energies Article
An Improved Commutation Prediction Algorithm to Mitigate Commutation Failure in High Voltage Direct Current Xinnian Li 1, *, Fengqi Li 2 , Shuyong Chen 3 , Yanan Li 4 , Qiang Zou 5 , Ziping Wu 6 and Shaobo Lin 3 1 2 3 4 5 6
*
School of Electrical Engineering, Beijing Jiaotong University, Beijing 100044, China State Grid Operation Company, Beijing 100052, China;
[email protected] State Key Laboratory of Power Grid Safety and Energy Conservation (China Electric Power Research Institute), Beijing 100192, China;
[email protected] (S.C.);
[email protected] (S.L.) State Power Economic Research Institute, Beijing 102209, China;
[email protected] NR Electric Co., Ltd., Nanjing 211102, China;
[email protected] Information Trust Institute, University of Illinois at Urbana-Champaign, Champaign, IL 61801, USA;
[email protected] Correspondence:
[email protected] or
[email protected]; Tel.: +86-010-8281-4193
Received: 20 July 2017; Accepted: 18 September 2017; Published: 25 September 2017
Abstract: Commutation failure is a common fault for line-commutated converters in the inverter. To reduce the possibility of commutation failure, many prediction algorithms based on alternating current (AC) voltage detection have already been implemented in high voltage direct current (HVDC) control and protection systems. Nevertheless, there are currently no effective methods to prevent commutation failure due to transformer excitation surge current. In this paper, an improved commutation failure prediction algorithm based on the harmonic characteristics of the converter bus voltage during transformer charging is proposed. Meanwhile, a sliding-window iterative algorithm of discrete Fourier transformation (DFT) is developed for detecting the voltage harmonic in real time. This method is proved to be an effective solution, which prevents commutation failure in cases of excitation surge current, through experimental analysis. This method is already implemented into TianShan-ZhongZhou (TianZhong) ± 800 kV ultra high voltage direct current (UHVDC) system. Keywords: commutation failure prevention (CFPREV); high voltage direct current (HVDC); inrush current; harmonic detection; predictive control
1. Introduction High voltage direct current (HVDC) is widely used for bulk power transmission with lower energy loss and flexible control ability [1–6]. Thus, in the last decade, more than 20 HVDC projects have been completed in China. It is inevitable for line commutate converter (LCC) thyristor valves to experience a commutation failure when a fault occurs around the inverter AC network, especially under conditions in which the inverter AC bus is 10% to 15% below the rated voltage [7]. Efforts have been made to study the reasons for commutation failure and the methods to mitigate such an issue [8,9]. Many methods regarding the prediction and prevention of commutation failures were presented in most of the literature [10–20], such as power component fault detection, direct-current predictive control, commutation area detection, and so on. A more detailed description of the commutation prediction method, which has been successfully used in HVDC projects, was proposed [21,22]. In general, the above commutation prediction methods can effectively detect voltage sags due to AC side ground fault and prevent the potential commutation failure or even continuous commutation failure via increasing the extinction angle proportionally to the voltage sag. Energies 2017, 10, 1481; doi:10.3390/en10101481
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However, the above methods are difficult to perform correctly when the converter bus voltage is affected by harmonics such as the transformer excitation surge current. TuanLin-FengJing ± 500 kV HVDC was tripped by commutation failure protection in 2013, when LianTang, a ultra high voltage alternating current UHVAC station nearby, was energizing its transformer [23]. GaoLing back-to-back HVDC went through four continuous commutation failures on 21 August 2015, when SuiZhong, an adjacent power plant, was energizing its boost transformer. The active power through TianShan-ZhongZhou (TianZhong) ± 800 kV UHVDC fluctuated between 3660 MW and 4040 MW for 13 s after GuanDu, a substation nearby, energized its transformer on 21 March 2016. The excitation surge current produced by energizing a large transformer could be damaging to the grid. Since an excitation surge current can be sustained for longer than 10 s and can be magnified after traveling along the power transmission line [24], it could be the most reasonable practical explanation for the continuous commutation failure of HVDC. Following the analysis of the response of the commutation failure prediction method applied to the TianZhong ± 800 kV UHVDC project on 21 March 2016, an improved commutation prediction method based on the harmonic characteristics of the converter bus voltage is proposed in this paper. This method is tested using an real-time digital simulator (RTDS) simulation platform, and the experimental results demonstrate its effectiveness. Moreover, this method is already implemented in the TianZhong ± 800 kV UHVDC system and will be applied to other HVDCs in the near future; it is especially suitable for those HVDCs that feed power into the same nearby grid. 2. Commutation Failure For a three-pulse thyristor bridge, during the commutation of the direct current from one valve to another, if the conducting valve cannot recover interdiction capability from the reverse voltage, the valve will continue to conduct while the succeeding valve fails to do so. This phenomenon is called commutation failure. Influencing Factors of Commutation Failure The following equation can be obtained from: 2Xr I γ = arccos( √ d + cos β) 3E2m
(1)
Equation (1) reveals the main impact factors of the converter commutation, which is characterized by the extinction angle (γ), including the commutating reactance Xr , direct current (DC) steady-state current at the inverter side Id , the amplitude of commutation voltage E2m , and the trigger advance angle β. The partial derivatives of the variables on the right side of the Formula (1) can be obtained:
∂γ √2Xr √ −1 ∂Id = 1−cos2 γ 3E2m 2Xr Id ∂γ √ √ 1 2 ∂E2m = 1−cos2 γ 3E2m 2I ∂γ − 1 √ d √ ∂Xr = 1−cos2 γ 3E2m ∂γ √ sin β ∂β = 1−cos2 γ
(2)
On the inverter side, the firing angle is greater than π/2 radians. The overlap angle varies roughly with DC current, which changes approximately inversely with the DC voltage on the inverter side. Equation (2) shows that commutation failures in HVDC systems are mainly caused by voltage magnitude reduction, DC current increment, triggering advance angle decrement, and commutation reactance increment.
