INSTITUTE OF INTEGRATED ELECTRICAL ENGINEERS
1
A CLOSER LOOK AT POWER FACTOR ADJUSTMENT, DOES NGCP REALLY NEED TO GET RID OF IT? R. F. Corpuz, G. R. Pagobo
Abstract—This study investigates Power Factor Adjustment as implemented by NGCP. It looks into the details of its formulation and concludes if it is profitable for NGCP to remove or carry on power factor adjustment.. Index Terms—Power Factor, Reactive Power, OATS Rules, Power Factor Adjustment
I. I NTRODUCTION HAT is power factor? In a power system, two types of power are actually transmitted. The first is the real power measured in MW and the second is the reactive power measured in MVAR. Ordinary loads such as heaters and incandescent bulbs consume real power (MW). Motors and transformers require reactive power (MVAR) in addition to MW. The vector sum of MW and MVAR is called apparent power measured in MVA. The relationship between the three power values is shown in Figure 1.
W
Figure 1.
II. R ATIONALE , W HY D OES NGCP C HARGE P OWER FACTOR A DJUSTMENT ? With reference to Figure 2 , each power value in the power triangle corresponds to a value of current. Thus a “current triangle” could be drawn. Figure 3 shows the current corresponding to each type of power.
Figure 3.
The Power Triangle
Power factor (pf) is the “factor” that is multiplied to the apparent power (MVA) to get the real power (MW). In mathematical terms: M V A × pf = M W
(1)
MW MV A
(2)
pf = cos(θ)
(3)
pf =
Figure 2. For the same MW, as MVAR is increased, θ increases. The power factor or cos(θ) decreases
So power factor is the cosine of the angle between the apparent power and the real power. With more motor loads connected to the system, more reactive power (MVAR) is needed. The net effect is an increase in angle θ and consequently, the MVA is increased. Figure 2 shows increasing angle θ as MVAR is increased. Note that as θ increases, cos(θ) or the power factor decreases. Low power factor degrades the quality of transmission line performance.
Currents Corresponding to Each Type of Power
Why does NGCP charge Power Factor Adjustment? The answer to that question can be found in Figure 3. Recall that NGCP’s power bill to its customers is based on MW or demand. Thus it is charging its customers payment for wheeling energy corresponding to IM W of current. Note from Figure 3 that the actual current that passed through the line was IM V A which is greater than IM W . . Thus to compensate for the extra current due to MVAR which passed though the transmission lines, Power Factor Adjustment is collected. III. T HE O RIGIN OF P OWER FACTOR A DJUSTMENT F ORMULA Since Power Factor Adjustment is intended to compensate for the extra current passing through the transmission lines, its quantity should be proportional to IM V A . From the Equation 3: pf = cos(θ) =
IM W MW = MV A IM V A
(4)
INSTITUTE OF INTEGRATED ELECTRICAL ENGINEERS
IM V A =
IM W cos(θ)
2
(5)
It can be seen from Equation 5 that “real current” IM W is IM W when power factor or cos(θ) is considered. increased to cos(θ) The quantity of current IM V A,pf which should be the basis of the Power Factor Adjustment is the actual current IM V A less IM W ). IM W (since IM W was increased to cos(θ) IM V A,pf = IM V A − IM W
Mi = Peak Demand within a Billing Period for a Load Customer’s particular Point of delivery. P Mi = Peak Demand within a Billing Period for all Points of Delivery of a Load Customer Observe that since pf = cos(θ) T DRC ×
(6) = T DRC × SP F
Substituting Equation 5 to Equation 6 IM W − IM W cos(θ) 1 = IM W −1 cos(θ)
IM V A,pf = IM V A,pf
(7) (8)
Thus the Power Factor Adjustment Charge should be based on the Total Demand Related Charge (T DRC) which is the amount charged for wheeling energy to IM W of corresponding 1 current multiplied by a factor of cos(θ) − 1 . As the power factor increases from 0 to 1, the “factor” abruptly decreases as shown in the Figure 4.
Figure 4.
