1. A Step Closer to Maintenance Free Gear
Dave Loucks, P.E., Eaton Corporation Senior Member IEEE
2010 Petroleum and Chemical Industry Committee Technical Conference
San Antonio, TX - September 20-22, 2010

2. Failure Modes
Electricity goes where it isn’t supposed to go (Insulation Failure)
Partial Discharge (Medium Voltage)
Ground Leakage Sensing / Arc Fault Detection
Electricity doesn’t go where it is supposed to go (Continuity Failure)
Energized Conductors with sufficient current
Temperature Measurement
De-energized Conductors
Inject current, measure voltage drop (ohmmeter) 2

3. Resistance Sensing Methods
Disadvantage
IR Photography
Expensive, visibility of objects required.
Non-Contact (Pyrometer)
Visibility of objects required. T = average in field of view. Multiple sensors required. I/O required
Contact (RTD, T/C, etc.)
Similar to pyrometer but also requires isolated I/O or battery power.
Fiber DTS
Expensive. Requires routing additional fiber over conductors
Low Resistance Ohmmeter
Difficult to perform on live system. Offline may require racking out device to test which could materially change results.

4. Direct Temperature SensingTC

5. Direct Temperature SensingTC
k Thermal conductivity of area touching between two objects
x Thickness of insulating material between two objects
A Surface area of the two objects touching one another
Thot Temperature of heated object
Tcool Temperature of heat sink or ambient
S Specific Heat of the cooling fluid
 Efficiency of heat sink
F Flow rate of cooling fluid
 Boltzmann Constant
 Surface Emissivity (0-1)
If:

6. Direct Temperature Sensing
Air flow
Ambient Temp
Surface cleanliness
Current flow
While “R” is one factor in Thot, other factors matter
Changes in ambient or air flow or cleanliness or current flow (per phase) will affect reading
Don’t assume that a low temperature means “no problem”
Low temperature could mean lower current or lower ambient or higher air flow or someone cleaned the dust out

7. Where Do You Measure Ambient?
Option 1: Add ambient temperature sensors at each bus temp sensor to factor out changes in ambient
Could be costly and difficult if you have many ambient sensors
Option 2: Measure ambient at some central point
Accuracy affected by variability of ambient sensed versus actual at conductor location.
Option 3: Derive ambient from the data
No extra sensors needed
Short of placing extra sensors near each bus temperature sensor, this will be the most accurate model

8. “Phase Difference” Analysis
Constant Ambient
Variable Phase A resistance
Constant Current
Constant Ambient
Variable Phase A resistance
Constant Current
Current
Phase-to-Phase Temp Differences
Analyze
Tc-Tb
Tc-Ta
Tc-Tb
Tc-Ta
non-zero
`slope - alarm!
resistance
Degrees
Difference between
Tb and Ta decreasing Tb Ta
Tb-Ta
Now
equal
Diff goes
negative
(Ta > Tb)
zero slope - OK
Tc-Ta
Tc-Ta
Tb-Ta
Tb-Ta
Time Ia
Tb-Ta Ib
Tc-Tb Ic
Tc-Ta
resistance
Degrees
Changing Ambient, Constant resistance
Constant Current
40 Degree Ambient
30 Degree Ambient
Time
Tc-Tb
same value
Changing Ambient, Constant resistance
Constant Current
Analyze
Current
Phase-to-Phase Temp Differences
Tc-Ta
same value
Tc-Tb
Tc-Ta
Tb-Ta
same value
Tb-Ta
zero slope - OK 8

9. How Do You Factor Current Changes?
T  f(i)
T  f(i)
f(i) = ?

10. Numerical Approximation
Independent variable: conductor temperature
Control variables: ambient, current
Measured Data
Numerical evaluation of energy balance equation
T = f(i) = f(i1) – f(i2)
T1 = [ H ln(i1)] T2 = [ H ln(i2)]

11. New Method Zxbus = Zx1 - Zx2 Ia Ib Ic Va1n Vb1n Vc1n Ia Ib Ic Va2n
Meter Ia Ib Ic
Va1n
Vb1n
Vc1n
Meter Ia Ib Ic
Va2n
Vb2n
Vc2n
Za1
Zb1
Zc1
Za2
Zb2
Zc2
Load
Za1 = Va1n / Ia
Zb1 = Vb1n / Ib
Zc1 = Vc1n / Ic
then
Za2 = Va2n / Ia
Zb2 = Vb2n / Ib
Zc2 = Vc2n / Ic
and
Zabus = Za1 - Za2
Zbbus = Zb1 - Zb2
Zcbus = Zc1 - Zc2
therefore
Since: Z = V / I

12. What R is a “Good” Junction?
Measured Per Pole Non-Fused Safety Switch
100A 181 
200A 132 
400A 87 
Measured Lug-to-Bus “Normal” (Good) Resistance
 (depending on ampacity of lug)
Heat Dissipation  Surface Area
Smaller products, less area for heat dissipation
Need lower and lower resistances to maintain low temp rise.
3rd party standards set limits for temperature rise

13. R Required for a “Bad” Junction
Recall our numerical solution of the temperature-versus-current relationship
Since Pin = i2R, then
Since T  ln(I) and Pin  R,