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magnitude reduction, DC current increment, triggering advance angle decrement, and commutation 3 of 16 reactance increment. There are two methods to prevent commutation failure: There are two methods to prevent commutation failure: (1) Increasing the γ angle in normal conditions, which will consume much more reactive power. (1) Increasing the γ anglefailure in normal conditions, consume much more reactive (2) When a commutation is detected in anwhich HVDCwill system, the inverter decreases the power. α angle in (2) order Whento a commutation failure is detected an HVDC system, the inverter increase the commutation margininand avoid commutation failure. decreases the α angle in order to increase the commutation margin and avoid commutation failure. 3. Analysis of the Response of the Commutation Failure Prediction Method for TianZhong 3. Analysis of the Response the Commutation Failure Prediction Method for TianZhong ± 800 kV Ultra High VoltageofDirect Current Project ± 800 kV Ultra High Voltage Direct Current Project Energies 2017, 10, 1481
3.1. The Commutation Failure Prediction Method in an High Voltage Direct Current Project 3.1. The Commutation Failure Prediction Method in an High Voltage Direct Current Project The basic reason for commutation failure is a lack of commutation margin and results over a The basic reason for commutation failure is a lack of commutation margin and results over a short short extinguishing time. If a high level γ angle is maintained, this will cost much more in terms of extinguishing time. If a high level γ angle is maintained, this will cost much more in terms of reactive reactive power. In the TianZhong ± 800 kV UHVDC control system, there is a model for commutation power. In the TianZhong ± 800 kV UHVDC control system, there is a model for commutation failure failure prediction (CFPREV), which can solve this antinomy preferably, as shown in Figure 1. This prediction (CFPREV), which can solve this antinomy preferably, as shown in Figure 1. This method method includes two parallel parts: one is based on a zero-sequence component for unsymmetrical includes two parallel parts: one is based on a zero-sequence component for unsymmetrical faults faults such as single-phase earth faults, and the other one is based on an abc-αβ transformation to such as single-phase earth faults, and the other one is based on an abc-αβ transformation to detect detect three-phase symmetrical faults. three-phase symmetrical faults.
Figure1.1.Structure Structureofof TianZhong ± 800 kV ultra-high voltage control Figure thethe TianZhong ± 800 kV ultra-high voltage direct direct currentcurrent (HVDC)(HVDC) control system. system.
If an AC system has a single-line ground fault, the converter bus voltage will contain a If an AC component, system has aZ,single-line ground fault, the converter bus voltage will containvoltage, a zerozero-sequence which is obtained by adding up the three-phase instantaneous sequence component, Z, which is obtained by adding up the three-phase instantaneous voltage, as as shown in Equation (3): shown in Equation (3): u0 = u a + u b + u c (3)
(3) = component + + As shown in Figure 2, if the zero sequence of the converter bus voltage is greater than a value, the commutation will switch a normal to Aspreset shown in Figure 2, if the zeroprediction sequence logic component of thefrom converter buscontrol voltagestrategy is greater another strategy in the 20 ms by immediately decreasing thefrom firinga angle forcontrol a certain degree, than a preset value, thenext commutation prediction logic will switch normal strategy to which is proportional to the maximal zero sequence component, and will maintain it for a certain time another strategy in the next 20 ms by immediately decreasing the firing angle for a certain degree, (12 ms in this case). Once the maximal AC fault has cleared, i.e., when and the zero component of which is proportional to the zerobeen sequence component, will sequence maintain it for a certain the converter bus voltage is lower than its setting, the prediction logic will increase the firing angle time (12 ms in this case). Once the AC fault has been cleared, i.e., when the zero sequence component order withbus a time constant of 30 ms,its enabling prediction logic to the of theslowly converter voltage is lower than setting,the thecommutation prediction logic will increase thereturn firingto angle normal control strategy smoothly.
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4 of 16 a time constant of 30 ms, enabling the commutation prediction logic to return to the normal control strategy smoothly. Ζ a b
t
a b Time
MAX HOLD Time
MAX
0
Transf orm
A B C
α β
U αβ
a b
t
a b Time
1
ACOS
MAX HOLD Time
0
Figure 2. A simplified logic diagram of commutation failure prediction (CFPREV). Figure 2. A simplified logic diagram of commutation failure prediction (CFPREV).
Another is based on abc-αβ to detect three-phase abc-αβ transformation Anotherpart part is based on transformation abc-αβ transformation to detect faults. three-phase faults. abc-αβ uses the rotatinguses vector expressvector three phase voltages: transformation thetorotating to express three phase voltages: 11 ( + ) 22 (4) + uc ) (4) uα == 3u a −− 3(u 3 3 b √√3 (5) = 3 ( − ) (5) u β = 3 (ub − uc ) 3 Inthe theabove aboveformula, formula,uuαand anduuβ correspond correspond to to the the projection projection of of the the vector vector on on to tothe theα-axis α-axisand and In α β theβ-axis β-axis in in the the αβ-plane. a rotating vector, ω,ω, which rotates in the the αβ-plane. By Bytransformation, transformation,we wecan canobtain obtain a rotating vector, which rotates in αβ-plane at a fixed speed. The magnitude of rotating vector αβ_SUM can be calculated using the the αβ-plane at a fixed speed. The magnitude of rotating vector αβ_SUM can be calculated using the formulabelow: below: formula q uαβ = uα 2 + u β 2 (6) = + (6) When a fault occurs in the AC system, αβ_SUM is compared to a filtered αβ_SUM, with a relatively When a faultasoccurs in the voltage. AC system, is between compared a values filteredisαβ_SUM, with large time constant the pre-fault If theαβ_SUM difference theto two greater than thea relatively large time constant as the pre-fault voltage. If the difference between the two values pre-defined level, the CFPREV model will kick in, convert the difference to an angle value, and finallyis greater than the pre-defined the CFPREV model will kick convert the difference to aan angle subtract the angle value fromlevel, the inverter α angle, advancing thein, firing instant, and leaving bigger value, and finally subtract the angle value from the inverter α angle, advancing the firing instant, and commutation margin. In the TianZhong HVDC project, the CFPREV program is installed in digital leaving a bigger commutation margin. In the TianZhong HVDC project, the CFPREV program signal processor (DSP), and the program step is 50 µs. For a 50 Hz AC system, this value is convertedis installed in value digitalofsignal (DSP), and the program is 50 µs. For a 50occurs. Hz AC system, this to the angle 0.9◦ . processor CFPREV can act immediately afterstep an AC system fault value is converted to the angle value of 0.9°. CFPREV can act immediately after an AC system fault 3.2. The Performance of Commutation Failure Prediction During the Disturbance occurs. The TianZhong ± 800 kV UHVDC project was operating with four bipolar converters rated at 3.2. The Performance of Commutation Failure Prediction During the Disturbance 4000 MW on 21 March 2016 before the disturbance occurred. Thewaveform TianZhongduring ± 800 kV project was in operating with bipolar converters rated at The the UHVDC disturbance is given Figure 3. Thefour output signal of the CFPREV 4000 MW on March 21, 2016 the unstable. disturbance occurred. system, α_CFPREV in the thirdbefore row, was It jumped 8.4◦ in 1 ms, remained stable for 20 ms, The decreased waveformback during thenext disturbance is given the in Figure The output of the CFPREV and then in the 20 ms, repeating process3.every 40 ms. signal Since the firing angle system, α_CFPREVquickly in the third was unstable. It jumped 8.4°angle in 1 ms, remained stable forover 20 ms, and order increased and row, decreased slowly, the extinction increased quite a lot a long then decreased back more in thereactive next 20 ms, repeating the process every 40 ms. Since firinglater angle period of time. Since power was consumed by the inverters, three the seconds a order filter increasedwas quickly and decreased the extinction angle increased quite a lot over a long period sub-bank switched in. Duringslowly, this process, the transmission power changed in the range between of time. moreMW reactive powerofwas consumedangle by thevariations. inverters, three seconds later a filter sub3660 MWSince and 4040 as a result the extinction bank was switched in. During this process, the transmission power changed in the range between 3660 MW and 4040 MW as a result of the extinction angle variations.
