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UNIT – 3 HVDC Converter Unit-03/Lecture-01 Components of EHV D.C. System Components of EHV D.C. System [RGPV/ Dec 2008/ 7]
The conversion from AC to DC and vice versa is done in HVDC converter stations by using three phase bridge converters. A converter has two types of circuits: 1. Main Circuits through which high power flows. The main circuit comprises of converter transformers, thyristor valves, bus bar, series reactor etc. 2. Control and Protection circuit for firing or blocking the valve in desired sequence, monitoring, etc. The main circuit of converters in HVDC terminal stations are made-up of series connected thyristor valves. The converter valves are connected between converter transformer and DC switchyard. Each thyristor valve is made up of several thyristor connected in series. Individual thyristor rating is about 1.5 kV, 600 to 4000 A. The configuration of rectifier and inverter is identical. The triggering pulse to thyristor gates are delayed by angle ′α′ called the delay angle, w.r.t. the instant of natural commutation i.e. the instant when the phase voltage across subsequently conducting valve exceeds that across the preceding value. The delay angle is varied by means of control circuit which gives the triggering pulse to the gates of the thyristor in each arm of the bridge in a definite sequence. With delay angle less than 90 o, the converter acts in the rectifier mode and with delay angle ′α′ between 90o and 180o the converter works as a inverter.
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Figure 1 HVDC System Connecting two AC systems S.NO RGPV QUESTIONS Q.1 Draw a schematic diagram of HVDC System identifying its main components. Explain about each components.
Year Dec 2008
Marks 20
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UNIT 3/ LECTURE 02 Rectifier used in HVDC System Three Phase Fully Controlled Rectifier in HVDC System [RGPV/ Dec 2008, Dec 2007/ 10] The three phase fully controlled bridge converter has been probably the most widely used power electronic converter in the medium to high power applications. Three phase circuits are preferable when large power is involved. The controlled rectifier can provide controllable output dc voltage in a single unit instead of a three phase autotransformer and a diode bridge rectifier. The controlled rectifier is obtained by replacing the diodes of the uncontrolled rectifier with thyristors. Control over the output dc voltage is obtained by controlling the conduction interval of each thyristor. This method is known as phase control and converters are also called “phase controlled converters”. Since thyristors can block voltage in both directions it is possible to reverse the polarity of the output dc voltage and hence feed power back to the ac supply from the dc side. Under such condition the converter is said to be operating in the “inverting mode”. The thyristors in the converter circuit are commutated with the help of the supply voltage in the rectifying mode of operation and are known as “Line commutated converter”. The same circuit while operating in the inverter mode requires load side counter emf. For commutation and are referred to as the “Load commutated inverter”. In phase controlled rectifiers though the output voltage can be varied continuously the load harmonic voltage increases considerably as the average value goes down. Of course the magnitude of harmonic voltage is lower in three phase converter compared to the single phase circuit. Since the frequency of the harmonic voltage is higher smaller load inductance leads to continuous conduction. Input current wave shape become rectangular and contain 5th and higher order odd harmonics. The displacement angle of the input current increases with firing angle. The frequency of the harmonic voltage and current can be increased by increasing the pulse number of the converter which can be achieved by series and parallel connection of basic 6 pulse converters. The control circuit become considerably complicated and the use of coupling transformer and / or interphase reactors become mandatory. With the introduction of high power IGBTs the three phase bridge converter has all but been replaced by dc link voltage source converters in the medium to moderately high power range. However in very high power application (such as HV dc transmission system, cycloconverter drives, load commutated inverter synchronous motor drives, static scherbius drives etc.) the
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basic B phase bridge converter block is still used. Operating principle of 3 phase fully controlled bridge converter A three phase fully controlled converter is obtained by replacing all the six diodes of an uncontrolled converter by six thyristors as shown in Fig. 15
Figure 14
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For any current to flow in the load at least one device from the top group (T1, T3, T5) and one from the bottom group (T2, T4, T6) must conduct. It can be argued as in the case of an uncontrolled converter only one device from these two groups will conduct. Then from symmetry consideration it can be argued that each thyristor conducts for 120° of the input cycle. Now the thyristors are fired in the sequence T1 → T2 → T3 → T4 → T5 → T6 → T1 with 60° interval between each firing. Therefore thyristors on the same phase leg are fired at an interval of 180° and hence cannot conduct simultaneously. This leaves only six possible conduction mode for the converter in the continuous conduction mode of operation. These are T1T2, T2T3, T3T4, T4T5, T5T6, T6T1. Each conduction mode is of 60° duration and appears in the sequence mentioned. The conduction