Unified Power Flow Controller (UPFC), as the representative of the third generation of FACTS devices, is by far the most comprehensive FACTS device. In the steady state of power system. It can implement power flow regulation, reasonable control of line active power and reactive power and improvement of the transmission capacity of power system. In the transient state of power system, it can realize fast-acting reactive power compensation, dynamical support of the voltage at the access point and improvement of system voltage stability. Moreover, it can improve the damping of the system and the stability of power angle.
For the first time in the world, NR Electric Co., Ltd realizes the UPFC technology based on Modular Multi-Level Converter (MMC) and provides a complete set of UPFC equipment.UPFC adjusts the output voltage phase angle and amplitude of the series transformer via the converter and control system to realize the optimal control of power flow and system voltage.
PCS-8200 system consists of the following main equipment:
Control and protection system
Thyristor Bypass Switch (TBS)
The control and protection system, as the core of UPFC, controls, monitors and protects all the equipment of system.
Adoption of embedded software and hardware technology, distributed architecture and object-oriented approach for dividing function.
Realization of all the operation modes of UPFC and switchover between different mode, including UPFC mode, Static Synchronous Series Compensator (SSSC) mode and Static Synchronous Compensator (STATCOM) mode.
Smooth start and stop function to solve the series-side converter charging problem and be able to put into the series transformer without stopping the transmission line.
Proposal of multi-inverter double-circuit line control strategy via automatic selection of the master control unit.
Reasonable partition of UPFC with the complete protection to realize no dead zone protection for the entire UPFC adapted to different operating modes.
TBS, one of the main controlled parts of UPFC, is installed in winding’s terminal of series transformer in valve side in parallel with the mechanical bypass switch. It is able to bypass the series converter quickly and ensure the AC line is not affected in case of converter fault. This also prevents the influence on the converter caused by AC line fault.
Adopting high-power thyristor valves and fast bypass-switch technology with short response time, with high withstand capability of short-term heavy current and high reliability, no needing the parallel of thyristor and the water cooling.
Utilization of high-power thyristor valves and fast bypass-switch technology with short response time, high withstand capability of short-term heavy current and high reliability, no needing the parallel of thyristor and the water cooling.
Adoption of natural cooing for TBS, damping resistance and static equalizing resistance to achieve convenient maintenance and high controllability.
Based on the rich experience on engineering, NR developed the UPFC IGBT valve with excellent performance, high reliability, small installation space and facility to be maintained.
MMC adopts the compact vertical integrated structure of converter valve tower with layered and split phase design to satisfy the low DC voltage applications of UPFC engineering. This favours electrical symmetry of three-phase arms of converter valve and reduces the external electromagnetic radiation and converter valve installation space.
According to IGBT valve’s charge characteristics during the start of UPFC, it uses ultra-low voltage of self-powered supply and active voltage-equalizing strategy to reduce the possibility of valve’s uncontrolled state and improve the reliability of IGBT valve’s startup and operation.
Adoption of ultra-low leakage inductance and digital drive technology in developing UPFC power valve module ensures operational safety margin of power devices. This also helps maximize the rational utilization of the power devices’ operating parameters, increase operational efficiency of converter valve and reduce the electrical stress of core power component.
Based on hardware detection and state prediction technology, it realizes the monitoring and protection of electrical and non-electrical quantities of IGBT valve in full range. It ensures that the operational converter valve can be monitored. The valve control system can send up the real-time status of converter valve and its sub-modules complying with IEC-61850 protocol to the HMI. The operator can check the electrical and non-electrical quantities of valve to reduce unplanned outage of converter valve.
The system adopts the valve control unit with microsecond control precision to increase the dynamic response capability of UPFC IGBT valve. It realizes perfect monitoring function of IGBT valve sub-module to achieve fast protection capability against sub-modules’ over-voltage and over-current states.
It utilizes the advanced separated module and component schema for maintenance. Pluggable boards in the module and standard interface terminals facilitate troubleshooting and maintenance.