Introducing an Effective Method for Teaching Power Electronics

in Marine Electrical Engineering Courses

J. Prousalidis

School of Naval Architecture and Marine Engineering

National Technical University of Athens (NTUA)

9 Heroon Politechniou St, 15773 Athens

GREECE

http://www.lme.naval.ntua.gr

Abstract: - This paper presents an introduction of a feasible method for teaching power electronic issues to non-electrical engineering disciplines. The method presented is founded on the switching function techniques, which as it is shown does not require any specific branches of knowledge. On these grounds, the topologies of certain fundamental power electronic devices, are analysed and presented. Furthermore, the ways that this methodology has permitted the introduction of power electronics to School of Naval Architecture and Marine Engineering of NTUA are discussed.

Key-Words: - Marine electrical engineering, power electronics, switching functions

1. Introduction

The extensive electrification of shipboard installations and the All Electric Ship concept is a rather appealing, if not unavoidable, perspective. This process seems to be, up-to-date, more significant and rapid in the warship domain following up the successful integration of the main and auxiliary electric propulsion schemes along with automation and weapon systems. Nevertheless, it is widely accepted that the electrification process would not have become possible but for the significant progress in the domain of power electronic devices. This argument is also reinforced considering that the Power Electronic Building Block (PEBB) concept, according to which all complicated power electronic topologies can be synthesized via fundamental modules, has been initiated in naval projects.

Evidently, the specifications for marine engineering curricula had to be adopted appropriately, keeping up with the aforementioned changes in naval industry. Thus, marine engineering courses have been enriched by electrical engineering courses comprising both power and control components applied to ship systems. On the other hand, the related courses had to be restricted to a certain limited number as the more conventional mechanical engineering core courses are of extreme importance. In this way, the problem emerged consisted in the necessity to offer significant amount of knowledge but in a limited number of courses. In School of Naval Architecture and Marine Engineering (SNAME) of National Technical University of Athens (NTUA), it has been decided to resolve this, by introducing coherent but feasible ways of teaching the required branches of knowledge [1-2].

Thus, concerning power electronics theory and applications, the switching function approach, [3-5], which is a handy tool at the design process of power electronic devices, has been introduced with very satisfactory results. The main advantage of this methodology is simplicity in conjunction with no extra background. Moreover, no dedicated computer package is required, while any mathematical tool can be used instead, facilitating, thus, teaching process even further.

2. Background

2.1 Power Electronics

Power electronics comprise actually circuits of semiconductor devices, which in contrast to ordinary electronics, operate in the non-amplifying region. Hence, they resemble the conventional circuit breakers with two distinct (binary) operating states, i.e. if they are considered of ideal behaviour :

(1)

On the contrary if the non-ideal behaviour of the electronic switch is taken into consideration, then input-output relationship is, see Fig.1:

(2)

Fig. 1. (a) Ideal and (b) non-ideal switch.

2.2 Ships and power electronic devices

In Figure a generic arrangement of a electric power plant of a ship with electric propulsion is presented.

a.  prime mover

b.  synchronous generator

c.  power transformer

d.  motor drive (frequency converter)

e.  motor (synchronous/inductive)

f.  propeller

g.  other load demands (pumps, winches, lighting etc)

Fig. 2. Generic arrangement of a shipboard electric power plant.

As it can be seen, the power plant serves, in general, all electric loads onboard comprising main and auxiliary propulsion, machinery auxiliaries, deck machinery, hotel loads, lighting etc. As already mentioned, almost all rotating machinery from a centrifugal pump up to main electric propulsion motor need to be properly driven in terms of speed and torque via power electronic devices.

2.3 Marine electrical engineering courses

Marine electrical engineering courses is only a small portion of the marine engineering stream at SNAME of NTUA, where marine engineering course (i.e. applied mechanical to ship systems) dominate. More specifically, electric power engineering courses comprise the following:

·  “Electroscience” with 4 hours per week, comprising fundamental electric circuit issues, see Table 1.

