1 Harmonics And Filters In HVDC Systems Ms. Tahoora Qureshi, Asst. Professor Department: B.E. Electrical Engineering Subject: HVDCT Semester: VII Teaching Aids Service by KRRC Information Section

2 INTRODUCTION Electrical energy transmitted through AC or DC transmission is to be delivered at consumers terminals at specified voltage level of constant magnitude without deviation from ideal waveform. Deviation from the perfect sinusoidal waveform is generally expressed in terms of harmonic components. Harmonics are defined as the content of electrical signals whose frequency is an integral multiple of the fundamental frequency produced by generators.

3 IMPORTANCE OF HARMONIC STUDYHVDC transmission system generated harmonic currents on AC side and harmonic voltages on DC side during operation. The harmonic currents generated at AC bus get transmitted to AC system and cause following adverse effects- Heating of equipments connected Instability of converter control Generates telephone and radio interference in neighboring communication line, thereby inducing harmonic noise. Harmonics can lead to over voltages due to resonance when filter circuits are employed.

4 GENERATION OF HARMONICSSources of harmonics are- AC generator Transformer Converter along with its control devices AC GENERATOR AS A SOURCE OF HARMONICS Although the waveform is usually good, an AC generator may be regarded as a source of balanced harmonic voltages because of non-uniform distribution of flux on the armature windings.

5 TRANSFORMER AS A SOURCE OF HARMONICS:Due to presence of DC component in transformer secondary, there is magnetic distortion and magnetic saturation which make transformers as sources of harmonic voltages. Magnitude of these harmonic depends on operating flux density. Converter transformers are usually operated at higher flux densities than conventional three phase transformers, therefore the possibility of generation of harmonics is more in former than in latter.

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8 HARMONICS DUE TO CONVERTERSCHARACTERISTIC HARMONICS: Under balanced operating conditions, the converter is supposed to produce fundamental and harmonics of the orders 5,7,11,13,etc. Normal current harmonic produced on AC side are of the order (np±1) Normal voltage harmonic produced on DC side are of the order (np) where ‘n’ is any positive integer and ‘p’ is pulse number. The above mentioned normal harmonics are called as characteristic harmonics.

9 HARMONICS DUE TO CONVERTERSCHARACTERISTIC AC HARMONICS

10 HARMONICS DUE TO CONVERTERSThe current IA= IA1+IA2 The line current of each converter bridge will be half of the total current mentioned above ie The rms value of which is,

11 HARMONICS DUE TO CONVERTERSFrom the above the maximum value of fundamental and harmonic currents will be, The second subscript “o” indicates that the overlap angle “u” is assumed to be zero.

12 HARMONICS DUE TO CONVERTERSWhen “u” is non-zero, the expression for Ih is given by, The above expressions are valid only for u≤60

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14 HARMONICS DUE TO CONVERTERSCHARACTERISTICS DC HARMONICS From the Fourier analysis of DC voltage waveform, we can obtain,

15 HARMONICS DUE TO CONVERTERSNON CHARACTERISTIC HARMONICS: The harmonics of the order other than characteristic harmonics are termed as non-characteristic harmonics. These are due to, Imbalance in the operation of two bridges forming the 12 pulse converter Firing angle errors Unbalance and distortion in AC voltages. Unequal transformer leakage impedances.

16 HARMONICS DUE TO CONVERTERSThe harmonics produced due to the 1st cause are termed as residual harmonics. These are majorly due to difference in the firing angles of the two bridges which lead to unequal cancellation of harmonics of order 5,7,17,19etc The last three cause can lead to generation of triple or even harmonics. (1) EFFECT OF FIRING ANGLE ERRORS: The major source of firing angle errors in modern converter stations is due to ripple in current f/b signal and not due to equipments. The tolerance in the firing pulses should not exceed ± 0.2º

17 HARMONICS DUE TO CONVERTERS(2) EFFECT OF UNBALANCED SYSTEM VOLTAGES: The presence of negative sequence component in AC voltage shifts the zero crossing of the commutating voltages. With IPC this introduces firing angle dissymmetry and results in non-characteristic harmonics. With 5% negative sequence voltage, the 3rd harmonic current generated can be as large 5% of the fundamental component.

18 HARMONIC SUPRESSION- FILTERSThe most easiest way to reduce harmonics is to use a passive LC resonant filter that offers low or zero impedance and bypasses the harmonic voltages to a level specified in technical specification. HARMONIC MODEL AND EQUIVALENT CIRCUIT:

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20 HARMONIC MODEL AND EQUIVALENT CIRCUIT:The harmonic currents of order “h” generated are divided into two components. (a) harmonic currents in filter branch (b) harmonic currents in AC network

21 HARMONIC ELIMINATION METHODSThere are two means by which harmonics can be reduced (1) Increasing pulse number (2) Use of filters INCREASING PULSE NUMBER: From converter analysis it is clear that the converter can be operated at higher pulse number preferably 12 pulse to reduce the harmonics. The pulse number can be increased from but it demands a more expensive transformer.

