MV transformers causing little or no interference with BPL devices, although there are exceptions.

Similar to other RF devices, BPL equipment requires a minimum signal-to-noise ratio

(SNR) to transmit and receive data reliably. The signal budget is affected by various factors such as noise levels and attenuation. As long as at least the minimum SNR is maintained, data communications can occur. The SNR impacts the data throughput between BPL devices (higher is better). In

At left, is a fairly healthy MV circuit with noisefloor around -45dBm; on the right, is a very noisy power line with several spikes over
^{other words, the less noise exists} +10dBm which is not usable for BPL (note the different scales on the y-axis, right graph is off set by 20 dBm).
on the power line the better the
performance of the devices.

In addition to components and systems causing noise and attenuation on the power lines, other factors such as the environment can aggravate the noise problem. RF behavior varies with temperature, moisture and sun activity among others. Typically, as the day warms, noise levels on the power line increase. For example, as more load is generated, arcing at a bad splice could intensify the emission of RF (in the 2-30 MHz range) that is particularly harmful to the performance of BPL devices. Also, the sun radiates a significant amount of RF energy that varies cyclically, causing additional noise on the MV power lines.

The BPL equipment employs a relatively wide spectrum in which it transmits and receives signals ( 10-30 MHz) making the receiver circuitry susceptible to strong signals even if they are not intended for it. These strong signals are caused by some of the noise sources described above (mostly from the first two). Close proximity to certain types of radio stations can cause the receivers to not be able to “hear” the weaker BPL signal and thus lose or not establish a link to its partner device. Fortunately, equip-

ment manufacturers have recognized this early on and are providing appropriate filters with their equipment. Even with the filters installed, there can still be some strong noise sources within the desired spectrum (frequency spectrum that filters allow to pass through), but that are well defined in spectrum width. Many BPL devices have additional filtering capability—referred to as notching—that can block a specific range of frequency spectrum (e.g. a 200 KHz wide spectrum).

Although these filtering methods can be effective for static noise, they are not very effective for random and/or time-variant noise. These types of noise can become quite a problem for BPL devices and can have a major impact on reliability of the data communications.

The sources for these types of noise can be from arcing caused by cracked insulators, bad or corroded cable splices, defective lightning arrestors, or other utility equipment on the circuit. With varying levels of humidity the noise levels can vary greatly, typically getting progressively worse with increased humidity. When it rains, however, it appears that the noise levels actually recede and performance improves.

The figure above shows a comparison between a fairly low noise circuit and one with severe noise issues. The BPL signal would literally get drowned out in the example on the right and would not be able to establish communications with its BPL peer. The measurements were taken phase-to-ground.

Noise sources on the power lines such as those originating from faulty utility equipment can be reduced or eliminated by repairing the bad components. Obviously, that could be quite a costly proposition, depending on the shape of a utility’s distribution network. Plus, it would be an on-going challenge to maintain the network to these high standards.

Another approach to solve the noise interference problem caused by utility equipment, is to develop coupling methods for the BPL equipment that don’t use the earth-ground as return for the RF signal, since most of the noise exists phase-to-ground. Phase-to-phase noise may be lower in comparison to phase-to-ground, and may present some additional advantages, although one drawback to this approach would be doubling of the number of overhead couplers for an installation.