resonancia en transformadores

The Online Journal on Electronics and Electrical Engineering (OJEEE) Vol. (3) – No. (2)

Reference Number: W10-0089 393

Detection of Parallel Resonance in the IndustryLevent Bilgili

Medical Metrology Services, 1348 S. No:17/205 Yenisehir Izmir Turkey, [email protected]

Belgin Emre TürkayDepartment of Electrical Engineering, Faculty of Engineering, Istanbul Technical University

Ayazağa Kampüsü, 34469 Maslak Istanbul, [email protected]

Abstract- With this study, it is aimed to developaccurate solutions with the detection of the causes ofcapacitor malfunction in buildings and industrialfacilities, where the ratio of total harmonic distortion inthe components of current and voltage is not in excessivevalue. in the facilities where the ratio of harmonicdistortion is measured not so high and thus unfilteredharmonic compensation system is used, it has beendetected in our measurements that when capacitor stepswere switched on, some harmonic values reached veryhigh ratings, thus especially small step capacitors wereunable to adjust to the new harmonic values and went outof order.

When reactive power of the capacitors that areswitched on merges with the inductive characteristics ofthe system, it causes parallel resonance. Our study pointsout the detection of this resonance with the samplemeasurements done in the field and deals with thecomputer environment studies that are necessary for thesecurity of the facility.

Keywords- parallel resonance, harmonics, power qualityanalyzer, measured effects of the harmonics in theindustry.

I. INTRODUCTION

In order to determine and analyze the system characteristicsin the power systems, the answer related to the frequency isexamined. When examining the answer related to thefrequency, not only the amplitudes of the existing harmonicsources in the system; but also the reaction of the system tothat harmonic amplitude is quite important. When examiningthe system characteristics, three points should be at first takeninto account: Impedance of the system, compensation amountin the circuit and the values of the resistive and harmoniccharges in the system [1].

Under normal conditions the power systems have inductivecharacteristics and have directional relations with thefrequency. In this case, the change in the impedance of thesystem and the interaction of the harmonics show adirectional characteristic before the insertion of capacitor, asin Figure (1).

Figure (1): Impedance-frequency diagram for inductivesystem

The current amplitude of a charge that containscomponents with harmonics in the frequencies excludingbasic frequencies can be illustrated as in Figure (2).

Figure (2): The distribution of harmonic amplitudesaccording to the frequency

In measuring the amplitudes of existing harmonics as inFigure (2), harmonics flow from a facility with a higherimpedance to a network with a lower impedance through thetransformer, as illustrated in Figure (3).

Figure (3): The direction of the currents with harmonics

To calculate the new characteristic of the system with theeffects of the compensation system that is attached to the buson the lower voltage side of the transformer as in Figure (3)and the load currents with harmonics, the equivalent circuit inFigure (4) can be used.

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Figure (4): The parallel resonance equivalent circuit that isseen by the harmonic circuit source

The equivalent impedance that is seen by the harmoniccurrent source is the impedance value that is found by theparallelism of the equivalent inductance of both the networkand the transformer and the capacitor inductance [2].

The harmonic level, which determines the highestdistortion value that is found by the calculation of system’sequivalent impedance difference related to the frequencywhich is calculated as from the basic frequency, showssimilarity to the harmonic which is found by the resonanceharmonic formula associated with the total switched oncompensation value belongs to the transformer andtransformer impedance of the transformer mentioned in theformula below.

In this case, if the compensation system, which is added tothe bus under the transformer, is simulated as in Figure (3);the impedance of the system moves away from the directionalstate shown in Figure 1 and acts as shown in Figure (5). Forinstance, when a 100 kVAr capacitor is connected parallel tothe bus, with each 100 kVAr addition the parallel resonancepoint slides to the lower frequencies and the impedanceamplitude decreases while resonance frequency is between600-800 Hz and the impedance is 0,150 ohm.