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Figure 3. The performance of the commutation failure prediction during the disturbance. (Note: Uac 3.The Theperformance performanceof ofthe thecommutation commutationfailure failure prediction during the disturbance. (Note: ac Figure3.3. The performance of prediction during the disturbance. (Note: Uac Figure during the disturbance. (Note: UUacis is the converter bus voltage inthe thecommutation inverter, IVY isfailure the Yyprediction transformer current in the valve side, α-CFPREV is the converter bus voltage in the inverter, I VYthe is the Yy transformer current in the valve side, α -CFPREV the converter bus voltage in the inverter, I is Yy transformer current in the valve side, α is VYIVY is the Yy transformer current in the valve side, isisthe bus voltage in the inverter, α-CFPREV theconverter output degree of commutation failure prediction (CFPREV), and α is firing angle.) -CFPREV isthe the output degree commutation failure prediction (CFPREV), and isfiring firing angle.) output degree ofofof commutation failure prediction (CFPREV), and αα isαis firing angle.) isthe output degree commutation failure prediction (CFPREV), and angle.)
3.3. Analysis of the Converter Bus Voltage 3.3.Analysis Analysisofof ofthe theConverter ConverterBus BusVoltage Voltage 3.3. Analysis the Converter Bus Voltage 3.3. The zero sequence component of the converter bus voltage, which repetitively changed between Thezero zerosequence sequencecomponent componentof ofthe theconverter converterbus busvoltage, voltage,which whichrepetitively repetitively changed between between The zero sequence component of the converter bus voltage, which repetitivelychanged changed The 60 kV at 20 ms with abundant harmonics, is shown in Figure 4. The peak value is around 60between kV. This 60kV kV 20 ms with abundant harmonics, isshown shown inFigure Figure 4.The The peakpeak value isaround around 60kV. kV.60 This 60 kVatat at20 20ms ms with abundant harmonics, is shown in Figure 4. peak The value is around kV. 60 with abundant harmonics, is in 4. value is 60 This explains the reason why the output of the commutation failure prediction also varied every 40 ms. explains the reason why the output of the commutation failure prediction also varied every 40 ms. This explains the reason the output ofcommutation the commutation failure prediction varied every 40 ms. explains the reason why why the output of the failure prediction alsoalso varied every 40 ms.
Figure 4. Zero sequence component of the converter bus voltage. Figure4.4. 4.Zero Zero sequencecomponent componentof ofthe theconverter converterbus busvoltage. voltage. Figure Zerosequence sequence component of the converter bus voltage. Figure
The converter bus voltage frequency analysis is conducted to understand the reason for the Theconverter converterbus busvoltage voltagefrequency frequencyanalysis analysisisisconducted conductedto tounderstand understandthe the reasonfor forthe the The The converter buscomponent. voltage frequency is that conducted to understand the reason for harmonic the above above zero sequence Figureanalysis 5 shows abnormal third, fourth, and reason fifth above zero sequence component. Figure 5 shows that abnormal third, fourth, and fifth harmonic above zero component. Figure shows that abnormal third, andsurge fifthvoltages harmonic zero sequence component. Figure showsis5that abnormal third, fourth, andfourth, fifth harmonic are voltages aresequence observed in phase B, 5which the key feature of transformer excitation current. voltages are observed in phase B, which is the key feature of transformer excitation surge current. voltages in phase B, which is the of key feature of transformer excitation surge current. observedare in observed phase B, which is the key feature transformer excitation surge current.
Figure 5. Frequency analysis of the converter bus voltage. Figure 5. Frequency analysis of the converter bus voltage. Figure5.5.Frequency Frequencyanalysis analysisofofthe theconverter converterbus busvoltage. voltage. Figure
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Further investigation demonstrates that the transformer was energized before the disturbance Further Furtherinvestigation investigationdemonstrates demonstratesthat thatthe thetransformer transformerwas wasenergized energizedbefore beforethe thedisturbance disturbance in the GuanDu substation, which is a 500 kV substation near the ZhongZhou converter station. ininthe GuanDu The GuanDusubstation, substation,which whichisisa a500 500kV kVsubstation substationnear nearthe theZhongZhou ZhongZhouconverter converterstation. station. The The connection between the GuanDu substation the ZhongZhou converter station is presented connection between the substation and the converter station isispresented inin connection between theGuanDu GuanDu substation andand theZhongZhou ZhongZhou converter station presented in Figure iseasy easy observe that the transformer surgecurrent currenttransferred transferredfrom fromGuanDu GuanDu Figure 6.6.It6.ItisIt tototo observe that the transformer surge Figure iseasy observe that the transformer surge current transferred GuanDutoto to ZhongZhou through the AC lines connecting them. ZhongZhou ZhongZhouthrough throughthe theAC AClines linesconnecting connectingthem. them.
Figure The wiring diagram of the AC/DC system. Figure Figure6.6.6.The Thewiring wiringdiagram diagramof ofthe theAC/DC AC/DCsystem. system.
