- Protection, Automation & Control
- Protection & Control
- Digital Substation
- Energy Management
- Power Stability Control
- Software Kit
Transmission Line Protection
PCS-221 Merging Unit
Merging units are used to sample AC analog values and transmit the sampled values(SV) to protection devices, bay control units(BCU) or electric meters through optical communication channels based on the IEC61850-9-2 protocol or IEC60044-8 protocol.
Input sampling signals, such as three-phase current and voltage, need to be synchronized if the merging unit is connected to several electronic CTs/VTs and electro-magnetic CTs/VTs. This can be solved either by GPS PPS, or by angle adjustment interpolation. The GPS PPS signal can gradually adjust the sampling interval of all intelligent sampling modules through the merging unit to realize synchronous sampling automatically. However, the synchronization accuracy of this method relies on the integrality and the accuracy of GPS PPS. Any loss of GPS PPS may directly influence the synchronization accuracy. The other way to synchronize the data sampling is using angle adjustment interpolation which does not rely on the GPS signal and thus has higher reliability. In practice, the mentioned two methods can be used in combination to achieve better accuracy.
Connection to Protection & Control Devices
Merging units and protection & control devices are usually connected through optical fibers based on IEC61850-9-1/2 or IEC60044-8 protocol. When using IEC60044-8 protocol, the merging unit only supports point-to-point communication with speed of 2.5Mbit/s or 10Mbit/s; when the IEC61850-9- 1/2 protocol is adopted, it is compatible with both Ethernet communication and point-to-point communication. The pointto- point communication is usually applied to the protection and control devices which have high requirements on real-time data sampling. The Ethernet network communication can be applied to bay control units and metering devices which have less requirements on real-time data sampling. In the point-to-point communication, it is much easier to use Manchester code than Ethernet technology. Thus, it is recommended to use IEC60044- 8 protocol to realize the communication between merging units and bay level devices in the practical application.