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Australasian Universities Power Engineering Conference, AUPEC 2013, Hobart, TAS, Australia, 29 September - 3 October 2013

U sing Rapid Development Tools to Design and

Construct a STATCOM with Active Harmonic

Cancellation Capabilities

K.S. NathandO, T.J. Summers2o, R.E. Betz30, D.R.H. Carter4t ° School of Electrical Engineering and Computer Science

University of Newcastle Newcastle, Australia tDenKinetic Pty Ltd

Darling Point, Australia 1 Kumaran.N [email protected], [email protected],

[email protected], [email protected]

Abstract-There is no doubt that the development of power electronics-based devices, such as STATCOMs, provides an invaluable learning experience for both undergraduate and postgraduate students. However, time constraints that typically surround university projects mean that students are usually forced to choose between a simulation-based study or a hardwarebased study. This paper investigates the use of low cost rapid development tools to facilitate very high level simulation-based and practical studies. The primary focus of this paper is on final year, electrical engineering capstone projects. A case study of such a project is presented, where a STAT COM that performs both power factor correction and active harmonic cancellation was modelled, simulated and constructed.

I. INTRODUC TION

Invaluable learning experiences can be provided to students

through the study of power electronics-based devices in their capstone university projects.This learning opportunity can be

greatly enhanced through the addition of experimental work to

typical simulation studies. These hardware-based projects can be extremely challenging due to the amount of work required

to create the base hardware (for example, a three-phase two

level voltage source converter). Due to these constraints, students often must choose between in-depth theoretical studies

or basic hardware projects. This problem can be alleviated through the use of rapid development tools, which allow

students to focus on controlling converters rather than building

them.

Rapid development tools are highly flexible and provide an easy method of implementation and prototyping. A very popu

lar rapid development tool is MATLAB's Simulink combined

with dSPACE, which allows MATLAB code to be interfaced with a physical system [1]. Many alternative rapid develop

ment tools exist, one of which is investigated in this paper. This rapid development tool, which is produced by Semikron

and DenKinetic, is considerably cheaper than dSPACE and

uses an industry standard Texas Instruments (TI) Digital Signal Processor (DSP).

The remainder of this paper outlines the development of a

STATCOM that was the subject of the first author's final year

engineering project. This project involves the full development

(both simulation and hardware) of a STATCOM that can

perform power factor correction and active harmonic filtering.

This STATCOM utilises many advanced control techniques,

including an enhanced implementation of Instantaneous Power Theory utilising phase-locked loops and deadbeat current con

trol. Space vector pulse width modulation (SVPWM) is also

used, which implements a modified 'limit hexagon' method that includes current angle preservation rather than voltage

angle preservation.

This project was initially developed using the Saber plat

form, with all control code written in C++ and compiled

into a single DLL file which interfaced with the simulation software. The STATCOM was then implemented in hardware

using rapid development tools produced by Semikron and DenKinetic. This hardware layout provided a three-phase

voltage source converter, gate drive circuitry, deadtime shoot

through protection, microcontroller, and anti-aliasing filters for current and voltage sensors. The bulk of the control

code written for the simulation could be easily reused in

the hardware controller with minimal modifications required. Both the learning outcomes and practical results achieved

are highlighted, demonstrating the significant benefits and

accomplishments possible using an approach involving rapid

development tools.

II. STATCOM

The STATCOM system constructed was a basic three bus

radial power system as shown in Fig. l. This topology is

commonly used when investigating STATCOM performance [2]-[4].

The topology was chosen as it represented a real world

scenario where the source and transmission network provide

a Thevenin equivalent of the entire network. The point of common coupling (PCC) connects the STATCOM to the

Australasian Universities Power Engineering Conference, AUPEC 2013, Hobart, TAS, Australia, 29 September - 3 October 2013 2

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Fig. 1. Basic test power system

STATCOM Load

vsc

Fig. 2. High level control block diagram for the STATCOM

network. The impedance between the PCC and the load is

negligible, and the load consists of both linear and non-linear

components.

Typical STATCOMs provide reactive power control for the purpose of power factor correction and voltage regulation,

however their functionality can also be extended to include

more advanced power conditioning techniques [5]. The STAT

COM developed in this project employs active harmonic

filtering in addition to power factor correction. To provide this

functionality, the inputs to the controller must include PCC voltages, load currents, STATCOM output currents and the

DC bus voltage.

A. Control Strategy

Fig. 2 provides a high level block diagram to give an

overview of the STATCOM controller. It can be seen that the

overall controller is divided into an outer loop controller and

an inner loop controller:

• Outer loop STATCOM controller: Generates the reference

currents the STATCOM should inject to achieve the over

all objectives of power factor correction and harmonic

cancellation, • Inner loop current controller: Aims to generate the

switching signals to force the STATCOM output currents to track the reference currents provided by the outer loop

controller.