Plot of
1 cos(θ)
SP F SP F − 1 = T DRC × −1 (10) pf cos(θ) 1 −1 cos(θ)
(11)
With reference to the previous section, the OATS Rules is consistent with charging the extra demand represented by the 1 current IM V A,pf since it considered the factor cos(θ) − 1 . The introduction of SP F in the numerator ensures that the customer is not penalized for having a power factor equivalent 1 2 to SP F . The inclusion of SP F in cos(θ) − 1 however, would result in collecting penalty charges which is lower than 1 the required theoretical charge given by cos(θ) − 1 . For all practical purposes however the collected penalty charge with SP F as a factor is acceptable as shown in Figure 5 where at power factors of 70%-90%3 , the curves are almost equal. Also notice that the deviation of the modified adjustment equation is most evident on lower power factors from 50% and below. This however should not be of concern since for most practical applications, power factor would not reach as low. In general, the equation is a good approximation for adjusting the demand to compensate for extra MVAR.
−1
Section F(AVIII)1.4 of the OATS Rules1 calculates Power Factor Adjustment (P F A) as: P F A = T DRC ×
SP F Mi −1 × P pf Mi
(9)
Where: T DRC =Total Demand-Related Charge which is the sum of Power Delivery Service Charge, System Operator Charge, Load Following and Frequency Regulation Charge, Black Start Service Charge and Intra-Regional Grid Cross Subsidy (in kW)for the Load Customer for the Billing Period SP F = Standard Power Factor. 85% if actual PF below 85%. Not Applicable (if pf is between 85%-91%). 90% - if actual PF is above 90% pf =Power Factor (actual) 1 The OATS rule is the basis of billing as per Transmission Service Agreement (TSA) signed by TransCo and its customers
Figure 5.
Effect of introducting SP F into the adjustment equation
Some municipal utilities such as Tacoma Public Utilities4 and Coon Rapids Electric Utilities5 and even the local MERALCO use the simple equation: 1 + (pf − SP F ) × Adjustment × kWdemand
(12)
2 If pf of customer = SP F , = (1 − 1) = 0 therefore customer is not charged 3 Typical power factor of induction motors. See http://www mytpu. org/ file_viewer.aspx?id=738 3 4 http://www.wyan.org/assets/electric/PowerFactor.pdf 5 http://crmu.net/PDF%20files/Rates/Electric/Power%20Factor%20 Adjustment%20Rate%20Schedule.pdf
INSTITUTE OF INTEGRATED ELECTRICAL ENGINEERS
Adjustment = 3% for bonus (that is pf > SP F ) and 6% for penalty. Note that Tacoma and Coon does not award bonus and that their SP F is quite high at 90% and 95% respectively as opposed to MERALCO where SP F = 0.85. At higher power factor, which is the usual case in of power the operation F − 1 as shown in system, (pf − SP F ) approximates SP pf Figure 6.
Figure 6.
(pf − SP F ) vs
SP F pf
−1
3
.
IV. L EGAL BASIS OF P OWER FACTOR A DJUSTMENT Except for the OATS Rules setting the required load power factor of customers to 85%, there are no other references or any other legal basis of Power Factor Adjustment both bonus and penalty. Even the Transmission Development Plan 2007 acknowledges this fact, to qoute: “However, the Grid Code is not strict on power factor and there is no mechanism, except through the revised OATS Rules, on enforcing high power factor to customers” V. R ECOMMENDATIONS Since NGCP billing is demand based, there is a need to impose the Power Factor Adjustment (penalty) to compensate for the extra current due to MVAR. This MVAR current equally degrades our lines as the MW current. It is sensible to consider removal of bonus for high power factors since the OATS Rules had already required the customers to maintain a minimum of 0.85 power factor. This means that along with the OATS Rules’ requirement for the customers, it requires NGCP to maintain its line capacity at customers’ power factor of 0.85. If investment on additional capacity would be more economical than the cost of the power factor bonus rewarded, then at some point we can consider keeping the power factor bonus. In general however, as with other utilities, NGCP’s lines are designed and built to be capable of handling currents at customer standard power factor of 0.85, hence, power factor bonus should be removed. Lastly, considerations should be made to increase the 0.85 power factor required by the OATS Rules in view of the practices of utilities in other parts of the world which sets their standard power factor higher than 0.90.
Rex F. Corpuz is a former Sr. VP of the National Transmission Corporation and a former President of National Grid Corporation of the Philippines (NGCP). He is presently the Sr. Technical Adviser at the Office of the President, NGCP. He is also a former Regional Governor of IIEE Region 1., e-mail: rfcorpuz@gmail.com
Gerald R. Pagobo is a Power System Consultant and formerly worked with the research group of NGCP. He received his MS in Electrical Engineering degree from the University of the Philippines in 2009. His research interests include reliability engineering, power system dynamic stability, electricity market optimization, and computer-based power system modeling and analysis. Email: grpagobo@gmail.com