 c/kWh (e.g. in regional Queensland), and lower during the off peak period when power is charged at 19 c/kWh. With larger solar arrays, an EMS can control the time at which loads come on- and off-line to concentrate loads to match high solar generation or spread / shift loads when power is most expensive. For a commercially available, off the shelf EMS, an expectation of 5% reduction in consumption (kWh) is reasonable; with the real gains being made when integrating renewables, embedded generation, and monitoring market forces to dictate load spreading/shedding, generation, and consumption. An EMS can then be thought of as the integral component that enables the highest value to be derived from a facility.
Turning motors speeds down, delayed starts, or turning motors / loads off via utilization of automated systems – refer EMSs above for more information.
VSDs are an example of a technology that enables motor speeds to be varied to match the output requirement (e.g. water pressure, ventilation air volumes per second, boiler combustion air, cooling requirements, tonnes per hour milling, etc). Taking fans as an example, the cube law relationship between speed and power means that reducing a fan’s speed in a variable torque load application by 20% can achieve energy savings of 49%.
Flow versus power draw curve for fixed and variable speed drives. The broken line indicates the power input to a fixed-speed motor and the solid line indicates the power input to a variable frequency drive. The shaded area represents the power saved by using a variable frequency drive for a given flow.
Shaping the power load to match the availability of lower cost power such as PV solar, stored PV solar, and off-peak power. Storage options include supercapacitors and batteries e.g. Li-ion, and can be charged using excess PV solar and/or low cost off-grid power then discharged during times of high power costs to reduce kW / kVA and kWh costs. Embedded generation such as diesel gensets can also be employed. Demand management can be integrated into automated systems – refer EMSs above.
Supercapacitors (supercaps) are positioned between capacitors and batteries in terms of electronic components and have the advantages of storing far more energy than a conventional capacitor, can turn on “instantaneously”, very long lifetimes regardless of the number of charge cycles (e.g. hundreds of thousands of charge/discharge cycles rather than thousands). Supercaps are heavy hence not used for transport or mobile devices but are well suited to stationary power requirements.
Battery options are now available at approximately 11 to 28 c/kWh over the warrantied period, depending upon scale and supplier, hence where current power prices exceed and are anticipated to be half this cost post-2020. There is an economic argument to not invest in batteries in the short term, however to ensure you are “battery ready” which includes ensuring room on a facility switch board, suitable covered / weather protected areas are available adjacent to a facility switch board and suitable data capture i.e. min data to enable associated data analytics for optimum sizing of batteries. A demand management system, utilizing embedded generation and load shedding is provided in the figure below. Refer EMS info above for further information.
Simplified diagram of a micro-grid adapted from Chen et al.. SC: super capacitor, which provides very rapid power load / power supply response and assists with synchronisation but has a smaller kWh storage than the battery bank (BAT). MGCC: microgrid control centre; LC: load control; EMS: Energy Management System; DSG: diesel gensets; LC: load control.
It has been observed that a large proportion of motors in a typical agribusiness site are direct on line (DOL) or star-delta starting (refer below). A DOL starter can draw up to ten times the normal running current during starting, which can be reduced by around 30% by a star-delta configuration, however this is relatively uncommon in Australia. DOL suits motors that need to run at full speed all of the time; if this is not required then there will be inefficiencies in power consumption, hence variable speed drives (VSD) may be used to better match motor requirements. Studies on VSDs show energy savings of 25 to 60%. Brief descriptions of different motor types is as follows:
Direct online (DOL): the motor is switched on in one operation, with a direct connection from the power source. The starting current can be up to ten times the normal running current of the motor. A contactor is generally used to switch power, and often a thermal or electronic overload relay is provided for motor protection. DOL is the simplest and cheapest method of motor starting, but consideration needs to be made for power supply limitations on starting current. DOL is well suited for motors that need to run at full speed all of the time.
Star-delta starting: the motor is first connected in ‘star’ configuration, which allows the motor to gather speed without drawing excessive current. Once the motor is up to speed (or a pre-set time is elapsed) it is then connected in the normal ‘delta’ configuration. This method can reduce the starting current demand by 30%, but is only suited to applications where the motor is starting without load (e.g. where a clutched gearbox is used). Star-delta starting is relatively uncommon in Australia.
Soft starting: an electronic device which regulates the voltage flowing to the motor at start-up. By slowly ramping up the supply voltage to the motor, a smooth start without excessive current flow can be achieved. Soft starters are more expensive than DOL or star-delta, but they are widely used due to their convenience and simplicity.
VSDs; also called Variable voltage / variable frequency (VVVF): an electronic device which allows complete control of the motor speed including starting and stopping. It operates by changing the frequency of the power supplied to the motor. VVVF is extremely versatile and often used in process applications where a constant flow needs to be maintained. In addition, because the motor can be run at a slower speed and hence use less energy, use of a VVVF can facilitate significant power savings. Variable speed drives are generally the highest capex motor starting type, but their versatility means they are very widely used. When operating at near full speed, there is a crossover point where VVVFs can use more energy than a DOL motor due to efficiency losses associated with VVVFs (i.e. heat losses; temperature control requirements); this is shown in the figure below. Hence, some larger systems may have both VVVF and DOL configurations.
Motors suitable for VSDs tend to spend a significant portion of their operation unloaded or at fractional load, with examples of suitable motor types including fans, conveyors, elevators, and augers. A quote was received for the 32 kW VSD motor at $7,759, as indicative of potential savings for other pieces of plant. Saving was conservatively estimated at 25%, saving $4,595 per annum, with simple payback 1.7 years.
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 http://engineering.electrical-equipment.org/power-quality/ac-power-back-to-basics.html, accessed 30 Apr 2019.
 Based upon http://buckles-smith.com/tech-blog/understanding-power-factor, accessed 30 Apr 2019.
 Savings from motor life and maintenance were not factored in cost benefit analyses here
 CapTech: https://www.captech.com.au/
 For a site with a typical maximum kW demand of 150 kW as observed over the 3-month average load data
 CapTech: https://www.captech.com.au/
 https://www.ergon.com.au/retail/business/tariffs-and-prices/large-business-tariffs, Tariff 45, accessed 30 April 2019.
 https://powerlogic.com.au/electrical-policies-and-procedures_105_2008376636.pdf, accessed 30 Apr 2019.
 ComAp: https://www.comap-control.com/
 Solar Choice Pty Ltd
 13.5 kWh Tesla Powerwall2 with 95% depth of discharge analysed over a 10 year warranty period.
 http://www.abb.co.uk/cawp/seitp202/c253ae5e6abf5817c1256feb0053baf7.aspx; https://www.designworldonline.com/how-to-calculate-compressed-air-savings; carbontrust.com
 Eaton (OEM): http://www.eatoncorp.com.au/Oceania/index.htm and Indratel (System Integrator): http://indratel.com.au/