1) Outer Loop Control: A block digram of the outer loop

controller is shown in Fig. 3. The outer loop controller is based on a variation of instantaneous power theory, also known

Fig. 3. Block diagram of outer loop control (STATCOM control)

as p-q Theory [6]. Instantaneous power theory involves a

time-domain analysis of voltages and currents to determine instantaneous real and imaginary powers [7]. Since the theory

is based on instantaneous voltages and currents, it is highly

generalised and applies equally to both steady and transient states [8]. The theory is therefore a very useful basis for

designing controllers of high performance power conditioning devices. Applying instantaneous power theory to a three-wire

system results in Equation I, where p is the real power, q is

the imaginary power, and Va, VfJ, ia, ifJ are obtained through power invariant Clarke transformations of the phase voltages

and currents.

] [ �; ] (I)

Both real and imaginary powers are further decomposed into

average and oscillating components, represented by p, q and

p, q respectively:

Real Power: p Imaginary Power: q

Average

p + q +

Oscillating

p q

A low-pass filter can then be used to separate the fundamental frequency components from the harmonic components.

The use of low-pass filters inherently introduces delays which negatively impact the STATCOM's performance [9]. Phase

locked loops (PLLs) were implemented to eliminate these

delays prior to Equation 1 being implemented. These PLLs are also able to predict future values which has serendipitous

benefits for the deadbeat current controller which will be

discussed in the following section on inner loop control.

2) Inner Loop Control: The voltage source converter can

be controlled to effectively act as a current source. Fig. 4

on the following page presents the block diagram for the STATCOM's current controller which consists of two main

blocks:

• A deadbeat controller to calculate the voltages required to force the currents to the desired values,

• A space vector pulse width modulation (SVPWM)

scheme to determine the switching times to synthesise

the previously calculated required voltages.

Following a derivation using an inductive output filter of inductance L f' the final deadbeat algorithm to determine the


SVPWlVI � Controller VSC � Deadbeat �/ "

" � Clarke �::---------' � Transi()rrn 1+"-----------'

Fig. 4. Block diagram of inner control loop (current control)

required STATCOM voltage for the [k, k + 1] interval is [!O]:

Vstatcom [k, k + 1] = Lf

(i* [k + 1] -i [k - 1]) + vpcc [k + 0.5] Ts + vpcc [k -0.5] -Vstatcom [k - 1, k]

In this equation, i* [k + 1] is the current desired at the end

of the next control interval and i [k - 1] is the previously

measured current. Similarly, vpcc [k + 0.5] is the predicted point of common coupling voltage and vpcc [k -0.5] is meas

ured. Vstatcom [k - 1, k] is the previously applied STATCOM

voltage.

Once the required converter voltage has been calculated,

it is necessary to compute the switching times for the six

IGBTs. This will result in the VSC producing a square voltage

waveform at each phase terminal, with an average value equal to the desired voltage. There are many methods to obtain

these switching times, the most highly regarded of which

is space vector pulse width modulation (SVPWM) [11]. The

most commonly used method to deal with space vectors that

lie outside the limit hexagon is to simply limit their magnitude

to the hexagon or circle [12], which maintains the angle of the

original desired voltage vector as shown by Viim,l in Fig. 5.

The problem in using the limit hexagon and limit circle method may not be obvious at first, so it is necessary to recall the

aim of the original reference voltage vector. This vector was

determined by calculating the voltage that needed to be applied across the filter, to generate a desired output current:

. Vstatcom - Vgrid Zstatcom =

Z filter Vfilter Zfilter

The angle of this desired current is dependent on both the voltage across the filter, Vfilten and the impedance of the filter,

Z filter, which is a constant. Fig. 5 shows the desired voltage across the filter relating to the grid voltage and the STATCOM

voltage. It can be seen that when the original reference vector

is limited using the previous methods, the angle of the filter voltage (and hence filter current) will change, yet the angle of

the desired vector will be preserved.

In fact, it is far more important that the angle of the

output current is preserved, rather than the angle of the

desired voltage vector [13]. This is a more complex method to implement, since it requires the additional knowledge of

the grid voltage vector, whereas the previous methods only

required the reference vector. The new reference voltage vector

can be obtained by limiting the filter voltage to the circle.

This new reference vector preserves the more important output current angle, rather than the STATCOM voltage angle.

Fig. 5. SVPWM voltage limiting techniques

It can be seen that there may be cases when it is impossible to preserve the current angle, such as when the grid voltage

vector also lies outside the limit circle. For these cases, the

previous limit circle method is utilised.

