Continued from technical article: Capacitor Banks In Power System (part three)
PF correction for loads connected on captive Diesel Generator (DG)
Let us consider that there is a captive diesel generator the rating of which is specified as 1000kVA and PF 0.85. Rating in kVA specifies the maximum current the alternator can deliver at the system voltage.
In the previous parts of this article we have seen that the role of power capacitors in improving the power factor and reducing total cost of electricity in an industrial installation is well established with regard to supply of power from the Utilitys/utilities.
Hence it seems logical to extend the above application of power capacitors when power is drawn from captive diesel generator to optimize their performance.
- Apprehension that the DG set may get over loaded due to the fact that the kVA rating or current delivered by the DG set is generally considered as the indicator of output of DG set. It is well known that use of capacitors will reduce the kVAR requirement from DG and hence kVA requirement will go down which in turn will reduce the current drawn from the DG set and could thus tempt the to add more loads on a given DG set.
- The other reason for such an opinion is related to the risks arising due to sustained leading power factor conditions that would occur with the use of fixed capacitors in variable load situations.
However with meticulous application of PF correction capacitor we can improve the overall efficiency of DG set operation and result in considerable economic benefits to the DG set user.
This article tries to analyze the same in the following paragraphs.
Diesel Generator Set Rating in kVA
As we have considered 1000kVA DG. This way of specifying the DG rating is very logical because specifies the maximum current the alternator can deliver at the system voltage.
DG set Rating in kVA at a particular PF
The diesel generator which we had assumed was of 1000kVA at 0.85PF. The relevance of PF in case of DG rating is as follows:
- To find mechanical power rating of a diesel engine for a particular diesel generator, first convert kVA to kW and thereafter kW to BHP. This can only be done if we assume a certain average Power Factor (PF) under which the DG set would operate.
- The power factor so assumed should be in line with the average power factor prevalent in the industry. A typical industrial load comprises of induction motors (typical PF of 0.8 to 0.85), non-linear loads (typical PF of 0.5 to 0.6) and combination of unity PF loads (Resistive heating and incandescent lighting). Hence assuming an average power factor of 0.85 for typical industrial loads is considered acceptable by convention.
- Consequently a power factor of 0.85 is used for calculating the kW, which is then converted to the BHP rating of the prime mover. BHP rating so obtained is the output of the prime mover. Considering suitable engine losses it becomes possible to calculate the power rating of the engine.
Now after understanding the DG set name plate rating parameter, let us come back to the question should we connect the Capacitor Banks in parallel to the loads conned to DG? Answer is YES, It is however, important to ensure that under actual operating conditions the kW loading and current loading should not be exceeded.
Power Factor of loads supplied by DG sets can therefore be improved closer to unity by use of suitable Reactive Power Compensation Systems keeping in view the rated current loading is not exceeded.
Let us consider an example for the same:
** Any industry has a 1000 kVA DG set which is loaded at an average of 600 kW at 0.7 PF. In addition, there are 125 kW of other loads within the same installation, which are not loaded on the DG set due to capacity restrictions that arise during occurrence of short-term peak loads, such as motor starting, and intermittent welding load. Due to this, productivity in the Industry is lowered when the DG Set is in operation.
During the period when Utility supply is available all loads can be operated. Is it possible to improve productivity when DG Set is in operation?
** A well designed power factor correction capacitor bank panel can improve the cost of electricity consumed from utility as well as improve productivity when DG Set is in operation.
• kW of load connected to DG = 600 kW
• Average load power factor in industry where DG is installed = 0.7
• kVA drawn at normal condition = 600 / 0.7 = 857 kVA
Hence percentage load on DG without Capacitor bank = 857 /1000 = 85.7%
Now if we connect the suitably sized and designed (already discussed in part1 to 3) capacitor bank in parallel to the loads connected to DG and improve the average overall load power factor from 0.7 to 0.85 than for the same percentage loading of 85.7% that is 857kVA the active power that can be drawn is = 857 x 0.85 = 728.45 kW
Hence one can see the moment capacitor bank is connected in parallel to the loads connected to the DG the additional requirement of 125kW is comfortably met without exceeding the percentage loading on DG.
During the period when the Industry is using supply from the Utility the Capacitor banks system can ensure consistently high PF, thereby achieving demand savings and reduction in losses and elimination of any PF penalty. Consequently, cost of electricity consumed from the EB will be minimized.
The same Capacitor banks system can be also used when the Industry is using supply from the DG set. The fast acting property of the Capacitor banks system will reduce the peak load requirements that are to be met from the DG set. This is achieved by providing instantaneous compensation from the Capacitor banks system during conditions when motors are started and / or welding machines are being operated. This will enable the Industry to transfer the 125 kW of additional load on to the DG set and ensure that productivity is improved when the DG set is in operation.
REACTIVE POWER COMPENSATION SYSTEMS by Capacitor Banks can enable D.G set users to reconfigure their loads / D.G sets to achieve better percentage loading and efficiency on the machines. As a result reduction in cost / kWh can be attained.
Impact of leading kVAR on generators
Now since we have very well established that a suitably designed Capacitor Banks can be connected in parallel to the loads connected to DG. However what is the impact if one keeps on improving the power factor and the power factor goes on leading side.
Some inherent characteristics of an alternator limit the amount of leading kVAR that can be absorbed by a DG. We cannot go on switching ON the Capacitor Banks as and when required, this can create over voltage condition in DG and subsequently over fluxing.
The ability of any generator to absorb the kVAR is termed as reverse kVAR limit. This ability is defined as reactive capability curve. Below figure shows typical generator reactive capability curve. X axis is the kVAR produced or absorbed (positive to the right). Y axis indicates the kW (positive going up). kVAR and kW are shown as per unit quantities based on the rating of the alternator (not necessarily the generator set, which may have a lower rating.
The normal operating range of a generator set is between zero and 100 percent of the kW rating of the alternator (positive) and between 0.8 and 1.0 power factor (green area on curve). The black lines on the curves show the operating range of a specific alternator when operating outside of normal range. Notice that as power factor drops, the machine must be de-rated to prevent overheating. On the left quadrant, you can see that near-normal output (yellow area) can be achieved with some leading power factor load, in this case, down to about 0.97 power factor, leading. At that point, the ability to absorb additional kVAR quickly drops to near zero (red area), indicating that the AVR is “turning off” and any level of reverse kVAR greater than the level shown will cause the machine to lose control of voltage.
A good rule of thumb for generators is that it can absorb about 20% of its rated kVAR output in reverse kVAR without losing control of voltage. However, since this characteristic is not universal, it is advisable for a system designer to specify the reverse kVAR limit used in his design, or the magnitude of the reverse kVAR load that is expected.
Note that this is not specified as a leading power factor limit, but rather as a maximum magnitude of reverse kVAR.