Chapter 18
RealTime Simulation for Time-Varying Harmonic Distortion Analysis: A NovelApproach
Y. Liu, and P. Ribeiro
18.1 - Introduction
A novelapproach to time-varying harmonic distortion assessement based on real-time (RT) hardware-in-the–loop (HIL) simulation is proposed. The sensitivity for power quality deviations of a variable speed drive controller card was tested in the platform. The successful experiment has contributed to the conceptive design of a universal power quality test bed, which would have the function of testing the immunity of electric components and equipment and the consequent impact on AC distribution systems.
Electromagnetic transient simulations or laboratory experiments are often used for power quality studies. The results of different simulation programs could be different because of the usage of different mathematic models. Also, the simulation results largely depend on the accuracy and complexity of those models. On the other hand, the disadvantages of the laboratory experiments are the high cost and the large amount of developing time. Also, the power quality impact on power systems is hardly achieved due to the difficulties of connecting a tested device to real power systems. 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 realtime (RT) hardware-in-the-loop (HIL) simulator is proposed. 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).
18.2 Description of the RT-HIL Platform
A RT-HIL platform is currently being established at the Center for Advanced Power Systems (CAPS) at FloridaStateUniversity, Tallahassee, Florida. The platform has been designed for research on NAVY all-electric ship power systems. Fig.18.1shows the diagram of the RT-HIL platform. The platform is composed of a digital simulator (RTDSTM), 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 Fig.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.
Figure18.1. Diagram of the RT-HIL platform at the CAPS, Tallahassee, FL
The distribution system of the US coast guard icebreaker “Healy” has been simulated in real-time to demonstrate the simulator’s capabilities. Fig.18.2shows a good agreement between the complete software simulation results and the field measurements taken during Healy’s commissioning trials in 1998.
Figure18.2. Measured AC bus voltage waveforms (broken gray lines) of US Coast Guard icebreaker (all four cycloconverter bridges operating) compared to the simulated waveforms (solid colored) using the digital simulator model (only one bridge of each of both cycloconverter drives operating)
18.3 Sample Case: Testing the Sensitivity of a Thyristor Firing Board to Poor Quality Power
An Enerpro FCOG 6100 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.
.
Figure18.3. Diagram of the simulated industrial distribution system and rectifier load (60 Hz, power base = 833kW)
To consider the application of the board on NAVY all-electric-ships, extreme conditions (e.g., significant frequency change) are simulated in the test. Table 18.1shows the RT-HIL simulation results.
Table 18.1 The RT-HIL simulation results for the firing board
PQ phenomena / Simulation resultsTHD /
- Tolerate THD up to 14.8% and higher
- Tolerance has no impact on distribution systems
Voltage sag /
- Tolerance depends on not only the time duration and voltage reduction, but also phase shift
- Tolerance results in DC voltage drop or blackout
Frequency change /
- Tolerate system frequency from 30 Hz to 80 Hz
Fig.18.4 and Fig.18.5show 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 a 0.1s DC blackout and about 1.5s transient process on both DC and AC systems. However, the sag without phase shifts only resulted in a DC voltage drop. This finding can not be discovered by using traditional laboratory tests (e.g., changing the amplitude of one phase voltage in a 3-phase vari AC).
Figure18.4. Single-phase voltage sag (0.1s duration, 40% voltage reduction, no phase shift) and its impact on the rectifier DC output (delay angle =7)
Figure18.5. Phase-shifted single-phase voltage sag (0.1s 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. Fig.18.6shows the diagram of the universal power quality test bed. A universalinterface 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.
Figure18.6. Diagram of universal power quality test bed
18.4 Conclusions and Future Work
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 conceptualdesign 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, “RealTimeDigital Harmonic Modeling and Simulation: An Advanced Tool for Understanding Power System Harmonics Mechanisms,” IEEE PES GeneralMeeting, Denver, USA, June 2004.