Electrical Data Logger Verification - Building Commissioning

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).

Single Phase AC Power •

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

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

Where pf = power factor Where Φ=phase angle between current and voltage

S (KVA) Q (KVAR) Φ P (KW)

Power Factor’s Practical Effects Q1 KVAR from Waveform Displacement

S (KVA)

Q2 KVAR from Waveform Distortion

Φ1 Φ2

P (KW) • • •

• •

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: 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 Per Phase: A: P = IVpf = 280V(100A).9 = 25200 W = 25.2KW B: P = IVpf = 280V(80A)1.0 = 22400 W = 22.4 KW C: P = IVpf = 280V(90A).8 = 20160 W = 20.2 KW Total 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

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 frontscreen – 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 Test 1 (Failed) Item 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Voltage A to Ground Voltage B to Ground Voltage C to Ground Voltage A to B Voltage A to C Voltage B to C Phase A Current Phase B Current Phase C Current Phase A PF Phase B PF Phase C PF

Power Quality Meter 277 277 276 481 479 480 205 170 222 0.94 0.92 0.91

Instrument 280 279 281 484 483 481 220 176 135 0.76 0.7 0.05

KVAR KVA KW

64 176 152

128.01 164.1 107

Electrical Sub-Meter Verification Report Test 2 (Passed) Fluke Power Quality Meter Instrument

Case 1 Test 1 (Failed)

Item 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16



Instrument 280 279 281 484 483 481 220 176 135 0.76 0.7 0.05

KVAR KVA KW

64 176 152

128.01 164.1 107


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

Case 1 Test 1 (Failed)

Item 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16



Instrument 280 279 281 484 483 481 220 176 135 0.76 0.7 0.05

KVAR KVA KW

64 176 152

128.01 164.1 107

Electrical Sub-Meter Verification Report Test 2 (Passed) Fluke Power Quality Meter Instrument 282.1 281.4 280.9

232 211 213 0.93 0.92 0.89

280 279 279 484 481 485 293 293 229 0.9 0.9 0.9

66.6 182 169.2

81 191.6 191.6

481 479 480

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


Power Quality Meter

Instrument

281 281 281


57 29 43 0.96 0.96 0.98

282 281 282 488 487 488 24 8 13 0.984 0.906 0.944

KVAR KVA KW

0.2 36.1 35

3 13 12



Power Quality Meter

Instrument

281 281 281


57 29 43 0.96 0.96 0.98

282 281 282 488 487 488 24 8 13 0.984 0.906 0.944

KVAR KVA KW

0.2 36.1 35

3 13 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.



KVAR KVA KW


Power Quality Meter

Instrument

281 281 281

57 29 43 0.96 0.96 0.98

282 281 282 488 487 488 24 8 13 0.984 0.906 0.944

60 30 44 0.96 0.96 0.98

282 281 282 488 487 488 59 30 44 0.96 0.96 0.98

0.2 36.1 35

3 13 12

0.2 36.5 35

0.3 36.3 35

281 281 281

Ultimately, we discovered that this was a wiring issue. This sub-meter was installed on a main switchgear, and the CTs were factory-installed to a set of shorting blocks. When the actual meters were installed, another set of shorting blocks were installed, but the original 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 submeters 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

Electrical Data Logger Verification - Building Commissioning

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