HARMONIC POWER QUALITY ON NAVAL SHIP ELECTRIC SYSTEMS

( OVERVIEW OF STANDARDS – FIELD MEASUREMENTS ON HN "MEKO" CLASS FRIGATES )

I.K. HATZILAU(1), AR. MAGOULAS(2), S. PERROS(3),

D. KAVOULAKOS(4), E. SAKIOTIS(4), E. CHRISTOFIS(4), F. MARTINOS(5), J. PROUSALIDIS(6)

(1) Prof. Dr. Ing., Chair of Electrical Engineering , Hellenic Naval Academy (HNA)

Tel: +3010-4581332, Fax: +3010-4181768, e-mail: ikx@snd.edu.gr (2) Dr., Lecturer, Chair Electrical Engineering, Hellenic Naval Academy (HNA) (3) Commander Hellenic Navy (HN), M.S.E.E. (4) Lieutenant Commander Hellenic Navy (HN), M.S.E.E. (5) Lieutenant Hellenic Navy (HN), MSc.M.E.S.E. (6) Dr., Lecturer, Naval and Marine Engineering Dept., National Technical University of Athens (NTUA)

9 Heroon Politechniou St, 15773 Greece. Tel: +3010-7722869, Fax: +3010-7721117, e-mail: jprousal@naval.ntua.gr

ABSTRACT: The main objective of this paper is a discussion on the power quality problems encountered on naval vessels with respect to the harmonic distortion limits imposed by relevant stan-dards, namely IEEE-519 which refers to all electric systems and STANAG-1008 which is dedicated to warships. A brief discussion is also made on the causes, consequences and remedies of har-monic distortion in the electrical plants, while measurements con-ducted onboard HN MEKO class frigates are presented. The evaluation of the measurements indicates that it can not be easily argued if the STANAG-1008 as well as IEEE-519 current restric-tions are violated or not, while the voltage distortion remains within the imposed limits.

Keywords: Naval vessel, power quality, harmonic distortion, STANAG 1008, IEEE-519. I. INTRODUCTION

During the last years there has been a deterioration to the problem of the harmonic distortion, observed in voltage and current, in the electrical plants of modern warships. This anomaly is caused by the increasing use of non- linear electrical loads and the fact that many of the loads are sen-sitive to a voltage supply with a high harmonic content. The problem is expected to become more severe in the fu-ture constructions due to the extensive use of devices with power electronics and the perspective of implementing electric propulsion utilizing electric converters feeding innovative A.C. motors. In brief, the problem is stated as follows [1-2]: The genera-tion of harmonic distortion has two causes. Synchronous generators produce - due to structural asymmetries - power with voltage harmonic distortion 1% approximately.

However, the main harmonic distortion is caused by the non-linear behavior of a number of electrical loads which, even though driven by a purely sinusoidal voltage, they absorb a deformed current (magnetic current of transform-ers, induction motors, fluorescent lamps and mainly static converters and power supplies of numerous electronic de-vices). The flow of these non-sinusoidal currents through the electric network creates voltage harmonic distortion in various points as a result of the distorted voltage drops on the internal impedance of all elements of the electric net-work. Furthermore, the main consequences of the high harmonic content in voltage and current are : • increase of iron-losses in electric machinery resulting

in efficiency decrease and overheating, • hums and vibrations in electric machinery, • malfunctions of electric devices, • over-voltage and over-current due to resonance result-

ing in insulation fatigue and breakdowns, • EMI/EMC problems to neighboring communication

systems. The electric system of a ship can be regarded as a non-interconnected one, comprising a reduced number of gen-erators and an increased number of non-linear loads of low or high voltage, e.g. 440V or 6.6 kV. Therefore, it can be argued that the standards issued for ordinary electric sys-tems can be applied to ships, too. On the other hand, spe-cific standards have been released covering the electric installations on shipboard taking into account their particu-larities, [3]. Similarly, referring to power quality problems, standards like IEEE-519 [1] can be applied to ships, al-though standards dedicated to ships have been issued, while particularly for warships the STANAG 1008 [4] is used. In this paper, a discussion is made concerning the restric-tions on voltage and current harmonic distortion implied by STANAG 1008. This discussion is supported by field measurements on HN MEKO class frigates, while some problems emerged from the application of the standards, are highlighted. Finally it is argued that IEEE-519 can resolve most of these difficulties.

