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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.