THE RTDS SIMULATION AND ANALYSIS ON CSG OPERATION CHARACTERS

THE RTDS SIMULATION AND ANALYSIS ON CSG OPERATION CHARACTERS Mengjun Liao, Weiqiang Han, Qi Guo, Yunwen Guo Electrical Power Research Institute, China Southern Power Grid, Guangzhou, P.R.China 510000 Abstracts: Nuozhadu and Xiluodu DC transmission system in operation makes the AC/DC hybrid scale of CSG enlarge to eight AC and seven DC transmission systems, which increases the complexity of AC/DC hybrid transmission system operation. It s indicates that the paralleled complex-circuit AC/DC hybrid transmission system of CSG cannot be simulated accurately and cannot be analysed on some deeper problems by only using the electro-mechanical transient simulation tools. This paper constructs the RTDS simulation platform of CSG above 220kV grid with 8 HVDC system, based on the closed loop connection and FDNE (Frequency Dependent Network Equivalent), which recurs the electro-magnetic transient and response of (U)HVDC control and protection system. At last the security risk assessment of multi-hvdcs commutation failures is present by this simulation platform in some extreme AC fault. Key words: Hybrid AC&DC power system, Real-time simulation, Interaction of AC&DC, Multi-HVDCs commutation failure 1. Overview China southern power grid (CSG) has institute eight HVDC and eight AC pattern in the West to East Power Transmission Project, while Nuozhadu HVDC project and Xiluodu HVDC project are operating in 2013. It operates one of the most sophisticated and technologically advanced in the world featuring long distance, extra high voltage and hybrid AC&DC operation. Multi-HVDC projects centre in the load area lead to complex interaction between HVDC system and AC system, such as a key issue of multi-hvdc commutation failure. Considering a key factor of nonlinearity of HVDC transmission system in the hybrid AC&DC operation, traditional TSA (Transient Stability Analysis) may be hard to simulate accurately in the electro-magnetic transience of interaction between HVDC and AC system and the response of control and protection system. Therefore, this paper indicates to build a real-time simulation platform of CSG to solve the analysis issues of hybrid AC&DC operation. At last, investigation about security risk of multi-hvdcs commutation failure is present. 2. Feasibility and Challenge of Large Scale Real-Time Simulation 2.1 Real-Time Simulation (RTS) RTDS, RT-LAB, HYPERSIM and ADPSS are typical RTS tools around the world. At the beginning of being designed, they are used to verify actual power system Control and Protection (C&P) devices with hardware-in-loop (HIL) technology instead of the traditional dynamic analog test. As an Electro-Magnetic Transient (EMT) algorithm, RTS simulates the real power system instant by building the power model with high order differential equation and partial differential equation. It is able to respond nonlinearity of power electronic accurately. Therefore, RTS presents significant advantages of accurate simulation for HVDC and HIL technology in hybrid AC&DC operation comparing with TSA. [1]~[4] 2.2 Challenge of Hybrid AC&DC Operation Real-Time Simulation In hybrid AC&DC operation simulation, two key issues below must be considered seriously: one is the accurate modelling of HVDC transmission system, including power electronic nonlinearity as well as C&P response of HVDC. The other is characteristic of large scale AC system. As