III. SIMUL ATIONS

The Saber simulation platform was used for the modelling

and simulation of the STATCOM as it is a proven platform

with excellent model accuracy and has an extensive device library [14]. Saber has support for digital control through

the use of dynamic-link libraries (DLL files), can perform a

range of analyses and results can be studied in-depth using CosmosScope.

The power electronic switches of the STATCOM are modelled as ideal controllable switches. The entire STATCOM

controller is implemented in a single DLL file to improve

simulation speed and reduce the possibility of simulation errors. A drawback of implementing the entire controller in a

single block is that it is not easy to view intermediate variables. For this purpose, 10 extra debug outputs are included, to which

any variables can be assigned and viewed. Another significant

inconvenience is time and difficulty involved if changes to the controller require modifications of the inputs/outputs from the

DLL file. A screenshot of the STATCOM controller sub-block

is shown in Fig. 6 on the next page.

IV. H ARDWARE

A. Converter

The power converter used is the Semikron Semiteach shown

in Fig. 7 on the following page. This product, which is

designed as an educational unit, is very flexible as it contains many components including a three-phase rectifier, three in

verter legs, a chopper leg, DC bus capacitors, snubber circuits

and IGBT drivers [15]. Due to the number of components and ease at which they can be accessed, the Semiteach is able to


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Fig. 6. Saber screenshot: STATCOM controller sub-block

V oltage source converter

Fig. 7. Semikron Semiteach (used as YSC) and filter inductors

simulate many industrial applications such as rectifiers, invert

ers, back-to-back converters and motor drives. The converter is

very useful as an educational device as it is transparent to allow visualisation of all components, and has short circuit protection

as well as built-in deadtime to prevent shoot-through of the

inverter legs [16].

For the STATCOM, three inverter legs and the DC bus

capacitors are used. Each inverter leg uses a single Semitrans module (SKM 50GB123D) consisting of 2 IGBTs and their

associated anti-parallel diodes. The DC bus contains two large

2200J-LF capacitors in series, with 22kD resistors across the

terminals of each capacitor for voltage balancing and to serve

as bleed resistors which discharge the capacitors when the converter is not in use [17]. The 23kVA Semistack used is rated

to 440Vac, 750Vdc, 30Arms, which is sufficient for this project

with additional overhead to allow for future modifications in specifications.

Fig. 8. PwrCON controller - outside and inside

Table I PWRCON PERIPHERAL CARDS USED

Card Use

AIVC Voltage feedback for DC bus and PCC voltages AIOA Analogue outputs connected to oscilloscope AICA Current feedback of STATCOM currents AICA Current feedback of load currents GDCE 15V IGBT drivers for the three inverter legs SERA.CAN Isolated CAN interface for computer communications

B. Controller

The controller used for the STATCOM is a DenKinetic 'PwrCON' unit which contains a Texas Instruments Delfino

F28335 microcontroller. The PwrCON box, shown in Fig. 8,

contains a base board which connects the microcontroller

card (TMS320F28335 ControICARD) to the various peripheral

cards. Table I lists the DenKinetic peripheral cards in the PwrCON box and details their usage.

The microcontroller was chosen because it is a digital signal

controller that has excellent floating-point performance and a

high clock speed processor, necessary for the computational

load of the STATCOM controller [18]. Another important

consideration when choosing this controller was the ability to perform simultaneous high precision ADC samples required

for the performance objectives of the controller. A significant benefit when using these rapid development

tools is the ability to essentially copy and paste the C++

code used in the simulations to the hardware controller. The

utilisation of classes and functions in the code allow other

students to use and enhance existing code which will provide a growing and higher quality library over time.

C. Sensors

Accurate voltage and current measurements are essential to the proper operation of the STATCOM. The DC bus and grid

voltages may be in excess of 100V, so an interface is needed

for connection to the ±lOV inputs of the AIVC card. The DenKinetic 'X002' card (four-channel voltage sensing board)

is used for this purpose which is essentially a voltage divider circuit. Fig. 9 on the next page shows this interface card with

the high voltage inputs on the right, and the low voltage signals

going to the controller on the left. The voltages on the card are potentially dangerous so the entire interface card is enclosed

in a box, which is also shown. Current measurements are provided by LEM LA 55-P Hall

effect current transducers shown in Fig. 9. These transducers

were chosen as they have a large frequency bandwidth (DC - 200kHz), appropriate current range (±70A), high current


Fig. 9. Voltage sensing board and current sensor

-,. -� --."'''''''''"'-'-'''''1 .t..dc.-.<"" _ _ "",,, .",,,,,,,,,,,",,,,-·""'",1

Fig. 10. Computer application: 'Advanced STATCOM Manager'

resolution, and a convenient interface with the DenKinetic 'AICA' current sensing peripheral cards.

