SYSTEM DIMENSIONING
(2nd of Two Parts)
(2nd of Two Parts)
THE LOAD FACTOR: Load factor is the ratio of the Average Load (AL) over that of the Peak Load (MD). Some time in 1996, Meralco & NPC declared rebates to industrial customers because of “improved heat rates due to improved system load factor”. Prior to this event, there was a campaign by utility companies urging customers to implement “Demand Side Management” – the purpose of which was to improve the over-all load factor. That brings us the question, what in real business & engineering sense, does “Load Factor” means?
If a plant for instance in Mindanao has a monthly load factor of, say 40%, why is an identical plant in Pasig City, 70%? Why is another identical plant in Cebu registering for instance, 65%? Experts in electrical engineering say that a load factor of 70% is ideal and a 60% for an industrial plant is already very good. What does it mean then to an industrial plant?
Load factor is a measure how the peaks and valleys of the system load behave. A low load factor means high peaks but at the same time, very low off-peak loads. It should be understood that the Average Load for a certain period is the energy consumed in kW-H divided by the number of hours inclusive of the period. If the kW-H consumption is low (so with the average load in KW), and if the peak is very high (the highest load within the period), the ratio between them is very low, too. To the generating plant, this scenario is a nightmare because the power plant has to run several generating units to watch and serve the peaks, plus the required spinning reserve capacity of at east equal to the capacity of the largest unit in operation. In the end, this makes the power plant operating in a very costly mode (requiring more fuel BTU per kW-H). With the utility rates regulated by ERC (Energy Regulatory Commission), the power plant will have to absorb the inefficiencies. While it is true that the utility company charges the customer a “demand charge” this alone can not pay nor offset the inefficiencies.
To the industrial plant, a ‘low’ Load Factor means that the peak MW is way above the average MW for the month or period. This means paying for higher demand charges. If the peaks can be controlled like proper scheduling of operation so as not to superimpose large loads, the peaks can be brought to the minimum, hence improving the load factor. Similar to the generating company’s predicament, a low load factor requires bigger substation capacities in preparation for large peaks while under-utilizing the system most of the time.
On top of the above, load factor is a reflection of the operation and ‘not-so-good-looking’ plant capacity utilization. For instance, if a manufacturing plant is only yielding 40% of plant production capacity, the plant operating loads may only be significant during the first three days of the work week. The rest of the week is off-peak, thereby bringing down the average load for the month. Please note that had the manufacturing plant operated 6 days a week, the peak demand would have been the same but the average load becomes higher because the wide valleys are eliminated thus, made narrow, i.e., during weekends only. If the plant is operating six (6) days a week, the load factor could be 70%. If the plant operates all the time even on Sundays, then the load factor becomes more than 80%. Hence, ‘high’ Load Factor bespeaks of good business, while a ‘low’ Load Factor means not-so-good business.
Load Factor therefore is influenced by the following circumstances:
a) OPERATING BEHAVIOR OF THE MANUFACTURING PLANT
If the manufacturing plant operates at or near its full production capacity, then the Load Factor will be close to 70%. High Load Factor, after all, means good business. Controlling the superimposing large loads can greatly help in making the load factor high. Test running a large motor for a few minutes after undergoing maintenance during production weekdays may not be desirable because it may mean PhP 100,000 penalty in terms of added demand charge. Typical Load Factors of brewery plants with large motors in its system viz-a-viz plant production rates can be seen as the following:
Plant Production Rate / Plant Load Factor
... 30% ... 0.35
... 40% ... 0.40
... 50% ... 0.48
... 60% ... 0.55
... 70% ... 0.60
... 80% ... 0.65
... 90% ... 0.68
... 100% ... 0.70
THE “DIVERSITY FACTOR” OR
“NON-SIMULTANEITY FACTOR”
“Diversity Factor” or “Non Simultaneity Factor” – is the ratio of the arithmetical sum of the maximum demands of all group loads over that of the system peak loads as seen by the main source. Diversity Factors normally end up greater than unity. Its reciprocal, the “Simultaneity or Coincidence Factor” must be less than unity. Diversity Factor means that the system peak load of an industrial plant is not the sum of the peaks of all departments or process plants. This is because the peaks of these group loads do not individually happen at the same time. Should the peaks occur at the same time, the “Coincidence Factor” or “Simultaneity Factor” or “Diversity Factor” should have been unity, which is in real scenario; this condition is statistically very difficult to happen.