table of Fig. 15 (b) shows voltage across different devices and the dc output voltage for each conduction interval. The phasor diagram of the line voltages appear in Fig. 15 (c). Each of these line voltages can be associated with the firing of a thyristor with the help of the conduction table-1. For example the thyristor T1 is fired at the end of T5T6 conduction interval. During this period the voltage across T1 was vac. Therefore T1 is fired α angle after the positive going zero crossing of vac. Similar observation can be made about other thyristors. The phasor diagram of Fig. 15 (c) also confirms that all the thyristors are fired in the correct sequence with 60° interval between each firing. Fig. 16 shows the waveforms of different variables (shown in Fig. 15 (a)). To arrive at the waveforms it is necessary to draw the conduction diagram which shows the interval of conduction for each thyristor and can be drawn with the help of the phasor diagram of fig. 15 (c). If the converter firing angle is α each thyristor is fired “α” angle after the positive going zero crossing of the line voltage with which it’s firing is associated. Once the conduction diagram is drawn all other voltage waveforms can be drawn from the line voltage waveforms and from the conduction table of fig. 15 (b). Similarly line currents can be drawn from the output current and the conduction diagram. It is clear from the waveforms that output voltage and current waveforms are periodic over one sixth of the input cycle. Therefore this converter is also called the “six pulse” converter. The input current on the other hand contains only odds harmonics of the input frequency other than the triplex (3rd, 9th etc.) harmonics. The next section will analyze the operation of this converter in more details.
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Analysis of the converter in the rectifier mode
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The input phase current i is expressed as a
From Fig. 16 it can be observed that i itself has a ripple at a frequency six times the input 0
frequency. The closed from expression of i , as will be seen later is somewhat complicated. 0
However, considerable simplification in the expression of i can be obtained if i is replaced by its a
0
average value I . This approximation will be valid provided the ripple on i is small, i.e, the load is 0
0
highly inductive. The modified input current waveform will then be i which can be expressed in a
terms of a fourier series as
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in particular i = fundamental component of i a1
a
From figure 16
Since displacement factor ∅ = 𝛼 displacement factor = cos 𝛼 Distortion factor
Power factor = Displacement factor × Distortion factor = 3cosαπ (13.16) The closed form expression for i in the interval 0
in this interval
can be found as follows:
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Since i is periodic over π/3 0
To find out the condition for continuous conduction it is noted that in the limiting case of continuous conduction.
then i is minimum at ωt = α. ∴ Condition for continuous conduction is 0ωt=αi≥. However 0
discontinuous conduction is rare in these conversions and will not be discussed any further.
S.NO Q.1 Q.2
RGPV QUESTIONS Year Justify the suitability of 3-phase full bridge converter for Dec 2008 HVDC application. Explain with current and voltage waveforms and deduce Dec 2007 relationship for output voltage and current considering the commutation angle.
Marks 10 10
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UNIT 3/ LECTURE 03 Inverter used in HVDC System Analysis of the converter in the inverting mode. In all the analysis presented so far it has been assumed that α < 90°. It follows from equation 16 that the output dc voltage will be positive in this case and power will be flowing from the three phase ac side to the dc side. This is the rectifier mode of operation of the converter. However if α is made larger than 90° the direction of power flow through the converter will reverse provided there exists a power source in the dc side of suitable polarity. The converter in that case is said to be operating in the inverter mode. It has been explained in connection with single phase converters that the polarity of EMF source on the dc side [Fig. 15(a)] would have to be reversed for inverter mode of operator. Fig. 17 shows the circuit connection and wave forms in the inverting mode of operation where the load current has been assumed to be continuous and ripple free.
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Analysis of the converter in the inverting mode is similar to its rectifier mode of operation. The same expressions hold for the dc and harmonic compounds in the output voltage and current. The input supply current Fourier series is also identical to Equation 13.8. In particular
For values of α in the range 90° < α < 180° it is observed from Fig. 13.3(b) that the average dc voltage is negative and the displacement angle φ of the fundamental component of the input ac line current is equal to α > 90°. Therefore, power in the ac side flows from the converter to the source. It is observed form Fig. 13.3(b) that an outgoing thyristor (thyristor T6 in Fig. 13.3(b)) after commutation is impressed with a negative voltage of duration β = π – α. For successful commutation of the outgoing thyristor it is essential that this interval is larger than the turn off
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time of the thyristor i.e, β ≥ ωtq, tq is the thyristor turn off time Therefore π-α ≥ ωtq or α ≤ π-ωtq. This imposes an upper limit on the value of α. In practice this upper value of α is further reduced due to commutation overlap.