The the GuanDu substation shown inin Figure Thetransformer transformerexcitation excitationsurge surgecurrent currentrecorded recordedatatthe theGuanDu GuanDu substation shown Figure The transformer excitation surge current substation isisis shown in Figure 7, 7,with phase high inrush current. 7,with with phase Bcontaining containing high inrush current.ItItagrees agreeswith withthe thefrequency frequencyanalysis analysisresult resultof ofthe the phase BBcontaining high inrush current. the frequency analysis result of the converter converterbus busvoltage voltageatat atthe theZhongZhou ZhongZhouconverter converterstation stationin inFigure Figure5.5.5. converter bus voltage the ZhongZhou converter station in Figure
Figure Figure7.7. 7.The Thetransformer transformerexcitation excitationsurge surgecurrent currentrecorded recordedatat atthe theGuanDu GuanDusubstation. substation. Figure The transformer excitation surge current recorded the GuanDu substation.
4.4. 4.AA ANovel NovelCommutation CommutationFailure FailurePrediction PredictionMethod MethodBased Basedon onthe theHarmonic HarmonicCharacteristics Characteristicsof ofthe Novel Commutation Failure Prediction Method Based on the Harmonic Characteristics of the Voltage theConverter Converter Bus Voltage Converter BusBus Voltage According above analysis, present commutation failure prediction fails Accordingto above analysis, the present commutation failure prediction method failstoto According totoabove analysis, thethe present commutation failure prediction methodmethod fails to correctly correctly handle the long term and periodically varying zero sequence component stimulated by correctly handle the long term and periodically varying zero sequence component stimulated by handle the long term and periodically varying zero sequence component stimulated by transformer transformer excitation current. To solve this problem, a new commutation prediction method based transformer excitation current. To solveathis a newprediction commutation prediction based excitation current. To solve this problem, newproblem, commutation method based onmethod the harmonic on characteristics the bus during onthe theharmonic harmonic characteristics theconverter converter busvoltage voltage during transformer chargingisis characteristics of the converter busofof voltage during transformer charging is transformer developed ascharging follows. developed developedasasfollows. follows.
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4.1. Harmonic Detection Based on the Sliding-Window Iterative Algorithm of Discrete Fourier Transformation (DFT) For any periodic signal x(τ) with a finite bandwidth, if its period is T, the bandwidth is the fundamental angular frequency ω to Nmax ·ω, and the DFT [25] is expressed as follows: Nmax
x (kτ ) =
∑
An cos(nωkτ ) + Bn sin An cos(nωkτ )
(7)
n =1
An =
Bn =
2 N
N −1
2 N
N −1
∑
x (iτ ) cos(nωiτ )
(8)
x (iτ ) sin(nωiτ )
(9)
i =0
∑
i =0
where k = 0, 1, 2, . . . , (N − 1), τ = T/N, and N is the periodic sampling frequency. It can be seen that the N sampling data for a complete sampling period, which is defined by the fixed starting point (i = 0), is simultaneously involved in these formulas, so the amount of calculation is huge. Obviously, these formulae are not suitable for the fast detection of instantaneous harmonic voltage. According to the real-time requirement of harmonic voltage detection, Formulas (8) and (9) are improved as follows: 2 Ncur − N +1 An = x (iτ ) cos(nωiτ ) (10) N i=∑ N cur
Bn =
2 N
Ncur − N +1
∑
x (iτ ) sin(nωiτ )
(11)
i = Ncur
where Ncur represents the latest sampled data points and x(iτ) represents the sampling data before i sampling period. By comparing these equations with the previous two, in the improved equation for An , Bn , (i = Ncur ) replaced (i = 0) and (Ncur − N + 1) replaced (N − 1); that is, the latest real-time sampling data is used for the calculation of harmonic voltage, while the oldest sampling data are eliminated accordingly. At the same time, the second harmonic component can be calculated by the following formula: x2 (kτ ) = A2 cos(2ωkτ ) + B2 sin(2ωkτ ) A2 =
B2 =
2 N
Ncur − N +1
2 N
Ncur − N +1
∑
(12)
x (iτ ) cos(2ωiτ )
(13)
x (iτ ) sin(2ωiτ )
(14)
i = Ncur
∑
i = Ncur
A2 and B2 must be calculated simultaneously through Formulas (13) and (14) during one sampling interval in order to obtain the desired second harmonic voltage in real time. The amount of computation is great when the sampling frequency of the system is high (i.e., N is relatively large). For this reason, a new computational model is developed to simplify the calculation process of the sliding-window iterative algorithm. It is shown in Figure 8.
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Figure Figure 8. 8. The The sliding sliding window window iterative iterative discrete discrete Fourier Fourier transformation transformation (DFT) (DFT)method. method.