The over-all peak DiF typical between main distribution feeders of medium size industrial plants is established to be in the vicinity of 1.15 - 1.35, as seen by the main substation or by the island generation power plant. This factor should be known for use in sizing the main substation or a power plant for that kind of specific industry. Please note that adding up all the individual maximum demands of load centers will end up to a very large size substation or power plant which is neither desired.
THE LOAD GROWTH FACTOR (LGF)
Electrical Engineering Practitioners usually consider load growth in sizing the system. It normally starts with the size of the sub-feeders & feeders to the transformers and substation. In the US, it is not unusual to provide a load growth factor of some 25-35% envisioned for the next five or seven years. As such, transformers are always oversized for same practical reasons.
b)THE PRESENCE OF LARGE MOTORS THAT INFLUENCE THE PEAKS
A Bottling Plant typically registers a 68% to 70% load factor, while a refrigeration plant at 49%. This is so because motor starts-up in a Bottling Plant, mostly by small motors (and a few 20 hp motors), as seen by the demand meter only represents ripples. While the entry of a large compressor into a system influences the peso value of the demand charge that the manufacturing plant is paying for. More often than not, the size of the substation or a power plant serving a beverage plant is greatly influenced by the block loads brought by the size by large refrigeration compressor motors. Note that a 1,000 Hp compressor is even larger than a Bottling Line or a Pharmaceutical or an Automotive Harness Plant, electrically. Starting up one thousand pieces of one hp motor is very different from starting up a single – 1,000 Hp motor.
Power Plant Generators and Substation Transformers react to large motor starting as manifested in voltage dips. In general terms, transformers in an electrical system are usually larger than the maximum demands they serve, in some instances even larger than the connected loads. In the industrial plant scenario, the obvious reason at first glance for this apparent oversizing is the anticipation for future load growth. Fine…
But more often than not, sizing the transformer with extra kVA capacity unwittingly addresses voltage sag problems, not for load growth for which it is intended originally. That’s why for newly constructed plants where load growth is not yet there, the problem of starting significantly large motors may not surface out. Why? Because the extra kVA capacity intended for load growth is taking care of it. (See related article of this topic in this blogsite).
For systems with large motors:
TRAFO KVA = (Maximum Demand kVA of Small Group Motors) + (Maximum Demand kVA of Other Loads Including Load Growth) + (kVA Ratings of all Large Motors) + (Additional Trafo kVA Capacity necessary to accommodate the inrush of the largest motor)
DOODS A. AMORA, PEE
(August 19, 2007)
If a plant for instance in Mindanao has a monthly load factor of, say 40%, why is an identical plant in Pasig City, 70%? Why is another identical plant in Cebu registering for instance, 65%? Experts in electrical engineering say that a load factor of 70% is ideal and a 60% for an industrial plant is already very good. What does it mean then to an industrial plant?
Load factor is a measure how the peaks and valleys of the system load behave. A low load factor means high peaks but at the same time, very low off-peak loads. It should be understood that the Average Load for a certain period is the energy consumed in kW-H divided by the number of hours inclusive of the period. If the kW-H consumption is low (so with the average load in KW), and if the peak is very high (the highest load within the period), the ratio between them is very low, too. To the generating plant, this scenario is a nightmare because the power plant has to run several generating units to watch and serve the peaks, plus the required spinning reserve capacity of at east equal to the capacity of the largest unit in operation. In the end, this makes the power plant operating in a very costly mode (requiring more fuel BTU per kW-H). With the utility rates regulated by ERC (Energy Regulatory Commission), the power plant will have to absorb the inefficiencies. While it is true that the utility company charges the customer a “demand charge” this alone can not pay nor offset the inefficiencies.
To the industrial plant, a ‘low’ Load Factor means that the peak MW is way above the average MW for the month or period. This means paying for higher demand charges. If the peaks can be controlled like proper scheduling of operation so as not to superimpose large loads, the peaks can be brought to the minimum, hence improving the load factor. Similar to the generating company’s predicament, a low load factor requires bigger substation capacities in preparation for large peaks while under-utilizing the system most of the time.
On top of the above, load factor is a reflection of the operation and ‘not-so-good-looking’ plant capacity utilization. For instance, if a manufacturing plant is only yielding 40% of plant production capacity, the plant operating loads may only be significant during the first three days of the work week. The rest of the week is off-peak, thereby bringing down the average load for the month. Please note that had the manufacturing plant operated 6 days a week, the peak demand would have been the same but the average load becomes higher because the wide valleys are eliminated thus, made narrow, i.e., during weekends only. If the plant is operating six (6) days a week, the load factor could be 70%. If the plant operates all the time even on Sundays, then the load factor becomes more than 80%. Hence, ‘high’ Load Factor bespeaks of good business, while a ‘low’ Load Factor means not-so-good business.