14. Example “Bad” Junction
RLug = 10 
90oC (363oK) nominal
How high must resistance rise to reach 130oC (403oK)?
403/363 = 1.11
e2.22 = 9.2
R must increase by 9.2 times (10   92  or +82 )

15. How Does “Normal” T Affect Z?
Cu  ( 0.4% / oC)
Based on 50oC rise on a 40oC ambient
? T if conductor load changes from no to full load?
(40oC, 313oK)  (90oC, 363oK)
50oC rise results in a 20% increase in R

16. How Many Ohms is 20%? Example: 200 A system R resistance (ohms)
Cross sectional area of a copper bus bar 6.35 mm (1/4 inch) x 25.4 mm (1 inch) is 1.61 x 10-4 m2 (0.25 in2)
Length = m (10 ft)
copper = 1.72 x 10-8 Hm
R resistance (ohms)
 resistivity (ohm-meter)
L conductor length (meters)
A conductor area (meters2)
325.6  * 0.2 = 65  from change in conductor temperature

17. Appears T Adds Significant Noise
82  from a failing junction
65  from ordinary conductor PTC effects!
How do we detect failing junctions from normal PTC effects?
Use the same “phase difference” analysis previously discussed
resistance
Degrees
Time
Constant Ambient
Variable Phase A resistance
Constant Current
Now
equal
Diff goes
negative
(Ta > Tb)
Tc-Ta
Tb-Ta
Tc-Tb
Difference between
Tb and Ta decreasing Tb Ta
Only works if you assume that the problem occurs on one phase at a time

18. Actual Equipment Testing
Junctions with +82  to baseline difficult to create
400A dropped across 82  = 0.033V (33 mV)
We could create a repeatable 500  increase.
?? amps across 500  = V  ?? = 66A
Vdrop_82 = 400 H82  = Vdrop_500 = 66 A H 500 
For test used 1.6 amps (2.5% of 66 A)
This is the equivalent of only 10A load on 400A circuit!
Would low-cost metering be sensitive enough to detect 82  at the equivalent of 10A load? ( A)

19. Experimentally Add 500  Zxbus = Zx1 - Zx2 Ia Ib Ic Va1n Vb1n Vc1n Ia
Meter Ia Ib Ic
Va1n
Vb1n
Vc1n
Meter Ia Ib Ic
Va2n
Vb2n
Vc2n
Za1
Zb1
Zc1
Za2
Zb2
Zc2
Load

20. Raw Impedance Calculation
Upstream Meter Z calculation
 Z
Added 500 
Unrelated load change on system
Downstream Meter Z calculation
Unrelated load change on system
Unrelated load change on system
Do you see a change in Z after adding 500 ?

21. Very Noisy Data Why so noisy? Do you see a 500  change? I don’t…
 8 m
 2 m
Z change occurred here
Why so noisy?
1) Waveforms captured different times, 2) Beyond A/D resolution limits
Do you see a 500  change? I don’t…

22. Filtered Z Extracting the Z data from noisy data was key requirement
Zmeasured(t1)  Zmeasured(t2)  Zmeasured(tn)
What is this noise then?
How do we separate a random, Gaussian noise from the real signal?

23. Step 1: Smooth Signal
If most recent sample is larger than rolling average
Increment rolling average by a magnitude equal to a gain times the most recent value
Aven = Aven-1 + (Sample * Gain)
If most recent sample is smaller than rolling average
Decrement rolling average by a magnitude equal to a gain times the most recent value
Aven = Aven-1 - (Sample * Gain)

24. Aven = Aven-1 + (Sample * Gain)
Z change occurred here
Looks like the trend is going up, but still quite subtle

25. Smoothed Trend Line: 3 Phase

26. Smoothed Trend Line: 3 Phase
Phase A increases
+500 
Phase B remains constant
Phase C remains constant

27. Convert Values To Percentages
Phase A is pulling away from Phase B and C

28. Unexpected Resolution
500  extra resistance
1.58 amps of current
V = IR = 1.58 (0.0005) = volts drop
But the meter used only resolves 0.12 volts / bit
We are resolving better than volts
/0.12 = 0.65% of a bit
We are reading signals smaller than 1/151 of a bit!

29. How Can This Be? Real world signals are noisy
This noise adds a broad spectrum forcing function on top of the underlying process variable
The magnitude of this forcing function is sufficient to slew past many bits of A/D resolution
The algorithm, rather than measuring the average value, instead calculates the probability that a particular bit is triggered
The bits closest to the real value are triggered more and have a higher probability of being triggered

30. Sensitivity Analysis Lower gain, higher sensitivity, slower response
Phase A is pulling away from Phase B and C
Delta 500  (1/151 bit)
Seems like more resolution is available
Lower gain, higher sensitivity, slower response
At least 1 more order of magnitude resolution (<10 70A)

31. Summary New method to detect failing pressure junctions
10  resolution in tests
Ambient and cooling insensitive
Detects failing junctions before glowing
Sense areas not convenient to IR scan
Supplement scheduled IR scan
Postpone if no issue (reduce cost)
Dispatch sooner if problem seen (improve reliability)