According to to the the calculation calculation model model in in Figure Figure 8, 8, N N point point sampled sampled data data in in the the power power frequency frequency According period is is stored stored in in the the continuous continuous data data space space after after being being multiplied multiplied with with the the appropriate appropriate rotation rotation period factors. The present sampled data storage location is determined by the loop pointer. After factors. The present sampled data storage location is determined by the loop pointer. After completing completing a full period of N points sampled data calculation, the pointer should point back to the a full period of N points sampled data calculation, the pointer should point back to the starting starting of position of thespace storage space so as startcycle the next cycle of replacement data simultaneously. position the storage so as to start thetonext of replacement data simultaneously. Therefore, Therefore, Equations (15) and (16) can be simplified as the calculation of Formulas (13) and (14): Equations (15) and (16) can be simplified as the calculation of Formulas (13) and (14): Ncur − N +1
∑
i = Ncur
Ncur − N
( ()ωiτ cos() =) = ∑ x (iτ ) cos
i = Ncur −1
) cos( ) −x (( N ( cur −−N ))τ ) cos cos( − N))τ)) x (iτ() cos (ωiτ ) − (ω ((Ncur −
(15) (15)
τ ) cos(ωN)cur τ ) ) cos( ( x ( Ncur + + Ncur − N +1
∑
i = Ncur
Ncur − N
( ()ωiτ x (iτ ) sin sin() =) = ∑
i = Ncur −1
) sin( ) −x (( N ( cur −−N ))τ ) sin x (iτ() sin (ωiτ ) − (ω ((Ncur − N))τ)) sin(
(16) (16)
τ ) sin(ωN)cur τ ) ) sin( + +( x ( Ncur The Formulas (13)(13) andand (14)(14) is reduced to a subtraction and anand addition by means Thesum sumoperation operationofof Formulas is reduced to a subtraction an addition by of the above transformation; the new data sum is restored to the unit of the old data sum storage means of the above transformation; the new data sum is restored to the unit of the old data after sum the computation. Therefore, theTherefore, whole process of theprocess iterative ofcalculation the sliding of window can storage after the computation. the whole ofcalculation the iterative the sliding be completed in the power frequency cycle at the initialization The summation operation can window can be completed in the power frequency cycle at thestage. initialization stage. The summation be completed absolutely one sampling period later. In this way, the operation duration of harmonic operation can be completed absolutely one sampling period later. In this way, the operation duration voltage detection will be significantly reduced by usingreduced the iterative sliding window algorithm, and the of harmonic voltage detection will be significantly by using the iterative sliding window real-time performance of suchperformance a system is also improved greatly. algorithm, and the real-time of such a system is also improved greatly. The sliding window iterative algorithm is much The sliding window iterative algorithm is much simpler simpler and and faster faster than than fast fast Fourier Fourier transformation (FFT) when the single harmonic voltage is calculated. It is very important transformation (FFT) when the single harmonic voltage is calculated. It is very important for for the the frequency dividing control when designing a relevant controller. frequency dividing control when designing a relevant controller. 4.2. A New Commutation Failure Prediction Method 4.2. A New Commutation Failure Prediction Method Through comparison with the present CFPREV logic used in the TianZhong ± 800 kV UHVDC Through comparison with the present CFPREV logic used in the TianZhong ± 800 kV UHVDC project, which is shown in Figure 2, an improved commutation failure prediction algorithm is proposed project, which is shown in Figure 2, an improved commutation failure prediction algorithm is based on the harmonic characteristics of the converter bus voltage during transformer charging. proposed based on the harmonic characteristics of the converter bus voltage during transformer The second, fourth, and fifth harmonics of the converter bus are calculated and compared with their charging .The second, fourth, and fifth harmonics of the converter bus are calculated and compared settings. If any of them are greater than their settings, the signal (Uac_Harm_Blk) is set, and the output with their settings. If any of them are greater than their settings, the signal (Uac_Harm_Blk) is set, of the prediction logic is held as its setting value (CFPREV_Harm ). The logic is shown in Figure 9. and the output of the prediction logic is held as its setting value (CFPREV_Harm). The logic is shown in Figure 9.
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9 of 16 9 of 16 9 of 16 Ζ Ζ
Ζ Ζ Ζ Ζ Ζ Ζ
Ζ Ζ Ζ Ζ Ζ Ζ
Figure 9. A simplified logic diagram of the new commutation failure prediction method to protect Figure 9.9. A A simplified simplified logic logic diagram diagram of of the the new new commutation commutation failure failure prediction prediction method method to to protect protect Figure against transformer excitation surge current. against transformer excitation surge current. against transformer excitation surge current. Uharm_max = MAX(Uac_2nd_Harm, Uac_4th_Harm, Uac_5th_Harm)
harm_max = MAX(Uac_2nd_Harm, Uac_4th_Harm, Uac_5th_Harm) UU harm_max = MAX(Uac_2nd_Harm, Uac_4th_Harm, Uac_5th_Harm) , > ( __ ∗ ) _ Uharm_max ∗ , > CFPREV_Harm= _ ∗ k , U > k CFPREV_Harm= set 1 harm_max k , 1 ≤ _ CFPREV_Harm = ≤ _ k,set , U harm_max ≤ k 1
(17) (17) (17) (18) (18) (18)
5. Experimentsand and Results 5. 5. Experiments Experiments and Results Results The experiment through RTDSisiscarried carried outto to reproducethe the aboveprocess process andverify verify the The The experiment experiment through through RTDS RTDS is carried out out to reproduce reproduce the above above process and and verify the the proposed algorithm. The RTDS RTDS experimental experimental system is shownininFigure Figure 10. The The topology topology ofthe the proposed proposed algorithm. algorithm. The experimental system is shown shown in Figure 10. 10. topology of of the TianZhong ± 800 kV UHVDC transmission system and the AC network of the ZhongZhou converter TianZhong TianZhong± ± 800 kV UHVDC transmission system and the AC network of the ZhongZhou ZhongZhou converter converter stationnearby nearby are modeled in detail. The model includes the main circuit with the primary station are modeled in detail. The model includes the main circuit with the primary equipment, station nearby are modeled in detail. The model includes the main circuit with the primary equipment, consisting of converters, transformers, DC lines, DC filter, AC filter, and a control and consisting converters, DC lines, DC control and protection equipment,ofconsisting of transformers, converters, transformers, DCfilter, lines,AC DCfilter, filter,and ACafilter, and a control and protection system in accordance with the practical system configuration. These detailed parameters system in accordance the practical system configuration. These detailed parameters shown in protection system in with accordance with the practical system configuration. These detailedare parameters are shown in Tables 1 and 2 and Figure 11. Tables 1 andin2Tables and Figure are shown 1 and11. 2 and Figure 11.
Figure 10. The real-time digital simulator experimental system. Figure 10. The real-time digital simulator experimental system. Figure 10. The real-time digital simulator experimental system.
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Table 1. The main parameters of Tianzhong ± 800kV kVultra ultrahigh highvoltage voltage direct direct current current (UHVDC). (UHVDC). Table ± 800
Serial Number Serial Number 1 2 3 4 5 6 7 8
1 2 3 4 5 6 7 8
Items Items
Units Units
Bipolar rated power
MW
The distance of DC lines
km
Bipolar rated power MW Rated voltage Rated voltage kV kV Rated current Rated current kAkA ◦ Rated firing angle at rectifier Rated firing angle at rectifier ° ◦ Rated extinction angle at inverter Rated extinction angle at inverter ° Smoothing reactor mH Smoothing reactor the commutating reactance % mH thedistance commutating reactance The of DC lines km %
Parameters Parameters 8000 8000 800800 5.05.0 1515 1717 6 × 50 6 × 50 24 242192 2192
Table filter of of TianZhong TianZhong ±±800 800kV kVUHVDC. UHVDC. Table 2. 2. The The AC/DC AC/DC filter
Items Items
Location Location TianShan TianShan filter DCDC filter ZhongZhou ZhongZhou TianShan TianShan filter ACAC filter ZhongZhou ZhongZhou
TypeType 1*HP2/39, 1*HP12/24 1*HP2/39, 1*HP12/24 1*HP2/39, 1*HP12/24 1*HP2/39, 1*HP12/24 4*BP11BP13, 4*HP24/36, 3*HP3,5*SC 4*BP11BP13, 4*HP24/36, 3*HP3,5*SC 8*HP12/24, 2*HP3, 9*SC 8*HP12/24, 2*HP3, 9*SC
Figure ± 800 Figure 11. 11. A A simplified simplified diagram diagram of of the the TianZhong TianZhong ± 800kV kVUHVDC UHVDCtransmission transmissionproject. project.
