Electrical Systems
description
Transcript of Electrical Systems
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: $14.45 /kVA-mo for all additional kVA Greatest average power during any 15-minute
period
Power factor: Implicit in kVA
Electrical Cost Breakdown
Interpreting Electricity Billing Data
0
500
1,000
1,500
2,000
Dem
and
(kW
)
0
5,000
10,000
15,000
20,000
25,000
Ener
gy (k
Wh/
day)
Demand (kW) Energy (kWh/day)
Interpreting Electricity Billing Data
0
20
40
60
80
100
120
140
160
180
200
12/22
/1994
2/24/1
995
4/26/1
995
6/26/1
995
9/26/1
995
11/22
/1995
1/25/1
996
3/26/1
996
5/24/1
996
7/26/1
996
9/25/1
996
11/22
/1996
Dem
and
(kW
)
0
200
400
600
800
1,000
1,200
1,400
1,600
Ener
gy (k
Wh/
day)
Actual Demand (kW) Billed Demand (kW) Energy (kWh/day)
Interpreting Electricity Billing Data
Use graphical analysis!
0
2,000
4,000
6,000
8,000
10,000
12,000
Jan-00
Feb-00
Mar-00
Apr-00
May-00
Jun-00
Jul-00
Aug-00
Sep-00
Oct-00
Nov-00
Dec-00
Dem
and
(kW
)
Electrical System Cost Saving Opportunities
Billing Errors– Reconcile billing error with utility
Meter Consolidation– Consolidate electrical meters
Purchasing Transformer– Purchase transformer and switch to primary service
Power Factor Correction– Correct power factor by downsizing over-sized motors– Correct power factor by adding capacitors
Demand Saving Potential– Reschedule operation of electrical equipment to reduce peak demand. – Use control equipment to shed loads to manage peak demand
Demand Response Program– Voluntarily reduce demand during demand emergency
Verify Billing Amounts
Date Days Consumption (kWh/period)
Avg Daily Consumption
(kWh/day)
Actual Demand
(kW)
Power Factor
Load Factor
Billed Amount
($/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
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10
20
30
40
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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
Elec
trica
l Dem
and
(kW
)
Meter 1 Meter 2
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10
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Hour of Day
Elec
trica
l Dem
and
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)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
Elec
trica
l Dem
and
(kW
)
Base load Shiftable Load
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20
40
60
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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
Elec
trica
l Dem
and
(kW
)Base load Shiftable Load
Demand Saving Potential
Good potential for uneven shifts
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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
Elec
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Base load Shiftable Load
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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
Elec
trica
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Demand Saving Potential
20 kW potential limited by first shift demand to 10 kW
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Base load Shiftable Load
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Hour of Day
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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
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on-peak off-peak
Time
Dem
and
(kW
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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.