Electrical Systems

Electrical Systems

This section discusses: – How utilities charge for electricity– How to calculate the avoided cost of electricity– How to use utility billing analysis to help identify cost

saving opportunities

Electric Rate Structures

Total cost of electricity in most rate structures is the sum of: – Service charge– Energy charge– Demand charge– Power factor charge

Energy Charge Based on total energy (kWh) consumed over entire month

Charge typically uses block structure with decreasing unit cost for larger amounts– Fixed Blocks

• $0.05 /kWh for the first 10,000 kWh• $0.04 /kWh for the next 100,000 kWh• $0.03 /kW for all remaining kWh

– Demand-dependent Blocks• $0.05 /kWh for first 250 kWh/kW• $0.04 /kWh for next 150 kWh/kW• $0.03 /kWh for all additional kWh

Fuel Cost Adjustments and Taxes: – Because the cost of fuel for a utility may vary over time, utilities sometimes modify

the energy costs in the rate schedule with a “fuel cost adjustment”.

Demand Charge

In addition to measuring total energy consumed over entire period, also measuring energy consumed over every 15 or 30-minute period during month. Find period with max energy use, convert to rate (kW), and charge monthly fee.

Demand charge may use block structure with decreasing unit cost for larger amounts– $15.00 /kW for the first 100 kWh– $12.00 /kW for all remaining kW

On Peak - Off Peak Rates: – ‘billing demand’ is calculated as the greater of:– the actual on-peak demand, or – 50% of the actual off-peak demand.

Seasonal Demand Charge: – the actual demand, or– 75% of the peak monthly demand during the previous 12 months.

Power Factor Charge

In inductive loads (i.e. motors), voltage lags current making some power unusable: reactive power (kVAr)

Supplied power (kVA) must compensate for reactive (kVAr) and usable (kW) power.

Power factor is ratio of usable power (kW) to supplied power (kVA) Most utilities charge for low power factor since supplied power (kVA)

> useable power (kW)

Example Rate Structure 1 Service:

– $95 /month Energy:

– $0.021 /kWh Demand:

– $13.86 /kW-month– Greatest average power during any 30-minute period– Greatest of:– 100% of on-peak (weekdays: 8 am to 8 pm)– 75% of off-peak (all other times)– 75% of max Jun, Jul, Aug, Dec, Jan, Feb in last 11

months Power Factor:

– $0.30 /kVAr-month

Example Rate Structure 2

Service:$100 /mo

Energy: $0.026 /kWh for first 250 kWh/kVA $0.011 /kWh for all additional kWh

Demand: $18.36 /kVA-mo for first 4,000 kVA: $14.45 /kVA-mo for all additional kVA Greatest average power during any 15-minute

period

Power factor: Implicit in kVA

Electrical Cost Breakdown

Interpreting Electricity Billing Data

0

500

1,000

1,500

2,000

Dem

and

(kW

)

0

5,000

10,000

15,000

20,000

25,000

En

erg

y (k

Wh

/day

)

Demand (kW) Energy (kWh/day)

Interpreting Electricity Billing Data

0

20

40

60

80

100

120

140

160

180

200

12/22

/199

4

2/24

/1995

4/26

/1995

6/26

/1995

9/26

/1995

11/22

/199

5

1/25

/1996

3/26

/1996

5/24

/1996

7/26

/1996

9/25

/1996

11/22

/199

6

Dem

and

(kW

)

0

200

400

600

800

1,000

1,200

1,400

1,600

En

erg

y (k

Wh

/day

)

Actual Demand (kW) Billed Demand (kW) Energy (kWh/day)

Interpreting Electricity Billing Data

Use graphical analysis!

0

2,000

4,000

6,000

8,000

10,000

12,000

Jan-00

Feb-00

Mar-00

Apr-00

May-00

Jun-00

Jul-00

Aug-00

Sep-00

Oct-00

Nov-00

Dec-00

De

ma

nd

(k

W)

Electrical System Cost Saving Opportunities

Billing Errors– Reconcile billing error with utility

Meter Consolidation– Consolidate electrical meters

Purchasing Transformer– Purchase transformer and switch to primary service

Power Factor Correction– Correct power factor by downsizing over-sized motors– Correct power factor by adding capacitors

Demand Saving Potential– Reschedule operation of electrical equipment to reduce peak demand. – Use control equipment to shed loads to manage peak demand

Demand Response Program– Voluntarily reduce demand during demand emergency

Verify Billing Amounts

Date DaysConsumption (kWh/period)

Avg Daily Consumption

(kWh/day)

Actual Demand

(kW)

