PLCC Components
LMU/ LMDU
The coupling device comprising the LMU & protective devices is used in coupling PLCC terminals with the transmission line. This is entirely a passive device and is fixed between the coupling capacitor’s LV terminal and the co-axial/balanced connection to the carrier terminal.
ROLE OF LINE TUNING UNITS IN PLCC LINK
LMU in combination with the coupling capacitor (Capacitive Voltage Transformer) is primarily, a bandpass filter with a frequency bandwidth of 40-500 kHz and Coupling Capacitor value from 4400pF to 12000pF.
The working mechanism of the line tuning unit is shown in the attached line diagram of a typical phase-to-phase PLCC link the line tuning unit is connected between the low voltage terminals of the coupling capacitor and station earth, the line tuning unit consists of the following components & each component performs the following function.
1. Matching Transformer
This component is required to efficiently transfer carrier power from the low-voltage side to the HT line via the co-axial cable and the coupling capacitor of the CVT with the surge impedance of the HT Line, 400Ω at phase-to-ground & 600Ω at phase-to-phase and the surge impedance of the co-axial cable is 75Ω.
This will not allow efficient transfer of carrier power from the low voltage end to the HT line energy transfer between two networks and is more efficient when the surge impedance is identical. Therefore, we need to match the HT line impedance to the co-axial cable impedance and it is achieved by employing a transformer with a suitable turn ratio so that the transformed impedance will be identical thus enabling efficient transfer of power from the low voltage end to the HT line.
2. High Pass Filter
This component is required to enable the line tuning unit to be selective so that only the desired signals are allowed to be transferred. This is basically a T-filter consisting of a polypropylene Capacitor that matches the value of the coupling capacitor of the CVT. The shunt arm of the filter is a suitably designed air-core inductor as an optional requirement and is not required for wideband design.
3. Drain Coil
This component is part of the three-element protective device employed for the safety of the personnel and the sensitive electronic components employed for communication and relaying. The drain coil is an inductor connected between the LV terminal of the coupling capacitor and the station earth. The inductor offers very low impedance to power frequency current.
4. Protective Arrester
The potential at the LV terminal of a coupling capacitor will reach dangerously high values during the system transient. This situation is further aggravated due to the presence of the drain coil. The protective arrester is connected to limit the value of this transient overvoltage to safe values.
5. Earth switch
This is a manually operated knife switch that can be operated along the LV terminal of the coupling capacitor, whenever maintenance work is contemplated at the site. All these components except the earth switch are located inside weatherproof FRP Housing.
MECHANISM
Coupling devices are used in PLCC links employed in H.V. networks.
It is connected between the low voltage terminal of the coupling capacitor/CVT and the station earth. The main functions of the coupling device are as follows.
- To safely ground the low voltage terminal of the coupling capacitor and at the same time limit high-frequency signals from draining to ground.
- To limit the transient over-voltages that are likely to appear at the low voltage terminal of the coupling capacitor.
- To safely ground the equipment during maintenance and emergencies.
- To enable the efficient transmission of the high-frequency signals generated from the carrier set
- to the high – voltage line using the high – voltage couplin capacitor.
- Provide discriminatory properties for the transmission of carrier signals by incorporating filters.
