Electrical Sub-Meter Commissioning

Joe Moroni

Cx Engineer, McKinstry Co.

Seattle, Wa.

Overview• Electrical Theory

– AC versus DC power

– Power Factor and what it means to you

• Electrical Sub-Meters

– How are outputs derived

– What can go wrong

• Measuring AC Power for verification

– Types of Instruments

– Electrical Safety Considerations

• Troubleshooting Exercise

Why Sub-Meters?

• Sub-metering is code driven

• Provides valuable data for owner

• Meeting the requirement isn’t really that hard… Meeting the intention of the requirement will take more understanding and training.

Electrical Theory

• Ohm’s Law: Current (I) flowing through a circuit is directly proportional to the applied voltage (V) and inversely proportional to the resistance (R)

V=IR• Electrical Power (P) is measured in Watts, and is—simply stated—the rate of doing

work.

DC Power: P = IV

AC Power Definition• AC means “alternating

current,” and the way this works is that current (and voltage) are transmitted in sinusoidal waveforms.

• As resistive or capacitive components are added to an AC circuit, the waveforms get of phase.

• The difference in phase between these is the phase angle (Φ).

Type Equation Units

Active Power (Real Power) P = IV pf KW

Apparent Power (Power delivered by Utility)

S = VI KVA

Reactive Power Q = VI sin Φ KVAR

Single Phase AC Power

• Thus, AC Power is more complex than DC power, and consists of three components:

Where pf = power factor

Where Φ=phase angle between current and voltage

S (KVA)

P (KW)

Q (KVAR)

Φ

Power Factor’s Practical Effects

Φ1

Φ2

S (KVA)

P (KW)

Q1 KVAR from Waveform Displacement

Q2 KVAR from Waveform Distortion

• Let’s call the figure above the load side power—what the client is using.• What this means is that the power they are able to use to run air handlers, power computers, or whatever

is the KW shown above.• But what they’re really getting from the utility is the KVA that they’re using. (This is the utility’s output KW,

because if we showed the same figure for their generators, the bottom leg of their triangle is what would be going out to the grid) Sometimes, the client will pay a power factor penalty to compensate for this—or pay for KVA outright.

• And what is causing this additional charge (and inefficiency) is the power factor, which is being caused by both displacement (Φ1 ) and distortion (Φ2 ) power factor.

• And finally, it is important for us to know all of this because we will be getting a lot of different data from the instruments we will be using and it is our job to translate that for the client.

Power Factor Expected Values

• “A practical measure of the efficiency of a power distribution system”

• By definition, must be a value between 0 and 1

• Normally, observed power factor values are between 0.7 and 1.0

• The lower the value, the more inefficient a system is

Multi-Phase AC Power

• Most Multi-Phase AC distribution systems are 3-phase systems

• To calculate power, you can either:

– Add the single phase powers together

– Use the three phase power equation, along with average current, voltage and power factor:

P = IV pf 1.73 (If using phase to phase Voltage)

P = 3 IV pf (If using phase to ground voltage)

Calculation Example

• You have a 3- phase system with the following readings:

• Power Per Phase:A: P = IVpf = 280V(100A).9 = 25200 W = 25.2KWB: P = IVpf = 280V(80A)1.0 = 22400 W = 22.4 KWC: P = IVpf = 280V(90A).8 = 20160 W = 20.2 KWTotal KW = 25.2+22.4+20.2 = 67.8 KW

• Three Phase Power Equation:P = 3*IVpf = 3*280V*90A*.9 = 68.0 KW P = 1.73*IVpf = 1.73*490*90A*.9 = 68.7 KW

Phase Voltage,Ph. to Ground

Voltage, Ph. to Ph.

Current pf

A 280 V 490 V 100 A 0.9

B 280 V 490 V 100 A 1.0

C 280 V 490 V 90 A 0.8

Power Sub-Meters

• As the previous calculations show, there are multiple ways to arrive at the same KW value

• Power meters arrive at final values by taking each phase individually and adding or by taking all three phases into account

• Thus, for verification purposes it is important to know how the meters you are verifying work so that you can effectively troubleshoot problems.

Power Meter Connection Verification

• Above is a schematic showing the connections of a 3-phase power meter.• The major components are:

• Current Transformers (CTs)• Voltage Connections (could use Voltage transformers, but usually line voltage)• Control Power to Meter• Communication cable to remote monitoring system

Current Transformers (CTs)

• Typically, are split-core type or rope type

• CTs are installed directly on the load wires, just like an amp clamp.

• Some CTs require shorting blocks for installation, while others do not. Specific data should be reviewed for individual CTs.

• CTs are directional, and if they are installed or wired in the wrong direction, current readings will not be correct (Negative value)

• Usually, an ammeter will not output negative Amp values, so another indication of backwards CTs is negative KW

• This can be fixed either at primary or secondary side

Voltage Connections

• Voltage connections will almost always be through a switch or breaker

• Meters will either use line voltage or a voltage transformer.