mentioned before, RTS presents significant advantages of accurate simulation for HVDC and HIL technology in hybrid AC&DC operation comparing with TSA. However, the simulation scale of RTS is limited to maintain the solution in real-time which ensures HIL requirement strictly. Taking Real-Time Digital Simulator (RTDS) as example, there are specific limitations of node, component and variable as well as interface channel number in each unit of RTDS. In the earlier investigation of power system, the key issues of system stability concentrated upon the generators electromechanical transient process to judge whether two groups of coherent generators would lose the synchronization during disturbance. Power system researches usually reduce the AC system scales in real-time simulation by equivalent models. Dynamic equivalence of coherent generator groups and network equivalence are two of the most common methods. However, as an increasing of system capacity and power electronic application, new challenge arises in real-time simulation of hybrid AC&DC system. One is the major factor of power system stability turns to HVDC electro-magnetic transience gradually from generators electromechanical transient. Traditional dynamic equivalence based on fundamental frequency may lose the interaction characteristic of high frequency. On the other hands, fast increasing of AC system scales make dynamic equivalence more difficulty than before. [5]~[6] Some references present a new technology of Frequency Dependent Network Equivalence (FDNE) as an accurate solution of real-time simulation for large scale power system. A wide-band multi-port equivalence will be used to instead of less critical power grid so that the simulation scales would be decreased but a wide-band electro-magnetic transience would be reserved in the simulation. In this way, hybrid AC&DC power system is able to be modelled in RTS [7~[9]. Considering advantages of RTS and FDNE technology, this paper has build a real-time simulation platform with whole transmission network above 220kV based on RTDS. Some important C&P devices of HVDC transmission projects and Special System Protection (SSP) are HIL in the RTS platform. 3. The RTS Platform with Whole Transmission Network of CSG above 220kV The RTS platform with whole transmission network above 220kV is shown as Figure.1. It includes eight HVDC transmission systems and their C&P systems. Two sets of C&P systems are HIL and the rest are used digital models in RTDS. Certainly, other C&P systems can be switched as HIL depending on different investigating objective. The AC system models are separated into two parts. One is electro-magnetic transient simulation model of full power system surrounding the converter stations and the West to East Power Transmission Project. The other part is an equivalence of less important power system using FDNE technology. By this way, it decreases the simulation scales but maintains a high simulation accuracy of hybrid AC&DC system. HVDC C&P model HVDC SYSTEM HVDC C&P DeviceClosed-loop HVDC Digital model BUS AC SYSTEM Full EMT simulation Grid Equivalence simulation Grid using FDNE Fig. 1 Schematic diagram of the CSG RTS platform with whole transmission network above 220kV 3.1 HIL of HVDC C&P System HVDC transmission system modelling is the most important section in the RTS platform. Power electronics are full electro-magnetic transient simulation models in RTDS, such as valve, converter transformer, filters and so on. The parameters refer to actual project. Set of C&P devices, including Station Controller, Pole Controller, Pole Protection and so on, are HIL with the specific GT interface cards. The analogs of voltage and current as well as status of breakers are sending to C&P devices by GT output interface cards. After C&P system calculation,

references of alpha angle and orders of breakers would be returned to the RTDS model for HVDC operation. of YG UHVDC, TG HVDC and GG2 HVDC are HIL in the RTS platform. Fig. 2 schematic diagram of HIL of HVDC C&P system 3.2 HIL of Special System Protection In order to verify the C&P response of actual power grid, some SSPs are necessary in the RTS platform. Out-off-step separator, a system special protection, is considered to be closed-in-the-loop, which is used to separate power system into two disconnected power grid to prevent instability of two groups of coherent generators during disturbance. Considering where oscillation between coherent generators centres in the AC transmission lines due to the multi-hvdcs commutation failure, out-off-step separator of stations between Guangxi power grid and Guangdong power grid are HIL in the RTS platform. 3.3 Comparison of Simulation Result and PMU Record These years, there are some post-disturbances in the multi-hvdcs infeed power grid leading to multi-hvdcs commutation failures. With comparison of simulation result and PMU record in actual case, the RTDS simulation platform is proved to be effective and high accuracy. Figure 3.1 to 3.3 show a post-disturbance simulation of 2012.8.11 failure. In this case, single phase short-circuit was located on the T-line from Zengcheng to Suidong Ⅱ which leaded to five HVDCs commutation failure. Firstly, it is necessary to adjust power flow and HVDC to meet the operation before post-disturbance in the CSG RTS platform with whole transmission network above 220kV. Then set a single phase short-circuit where it was in 2012.8.11 failure. In this case, C&P systems Fig. 3.1 The comparison of simulation result and PMU record YG HVDC power, voltage and current Fig. 3.2 The comparison of simulation result and PMU record TG HVDC power Fig. 3.3 The comparison of simulation result and PMU record power of AC transmission line