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UNIT 3/ LECTURE 04 Harmonics Generation in the Converter Harmonic Generation in the Converter [RGPV/ Dec 2007, Dec 2008/10] There are different types of loads that generate harmonics in power systems. The linear time-invariant loads are designed such a way so that the sinusoidal voltage results in a sinusoidal flow of current. These loads have constant steady-state impedances during the applied sinusoidal voltage. When the voltage is increased, the current increases in direct proportion. The transformers and rotation machines are the examples of this kind of loads when operated in normal condition. In nonlinear load, the applied sinusoidal voltage does not result in a sinusoidal flow of current. These loads are not constant impedances during the entire cycle of the applied sinusoidal voltage. For example, wind and solar power generation, switching mode power supplies, computers, copy machines and television sets. In utility distribution feeders and industrial plant power systems, the main tendency is for the harmonic currents to flow from the harmonic producing load to the power system source. This is shown in Figure 17. The impedance of the power system is normally the lowest impedance seen by the harmonic currents. That means the bulk of the current flows in to the source. The source of harmonics can be located by using this general tendency of the harmonic current flow. The power quality meters can be used to measure the harmonic currents in each branch starting at the beginning of the circuit and trace the harmonics to the source.
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Figure 17 General flow of harmonic currents in a power system The power factor correction capacitors can alter this flow pattern. For example, adding a capacitor to this circuit as shown in the following circuit may draw a large amount of harmonic current into that portion of the circuit as shown in figure 18.
Figure 17 Power factor capacitors can alter the direction of flow of the harmonic component of the current. S.NO RGPV QUESTIONS Q.1 What are the causes of harmonics in HVDC system. Q.2 Write a short note on “Causes of harmonics in HVDC Converter”.
Year Dec 2007
Marks 10
Dev 2008
05
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UNIT 3/ LECTURE 05 Adverse Effect of Harmonics in the Converter Effect of Harmonics [RGPV/ June 2008, June 2010, Dec 2010, June 2007, Dec 2009/ 10] Harmonics practically effect to every equipment in the power system. The effect of voltage distortion is divided in three major categories, the thermal stress, the dielectric stress and load disruptions. Heating effects: Harmonic current flowing in the circuits cause heating effects in the conductors. Especially eddy current losses are proportional to the square of the frequency. Some harmonics, notably the 5th, are negative sequence or backward rotating and tease can increase losses by inducing even higher frequency currents in machine rotors. Interference: Harmonics can cause interference to communications systems, protections systems and signaling circuits due to electromagnetic induction or to the flow of the ground currents. Resonance: Harmonics generated in one part of circuit may increase the resonance effects in another part of the circuit. Some resonance can be dangerous if the magnification is large because of high circuit Q-factor or low damping. Even harmonics: Even harmonics may cause asymmetrical magnification and can lead to saturation. Some more adverse effects of harmonics listed as follows: - Malfunction in electronics devices and computer equipments - Errors in measurements - Overheating and over stressing of insulations - Lamp flicker when harmonic pulses involved. - Sometimes machine vibrates
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- Blowing out of small auxiliary devices like fluorescent lamp capacitors. Harmonics Analysis The first step in solving harmonic related problem is to perform an analysis to determine the specific needs of power system. The analysis then applied to study of resonant conditions and harmonic filter design. The in-depth study is involved because of the interaction between harmonics producing source and power system, the limitations of modeling equipments in the power system and need to check for the accuracy.
S.NO RGPV QUESTIONS Q.1 Discuss the problems associated with the harmonics introduced by the HVDC Converters. Q.2
Write a short note on “Adverse affects of harmonics in HVDC System”.`
Year June 2008 June 2010 Dec 2010 June 2007 Dec 2009
Marks 10
10
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UNIT 3/ LECTURE 06 Harmonics Mitigation and Filters
Harmonics Mitigation and Filters [RGPV / Dec 2010, Dec 2009, Dec 2007, June 2010, Dec 2004/ 10] The harmonics is becoming a bigger concern now a day with the increase nonlinear load in the power system. There are multiple ways to control the harmonics as follow: - Find the nonlinear load and reduce the harmonic current - Add filter to remove the harmonic current or block the harmonic current from
entering
to the system - Modify the system frequency response to avoid harmful interaction with harmonic current. Passive filters Nonlinear load produces harmonic currents that can travel to other part of the power systems and eventually goes back to the source. As we review that harmonics current can damage power systems many ways. One of the ways to block this unwanted characteristic of the system is to block it by using filters. There are two types of filters, active and passive filter. The interest of this project is to design a single tuned “notch filter” since it is sufficient for the application and importantly it is inexpensive. Figure 19 shows configuration of the filter, equivalent circuit of the filter and the frequency response of the filter. This filter has two advantages, it suppresses the harmonics and increases power factor. This filter is tuned slightly lower than the harmonic to be filtered to provide a safely margin in case there is some change in the system parameters that may raise the notch frequency.