Power Factor

Load Factor

Billed Amount

($/period)

Unit Cost ($/kWh)

Calculated Amount

($/period)11/20/01 32 1,743,914 54,497 6,731 93% 0.34 $110,757 $0.064 $110,75812/20/01 30 1,526,951 50,898 6,610 93% 0.32 $103,913 $0.068 $103,9141/21/02 32 1,404,734 43,898 6,699 93% 0.27 $102,091 $0.073 $102,0932/20/02 30 1,515,385 50,513 4,131 88% 0.51 $95,426 $0.063 $95,4273/20/02 28 1,325,472 47,338 3,945 87% 0.50 $90,469 $0.068 $90,4704/19/02 30 1,334,098 44,470 3,734 88% 0.50 $90,694 $0.068 $90,6955/20/02 31 1,241,993 40,064 3,548 87% 0.47 $88,291 $0.071 $88,2936/20/02 31 1,335,909 43,094 3,758 86% 0.48 $90,741 $0.068 $90,7427/19/02 29 1,197,403 41,290 3,596 85% 0.48 $87,128 $0.073 $87,1308/20/02 32 1,357,669 42,427 3,467 88% 0.51 $84,359 $0.062 $84,3619/20/02 31 1,248,546 40,276 3,256 86% 0.52 $81,513 $0.065 $81,51410/21/02 31 1,260,806 40,671 3,321 86% 0.51 $81,833 $0.065 $81,834

Tot/Avg 367 16,492,880 44,953 4,400 88% 0.43 $1,107,214 $0.067 $1,107,215

Meter Consolidation

2 Meters: ED = 80 + 50 = 130 kW 1 Meter: ED = 100 kW

0

10

20

30

40

50

60

70

80

90

100

110

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Hour of day

Ele

ctri

cal

Dem

and

(kW

)

Meter 1 Meter 2

0

10

20

30

40

50

60

70

80

90

100

110

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Hour of Day

Ele

ctri

cal

Dem

and

(kW

)Meter 1 Meter 2

Purchase Transformer And Switch To Primary Service

Primary Service: customer owns and maintains the transformer Secondary Service: utility owns and maintains the transformer Lower electricity rates for primary service, since the customer must purchase and

maintain the transformer. Advantageous for customer to purchase transformer when demand > 1,000 kVA.

Correct Power Factor

Low PF has three adverse effects:– Utilities charge for low PF – Increased supplied power (kVA) increases line losses, line size,

transformer size– May cause sensitive electrical equipment to malfunction.

Good practice to maintain PF > 90% by– Right-sizing under-loaded motors– Adding capacitors that absorb kVAr, decrease kVA and increase PF

Pa = Active Power (kW)

Pt = Total Power (kVA)

Pr =Reactive Power(kVAr)

PF = Pa (kW) / Pt (kVA)

q

Demand Saving Potential

Low potential for 1-shift and 3-balanced shift operations

0

20

40

60

80

100

120

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

Hour of day

Ele

ctri

cal

Dem

and

(kW

)

Base load Shiftable Load

0

20

40

60

80

100

120

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Hour of day

Ele

ctri

cal

Dem

and

(kW

)Base load Shiftable Load

Demand Saving Potential

Good potential for uneven shifts

0

20

40

60

80

100

120

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Hour of day

Ele

ctri

cal

Dem

and

(kW

)

Base load Shiftable Load

0

20

40

60

80

100

120

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Hour of Day

Ele

ctri

cal

Dem

and

(kW

)Base load Shiftable Load

Demand Saving Potential

20 kW potential limited by first shift demand to 10 kW

0

20

40

60

80

100

120

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Hour of Day

Ele

ctri

cal

Dem

and

(kW

)

Base load Shiftable Load

0

20

40

60

80

100

120

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Hour of Day

Ele

ctri

cal

Dem

and

(kW

)Base load Shiftable Load

Demand Saving Potential

Savings larger with “off-peak demand rates”: Savings = 500 kW for moving demand from 1st to 2nd shiftSavings = 1,000 kW for moving demand from 1st to 3nd shift

3,000

2,000

1,000

0

on-peak off-peak

Time

Dem

and

(kW

)

Stagger Startup of Barrel Heaters

Sunday barrel pre-heat would set peak if hydraulic motors on during startup.

Demand Response Programs

Many utilities and independent companies offer demand response programs

Demand response compensates customers for having ability and willingness to curtail load during critical times on the grid.

Demand response payments ~ $50 /kW-year. No “demand emergencies” in 13-state PJM territory in

last two years. Example: agree to reduce demand by 100 kW and

receive ~ $5,000 per year.