D. Computer Interface

The DenKinetic 'SERA.C AN' peripheral card in the Pwr

CON unit provides a CAN (Controller Area Network) commu

nications interface, which is a standard designed to communicate with other CAN ports (and not specifically with PCs).

This protocol is capable of a IMbps throughput, enabling

real-time transmission of large amounts of data. To facilitate communications with a PC, a special CAN-USB interface

device is required.

A computer program was designed and coded in C# to

manage the STATCOM. Two-way communications with the

PwrCON box enabled control of the STATCOM, as well as real-time feedback of system information.

The application, shown in Fig. 10, provides the ability to

control the objectives and functionality of the STATCOM,

as well as view real-time data of the power system and

controller. The graphical user interface shows real-time graphs

of the point of common coupling voltage, grid frequency, DC bus voltage, power factor and PLL statuses. The func

tions of power factor correction and harmonic filtering can

be independently enabled or disabled, with each harmonic

independently controllable. Each of the outputs connected to

the oscilloscope can also be configured from an extensive list

of options, to make the debugging and fault-finding process

considerably easier and more streamlined.

V. RESULTS

To demonstrate the capabilities and effectiveness of the STATCOM, three experiments were conducted:

1) Power factor correction only,

2) Active harmonic cancellation only,

3) Simultaneous power factor correction and active harmonic cancellation.

Fig. 11. Results - Power factor correction

For each experiment in this section both simulation and ex

perimental results are shown. This is to provide validation and

highlight the consistency of the simulated results compared

with hardware results.

For the hardware results in this section 4 traces are shown

(all phase 'a' ):

• Yellow trace: point of common coupling voltage, • Green trace: source current,

• Purple trace: load current, • Pink trace: injected STATCOM current.

A. Power Factor Correction

The load for the demonstration of the power factor cor

rection capabilities is a set of three variable inductors (with an adjustable ferrite core) connected to a three-phase resistive

load bank. This type of load results in a purely fundamental

frequency current draw with adjustable real and reactive com

ponents (i.e. adjustable current magnitude and phase shift).

The results in Fig. 11 show that the source current is in

phase with the PCC voltage and actually supplies a smaller

magnitude current than what the load is consuming. It can

also be seen that the STATCOM current is 90 degrees out of

phase with the PCC voltage, indicating purely reactive current injection. The source power factor increased from 0.71 to 0.99.

B. Active Harmonic Cancellation

The load for the demonstration of the active harmonic cancellation capabilities is an uncontrolled three-phase rectifier

with a rheostat and speed-controllable DC motor connected.

This type of load has very large 5th and 7th harmonic currents

[19]. The load currents are in phase with the PCC voltages,

so the STATCOM injections are purely harmonic.

Fig. 12 shows a heavily distorted current drawn by the

uncontrolled rectifier, but a source current that is almost purely

sinusoidal. This is because the STATCOM current is injecting

all the 5th and 7th harmonic components of the load. The total harmonic distortion (THD) of the source is a mere 5%

compared with the load's 60%.

C. Combined P FC and AHF

The load for this experiment is a combination of the

previous two loads in parallel to show that both objectives

(power factor correction and active harmonic cancellation) can be achieved simultaneously.


Fig. 12. Results - Active harmonic cancellation

Fig. 13. Results - Combined PFC and AHF

The results in Fig. 13 show that even though the load current

has significant harmonic distortion and phase shift, the current

supplied by the source is almost purely sinusoidal and in phase with the PCC voltage, essentially making the overall load look

like a pure resistor. The THD reduces from 40% to 5% and the power factor increases from 0.87 to 0.99.

VI. CONCLUSIONS

The use of rapid development tools provided by Semikron and DenKinetic have enabled this electrical engineering final

year project to be completed in a mere 9 months, which also

included researching current state of the art control techniques and writing the thesis. The converter, controller and sensors are

all configured to work together, allowing students to focus on

control aspects by requiring minimal effort for the physical construction and interfacing of these devices. The ability to

move back and forth between simulation and hardware easily provided many significant benefits to quickly find problems

and investigate potential improvements.

The final results obtained demonstrate that the STATCOM

worked remarkably well, with an increase in power factor to 0.99 and up to a 55% reduction in source current total

harmonic distortion. The high correlation and consistency

of results between simulation and hardware studies gives confidence in the rapid development tools used.

It is important to remember that there are other rapid de

velopment tools available, such as MATLAB's Simulink com

bined with dSPACE. The tools used in this project however, offer advantages in terms of significant cost savings and also in

terms of using equipment which is closely aligned to industry.

By using these tools, students are able to achieve more than

ever before in their power electronics-based capstone projects,

greatly enhancing their learning experience and strengthening

their skill sets for careers in both academia and industry.

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