Introducción al estudio de capa límite 0.4 Aproximación de capa límite

Power-line carrier equipment is used in many protection and control applications; refer to IEEE Std 643^TM. The most common applications include pilot transmission-line protection, such as blocking or unblocking schemes, direct transfer tripping, and phase comparison. Other less common applications include voice and data traffic in addition to protection, based on the bandwidth a trap is designed for, or the type of coupling (single phase or multiple phase) to the transmission line. For the purpose of system testing, the following discussion covers single-phase coupling. The same techniques are used in phase-to-phase and three-phase coupling.

The types of tests covered in this clause include the following:

⎯ Line trap

⎯ Carrier transmit measurement terminated and bridged

⎯ Carrier receive measurement terminated and bridged

⎯ Measurement specification

⎯ Amplifier impact of power-line carrier testing 7.1.1 Line trap

This test requires the transmission or distribution line to be deenergized. Figure 28 shows a typical test setup. The trap tuning (resonant frequency set point) is verified by checking impedance versus frequency.

In Figure 28, the center frequency is tuned to the maximum impedance, which is measured using the impedance meter and signal generator. When verifying the trap frequency set point or points, it is best to use the highest impedance possible for the used spectrum. The typical minimum impedance value for wide band traps is 600 Ω. For single-frequency traps, the acceptable minimum impedance value may be 1000 Ω.

These ohmic values are examples. The manufacturers’ instruction manuals provide the minimum acceptable impedances.

1000 OHMS Minimum

WAVE TRAP

Impedance Meter Signal Generator

CF OHMS

Frequency (kHz)

Figure 28 —Power-line carrier wave trap frequency set point verification Table 7 describes the general connection terminals between test and measurement equipment.

Table 7 —Typical connection terminals between test and measurement equipment for wave trap frequency set point verification

Wave trap frequency set point verification connections Signal generator output

(high-level very low frequency (VLF) signal source]

Impedance magnitude (level) meter

Reflected frequency (RF) out Signal input from the RF generator High-impedance inputs (Hi Z and Gnd) Connected to wave trap 7.1.2 Carrier transmit and receive measurement terminated and bridged

This test verifies that the transmitted power from the power-line carrier equipment is efficiently coupled to the line. The test measures the standing wave ratio (SWR) and/or the reflected power. Note that the ratio mismatch would be excessively high and recognizable for defective or poorly adjusted equipment. It is possible not to achieve a precise 1:1 ratio match, for example, for multitap transmission lines. Also, care should be exercised for multitap transmission lines at the tap points to allow signals not to be attenuated excessively at the tap point and for the signals to get through to all the remote station terminals of the line.

In the transmit mode, the test equipment is connected to the carrier transmitter, as shown in Figure 29. In the bridge test, the impedance matching transformer of the line tuner is used to match the impedance of the carrier equipment to the impedance of the line. These measurements generally identify the types of problems that can produce improper readings, such as failed coax cable, misadjusted line tuner, or failed component. Also, it is important to verify on either side of the frequency spectrum in the case of hybrid arrangement.

With the carrier receive test, the signal level received from the remote terminal is verified. In the receive mode, the test equipment is connected to the line tuner.

Figure 29 —Setup for power-line carrier testing—equipment setup for transmit test (bridged) reflected power and SWR

Table 8 describes the general connection for measuring the power-line carrier transmit signal.

Table 8 —Typical connection for measuring and testing transmit frequency for Figure 29 Set-up transmit measurement using automatic VLF power standing wave ratio (SWR) test set

SWR meter Frequency selectable voltmeter

Voltage sampling coaxial terminal (e.g., –20 db signal)

Internal attenuator

RF in PLC output

RF out (transmitter) Line tuner input

The proper connections

As is the case for any type of test, proper connections and verification prior to turning on any test equipment are critical. In the case of power-line carrier measurement test equipment, it is also important to realize that at times power amplifiers may be used or to be aware of equipment limitations to minimize potential damage to equipment or the calibration of the test sets. For example, the user should be careful to connect the PLC transmitter output to the SAMPLE –20 dB connector in the case of the SWR meter. In most cases, this output port can only tolerate a small power (in the range of 100 mW) before blowing a protective fuse. Once the fuse is blown, the sample port becomes inoperative and the voltage sample feature is lost until the fuse is replaced. Damage to the instrument may be limited to the blown fuse if a protective fuse is provided.

7.1.3 Example for typical measurement specification for protection communication using power-line carrier

The following values illustrate the types of frequency ranges for different power-line carrier applications:

a) Permissive carrier output = 10 W or +40 dbm b) Blocking carrier output = 100 W or +50 dbm

c) Carrier receive level = 1 W or +30 dbm

d) Percentage of reflected power = 5% (or SWR of 1.6:1) e) Wavetrap impedance 1000 Ω at carrier frequency.

These numbers are examples only. The maximum permissible PLC can be 100 W, the minimum line trap impedance can be less than 1000 Ω, and the maximum reflected power of 5% may be difficult to obtain on short lines, maybe closer to 20%.

For multiple signals coupled to the same line or to different phases (multiphase coupling), additional tests may be needed to determine that a given receiver is not subject to interference from other signals and to validate proper frequency selection. Additional interference may be caused by other carrier transmitters connected to adjacent lines connected to the bus or on the same right of way. Interference from a parallel circuit on the same tower is a notable example. Misadjusted or failed traps can cause elevated levels of interference to other carrier channels, as well as the reduction of the signal strength at the intended receiver.