• Again, review submittal data or operation manuals to ensure meters set-up properly

Other Connections

• Control power

– May be either high or low voltage connections

• Communications Wiring

– To remote monitoring system or other front-screen

– If required, should be properly scaled (i.e. PTP)

Troubleshooting Exercise

• The following slides are from one of my projects and show a wide range of problems

Case 1

Electrical Sub-Meter Verification Report

Test 1 (Failed) Test 2 (Passed)

Item Power Quality Meter InstrumentFluke Power

Quality Meter Instrument1 Voltage A to Ground 277 2802 Voltage B to Ground 277 2793 Voltage C to Ground 276 2814 Voltage A to B 481 4845 Voltage A to C 479 4836 Voltage B to C 480 4817 Phase A Current 205 2208 Phase B Current 170 1769 Phase C Current 222 135

10 Phase A PF 0.94 0.7611 Phase B PF 0.92 0.712 Phase C PF 0.91 0.051314 KVAR 64 128.0115 KVA 176 164.116 KW 152 107

Case 1Electrical Sub-Meter Verification Report

Test 1 (Failed) Test 2 (Passed)

Item Power Quality Meter InstrumentFluke Power

Quality Meter Instrument1 Voltage A to Ground 277 2802 Voltage B to Ground 277 2793 Voltage C to Ground 276 2814 Voltage A to B 481 4845 Voltage A to C 479 4836 Voltage B to C 480 4817 Phase A Current 205 2208 Phase B Current 170 1769 Phase C Current 222 135

10 Phase A PF 0.94 0.7611 Phase B PF 0.92 0.712 Phase C PF 0.91 0.051314 KVAR 64 128.0115 KVA 176 164.116 KW 152 107

Here, Phase C current seems to be driving the power factor discrepancy for all three phases and in turn the power readings.

Case 1Electrical Sub-Meter Verification Report

Test 1 (Failed) Test 2 (Passed)

Item Power Quality Meter InstrumentFluke Power

Quality Meter Instrument1 Voltage A to Ground 277 280 282.1 280

2 Voltage B to Ground 277 279 281.4 279

3 Voltage C to Ground 276 281 280.9 279

4 Voltage A to B 481 484 481 484

5 Voltage A to C 479 483 479 481

6 Voltage B to C 480 481 480 485

7 Phase A Current 205 220 232 293

8 Phase B Current 170 176 211 293

9 Phase C Current 222 135 213 229

10 Phase A PF 0.94 0.76 0.93 0.9

11 Phase B PF 0.92 0.7 0.92 0.9

12 Phase C PF 0.91 0.05 0.89 0.9

1314 KVAR 64 128.01 66.6 81

15 KVA 176 164.1 182 191.6

16 KW 152 107 169.2 191.6

Troubleshooting revealed that phase B and C connections from CT to instrument were swapped.

Case 2

Electrical Sub-Meter Verification Report

Test 1 (Failed) Test 2 (Passed)

Item Power Quality Meter InstrumentFluke Power

Quality Meter Instrument1 Voltage A to Ground 281 282

2 Voltage B to Ground 281 281

3 Voltage C to Ground 281 282

4 Voltage A to B 488

5 Voltage A to C 487

6 Voltage B to C 488

7 Phase A Current 57 24

8 Phase B Current 29 8

9 Phase C Current 43 13

10 Phase A PF 0.96 0.984

11 Phase B PF 0.96 0.906

12 Phase C PF 0.98 0.944

1314 KVAR 0.2 3

15 KVA 36.1 13

16 KW 35 12

Case 2Electrical Sub-Meter Verification Report

Test 1 (Failed) Test 2 (Passed)

Item Power Quality Meter InstrumentFluke Power

Quality Meter Instrument1 Voltage A to Ground 281 282

2 Voltage B to Ground 281 281

3 Voltage C to Ground 281 282

4 Voltage A to B 488

5 Voltage A to C 487

6 Voltage B to C 488

7 Phase A Current 57 24

8 Phase B Current 29 8

9 Phase C Current 43 13

10 Phase A PF 0.96 0.984

11 Phase B PF 0.96 0.906

12 Phase C PF 0.98 0.944

1314 KVAR 0.2 3

15 KVA 36.1 13

16 KW 35 12

• Here, Current on all three phases is off. Phase a is ~ ½ expected, phase B and C ~ ¼.• Power readings are off by roughly 1/3.

Case 2Electrical Sub-Meter Verification Report

Test 1 (Failed) Test 2 (Passed)

Item Power Quality Meter InstrumentFluke Power

Quality Meter Instrument1 Voltage A to Ground 281 282 281 282

2 Voltage B to Ground 281 281 281 281

3 Voltage C to Ground 281 282 281 282

4 Voltage A to B 488 488

5 Voltage A to C 487 487

6 Voltage B to C 488 488

7 Phase A Current 57 24 60 59

8 Phase B Current 29 8 30 30

9 Phase C Current 43 13 44 44

10 Phase A PF 0.96 0.984 0.96 0.96

11 Phase B PF 0.96 0.906 0.96 0.96

12 Phase C PF 0.98 0.944 0.98 0.98

1314 KVAR 0.2 3 0.2 0.3

15 KVA 36.1 13 36.5 36.3

16 KW 35 12 35 35

Ultimately, we discovered that this was a wiring issue. This sub-meter was installed ona main switchgear, and the CTs were factory-installed to a set of shorting blocks. Whenthe actual meters were installed, another set of shorting blocks were installed, but theoriginal were never removed.

Other Observed Issues• Counterclockwise phase rotation

– Sub-meters may not be programmable, and if a meter only looks at one phase and adds values for total power, this could give incorrect (leading) power factor. This will affect magnitude of your KVAR reading.

• Sub-meter panels might have only one voltage reference. Need to ensure that the correct voltage is connected to each instrument.– In my experience, each floor of a building got 480V

power and stepped it down to 120V. The power sub-meters only referenced one floor for 120V, however. This caused pf values to be wrong for the affected floors.

Summary

• Sub-Metering is the wave of the future

• Something relatively simple in concept is often fumbled on delivery

• Understanding electrical theory and the specific meter installed will help us better serve the customer