II. POWER QUALITY STANDARDS AND MEAS-UREMENTS A. STANAG-1008/ Edition 8 The specifications of the electrical power plants in NATO naval vessels are laid down in STANAG-1008 [4]. Ac-cording to Editions 7 and 8 which hold for about fifteen years, the Total voltage Harmonic Distortion (THD) of a 60Hz and 400Hz network must not exceed 5%, while no harmonic should exceed 3% of the fundamental. To achieve the above, STANAG 1008 provides design constraints for current harmonics produced by each sepa-rate load presented in Table I. The constraints refer to the %portion of the ”rated full load (nominal) fundamental current”, I1fL, of each load. Table I. Current harmonic constraints by STANAG 1008 / Edition 8

Harmonic order n Acceptable level of harmonic value in % of the ”rated full load (nominal) fundamental current”, I1fL

Loads < (1kVA/60Hz or 0.2kVA/400Hz and 2A/115V/400Hz)

n >1 100/n %

Loads > (1kVA/60Hz or 0.2kVA/400Hz and 2A/115V/400Hz)

2 ≤ n < 33 3 % n ≥ 34 100/n %

The validity of this constraint has been more and more un-der reconsideration for the last years. The NATO AC/141(NG/6)SG/4 Group which is the custodian of STANAG 1008, considers the limitation of this STANAG with skepticism, since theoretical estimations and meas-urements in the electrical plants of naval ships indicate that STANAG 1008 current constraints appear too strict on the harmonic distortion allowed. Moreover, the reference term ”rated full load fundamental current”, I1fL has occasion-ally lead to misinterpretations as well as difficulties to vali-date the STANAG constraints on shipboard, [5], [6], [7], [8]. Thus, the equipment rated power is often overesti-mated so that its “actual load” is lower - and much lower in some cases - than the “rated full load”. It is worth noting, that, in general, the harmonic spectrum in “actual load” could be considerably different from that in “rated full load”. This remark holds and is further supported by the field measurements presented in the following. B. FIELD MEASUREMENTS AND RESULTS During a project investigating electric power quality in Hellenic Navy ships, several measurements on voltage and current harmonics have been conducted onboard HN MEKO frigates, [9]. The electric plant (Fig. 1) in these ships is served by four generators (775KVA, 450V/60Hz, 3-phase), laid in two

separate engine-rooms. The generators feed two main switchboards supplying thirteen Load Centers (LCi, i = 1,...,13). These LC distribute power (440/220/115V, 60Hz, 3-phase/1-phase and 24V DC) to all separate loads, see ref. [9],[10]. The instruments ‘DRANETZ 658 Power Quality Analyzer’ and ‘ELCONTROL VIP SYSTEM3' Energy analyzer’, which can analyze current and voltage measurements up to the 50th and 25th harmonic respectively (for 50 and 60 Hz systems), have been used. Measurements have been taken at load inputs (440V 115V), transformer (440/115V) primary and secondary coils, 60/400Hz converter inputs (440V), LC inputs, and generator outputs, while generators were operating in nor-mal power levels. Representative results are shown in Fig. 2. The spectrum of the current harmonics in these meas-urements, and the respective limits according to STANAG-1008/Edition 8 are shown in Fig. 3. In Fig. 3(A) single consumers' current appears, while in Fig. 3(B) grouped consumers' current is presented (e.g. Load Centers, etc.) and finally in Fig. 3(C) the corresponding voltage har-monic spectrum is shown.

From the measurements performed, it is evident that: a) The highest harmonics of current are of the order 3, 5, and 7 with their values being fairly large. b) The current harmonics measured appear to be of consid-erable portion of the corresponding (“actual load”) funda-mental. These figures appear to exceed by far the con-straints of STANAG 1008/ Edition 8. Similar sets of meas-urements in ships of several NATO Navies have lead to the impression that these constraints are too strict. However, in the authors opinion no safe conclusion according to Edition 8 stipulation, can be drawn based only on “actual load” spectra as few electric loads operate near to their “rated power”. Typical examples of this category are power trans-formers and power electronic converters, with their THD being a function of their demand in electric power. For instance, measurements at 60/400 Hz frequency converter of MEKO frigate performed in the usually operating power range (“actual load”) indicate excessive current harmonic distortion, as shown in Fig. 4. However, STANAG con-straints are not necessarily violated, as, according to the trend of the measurement points presented in Fig. 4, - which trend is also confirmed by the calculations according to IEEE-519 - the more operating power increases towards its rated value (“rated power”), the more harmonic distor-tion levels decrease, reaching eventually the permissible limits. Therefore in many cases, it is difficult to assess the distortion of currents of a stand-alone equipment, as de-fined by STANAG, without resorting to manufacturers’ test results or laboratory experiments performed in rated load conditions, which in current practice are rarely avail-able.

c) Nevertheless, no matter what the relevant values of the measured current harmonics are, the measured voltage dis-tortion - referring to either the THD or the individual har-monics - is well beyond the imposed marginal values of STANAG.