5.1. with the the Traditional Traditional Commutation Commutation Failure Failure Prediction PredictionStrategy Strategy 5.1. To To Reproduce Reproduce the the Process Process with After Figure 7 is7 injected intointo the the busbus bar bar of the After the the transformer transformerexcitation excitationcurrent currentrecorded recordedinin Figure is injected of GuanDu substation, the same transient behavior is observed in HVDC according to Figure 12. The the GuanDu substation, the same transient behavior is observed in HVDC according to Figure 12. output of the failure prediction (α_CFPREV ) swings in theinrange between nine nine and 0° at0a◦ The output ofcommutation the commutation failure prediction (α_CFPREV ) swings the range between and periodicity of 40 ms, resulting in a disturbance of both the DC voltage and current. at a periodicity of 40 ms, resulting in a disturbance of both the DC voltage and current.
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Figure 12. Reproduction disturbancedue due to to the excitation current recorded at theat the Figure 12. Reproduction of of thethe disturbance thetransformer transformer excitation current recorded GuanDu substation commutation failure prediction algorithm. Figure 12. Reproduction of traditional the disturbance due to the transformer excitation current recorded at the GuanDu substation andand thethe traditional commutation failure prediction algorithm. GuanDu substation and the traditional commutation failure prediction algorithm.
5.2. To Verify the Proposed Commutation Failure Prediction Strategy under the Same System Configuration 5.2. To5.2. Verify the Proposed Commutation Failure Prediction Strategy under the Same System Configuration To Verify the Proposed Commutation Failure Prediction Strategy under the Same System Configuration The proposed commutation failure prediction strategy is implemented into the control system The proposed commutation failure prediction strategy implemented into the control system The proposed commutation failure prediction strategycurrent isisimplemented into theof control system and evaluated by injecting the same transformer excitation into the bus bar the GuanDu and evaluated by injecting the same transformer excitation current into the bus bar of the GuanDu and evaluated injecting the same transformer excitation current substation. The by RTDS experimental results are shown in Figure 13. into the bus bar of the GuanDu substation. The The RTDS experimental results 13. substation. RTDS experimental resultsare areshown shown in in Figure Figure 13.
Figure 13. Reproduction of the disturbance due to the transformer excitation current recorded at GuanDu substation and theofproposed commutation prediction algorithm. Figure 13. Reproduction disturbance due tofailure the transformer excitation current recorded at Figure 13. Reproduction of thethe disturbance due to the transformer excitation current recorded at GuanDu substation and the proposed commutation failure prediction algorithm.
GuanDu substation and the proposed commutation failure prediction algorithm. The output of the commutation failure prediction changes from 0° to 9° as soon as the The output the commutation prediction from component 0° to 9° asissoon as the transformer of the of GuanDu substation isfailure energized and the changes zero sequence detected at ◦ as soon asisthe The output of the commutation failure changes from 0◦ tofor9component transformer transformer ofbus theof GuanDu substation is prediction energized and the sequence detected at the converter the ZhongZhou converter station. It iszero beneficial the inverter to prevent the converter bus ofdue the ZhongZhou converter station. It is component beneficial forisnearby the inverter prevent failure the converter bus distortion caused by the excitation surge of thecommutation GuanDu substation istoenergized and thevoltage zero sequence detected attothe converter commutation failure due to thestation. converter bus voltage distortion caused by nearbycommutation excitation surge bus ofcurrent. the ZhongZhou converter It is beneficial for the inverter to the prevent failure current. Different from Figures 12 and 13, α _CFPREV remained at 9° the whole time, until the excitation due to the converter bus voltage distortion caused by the nearby excitation surge current. Different from transformer Figures 12 and 13, α_CFPREV 9° the whole time, excitation current of GuanDu’s to anremained acceptableatlevel. theuntil entirethe process, the Different from Figures 12 anddecreased 13, α_CFPREV remained at 9◦ Throughout the whole time, until the excitation current of GuanDu’s transformer decreased DC voltage and DC current remained stable.to an acceptable level. Throughout the entire process, the current of GuanDu’s transformer decreased to an acceptable level. Throughout the entire process, DC voltage and DC current remained stable. the DC DC current remained stable. 5.3.voltage To Verifyand the Proposed Commutation Failure Prediction Strategy Under 1000 kV Ultra High Voltage 5.3. To Verify the Proposed Commutation Failure Prediction Strategy Under 1000 kV Ultra High Voltage Transformer Charging 5.3. ToTransformer Verify the Proposed Charging Commutation Failure Prediction Strategy Under 1000 kV Ultra High Voltage With the increase in the AC voltage level, the capacity of the 1000 kV UHV AC transformer Transformer Charging With developed the increase in the AC voltage level,MVA. the capacity of UHV the 1000 kV UHV AC transformer gradually from 3000 MVA to 4500 When the Transformer is charged, the With the increase infrom the AC the capacity of UHV the 1000 kV UHVis AC transformer gradually developed 3000voltage MVA tolevel, 4500 MVA. When the Transformer charged, the
gradually developed from 3000 MVA to 4500 MVA. When the UHV Transformer is charged, the inrush current amplitude is larger and lasts longer, and the problem of harmonic voltage distortion and voltage reduction becomes more and more serious.