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Figure 19 Creating a fifth-harmonic notch filter and its affect on the system response S.NO RGPV QUESTIONS Q.1 Write a short note on “tuned power filter”. Q.2 Explain different types of AC filters giving their configuration and impedance characteristics. Q.3 Explain the remedial measures of harmonics. Q.4 Explain DC filters, mentioning the criteria by which the effectiveness of the DC filters can be judged.
Year Dec 2004 June 2010
Marks 10 10
Dec 2010 Dec 2007 Dec 2009
10 10
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UNIT 3/ LECTURE 07 HVDC Characteristics HVDC Characteristics. An important factor when considering multiple inverters operating into a common AC system is the static characteristics of each inverter. These characteristics (normally plotted as DC current Versus DC voltage) represent the converters dynamic response to transient behaviour in the AC system. They can be considered as the cerebral cortex of the HVDC controller as, in the same way as if you touch something hot you will undoubtedly move your hand rapidly without higher brain functions being involved, the HVDC converter static characteristics define the way in which the converter will respond without involving higher level control functions. The selection of static characteristics will influence, to some extent, the susceptibility of a HVDC converter to a commutation failure. As an example, the static characteristics can be configured to maintain the DC voltage at a constant value for small reductions in the AC system voltage, As shown in Figure 20. This will help to mitigate the increase in DC current flowing from the stored energy of the DC conductor into the inverter as a consequence of an AC system dip. However, when considering multiinfeed HVDC converters the static characteristics can have a significant impact on the converters ability following a commutation failure.
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Vd
Rectifier
Inverter
}
Id
Constant DC Voltage
FIGURE 20 Static Characteristics of a HVDC scheme including a constant DC voltage operating range Consider only one inverter feeding into an AC system. The speed of recovery of the inverter following a commutation failure can be optimised against the inertia and performance of the AC system synchronous machines in order to optimise the system stability. However, when two or more inverters are attempting to recover into the same AC system they can interact such that subsequent commutation failures on one or more of the inverters is invoked; potentially leading to an instability in the AC power system. It may, therefore, be more appropriate to actually slow down the recovery of one converter with respect to another so that the total power flow into the receiving AC system is achieved more stably and hence quicker. Figure 21 shows two different scheme static characteristics. From Figure 21 it can be seen that in Scheme 1 Area A is much smaller than Area B of Scheme 2, hence the recovery charging current and the speed in which the converter angles respond will be different so, for physically identical schemes, Scheme 2 will recover faster than Scheme 1. Moreover, in Figure 2 an approximate constant reactive power absorption (constant Q) line is plotted. It can be seen that for Scheme 1 the recovery characteristic takes the converter operating point to the right of the constant Q line and hence the converter will absorb more reactive power during recovery, leading to an AC voltage dip which could cause a commutation
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failure in neighbouring inverters. Now consider Scheme 2; its recovery characteristic passes to the left of the constant Q curve and hence will always absorb less reactive power during recovery, consequently leading to an AC overvoltage and therefore less likely to initiate a commutation failure in nearby inverters.
Scheme 2
Scheme 1 Vd
Vd
Rectifier
Rectifier Constant Q
Constant Q Inverter
Area A
Inverter
Area B
Id
Id
FIGURE 21 Static Characteristics of two HVDC schemes which will have different recovery times and different reactive power absorption from the AC system during recovery
Whilst it may not be practical to have all HVDC schemes causing an AC system overvoltage following an AC system disturbance which resulted in a commutation failure there maybe some system benefits to having a combination of different characteristics in a multi in feed system. It is therefore suggested that, whenever possible, the static characteristics of any existing HVDC schemes are established and modelled as accurately as possible in order to obtain the most realistic representation of the expected dynamic behaviour of the HVDC schemes.