22 USE OF FILTERS A filter is a network that responds more vigorously at some frequencies than others and reduces the amplitude of one or more fixed frequency currents or voltage. They normally consist of one or more LC tuned circuits with a fairly high Q(40), and can comprise a high pass filter.

23 DESIGN OF AC FILTERS AC filters are designed to reduce telephonic interferance. This can be measured by one of the following performing indices. HARMONIC DISTORTION: Where, Ih-harmonic current Zh-harmonic impedance V1- fundamental component of line to neutral voltage m- highest harmonic considered

24 DESIGN OF AC FILTERS TELEPHONE INFLUENCE FACTOR (TIF):ph- C message weighing (this reflects the frequency dependent sensitivity of human ear and has a max value at frequency of 1000hz)

25 DESIGN OF AC FILTERS TELEPHONE HARMONIC FORM FACTOR (THFF):IT PRODUCT:

26 FILTER CONFIGURATIONSTypes of AC Filters  The different types of AC filters used inHVDC systems were as follows 1. Band pass filter 2. Single tuned filter 3. Double tuned filter 4. High pass Filter i. Second order filter ii. C type filter

27 TYPES OF AC FILTERS Single Tuned Filter Double tuned filter AdvantagesIt was to filter out the characteristic harmonics of single frequency. It was used to filter out 11th and 13th harmonics in a converter system. Double tuned filter It was used to filter out two discrete frequencies. It was also used to eliminate11th and 13th harmonics. Advantages 1. Only one inductor was subjected to full line impulse voltage 2. Reduced power loss at the fundamental frequency

28 TYPES OF AC FILTERS High Pass Filter Second order High pass filter It was used to filter out the higherfrequencies. Tuning of these filters was not critical. C type High pass filter  It was used to reduce power losses at the fundamental frequency. Capacitor was connected in series with inductor to provide low impedance path to the fundamental component of current. A third harmonic filter was used to filter out the non-characteristic harmonic of third order. All the filter branches were capacitive at fundamental frequency and supply reactive power.

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30 DC FILTERS The DC voltage on the converter side contains both characteristic and non-characteristic and non- characteristic harmonics. These harmonics result in current harmonics in the DC line and produce noise in telephone circuits. Effectiveness of DC filter is judged by one of the following, Maximum TIF value on DC high voltage bus Maximum included noise voltage (INV) in millivolts/km in a parallel test line situated one km away from HVDC line Max permissible noise to ground in Db on telephone lines close to HVDC lines.

31 DC FILTERS The DC filter used maybe of single tuned or double tuned type to filter out 6th or 12th harmonic voltages and high pass filters are used to filter out higher order harmonics. DC filters are designed only to eliminate DC harmonic voltages and there is no need for filters to supply reactive VARs as in case of AC filters.

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33 POWER LINE COMMUNICATION AND RI NOISEHVDC converters produce high levels of electrical noise whose frequencies range in the carrier frequency band from 22Khz to 500 khz They also generate radio interference (RI) noise of frequencies in mega hertz Power line communication and radio interference (PLC- RI) filters are used to minimize the interference with power line carrier communication system.

34 POWER LINE COMMUNICATION AND RI NOISE

35 ACTIVE FILTERS The active filter used in DC filtering is usually a hybrid active filter i.e an active filter in series with shunt passive filter. Here, a double tuned filter (for12th and 24th) is connected in series with VSC-based active filter. IGBT devices are used in VSC with PWM. The transformer provides galvanic separation between VSC and HVDC line, it helps in raising voltage to desired level. The control strategy of active filter is to inject harmonic voltage of appropriate magnitude and phase angle to cancel harmonic currents flowing in the line.

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37 OTHER METHODS OF HARMONIC ELIMINATION(1) ELEMINATION BY MAGNETIC FLUX COMPENSATION

38 OTHER METHODS OF HARMONIC ELIMINATION(2) HARMONIC INJECTION

39 OTHER METHODS OF HARMONIC ELIMINATION(3)DC RIPPLE INJECTION: In this a triple frequency current wave is generated such that the magnitude is a function of the magnitude of DC currents and wave shape. This current is injected into neutral of the main transformer secondary and flows through the conducting transformer winding. The modified currents in the transformer phases will then contain only 12 pulse related current harmonics