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
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
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
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
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.
Case 2
Test 1 (Failed)
Item
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
Power Quality Meter
Instrument
281
281
281
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
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
Electrical Sub-Meter
Verification Report
Test 2 (Passed)
Fluke Power
Quality Meter
Instrument
Case 2
Test 1 (Failed)
Item
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
Power Quality Meter
Instrument
281
281
281
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
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
Electrical Sub-Meter
Verification Report
Test 2 (Passed)
Fluke Power
Quality Meter
Instrument
• 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 2
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
KVAR
KVA
KW
Electrical Sub-Meter
Verification Report
Test 2 (Passed)
Fluke Power
Quality Meter
Instrument
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