The key to detecting broken conductors



 

Detecting broken power line conductors has been made easier by the implementation of a new American National Standards Institute (ANSI) standard in modern recloser controllers. This article describes the process.

The latest implementation of the ANSI 46BC standard protection code provides for additional capabilities in broken conductor detection. Given the simplicity of the ANSI protection scheme, negative phase sequence protection techniques can now be applied by any user of NOJA Power’s RC10 controller to accomplish protection for broken conductors. Negative phase sequence (NSP) protection has been a mainstay in the protection suite offered by the company for many years, but the latest evolution of this functionality is the implementation of ANSI 46BC protection.

The application of NPS protection is an excellent detector of phase imbalance, however the values of I2 can be sensitive to adjustments of real load due to practical limits in achieving balanced networks. ANSI 46BC broken conductor protection compares the ratio of positive phase sequence (PPS) current (I1) to NPS current (I2), rather than each protection element in isolation.

Fig. 1: Installation example of a NOJA Power OSM recloser.

This assists in mitigating the variation of NPS values across the scope of energy delivery to customers while, at the same time, determining if there is a genuine phase discontinuity present.

The effect of discontinuity on power systems

When three-phase power lines experience a discontinuity, the effective balance of current and voltage across the three phases is compromised. This discontinuity could be a broken conductor either landing on the ground, or a blown single-phase fuse on one or two of the phases. These discontinuities may not necessarily result in higher over-currents, as the leakage to earth may be high or approaching infinite (floating discontinuous cable still attached to an insulator). When applying NPS protection to any particular feeder, the protection engineer must understand the principle of NPS impedance, and then apply the correct settings based upon multiple fault cases.

Positive sequence current is typically associated with the load placed upon a feeder, and is usually a measure of the “good stuff” being supplied to a customer. Too much positive sequence current flowing through a cable implies that there is an overload of customer usage (or usually a symmetrical three-phase fault) and that the current should be interrupted to preserve the asset.

Negative sequence current is essentially an imbalance in currents across the three phases, which can arise from multiple issues ranging from unbalanced single-phase connections, heavy use on a particular line, or the occasional connection style of “short arm syndrome” where customer connections are only made to the easily accessible outside A and C (red and blue) phases.

NPS protection alone is typically set around 30% of the overcurrent configuration, and if this was applied to the feeders in Table 1 it could be reasonably assumed that the broken conductor would not be detected. This is especially prevalent in low-load conditions. Table 1 demonstrates a case where a broken conductor could be detected with a high degree of confidence, despite a load current as low as 10 A.

The advantage of the ANSI 46BC implementation is that the NPS value is not examined in isolation, but compared to the value of the positive sequence current. Normal load variations can cause changes in the practical amount of NPS current seen by any particular relay, as demonstrated in Table 1.

Table 1: NPS (I2) variations.
Example Load variation Broken conductor (single discontinuity – floating)
Low load High load Low load High load
Positive sequence (I1) 10 A 500 A 10 A 500 A
Negative sequence current (I2) 1 A 10 A 5 A 250 A
Ratio of I2/I1 10% 2% 50% 50%

Variations in load during the course of a normal utility supply-day could imply higher NPS current, but this does not necessarily imply a conductor discontinuity. The challenge arises however when there is a break in one or two of the cables, as this does not necessarily increase the positive sequence current, meaning that conventional over-current/earth-fault relays may not detect the output. I2/I1 is relatively constant for any variation in load demand on three phases, but as soon as a discontinuity arises the ratio grows rapidly.

Table 2 outlines an exploration of some of the typical ratios experienced in a three wire three-phase distribution system.

Although the presence of NPS on any line implies imbalance, the practical truth is that the majority of distribution feeders will exhibit some level of NPS current. Configuring a protection element which only acts on this NPS current in isolation can be challenging as the protection should be sufficiently sensitive to detect genuine fault imbalances, but also sufficiently large so that nuisance tripping does not occur.

From Table 2, it is apparent that a healthy line has a I2/I1 ratio of approximately zero, but when a discontinuity appears on one or two of the lines, the ratio of I2/I1 rises above 20%. Considering that for most residential loads the positive and negative sequence impedance would be similar, it can be inferred that the ratio between I2 and I1 removes the need for impedance calculations as the impedances are cancelled out.

The implementation of ANSI 46BC in the recloser allows protection engineers to configure the ratio value at which a pickup is registered, with an appropriate definite timer to remove spurious tripping. Better still, the global configuration of 20% is a useful reference figure to indicate the presence of a broken conductor.

Table 2: Broken conductor ratios.
Fault condition Ratio of  I2/I1
Three phase balanced ~0
One phase interrupted and floating (not on the ground) 50%
Two phases interrupted and floating 100%
Broken conductor fallen on the load side Typically 25 to 50%
Discontinuity in an underground cable system Typically 25 to 50%

In the scenarios where NPS impedance differs from the positive sequence impedance, this 20% reference value can be adjusted accordingly. This implementation greatly reduces the complexity associated with implementing an NPS protection scheme, providing access to the benefits of the protection to a far larger group of users.

This implementation is useful for utilities and private entities looking to reduce their bushfire risk. With international investment in bushfire risk reduction strategies growing significantly, these users now have a simple solution to determine phase discontinuities and detect downed conductors.

Historically, the challenge associated with sensitive detection of small imbalances was significant, however with the use of modern semiconductor relays, paired with field protection devices, it is now possible to detect imbalanced conditions with far greater certainty.

Contact Jeremy Wood, RWW Engineering, Tel 011 433-8003, jwood@rww.co.za

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Source: EE plublishers

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