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Chapter 18 Real Time Simulation for TimeVarying Harmonic Distortion Analysis Y. Liu, and P. Ribeiro 18.1 - Introduction Emulating conditions existing on a real power system has always been a critical requirement for myriad applications, including testing new control and protection equipment. Historically, this requirement has been met by Transient Network Analyzers (TNAs) which were built using scaled-down analog models of power system equipment interconnected in their original configuration. Although TNAs were inherently real-time in nature, they soon ran into problems of complexity, inadequate scaling, accuracy, and cost. These limitations motivated the introduction of off-line digital simulators such as the Electromagnetic Transient Programs (EMTP, which model the system mathematically and solve it numerically in the time-domain. Nevertheless, their main drawback is the lack of real-time interaction with the equipment under test. Rapid advances in modern computers and digital signal processing hardware has finally led to the development of fully digital real-time simulators that are capable of simulating adequately detailed power system models with sufficient speed to meet the output bandwidth requirement for representing real network conditions. The chief characteristic of such simulators is their ability to interact in real-time with actual hardware connected in closed-loop. Therefore, many tests that cannot be performed on a real system can be safely and efficiently done on a real-time digital simulator. In the power quality (PQ) and harmonics research area real-time digital simulators can play a vital role. Traditionally, time-varying harmonics were studied indiscriminately using statistical and probabilistic methods for periodic harmonics. However, the practice could not accurately describe the random characteristics of the time-varying processes, or capture the reality of physical phenomena giving rise to such harmonics. To precisely interpret the time- varying processes, a time-dependent spectrum is needed to compute the local powerfrequency distribution at each instant of time. With real-time digital simulators, this intense requirement on computational power can be easily satisfied. The concern of PQ has also led to significant advances in the equipment development for PQ measurement, waveform generation, disturbance detection, and mitigation. Several PQ measurement and monitoring devices are currently being developed using both DSP and general purpose microprocessor technologies. There is a need for testing these fast acting apparatus and their controllers to evaluate their response to typical PQ disturbances such as voltage sags, swells, harmonics, impulses, transients, flicker, unbalanced operation and interruptions. A real-time digital simulator can be used to simulate such disturbances and apply them to the tested device under closed-loop conditions [1-6]. To achieve better accuracy on the power quality studies of large and complex power systems in an economic way, a novel power quality assessment method based on a real time (RT) hardware-in-the-loop (HIL) simulator can be used. Hardware-in-the-loop is an idea of simultaneous use of simulation and real equipment. Generally, a HIL simulator is composed of a digital simulator, one or more hardware pieces under test, and their analog and digital signal interfaces (e.g., high performance A/D and D/A cards). This chapter describes an example of the analysis of the sensitivity for power quality deviations of a variable speed drive controller card using real time simulation and hardware in the loop. The experiment has contributed to the design of a power quality test bed, which can be used to test the immunity of electric components and equipment and the consequent impact on AC distribution systems. 18.2 Description of the RT-HIL Platform Figure 18.1 shows the diagram of the RT-HIL platform. The platform is composed of a digital simulator, tested hardware, and their interface (e.g., power amplifiers and transducers). The simulator can be used either as an independent simulation system (e.g., no hardware in the loop), or with tested hardware. In Figure 18.1, a real power electronic device is connected to the simulated power system through D/A adaptors and power amplifiers. The supply current of the AC/DC converter is measured and fed back into the system at the common coupling point through transducers and A/D adaptors. In fact, any component (e.g., controllers of power electronic devices, and control and protection equipment) in power systems could be tested in the platform. DIGITAL SIMULATOR D/A AC loads A/D D/A Voltage reference A/D D/A Generation and distribution systems Current injection A/D DSP hardware Ethernet PC: GUI and real-time control Current probe Amplifier Transducer Amplifier Transducer Amplifier Transducer Power electronic device and its controllers TESTED HARDWARE AND ITS INTERFACE Figure 18.1. Diagram of the RT-HIL platform A real distribution system of a shipboard power system was used for demonstrating the simulator’s capabilities. Figure 18.2 shows a good agreement between the complete software simulation results and the field measurements. HV Bus PT output voltage [V] 200 Phase A Phase C 150 100 Measured waveform event at 05/13/00 22:13:12.64 50 0 Simulated