Table 1 shows the detailed difference of HVDC commutation failure between simulation and actual record. According to the simulation result and PMU record, commutation failure of three HVDC projects during post-disturbance present approximately same response in power loss and recovery. Then, prony method is taken to analyse the system oscillation frequency and damping. The difference of oscillation between simulation and actual record are shown as Table 2. Table 1 The detailed difference of HVDC commutation failure HVDC Minimun power Recovery time PROJECT Simulation Record Simulation Record substation is shown as figure 4. Two transmission lines or transformers are connecting to the electrical buses of substation by three breakers. If a short-circuit fault locates in LINE 2, breaker 5021 and 5022 will be open to clear the fault. However, if the mid-breaker 5022 doesn t open as well as expected, the Breaker Failure Protection will be active after a delay to open breaker 5021 and 5023. At the same time, the relevant breakers of the other side of LINE 2 and LINE 5 will be open, too. In this situation, two transmission lines would be power off. LINE 1 LINE 2 LINE 3 A Short-circuit fault 5011 5021 5031 #1 Bus YG UHVDC 0MW 0MW 177ms 158ms 5012 5022 5032 TG HVDC 0MW 280MW 132ms 142ms GG2 HVDC 1600MW 1800MW 58ms 22ms 5013 5023 5033 #2 Bus Table 2 The detailed difference of oscillation Oscillation Frequency Damping Transmission Line Simulation Record Simulation Record Wuzhou Luodong I 0.37 Hz 0.36 Hz 6.6% 11.5% Hezhou Luodong I 0.36 Hz 0.37 Hz 6.5% 12.5% Guilin - Xianlingshan I 0.37 Hz 0.36 Hz 7.3% 11.6% 4. Power system security risk of multi-hvdcs commutation failure As it shows above, short-circuit fault in multi-infeed power system may lead to multi-hvdcs commutation failure. As if this fault is not able to be clear in time, multi-hvdcs commutation failure will last longer and system disturbance becomes more critical. Even, it will cause system instability and damage. In order to investigate the power system security risk of multi-hvdcs commutation failure, this section simulates largest load and power generation of strict operation in RTS platform and sets a series of faults about single phase short-circuit and mid-breaker failure located in important 500kV substations of Guangdong power grid. The main electrical connection of 500kV LINE 4 LINE 5 LINE 6 Fig. 4 The main electrical connection of 500kV substation Take Beijiao 500kV substation as example. According to the main electrical connection of Huad substation, if a short-circuit fault and mid-breaker failure locate in Beijiao-Zengcheng I transmission line, Breaker Failure Protection acts after 300ms and cut-off Beijiao-Zengcheng I transmission line and #1 transformer. As Figure 5 and Table 3 shown below, eight HVDC projects present commutation failure at the same time during this disturbance. The minimum power of four HVDC projects reach 0 MW and all HVDC projects take more than 344 micro-second to be recovery. Because of the long-time multi-hvdcs commutation failure, system becomes instable and out-off-step oscillation is located in the AC transmission lines of Guangdong section. As Figure 6 shown, about 3 seconds after the short-circuit fault, Special System Protection acts and cut off the AC transmission lines. Power grid is separated into two disconnected parts. Besides, system instability will be caused by a short-circuit fault and mid-breaker failure in another

nine substations of Huadu, Suidong and so on. Fig. 5 Simulation result of HVDC power Minimum Power HVDC Project of HVDC Recovery (MW) XLD HVDC 0 400ms NZD UHVDC 820 462ms YG UHVDC 0 618ms GG2 HVDC 422 517ms GG1 HVDC 0 344ms TG HVDC 234 548ms SG HVDC 0 370ms Fig. 6 Simulation result of AC transmission lines of Guangdong section 5. Conclusion CSG operates one of the most sophisticated and technologically advanced in the world featuring long distance, extra high voltage and hybrid AC&DC operation. Multi-HVDC projects centre in the load area lead to complex interaction between HVDC system and AC system. Facing the difficulty and challenge of simulation of hybrid AC&DC power system, this paper investigates to build a real-time simulation platform for the whole transmission network of CSG above 220kV. With comparison of simulation result and PMU record in actual case, the RTDS simulation platform is proved to be effective and high accuracy. Another investigation of this paper is power system security risk of multi-hvdcs commutation failure. Multi-HVDCs commutation failure will last longer, or even lead to system instability if post-disturbances in the multi-hvdcs infeed power grid does not be clear in time. A series of single phase short-circuits and mid-breaker failures in important 500kV substations of Guangdong power grid have been simulated in the RTS platform.

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