Getting to know PLC (POWER LINE CARRIER)
Basic Principles of PLC
The Power Line Carrier (PLC) system used by an electricity company utilizes High Voltage Overhead Lines and Extra High Voltage Overhead Lines as its transmission medium. In PLC, the signals transmitted are voice communication, data communication, and teleprotection. The system operates at frequencies ranging from 50 KHz to 500 KHz.
Essentially, PLC is a radio network connected through a power grid that acts as its antenna. What is needed in PLC is the conductor, not the voltage present in it.
Therefore, even if the conductor is not energized, the PLC can still function as long as the conductor is not disconnected. Special equipment is needed to input and extract signal information and electrical energy at the ends of the conductor. The block diagram of PLC is shown in Figure 1.
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| Figure 1: Block Diagram of PLC |
Coupling Equipment
To allow high-voltage line conductors to be used as a medium for propagating information signals, coupling equipment is required, which functions to:
- Carry a carrier frequency band from the PLC terminal to the high-voltage line and vice versa, ensuring minimal signal attenuation losses.
- Protect communication equipment from excessive voltage.
- Provide high impedance to the high-frequency carrier to prevent interference from equipment at the substation.
Coupling Capacitor
A high-voltage coupling capacitor serves as a connecting device between the high-frequency carrier signal equipment and the high-voltage phase wire conductor. It is also used for low-voltage measurements. Physically, it consists of a series of mica or paper capacitor elements submerged in oil. The elements and oil are enclosed in a dielectric porcelain cylinder designed with fins to prevent rainwater from flowing directly from the high-voltage side to the low-voltage side or the ground, which could cause a short circuit.
The cross-section of the coupling capacitor is connected to potential transformer equipment. This type of capacitor is also known as a Capacitor Voltage Transformer (CVT), used for voltage measurement and connected to a voltmeter on the control panel.
The output voltage from the transformer’s secondary winding is 220 volts, a conversion from the high voltage. For PLC purposes, only the capacitor is needed, while the potential transformer is used for power measurement.
A capacitor has a low impedance for high frequencies and a high impedance for low frequencies. Therefore, the coupling capacitor passes the high frequency generated by the PLC terminal and blocks the 50 Hz power grid frequency that carries electrical energy. If any 50 Hz frequency passes through the coupling capacitor, it is grounded via protection equipment. The capacitance of the coupling capacitor depends on the voltage class of the power transmission line used.
Wave Trap (Line Trap)
The Wave Trap, also known as Line Trap or Blocking Coil, is installed in series with the transmission line. Its main function is to block high frequencies emitted by the PLC terminal or received from the opposite station, preventing them from flowing into substation equipment, such as transformers or control panel measurement devices grounded to the earth. It is the opposite of the coupling capacitor, which allows high-frequency signals to pass to the substation equipment.
The Wave Trap’s impedance must allow the 50 Hz - 60 Hz low-frequency current to pass while providing high impedance to the high-frequency signals carrying information.
Since the Wave Trap is installed in series with high-voltage transmission lines, it must be able to carry the electrical current of the conductor (see Figures 2, 3, 4, 5, and 6). Additionally, it must withstand mechanical and thermal stress from the large working current or short-circuit current that may occur.
The Wave Trap is made of a coil that can hold energy at high frequencies but easily passes low-frequency energy. The Wave Trap’s current rating must match the current rating of the conductor on which it is installed. Common current ratings include 200 A, 400 A, 600 A, 800 A, 1250 A, 2000 A, and 3500 A.
Figure 2: Wave Trap 150 kV
Figure 3: Wave Trap or Line Trap at GI 150 kV
Figure 4: Wave Trap or Line Trap at GI 150 kV
Figure 5: Wave Trap
Figure 6: Wave Trap 500 kV
Figure 7: Line Trap at GI Titi Kuning 150 kV
Basic Working Principle of Wave Trap
The basic principle of the Wave Trap is a parallel L-C circuit, consisting of three main components: L, C, and LA, as shown in Figure 8.
Figure 8: Wave Trap Circuit Diagram
Main Coil (L):
Usually made of aluminum or copper, with various current capacities, it functions to transmit the 50 Hz current.
Tuning Capacitor (C):
It adjusts the desired frequency band and blocks unwanted frequencies.
Lightning Arrester (LA):
It protects the tuning capacitor and coil from lightning disturbances.
From the circuit shown in Figure 8, an impedance curve can be generated as a function of frequency. To determine the resonance frequency for blocking the specific frequency from the PLC terminal, the following formula is used:
The resonance frequency can be adjusted by tuning the capacitor, while the inductance value remains constant. Common inductance values are 0.2 mH, 0.3 mH, 0.4 mH, 0.5 mH, and 1 mH. Tuning capacitor values vary from 1.2 nF to 24 nF, with high breakdown voltages between 7,000 V and 20,000 V. The Wave Trap is also protected from lightning and short circuits by a parallel-connected arrester.
Line Matching Unit (LMU)
The Line Matching Unit connects the coupling capacitor (CC), which has an impedance of 300-600 Ohms, to the PLC terminal with 75 Ohm impedance.
The functions of LMU are:
- To match the line impedance with the coaxial impedance leading to the PLC terminal.
- To ensure the capacitive reactance of the coupling capacitor provides a resistive load for the carrier signal transmitter.
The LMU consists of components like a balancing transformer, coils, protection equipment, capacitors, and hybrids.
Figure 9: LMU for a Single Frequency
The transformer (T) balances the impedance of the high-voltage line and the coaxial cable. The inductance coil (L) and high-frequency capacitor (Cs) ensure that the carrier wave’s resistive load is adjusted accordingly. The inductance of L can be adjusted to counteract the capacitive reactance from the coupling capacitor (Xc). The Cs capacitor prevents 50 Hz frequencies from passing through the coil (L).
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