waveform dt = 65 s -50 -100 Phase B -150 -200 22:13:12.640 t [h:m:s] 22:13:12.645 22:13:12.650 22:13:12.655 22:13:12.660 Figure 18.2. Measured AC bus voltage waveforms (broken gray lines) of a real shipboard distribution system (with only one bridge of each cycloconverter operating) 18.3 Sample Case: Testing the Sensitivity of a Thyristor Firing Board to Poor Quality Power A three-phase thyristor firing board was tested in the platform for its sensitivity to poor quality power. Also, the impact of its sensitivity on the DC load and the AC distribution systems is considered. The schematic of the simulated AC distribution system is shown in fig. 18.3. . Power grid VL-L=12.47 kV Distribution transformer 12.47 kV/480 V 6-pulse thyristor rectifier + Industry DC load YY Lline=0.05 p.u. Rline=0.005 p.u. LT=0.05 p.u. RT=0.005 p.u. RL=0.48 ohm LL=1 mH Firing pulse board Figure 18.3. Diagram of the simulated industrial distribution system and rectifier load (60 Hz, power base = 833 kW) To test the application under extreme conditions the cases shown in Table 18.1. Table 18.1 The RT-HIL simulation results for the firing board PQ phenomena THD Voltage sag Simulation results Tolerate THD up to 14.8% and higher Tolerance has no impact on distribution systems Tolerance depends on not only the time duration and voltage reduction, but also phase shift Frequency change Tolerance results in DC voltage drop or blackout Tolerate system frequency from 30 Hz to 80 Hz Figure 18.4 and Figure 18.5 show the single-phase voltage sag with and without any phase shift and their impact on the DC output voltage of the rectifier. The sag with a phase shift resulted in the reboot of the firing board. The reboot then resulted in a 0.1 s DC blackout and about 1.5 s transient on both DC and AC systems. However, the sag without phase shifts only resulted in a DC voltage drop. This finding cannot be discovered by using traditional laboratory Primary voltage (kV) tests. 10 5 0 -5 -10 DC voltage (kV) 0.05 0.1 0.15 0.2 0.25 0.1 0.15 Time (s) 0.2 0.25 0.4 0.3 0.2 0.1 0 0.05 Figure 18.4. Single-phase voltage sag (0.1 s duration, 40% voltage reduction, no phase shift) and its impact on the rectifier DC output (delay angle = 7) Primary voltage (kV) 10 5 0 -5 -10 DC voltage (kV) 0.05 0.1 0.15 0.2 0.25 0.1 0.15 Time (s) 0.2 0.25 0.4 0.3 0.2 0.1 0 0.05 Figure 18.5. Phase-shifted single-phase voltage sag (0.1 s duration, 40% voltage reduction) and its impact on the rectifier DC output (delay angle = 7) The successful test leads to a design of a universal power quality test bed. Figure 18.6 shows the diagram of the universal power quality test bed. A universal interface is built to easily connect any firing board. Test systems and power quality phenomena can be selected from the existing ones in the digital simulator or self designed for a special purpose. Digital simulator Self designed Test system N Start Selecting Tested systems Selecting PQ phenomena Print results Test system 2 Test system 1 Frequency change Voltage sag THD Firing board Firing pulses Universal interface (e.g., power amplifiers, and transducers) End Figure 18.6. Diagram of universal power quality test bed 18.4 Conclusions and Future Work Reference voltages A novel power quality assessment method was proposed. The method is applied in the RT-HIL platform to test an industry firing board. The successful initial test results show that the tested board can tolerate highly distorted voltages, significant sudden frequency change, and three-phase voltage sags, but it cannot tolerate certain short-term phase-shifted single-phase voltage sags. This result which could only be revealed through the proposed RT-HIL method is helpful for future product improvements. The successful experiment has contributed to the conceptual design of a universal power quality test bed, in which any kind sensitivity of power quality deviation could be revealed. 18.5 References [1] A. J. Grono, “Synchronizing Generators with HITL Simulation,” IEEE Computer Applications in Power, Vol. 14, No. 4, October 2001, pp. 43-46. [2] M. Steurer, S. Woodruff, “Real Time Digital Harmonic Modeling and Simulation: An Advanced Tool for Understanding Power System Harmonics Mechanisms,” IEEE PES General Meeting, Denver, USA, June 2004. [3] Lok-Fu Pak, Dinavahi, V., Gary Chang; Steurer, M., Ribeiro, P.F. , “Real-Time Digital TimeVarying Harmonic Modeling and Simulation Techniques, IEEE, Transactions on Power Delivery, v 22, n 2, April 2007, p 1218-27. [4] J. Langston, S. Suryanarayanan, M. Steurer, M. Andris, S. Woodruff, and P. Ribeiro, “Experiences with the Simulation of a Notional All-Electric Ship Integrated Power System on a Large-Scale Electromagnetic Transient Simulator,” 2006 IEEE Power Engineering Society General Meeting, Montreal, Quebec, Jun. 2006. [5] W. Ren, M. Steurer, S. Woodruff, and P.F. Ribeiro, “Augmenting E-Ship Power System Evaluation and Converter Controller Design by Means of Real-Time Hardware-in-Loop Simulation,” in Proc. of 2005 IEEE Electric Ship Technologies Symposium, July 25-27, Philadelphia, USA, pp. 171-175. [6] S. Suryanarayanan, W. Ren, M. Steurer, P. Ribeiro, and G. T. Heydt, “A real-time controller concept demonstration for distributed generation interconnection,” accepted for presentation at the 2006 IEEE Power Engineering Society General Meeting, Montreal, QC, Canada, Jun. 2006.