Table 1. “Electroscience” course contents

Subject / %
Signals and Systems / 4/52
Electric circuits and components / 4/52
Kirchoff’s laws and solution of electric networks / 6/52
Equivalent circuit theorems / 6/52
Sinusoidal steady state / 8/52
Electric power / 4/52
Three phase networks (symmetrical and unbalanced) / 8/52
Solving electric networks via Laplace transform / 4/52
Electricity hazards / 4/52
Magnetic circuits / 4/52
Total Sum / 52 (13 weeks)

·  “Applied Electroscience and Electric Laboratory” with 5 hours per week. This course has started being reformed during a Curriculum Reformation programme within the “Operational Programme for Education and Initial Vocational Training - EPEAEK” funded by both the European Union and the Greek Government in 2001. Thus, in Table 2 the old course contents, while in Table 3 the new and reformed ones are presented. As it can be seen, after the reformation, significant amount of teaching material has been added. This has been achieved on the one hand by rebalancing the teaching hours in conjunction with the incorporation in the teaching process of feasible instruction tools.

Table 2. “Applied Electroscience in Naval Systems and Electric Laboratory” course contents before “EPEAEK” reformation

Subject / Teaching Hours
Magnetic circuits / 7/65
Single-phase Transformers / 6/65
Three-phase Transformers / 6/65
Rotating machinery / 6/65
AC synchronous machines / 10/65
AC asynchronous machines / 10/65
DC machines / 10/65
Naval applications of electric machinery on shipboard installations / 10/65
Total Sum / 65 (13 weeks)

Table 3. “Applied Electroscience in Naval Systems and Electric Laboratory” course contents after “EPEAEK” reformation

Subject / %
Magnetic circuits / 4/65
Power Transformers (1- and 3-phase) / 6/65
Rotating machinery / 6/65
AC synchronous machines / 6/65
AC asynchronous machines / 7/65
DC machines / 5/65
Computer programs for electric network studies / 8/65
Computer Programs for electric network analysis - PSCAD / 4/65
Power electronics / 5/65
Electric motor drive control / 5/65
Specific type motors / 4/65
Naval applications of electric machinery on shipboard installations / 5/65
Total Sum / 65 (13 weeks)

Thus, teaching of power electronic core knowledge has become possible via the exploitation of switching functions. Furthermore, the usage of computer programmes for analysis of electric networks (especially for rather complicated arrangements) has also been adopted in an attempt to facilitate both instruction and comprehension. After a feasibility study comparing several computer programmes used for both educational and research purposes, PSCAD, the programme released by Manitoba HVDC Research Center has been selected as the main computer tool. Thus, students spend some time in the School PCLAB getting acquainted with this programme.

3. Switching functions

As already mentioned, switching functions are mainly used at the design process of power electronic devices. According to their fundamental cencept, the electronic switch is considered to be ideal having two possible states of operation, ON and OFF, i.e. :

·  the ON (logic 1) , where the output (voltage or current) equals the input , and

·  the OFF (logic 0), where the output (voltage or current) is equal to 0.

Considering for instance voltage quantities, this binary logic can be summarized in the following equation:

(3)

where Sw is the switching function representing switch operation state in a Boolean algebra manner, i.e.:

(4)

Evidently, the changeovers between the two states are specified by the switching technique of each power electronic device, e.g. PWM, SPWM etc. Moreover, via this approach, the main theory of power electronic devices can be easily taught and assimilated as the background required is elementary, i.e.:

·  logic (boolean) algebra

·  elementary circuit theory

It is worth noting, that via this method the voltage and current quantities can be expressed in per unit (p.u.) system enabling, thus, their further mathematical elaboration, e.g. performing FFT analyses, etc.

In the following, the input-output relationship of fundamental power electronic schemes is expressed via switching functions. Moreover, in the Appendix, the corresponding equations of a 6-pulse cyclo-converter are presented.

3.1 6-pulse Rectifier

Fig. 3. Generic 6-pulse rectifier scheme.