Load Factor therefore is influenced by the following circumstances:
a) OPERATING BEHAVIOR OF THE MANUFACTURING PLANT
If the manufacturing plant operates at or near its full production capacity, then the Load Factor will be close to 70%. High Load Factor, after all, means good business. Controlling the superimposing large loads can greatly help in making the load factor high. Test running a large motor for a few minutes after undergoing maintenance during production weekdays may not be desirable because it may mean PhP 100,000 penalty in terms of added demand charge. Typical Load Factors of brewery plants with large motors in its system viz-a-viz plant production rates can be seen as the following:
Plant Production Rate / Plant Load Factor
... 30% ... 0.35
... 40% ... 0.40
... 50% ... 0.48
... 60% ... 0.55
... 70% ... 0.60
... 80% ... 0.65
... 90% ... 0.68
... 100% ... 0.70
THE “DIVERSITY FACTOR” OR
“NON-SIMULTANEITY FACTOR”
“Diversity Factor” or “Non Simultaneity Factor” – is the ratio of the arithmetical sum of the maximum demands of all group loads over that of the system peak loads as seen by the main source. Diversity Factors normally end up greater than unity. Its reciprocal, the “Simultaneity or Coincidence Factor” must be less than unity. Diversity Factor means that the system peak load of an industrial plant is not the sum of the peaks of all departments or process plants. This is because the peaks of these group loads do not individually happen at the same time. Should the peaks occur at the same time, the “Coincidence Factor” or “Simultaneity Factor” or “Diversity Factor” should have been unity, which is in real scenario; this condition is statistically very difficult to happen.
The over-all peak DiF typical between main distribution feeders of medium size industrial plants is established to be in the vicinity of 1.15 - 1.35, as seen by the main substation or by the island generation power plant. This factor should be known for use in sizing the main substation or a power plant for that kind of specific industry. Please note that adding up all the individual maximum demands of load centers will end up to a very large size substation or power plant which is neither desired.
THE LOAD GROWTH FACTOR (LGF)
Electrical Engineering Practitioners usually consider load growth in sizing the system. It normally starts with the size of the sub-feeders & feeders to the transformers and substation. In the US, it is not unusual to provide a load growth factor of some 25-35% envisioned for the next five or seven years. As such, transformers are always oversized for same practical reasons.
b)THE PRESENCE OF LARGE MOTORS THAT INFLUENCE THE PEAKS
A Bottling Plant typically registers a 68% to 70% load factor, while a refrigeration plant at 49%. This is so because motor starts-up in a Bottling Plant, mostly by small motors (and a few 20 hp motors), as seen by the demand meter only represents ripples. While the entry of a large compressor into a system influences the peso value of the demand charge that the manufacturing plant is paying for. More often than not, the size of the substation or a power plant serving a beverage plant is greatly influenced by the block loads brought by the size by large refrigeration compressor motors. Note that a 1,000 Hp compressor is even larger than a Bottling Line or a Pharmaceutical or an Automotive Harness Plant, electrically. Starting up one thousand pieces of one hp motor is very different from starting up a single – 1,000 Hp motor.
Power Plant Generators and Substation Transformers react to large motor starting as manifested in voltage dips. In general terms, transformers in an electrical system are usually larger than the maximum demands they serve, in some instances even larger than the connected loads. In the industrial plant scenario, the obvious reason at first glance for this apparent oversizing is the anticipation for future load growth. Fine…
But more often than not, sizing the transformer with extra kVA capacity unwittingly addresses voltage sag problems, not for load growth for which it is intended originally. That’s why for newly constructed plants where load growth is not yet there, the problem of starting significantly large motors may not surface out. Why? Because the extra kVA capacity intended for load growth is taking care of it. (See related article of this topic in this blogsite).
For systems with large motors:
TRAFO KVA = (Maximum Demand kVA of Small Group Motors) + (Maximum Demand kVA of Other Loads Including Load Growth) + (kVA Ratings of all Large Motors) + (Additional Trafo kVA Capacity necessary to accommodate the inrush of the largest motor)
DOODS A. AMORA, PEE
(August 19, 2007)
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