Slide 1

Detecting Open Phase ConductorsBob Wilson – ABB Inc.

2015 Texas A&M Relay Conference – March 31, 2015

Slide 2

Effects of Open Phase ConductorsModerate to Severe Neutral Current

All three sequence networks are involved Creates a phase imbalance resulting in

moderate to heavy neutral current in grounded wye systems

P on

xo oy

N on

xo oy

0 on

xo oy

Slide 3

Effects of Open Phase ConductorsUnbalanced Currents and Voltages

Imbalance in loads results in downstream imbalanced voltage

Imbalanced voltages cause excessive heating in three phase motors and transformers

Slide 4

Open Phase ConductorsConventional Protection is Largely Ineffective

Ground protection is based on 3Io magnitude Ground protection must not trip for worst-case

natural load unbalance, usually under peak load conditions

Ground pickup settings may be only slightly less than phase pickup settings.

Complete loss of one phase may be undetectable except at full load conditions

A

B

C

3I0

Ia + Ib + Ic = 3Io

A

B

C

3I0

Slide 5

Open Phase ConductorsWorst Heavy Load Unbalance Dictates Ground Pickup

Ia + Ib + Ic = 3Io

A

B

C

3I0

Neutral current pickup must be above 250A in this example

Heavy Load Assumptions (50% unbalance)Ia = 250 ∠ 0Ib = 500 ∠ 240Ic = 500 ∠ 120I1= 416.67 ∠ 0I0 = 83.33 ∠ 180 I2 = 83.33 ∠ 1803I0 = 250A

Slide 6

Open Phase ConductorsConventional Ground Protection is Largely Ineffective

Ia + Ib + Ic = 3Io

A

B

C

3I0

Light Load Assumptions (100% unbalance)Ia = 0 ∠ 0Ib = 200 ∠ 240Ic = 200 ∠ 120I1 = 133.33 ∠ 180 I0 = 66.67 ∠ 180 I2= 66.67 ∠ 180

A complete loss of one phase under light-to-medium load conditions does not produce enough zero sequence current to detect an open conductor condition

3I0 = 200A

Slide 7

Open Phase ConductorsNegative Sequence also Ineffective

Ia + Ib + Ic = 3Io

A

B

C

3I0

Light Load Assumptions (100% unbalance)Ia = 0 ∠ 0Ib = 200 ∠ 240Ic = 200 ∠ 120I1 = 133.33 ∠ 180 I0 = 66.67 ∠ 180 I2 = 66.67 ∠ 180

In a grounded wye system, the negative sequence current is no larger than the zero sequence current

Slide 8

Open Phase Conductors46PD Phase Discontinuity Protection

50% Unbalance

3I0 = 250A

= 0.20

Heavy Load Conditions

46PD is based on I2/I1 ratio

Slide 9

Open Phase Conductors46PD Phase Discontinuity Protection

Light Load Conditions

The 46PD responds to percentage unbalance and therefore provides improved sensitivity under less than maximum load conditions.

Slide 10

Open Phase ConductorsConclusions

Detection of open phase conductors is a challenge Zero sequence and negative sequence elements are mostly ineffective 46PD (percentage unbalance) offers additional sensitivity when load

levels decrease Even 46PD cannot detect all open phase events

Slide 11

Open Phase Conductors

Questions?