Figure (5): The change of parallel resonance frequency andthe impedance of the system with the addition of capacitor

It is known that the compensation systems that are built tosatisfy the reactive power need increases the amplitudes ofthe current and voltage harmonics in the system. If theinteraction of the system impedance in Figure (5) with thecurrents of the harmonic charges fed from the transformer asillustrated in Figure (2), it is expected that the currents withharmonics are to flow over the bus to the network or to goforward to a point with lower impedance.

In the amplitudes of the harmonic components that aremeasured from the secondary of the transformer, it is normalto observe an increase to a certain ratio as a result ofcompensation addition; whereas quite high amplitude valuesarise in the harmonic component that goes into resonance.These harmonic currents flow to the capacitors or to thecharges in the system with lower impedance and causeproblems such as breakdowns, explosions, fire etc. [3]

II. THE DETECTION OF THE PARALLEL RESONANCEWITH THE POWER-QUALITY ANALYZER

As a sample application, a measurement done by thesecondary of the transformer of a facility was analyzed. Thereis an unfiltered harmonic compensation system in the facility.From time to time, some capacitors and safety fuses areharmed in this power panel, which is attached to the same busas directional and harmonic charges and is controlledautomatically by a reactive power control relay. In normalworking conditions, because the total distortion ratio isthought to be not so high while the compensation is in or outof the circuit, the necessity of the harmonic filtering could notbe realized by the users before this analysis.

Figure (6): The facility where the analysis was conducted

In the charge measurements done in the main distributionswitch that is on the low voltage side of the transformer, it ispossible to examine the harmonic charges that go over thetransformer and the change of the harmonic voltages in thebus according to the compensation value in the circuit, withthe help of a simple analyzer that is connected as shown inFigure (6) and can be connected to a computer [4].

Figure (7): Voltage-Current Diagram

The changes in the voltage and the current are observed inFigure (7) when the compensation is switched off for a coupleof minutes.

When the compensation system is switched off, it ismeasured that it satisfies an inductive reactive power need of

(1)



approximately 250 kVAr during the measurement and gradualswitch-ons are examined for the moment when it is switchedon again.

Figure (8): Inductive reactive power diagram

The performance of the total harmonic distortion during themeasurement has been examined in Figure (9) and parallelresonance condition has been observed in this diagram.

Figure (9): Total harmonic distortion diagram

The total harmonic distortion, which had not exceed 12 %in our measurement that started while the compensationsystem was switched on, has declined to 8% when thecompensation system was switched off; but during theswitching-on of the graduals as in Figure (10), the totalharmonic distortion has exceeded the value of 24% in thecurrent and the value of 7% in the voltage in the moment thatcorresponds to a 159 kVAr need.

Figure (10): The inductive reactive power value for thecondition in which the harmonics are the highest in

percentage

When the harmonic spectrum for this system is examined,Figure (11) states the condition in which the compensation isswitched on in the system. When the highest values are takeninto account as amplitude, the voltage harmonics are under2% and the current harmonics are under 8 %.

Figure (11): Harmonics while 250 kVAr compensation isswitched on

When the behavior of the resistive and harmonic chargesfor the condition where the compensation is switched off isexamined in Figure (12), the total harmonic distortion staysunder 1.5 % for the voltage and 5% for the current.

The point to pay attention here is that the highest valueamong the current harmonics is the level of 11th harmonic.

Figure (12): Harmonics while the compensation isswitched off

Figure (13): Harmonics for the moment of parallelresonance

In the condition where the capacitors are graduallyswitched on with the help of reactive power control relay, the11th harmonic component which has the biggest amplitude ofall charge harmonic components, reaches the maximum valuefor the grade that corresponds to the need of 159 kVAr inreactive power.

While the ratio of the 11th voltage harmonic to the basiccomponent as shown in Figure (12) is 1.5% when thecompensation is switched off, in Figure (13) it rises up to 6-7%; and while the first state of the current harmonics inFigure (12) is 4-5%, for the parallel resonance it rises up tovalues of 25 % in Figure (13).