The output vs input voltage relationship is :

(5)

Similarly, the corresponding input vs output current relationship is:

(6)

While the switching functions involved in equations (5) and (6) are actually interrelated as shown in the following:

(7)

3.2 6-pulse Inverter

3.2.1 6-pulse Voltage Source Inverter (VSI)

Fig. 4. Generic 6-pulse VSI scheme.

In the VSI case, see Fig. 4, the output vs input voltage relationship is:

(8)

Furthermore, the input vs output current relationship is:

(9)

3.2.2 6-pulse Current Source Inverter (CSI)

In the CSI-case, see Fig. 5, the output vs input current relationship can be expressed in matrix form as:

(10)

Fig. 5. Generic 6-pulse CSI scheme.

while the corresponding input voltage vs output voltage relationship is:

(11)

3.3 Rectifier-Inverter Combination

Evidently as the DC input of an inverter can be the output of a rectifier, equations (5-11), where switching functions have been incorporated, can be combined resulting in a total AC/AC input – output relationship, either for voltages or currents. Thus, in Figs. 6-7, the simulated input/output voltage curves of such a combination in the case of a 6-pulse VSI with an uncontrolled rectifier are presented. The VSI considered is driven by an SPWM controller and supplies a 6kV ship propulsion motor. The switching sequence of SPWM technique is taken inherently into account by the switching functions of the inverter.

Fig. 6. Voltage curves (input and output) of uncontrolled rectifier (voltage in pu-system).

Fig. 7. Voltage curves (input and output) of a 6-pulse VSI supplied by the output of Fig. 6.

4. Switching functions with Non-ideal switches

For reasons of simplicity let’s consider a non-ideal electronic switch, i.e. a device with a voltage drop, see Figure 1b. This voltage drop actually consists of a constant component Ve in series with a current dependent one, i.e. a resistive element Re. In this case, the input output relationship do not purely refer only to voltage or current respectively, but there is a voltage – current relationship at the output terminal instead, see also equation (2):

(12)

while the current relationship is given by:

(13)

Thus, voltage equation becomes:

(14)

Moreover, the output voltage and current are related via the converter load (Ohm’s Law), i.e:

(15)

or

(16)

In this way, substituting equation (16) into (14) the output voltage versus input voltage relationship becomes:

(17)

which finally turns into:

(18)

In this final equation no current is directly involved, while it is elicited that the input-output voltage relationship is a rather more complicated, but still a linear-algebraic, like in the case of an ideal switch.

5. Conclusions

This paper presents an introduction of a feasible method for teaching power electronic issues to non-electrical engineering disciplines. The method presented is founded on the switching function techniques while it is shown that it does not require any specific branches of knowledge. Moreover, this methodology can incorporate even the non-ideal behaviour of power electronic switches. This approach has successfully integrated power electronic core course in marine electrical engineering studies of School of Naval Architecture and Marine Engineering of NTUA.

Acknowledgements

The author wishes to thank the European Union and the Greek Ministry of Education for the financial support on his efforts to reform the marine electrical engineering courses of the marine engineering curriculum of his School within the “Operational Programme for Education and Initial Vocational Training - EPEAEK”-project. The Project is co-funded by the European Social Fund (75%) and National Resources (25%).

References:

[1]  J.M. Prousalidis, " Simulation tools for ship electric power and control system studies ", Proceedings of 6th International Naval Exhibition and Conference (INEC2002), Glascow (UK), pp. 263-275 (23-24 April 2002).

[2]  J. Prousalidis, I.K. Hatzilau, Cdr S. Perros, P. Buchanan, D. Muthumuni, " Introducing a COTS simulation tool for ship electric power quality studies ", Proceedings of International Naval Exhibition and Conference, Amsterdam (The Netherlands), Vol. II, pp. 23-34 (16-18 March 2004).

[3]  T. Skvarenina, " The Power Electronics Handbook ", CRC Press, USA, 2002.

[4]  J. D. Irwin" The Industrial Electronics Handbook ", CRC Press, USA, 1997.

[5]  B. Bose, " Power Electronics and Variable Speed Drives ", IEEE Press, 1996.