Preamble
From the last four blogs, we now have a good understanding of power flow in AC systems. We found that there is a component of AC power flow which is oscillatory. In other words, a part of the power that flows back and forth from the source and load every cycle. We called it the “Reactive Power”.
The reactive power plays a major role in the design of AC transmission systems and in the economic operation of AC power systems. Interestingly, the reactive power is not only relevant for electric utilities, but it is also relevant for the consumer.
Consumers can reduce their power bills by reducing the ‘consumption’ of reactive power. Quite often, many consumers are unaware or unconcerned about this issue. This may be because of the simpler tariff structure used by some utilities to reduce metering and billing costs. In other words, the consumer is billed only for active power (kW) demand and active energy (kWh) consumption. In such cases, the utility absorbs the cost of inefficiency due to reactive power flows.
The modern ‘market-based’ electric utilities cannot afford to absorb the cost of consumer inefficiency. Hence, the tariff structure is designed to penalise the low power factor loads. This provides an incentive for the customers to improve the load power factor.
In addition, in the case of large industrial sites such as mines, it is possible to reduce the power loss within their distribution system by locating the power factor correction (PFC) equipment as close to the loads as possible.
This blog illustrates the advantages of installing PFC capacitors with a practical case study.
Mine power system case study
I developed a user-friendly power system software on IBM personal computers in 1985. A power system workshop based on the software was conducted in 1987 at Capricornia Institute of Advanced Education (CIAE) Rockhampton, Australia. At this course, a young electrical engineer from a mining site in the Northern Territory, Australia, attended the workshop. He had a problem at the site.
The Problem
The power factor correction capacitors installed at the mine’s substations were old and the electrolyte was leaking. The electrolyte used in the capacitors was Polychlorinated Biphenyl (PCB). The PCB is carcinogenic and is known to cause various types of cancer. As it was a major safety issue, the management had decided to remove all the capacitors. It was his responsibility to ensure the feasibility of operating the power system without capacitors. The main concern was the voltage levels in the system. It was also necessary to ensure feasibility of starting the crusher (largest) motor.
The power system
The mine power system was independently operated and supplied by 2 x 50 MVA generators at the mine site. The total load on the generators was about 60 MVA. The mine load relevant to the study was about 15 MVA. The rest of the generation was supplying a neighbouring system and was treated as a lumped load.
The mine distribution system consisted of about 30 km of 22 kV overhead lines. The system load consisted of one 6.6 kV crusher substation, three 415 V conveyor substations, and the township electric supply modelled as a lumped load.
The system was not connected to an external utility supply. Hence, electricity tariffs which penalise the reactive power flows was not an issue.
The total rating of the capacitors to be replaced was about 1.7 Mvar. This could be easily be met by the available local generation capacity. Hence, the main concern of the electrical engineer was to ensure adequate voltage levels and feasibility of the starting of the crusher motor.
The smart young engineer had done ‘hand’ calculations to establish the voltage levels for normal load conditions and for crusher motor starting! He had also calculated the power loss figure! As per the studies, the voltage levels were well within the limits and the starting of the crusher motor was feasible.
The young engineer attended the workshop mainly to confirm his study results with the software used at the workshop. He stayed in Rockhampton for a few extra days to conduct the required load flow studies. He was pleased to find that his load flow results had almost an exact match with the results obtained by the software used in the CIAE power system workshop!
He decided to return to his office and finalise the report with the recommendation to remove the capacitors.
Power loss with the capacitor in service?
Just out of curiosity, I decided to run another load flow study with all capacitors in service. Once the base system data has been entered into the load flow software, conducting additional incremental studies is a breeze. With modern software and computers, the results can be obtained literally at the click of a few buttons! In fact, for an inquisitive power system engineer, this is more fun than work!
We were surprised to look at the power loss figures. The figures are as given below.
Total power loss with capacitors out of service = 727 kW
Total power loss with capacitors in service = 681 kW
Reduction in power loss with capacitors in service = 46 kW
The total generation was about 48,700 kW. Hence, the savings in power loss of 46 kW when compared with generation is exceedingly small. In other words, the extra fuel spent to generate the additional power is relatively negligible. However, it is a different story if the cost of energy or savings is calculated over a period of one year.
Annual savings = 46 kW x 16 hr/day x 365 day/year x $0.10/kWh
= $ 26,865 / year
The cost estimate for replacement of all capacitors was as below.
Capacitor cost $ 27,000
Installation cost $ 8,000
Total cost $ 35,000
The cost of replacing the capacitors could be recovered in less than 2 years!
In most large industrial systems, it is possible to recover the cost of capacitors in about 1 to 3 years, depending on the load conditions.
Revised Recommendations
The young engineer was pleasantly surprised by the above cost figures, so, the report was revised with the following recommendations.
- There are no technical reasons why capacitors cannot be removed and not replaced.
- Economic considerations justify the capacitors to be replaced.