III. THE CONFIRMATION OF THE ANALYZERSOLUTIONS WITH THE HELP OF A COMPUTER

When the combined steps in compensation panels are takeninto account, it is hard to presume with which step a systemgoes into resonance. The capacitor switching algorithms of 12degree relays that are physically available in the market canchange according to the brand and the model. So that theelectrical grading possibility and the condition of reactivepower need supply can not be examined by the manualmethods.

Only by utilizing a simple calculation table and transformerpower and short circuit voltage, the system’s answer to theharmonics can be examined from the equivalent circuitcalculation in Figure (4).

Table (1): The amount of risky compensation correspondentto the transformer label value according to the harmonic

degrees

In the measurements done with the power quality analyzer,the amplitudes that belong to the charge harmonics in theharmonic degrees shown in Table (1) will reach maximum atthe values very close to the aforementioned parallelresonance kVAr values. By comparing this simple calculationand the measurements in the field, the information about theharmonic degrees that has to be focused on, the capacitoraddition and which behavior the system’s answer will showcan be obtained.

To give an example for our application; if we add the 50kVAr constant step compensation that is attached to ourmeasuring point on the main distribution panel to theoperation:a) When the 11th harmonic is at its highest value, the total

capacitor power in the circuit is 50 kVAr + (250kVAr –159 kVAr) = 141 kVAr. When taken into account that11th harmonic has amplitude among the charges, themost risky level is a value close to the peak valueapproximately 148 kVAr, as shown in Table-1.

b) According to the fact that under the transformer there is atotal capacitor power of 250 kVAr + 50 kVAr = 300kVAr while the compensation is switched on; whencontrolled from the values on Table-1, if there exist 7thand 8th harmonics among charge harmonics, theamplitudes of these harmonics will obviously be affectedmore.

While in Figure (12), the 7th and 8th harmonics are notdominant relative to the 11th harmonic in the beginning, inFigure (11) they reached their maximum when thecompensation was switched on. It can be assumed that thepeak value of the parallel resonance in that moment isbetween 7th and 8th harmonic degrees.

When the compensation system is switched on in themeasurement model, the ratio of load current to the basiccomponent that is the 11th harmonic degree was 4.5 % atfirst, but then it reached an amplitude of up to 25 %. Witheach new grade that is switched on, parallel resonancefrequency has slide to lower harmonic degrees and the 11thharmonic has returned to average values. As long as thecapacitor power changes according to the need and thecapacitors lose in value, the parallel resonance condition cancoincide again in another frequency.

IV. CONCLUSION

This study has dealt with the detection of the parallelresonance risk with the measurements done by power qualityanalyzers in industrial field and its confirmation with thecomputerized calculation methods.

In this study, the points that have to be taken into accountfor the analysis of the problems such as malfunctions, fuseexplosions, fires etc. which happen in the facilities that triggercapacitor without using harmonic filtered compensationsystem because the total harmonic distortion amount isgenerally below the value determined in IEC 61000-4-7 andEN 50160, have been discussed. Accordingly, the value lossof the grades of compensation panel can bring some harmonicdegrees to a level very high and risky for the system, even theelectrical and physical grading difference has a small valueamong the existing charge harmonics. For this reason, themeasurements and the probable values that can occur out ofthe general performance of the system should be compared bybeing calculated in a computerized environment.

REFERENCES

[1] Dugan Roger C., McGranaghan Mark F., SantosoSurya and Beaty H.Wayne, Electrical Power SystemsQuality Second Edition, McGraw-Hill, 2004

[2] Schneider Electric Publications, Industrial ElectricalNetwork Design Guide, T&D 6 883 427/AE, , 2000

[3] Collombet C., Lupin J.M., Schonek J., Harmonicdisturbances in networks and their treatment, CahierTechnique Schneider Electric No:152

[4] Schneider Electric Publications, Electrical InstallationGuide, 2005

resonancia en transformadores

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