440V 60Hz

LC1

2.1 2.2 .... 2.i T2.1 . . . 24VDC

LC3 . . . LC7LC7

C2.1 . . C2.i CT2.1 . . .

440V 400Hz

115V400Hz

LC8 . . . LC11

G1 G3 G4G2 SC SC

LC2

LC13

LC12

115V 60Hz

T.L

2sT2pT

Fig. 1. Schematic Presentation of the Electrical Plant of F/G MEKO.

B.1 B .2

-150

0:34.735 0:34.740 0:34.745 0:34.750

V olts

-750

-500

-250

0

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750

Am ps

-100

-50

0

50

100

150

sec Thd H05 H10 H15 H20 H25 H30 H35 H40

% FND

0

10

20

30

40

50

60

Phase

0

50

100

150

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250

300

350

Thd H05 H10 H15 H20 H25 H30 H35 H40 H45

% FND Phase

050

100

150

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350

400125

100

75

50

25

00:36.810 0:36.815 0:36.820 0:36.825

Volt

-750

-500

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0

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500

750

Am p

-15

-10-5

0

5

10

15

sec

V I

V I

A.2A.1

Fig. 2. Voltage (V) and current (I) waveshape as well as Current magnitude and phase harmonic spectrum of a 100 kW rated power load on the 440 V/60 Hz system, in two different operating conditions (A and B).

Fig. 3. (A) Current harmonic spectra for independent loads (power ⊇1 kVA). (B) Current harmonic spectra for groups of loads (e.g. LC inputs etc.) (power ⊇1 kVA).

(C) Respective voltage harmonic spectra.

Fig. 4. Current THD and 5th HD of the 60/400 Hz converter as a function of operating power

C. STANAG-1008/ Edition 9 As mentioned above, STANAG-1008 / Edition 8 is to be replaced by Edition 9, being currently under ratification. In this edition, the voltage distortion constraints still hold, whereas the current constraints have been completely re-moved and substituted by (see also Table II): “If the sum of the power of all loads which distort the cur-rent waveform connected to the supply system, Σ Pdist is less than 1% of the short circuit power of the generation capacity, Ssc, with the largest single distorting load less than 0.5%, normally no measures are necessary to reduce current harmonics. This limit can be further extended to 2% of Ssc, if this load is due to the sum of only small dis-torting equipment, each less than 0.1% of Ssc. If any of these limits is reached or exceeded by the distorting load, analyses to whether STANAG-1008 requirements are still met with respect to voltage harmonics and an early co-ordination between the system design authority and the equipment manufacturer should be undertaken”. Table II. Current harmonic constraints by STANAG 1008 / Edition 9

Condition Action required

sck

kdist SP %.1, =∑ with

sckdist SP %.5.0)(Max ,k =

No action

sck

kdist SP %.2, =∑ with

sckdist SP %.1.0)(Max ,k =

No action

Otherwise Analysis Pdist,k: power of devices distorting current waveforms Ssc: short circuit power level of the supply system

Therefore, it is stressed that after this amendment, no tan-gible limitation on current (or any other quantity) distortion for each stand-alone power consumer is actually set in an effort to restrict the voltage distortion within the specified limits. On the contrary, voltage distortion has to be inves-tigated by performing a plethora of costly power quality analyses (simulations of operation of the entire electric system and/or field measurements on it), where all alterna-tive solutions from various manufacturers and at several operating points must be taken into account whenever a piece of equipment is to be installed or replaced. D. IEEE-519 According to IEEE-519 [1,11], the acceptable levels of current and voltage distortion are defined at the Point of Common Coupling (PCC), i.e. that busbar defined as the border between the power generation and power consump-tion sections. The limit values depend on the voltage level of the electric system studied and the ratio of short circuit current over maximum load current, the latter being the “average of maximum loads achieved on a 12-monthly

basis”. This definition is much more readily applicable to any electric installation including the shipboard ones. Moreover, at an initial approximation, the average maxi-mum load can be estimated by the service load simultane-ous factors fs, which are extensively used at the electric balance of the ship power plant and are often readily avail-able for a great variety of ship types. Factors fs are defined as the ratio of the actual energy required on a daily basis over the energy corresponding to the rated power for the same daily interval, therefore:

(average maximum load) = fs * (rated load) Furthermore, the ship electric networks drop into the cate-gory between 120 V and 69 kV (section 10.2), while the short circuit current over maximum load current ratio is normally less than 20 (ship grounding systems are de-signed so that short circuit currents are just few times greater than the maximum load currents [12]). Consequently : identically to STANAG 1008, the voltage distortion is limited to 5% while no harmonic should ex-ceed 3%. Concerning current distortion, this is limited as explained in the Table III. Table III. Current harmonic constraints by IEEE-519

Harmonic order n

Acceptable level of harmonic value in % of the maximum load current (fundamental), I1fL, at PCC

n< 11 <4% 11<=n<17 <2% 17<=n<23 <1.5% 23<=n<35 <0.6% 35<=n <0.3%

However, it has to be stressed that even in the IEEE-519 case, there have been discussions on the interpretation of the definitions of PCC, short circuit current and maximum load current values considered, as they may result in erro-neous conclusions [11]. III. DISCUSSION Taking into account the aforementioned points, it is under-lined that all standards require that the voltage total har-monic distortion is less than 5%, a constraint proven to hold almost by any measurements onboard. However, there seems to be no clear way of estimating this voltage distortion e.g. by investigating the current distortion caused by each separate load. More specifically: a) STANAG 1008/Edition 8, makes the effort to set con-straints on current harmonics so that voltage distortion is kept within acceptable limits. However, these current dis-tortion constraints are, on the one hand, difficult to inter-pret and finally measure, whereas on the other hand, it can be argued they are often exceeded without, though, leading to excessive values of voltage distortion. b) STANAG 1008/Edition 9, which is planned to succeed Edition 8, does not offer an actual solution to the problem,

by clarifying or even resetting the current harmonic distor-tion constraints. On the contrary, Edition 9 simply states that several analyses must be performed at the ship design stage or at any revamp to reassure that voltage distortion does not exceed the limit of 5%, without setting any guid-ing constraints for each separate manufactured equipment. c) Finally, IEEE-519, which could be applied to shipboard installations, appears more readily applicable referring more clearly to certain operating conditions and measure-ments. Therefore, it is believed, that IEEE-519 constraints on current could be applied onboard despite the difficulty to assess the average of maximum loads on a 12-monthly basis, as, even at the design stage, the service load simulta-neous factors fs can be used. IV. CONCLUSIONS In this paper a discussion is made on the power quality harmonics on shipboard electric systems. The discussion is supported by field measurements on HN MEKO class frig-ates. Comparing the measurement results with the direc-tives of STANAG 1008 / Edition 8, in many cases it is rather difficult to identify whether the suggested current harmonic limits are violated or not, as they refer to often unattainable operating points. On the other hand, the volt-age harmonic distortion in operating plants, which is the ultimate objective, is measured to be well below the limits set by either STANAG-1008 or IEEE-519 (voltage THD less than 5%, each voltage harmonic very less than 3% ). STANAG 1008 / Edition 9, which will replace Edition 8, practically sets no limits on current harmonics of each separate or group of loads. On the contrary, IEEE-519 sets current distortion limits for the installed equipment and seems to be more readily applicable, as it refers to actual operating points. V. REFERENCES [1] IEEE Std 519-1992 : "IEEE Recommended Practice and Require-ments for Harmonics Control in Electrical Power Systems", IEEE, NewYork, 12 April 1993. [2] Vokas, G. ,Gazithellis, D. , Katsikaris, C., Peponis, G., "Electric Power Quality ", Technical Chamber of Greece, Athens, May 1993 (In Greek). [3] IEEE Standard 45-1993, "Recommended Practice for Electrical Instal-lations on Shipboard", IEEE Press, New York, 1993 [4] STANAG 1008 : "Characteristics of Shipboard Electrical Power Systems in Warships of the North Atlantic Treaty Navies", NATO , Edi-tion 8, 21 Febr. 1994./ Edition 9 under ratification. [5] NATO AC/141(SG/6)SG/4 FRANCE paper 93/08 (44)-8 Sept. 93, : "Harmonic pollution on Ship electric side circuit - Limitation of harmonic currents injected into side circuit" , DGA Note technique No 353/93 SY/SEP, Paris 8 Sept. 1993. [6] NATO AC/141(SG/6)SG/4 GERMANY paper (02)96/10-03.04.96 : "STANAG 1008, Harmonic Distortion, Compilation of Replies to Ques-tionnaire" [7] NATO AC/141(SG/6)SG/4 GERMANY paper (07)97/24-12.12.97 : "Harmonic Distotion". [8] NATO AC/141(SG/6)SG/4 GREECE paper (03)00/28-03.04.00 : "Interpretation of Current Distortion Limits of STANAG 1008 Ed. 8”