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inrush current current amplitude amplitude isis larger larger and and lasts lasts longer, longer, and and the the problem problem of of harmonic harmonic voltage voltage distortion distortion inrush
Figure 14 shows the waveforms ofand the more charging inrush current of the 1000 kV UHV Transformer andvoltage voltage reduction becomesmore more and moreserious. serious. and reduction becomes affecting the HVDC transmission system. The commutation failure the inverter occurs at the same Figure14 14shows showsthe thewaveforms waveformsof ofthe thecharging charginginrush inrushcurrent current ofof the 1000 kVUHV UHV Transformer Figure of the 1000 kV Transformer time,affecting which isthe caused by the charging inrush of thefailure UHVof Transformer. The CFPREV model affecting the HVDC transmission system. Thecurrent commutation failure of theinverter inverteroccurs occurs atthe thesame same HVDC transmission system. The commutation the at ◦ . As time, which causedsignal by the theof charging inrush current ofthe the UHV UHV Transformer. Transformer. The CFPREV CFPREV model which caused by charging inrush of The model kickstime, in, and theisis output this model is current 20.6 inrush current attenuates slowly, the zero kickscomponent in,and andthe theoutput output signal ofthis this modeliscommutation is20.6°. 20.6°.As Asthe theinrush inrush current attenuates slowly,the thezero zero kicks in, signal model current attenuates slowly, sequence voltage of of the original failure prediction is greater than the set ◦ sequence component voltage of the original commutation failure prediction is greater than the set value sequence component voltage of the original commutation failure prediction is greater than the set value value for a long time, and the extinction angle is maintained near 41 . The DC voltage drops to 561 kV, for long time, andthe theextinction extinction angle maintained near41°. 41°. TheDC DCvoltage voltage drops to561 561kV, kV, which 15), for aalong angle isismaintained near The to which leads totime, the and voltage dependent current order limiter (VDCOL) actingdrops (as shown in which Figure leads to to the the voltage voltage dependent dependent current current order order limiter limiter (VDCOL) (VDCOL) acting acting (as (as shown shown in in Figure Figure 15), 15), and and leads and the DC current is limited at 0.88 p.u. (DC power is 0.64 p.u.). In 2.4 s, the CFPREV model based on theDC DCcurrent currentisislimited limitedat at0.88 0.88p.u. p.u.(DC (DCpower powerisis0.64 0.64p.u.). p.u.).In In2.4 2.4s,s,the theCFPREV CFPREVmodel modelbased basedon onthe the the the harmonic detection function is put into operation, and the output signal of this model drops to harmonic detection detection function function isis put put into into operation, operation, and and the the output output signal signal of of this this model model drops drops to to 6°. 6°. harmonic ◦ ; the DC voltage rises to 694 kV, which is less than the 6◦ . The is maintained near 22the Theextinction extinctionangle angle is is maintained maintained near near 22°; the DC DC voltage voltage rises rises to to 694 694 kV, kV, which which isis less less than than the the The extinction angle 22°; action value of VDCOL; and the DC current up to 1.06p.u. p.u.(DC (DC power is 0.93 p.u.). Therefore, action value of VDCOL; VDCOL; and the DC currentgoes goes up up to to 1.06 1.06 p.u. (DC power 0.93 p.u.). Therefore, action value of and the DC current goes power isis 0.93 p.u.). Therefore, our model can can not only prevent continuous failurefrom from transformer excitation our model model can not not only only prevent continuouscommutation commutation failure failure from transformer excitation surgesurge our prevent continuous commutation transformer excitation surge current butalso also can mitigate the sharp decreaseof ofDC DCpower. power. current but mitigate sharp decrease of current but also cancan mitigate thethe sharp decrease power.
Figure 14.Reproduction Reproduction ofthe the disturbancedue dueto toaaa1000 1000kV kV UHV AC transformer excitation current Figure 14. disturbance due to AC transformer excitation current Figure 14. Reproduction of of the disturbance 1000 kVUHV UHV AC transformer excitation current andthe theproposed proposedcommutation commutationfailure failureprediction predictionalgorithm. algorithm. and and the proposed commutation failure prediction algorithm.
Figure 15. The static characteristic dependent current order limiter (VDCOL). Figure 15.The The static characteristicof ofthe thevoltage voltagedependent dependentcurrent current order limiter (VDCOL). Figure 15. static characteristic of the voltage order limiter (VDCOL).
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5.4. To Verify the Proposed Commutation Failure Prediction Strategy in TuanLin-FengJing ± 500 kV High Voltage Direct Current 5.4. To Verify the Proposed Commutation Failure Prediction Strategy in TuanLin-FengJing ± 500 kV High TuanLin-FengJing Voltage Direct Current (LinFeng) ± 500 kV HVDC was tripped by commutation failure protection in 2013, when LianTang, a UHVAC station nearby, was energizing its transformer [23]. In reference [23], TuanLin-FengJing (LinFeng) ± 500 kV HVDC wasa tripped commutation failure in the LinFeng HVDC model was established by using detailedby model consistent with protection the practical 2013, when LianTang, a UHVAC station nearby, was energizing its transformer [23]. In reference [23], project. Through simulation studies, we put forward the measures to optimize the pole control the LinFeng model was by using a detailed model consistent system such HVDC as (1) to cancel theestablished instantaneous current compensation function inwith the the polepractical control project. Through simulation studies, we put forward the measures to optimize the pole control system and (2) to optimize the functions of VDCOL and DC controller commutation system failure such as (1) to cancel the instantaneous current compensation function in the pole control system and prediction. (2) toIn optimize functions of VDCOL and DC controller commutation failure order tothe verify the proposed commutation failure prediction strategy inprediction. LinFeng HVDC, the In order to verify the proposed commutation failure prediction strategy infailure LinFeng HVDC, LinFeng HVDC model was established in RTDS, and an improved commutation prediction the LinFeng HVDC model was established in RTDS, and an improved commutation failure prediction algorithm based on the harmonic characteristics of the converter bus voltage is applied. In order to algorithm based on the harmonic characteristics of the converter bus voltage is applied. order to analyze a cyclical commutation failure problem, the practical magnetizing surge current ofInLianTang analyze a cyclical commutation failure problem, the practical magnetizing surge current of LianTang first needs to be calculated by the simulation (the initial value is 4699 A). Next, the same magnetizing first needs to beiscalculated by the (theatinitial value is 4699 Next, theso same magnetizing surge current injected into thesimulation AC system the inverter side A). of LinFeng as to study the surge current is injected into the AC system at the inverter side of LinFeng so as to study the commutation failure of LinFeng HVDC under the magnetizing surge current distortion and propose commutation failure of LinFeng HVDC under the magnetizing surge current distortion and propose a solution for periodic commutation failure accordingly. Figure 16 is the waveform before the aoptimization solution forofperiodic commutation failure accordingly. Figure commutation 16 is the waveform the commutation failure prediction. Five periodic failures before happenthe at optimization of the commutation failure prediction. Five periodic commutation failures happen the inverter, which behaves in the same way as in [23]. at the inverter, which behaves in the same way as in [23].