[9] Hatzilau, I.K., "Verification of Electric Power Quality on Shipboard ", Research Project Progress Report, Hellenic Naval Academy (HNA), July 1999 (In Greek) [10] "MEKO 200 General Purpose Frigate (HN) "HYDRA" Class" - "System Manual, Electrical System and Automation" , Doc. No. 0720-13-00 File 01 [11] McGranahan, M., “Overview of the Guide for Applying Harmonic Limits on Power Systems-IEEE 519A’’, Proceedings of 8th International Conference of Harmonics and Quality Power, Athens (Greece), 16-18 October 1998, pp. 462-469. [12] Bal, R.,Stephens, G.W., “Neutral Earthing of Marine Electrical Power Systems”, Trans I Mar E (TM), Vol. 95, Paper No 32, 1982. VI. BIOGRAPHIES Prof. Dr. Ing. I.K. HATZILAU (Electrical & Mechanical Engineer from NTUA/1965, Dr. Ing. from University of Stuttgart/1969). After few years in the industry, he joined the Academic Staff of HNA where he has been for 25 years. He is also representative of HN in NATO AC/141(NG/6)SG/4 dealing with electric systems in warships. Dr A. MAGOULAS, (Electrical Engineer from NTUA/1985, PhD from NTUA/2000). He has been Lecturer at the HNA since 1990 and is inter-ested in Electromagnetic Scattering problems and also in Electrical Power systems. Cdr. S. PERROS, (Engineer Officer from the HNA/1984, M.Sc in Elec-trical Engineering from Naval Postgraduate School in Monterey Califor-nia, USA/ 1992). After many years of service in HN warships (including Frigate MEKO HN type), he is assigned now in HN General Staff. He is also an associate of the El. Engineering Chair of the HNA and is interested in the naval ships electric systems and the electrical power quality prob-lems. Lt. Cdr. D. KAVOULAKOS, (Deck Officer from the HNA/1988, M.Sc in Electrical Engineering from Naval Postgraduate School in Monterey California, USA/ 1996). After many years of service in HN warships (in-cluding Frigate MEKO HN type), he is assigned now in HN General Staff. He is interested in Communication Systems and also in Electric Power Quality problems on ship electric systems. Lt. Cdr. E. SAKIOTIS, (Engineer Officer from the HNA/1987, M.Sc in Electrical Engineering from Naval Postgraduate School in Monterey Cali-fornia, USA/ 1996). He has served in many HN warships (including Frig-ate MEKO HN type) and is now serving in Souda Naval Base as head of Energy Division and Electrical & Electronic Workshops Manager. He was also an associate of the El. Engineering Chair of the HNA. Lt. Cdr. E. CHRISTOFIS (Deck Officer from the HNA/1988, M.Sc in Electrical Engineering from Naval Postgraduate School in Monterey Cali-fornia, USA/ 1996). After many years of service in HN warships (includ-ing Frigate MEKO HN type), he is assigned now in HN General Staff. He is interested in Communication Systems and also in Electric Power Quality problems on ship electric systems. Lt. F. MARTINOS (Engineer Officer from the HNA/1991, M.Sc in Mili-tary Electronic Systems Engineering from Royal Military College of Sci-ence of Cranfield University ,UK/1997). At present he is positioned as a Director of the Electronics Department (Electronics Officer) in H.S. SPETSAI (Frigate MEKO HN type) and he is an associate of the El. Engi-neering Chair of the HNA and he is interested in the naval ships electrical systems and the Electric Power Quality problems. Dr. J. PROUSALIDIS (Electrical Engineer from NTUA/1991, PhD from NTUA/1997). Currently, he is Lecturer at the Naval Architecture and Marine Engineering Department of National Technical University of Ath-ens, dealing with electric energy systems and electric propulsion schemes on shipboard focusing on electric power quality problems and analysis of transients.