Figure 16. 16. The The test test waveform waveform before before the the optimization Figure optimization of of the the commutation commutation failure failure prediction. prediction.
Figure 17 is a test waveform applied to an improved commutation failure failure prediction prediction algorithm. algorithm. In the and fifth harmonic voltage of the bus are than the third thirdsecond, second,the thesecond, second,fourth, fourth, and fifth harmonic voltage of converter the converter busgreater are greater ◦ . The their settings, and the signal of this model (CFPREV_ ) increases than their settings, andoutput the output signal of this model (CFPREV_ Harm ) increasestoto66°. The firing firing angle Harm ◦ , and the extinction angle rises up to 23 ◦ , which increase the is maintained near 144 144°, 23°, the commutation commutation margin. prevent the continuous commutation failure caused transformer excitation margin. Therefore, Therefore,it can it can prevent the continuous commutation failure by caused by transformer surge current. method in this paper is more and effective in cases in excitation surgeThe current. Theproposed method proposed in this paperstraightforward is more straightforward and effective which commutation failure isfailure causedisby inrushbycurrent. cases in which commutation caused inrush current.
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Figure waveform when the improved commutation failurefailure prediction algorithm is applied. Figure 17. 17.The Thetest test waveform when the improved commutation prediction algorithm is (Note: I is the Yd transformer current in the valve side; CFPREV_ is the output of CFPREV VD Harm applied. (Note: IVD is the Yd transformer current in the valve side; CFPREV_Harm is the output of based on based harmonic detection, detection, Uac_Harm_Blk is the action signal of CFPREV on harmonic CFPREV on harmonic Uac_Harm_Blk is the action signal ofbased CFPREV based on detection.). harmonic detection.).
6. 6. Conclusions Conclusions The The traditional traditional prediction prediction algorithm algorithm of of commutation commutation failure failure will will be be inappropriate inappropriate when when the the long-term voltage distortion of a converter bus is caused by the energizing transformer long-term voltage distortion of a converter bus is caused by the energizing transformer near near the the inverter inverter station, station, and and the the distortion distortion consequently consequently leads leads to to continuous continuous commutation commutation failures. failures. This This is is potentially dangerous to AC/DC systems. potentially dangerous to AC/DC systems. In In this this paper, paper, an an improved improved commutation commutation failure failure prediction prediction algorithm algorithm is is proposed proposed based based on on the the harmonic characteristics of the converter bus voltage during transformer charging. Meanwhile, harmonic characteristics of the converter bus voltage during transformer charging. Meanwhile, a asliding-window sliding-windowiterative iterativealgorithm algorithmofofdiscrete discrete Fourier Fourier transformation transformation (DFT) (DFT) is is developed developed for for detecting detecting the the harmonic harmonic voltage, voltage, which which is is simpler simpler and and faster faster than than FFT FFT when when the the single single harmonic harmonic voltage harmonic voltage is greater than the settings, the correct minimal voltageisiscalculated. calculated.If the If the harmonic voltage is greater than the settings, the additional correct additional extinction angle is held until the excitation current decreases to an acceptable level. Therefore, can minimal extinction angle is held until the excitation current decreases to an acceptable itlevel. not only prevent commutation failurecommutation due to transformer surge current but can Therefore, it cancontinuous not only prevent continuous failureexcitation due to transformer excitation also mitigate the sharp decrease of DC power. This method has been proved to be effective through surge current but can also mitigate the sharp decrease of DC power. This method has been proved to RTDS experiments in RTDS the LinFeng and TianZhong HVDCand systems. Moreover, it is systems. already implemented be effective through experiments in the LinFeng TianZhong HVDC Moreover, it in the TianZhong ± 800 kV UHVDC system. is already implemented in the TianZhong ± 800 kV UHVDC system. Acknowledgments: The research is supported by “science and technology project of the State Grid Corporation Acknowledgments: The research is supported by “science and technology project of the State Grid Corporation of China (FX71-16-003) and (XT71-15-045)”. of China (FX71-16-003) and (XT71-15-045)”. Author Contributions: Xinnian Li, Fengqi Li, Yanan Li, and Shuyong Chen proposed the core idea and developed Author Contributions: Xinnian Li, Li, Yanan Li, and Shuyong proposed the Ziping core idea and the models. Qiang Zou performed theFengqi experiments, exported the results, andChen analyzed the data. Wu and Shaobo Lin revised the paper. Li and Fengqi contributedexported to the design the models and the writing of developed the models. QiangXinnian Zou performed the Li experiments, the of results, and analyzed the data. this manuscript. Ziping Wu and Shaobo Lin revised the paper. Xinnian Li and Fengqi Li contributed to the design of the models and the writing of thisThe manuscript. Conflicts of Interest: authors declare no conflicts of interest.
Conflicts of Interest: The authors declare no conflicts of interest.
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Abbreviation The following abbreviations are used in this manuscript: γ α Xr Id E2m β IVY IVD CFPREV α_CFPREV u0 uα uβ uαβ ua , ub , uc Uac_COMP Ncur x(iτ) TianZhong UHVDC DFT FFT CFPREV_Harm Uac_Harm_Blk RTDS HP BP SC
Extinction Angle Firing Angle Commutating Reactance Dc Current Commutation Voltage Trigger Advance Angle YY Transformer Current in Valve Side YD Transformer Current in Valve Side Commutation Failure Prediction The Output of CFPREV Zero Sequence Component uαβ in α axis uαβ in β axis The Magnitude of Rotating Vector Three-phase Instantaneous Voltage of Converter Bus Magnitude of Uac Before Fault The Latest Sampled Data Points Sampling Data Before i Sampling Period TianShan-ZhongZhou Ultra High Voltage Direct Current Discrete Fourier Transform Fast Fourier Transformation The output of CFPREV based on harmonic detection Action signal of CFPREV based on harmonic detection Real-time Digital Simulator High-pass Filter Band-pass Filter Shunt Capacitor
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