A new method of converter transformer protection without commutation failure

Transcription

1 IOP Conference Series: Earth an Environmental Science PAPER OPEN ACCESS A new metho of converter transformer protection without commutation failure To cite this article: Jiayu Zhang et al 08 IOP Conf. Ser.: Earth Environ. Sci View the article online for upates an enhancements. This content was ownloae from IP aress on 0/04/09 at :46

2 A new metho of converter transformer protection without commutation failure Jiayu Zhang, Bo ong, Mingchang Liu, Jun Zhang, Jianhong Guo, Xu Jing School of Electrical Engineering Northeast Electric Power niversity, Jilin 0, China State Gri Tongliao Power Supply Company, Tongliao 08000, China Jilin Power Supply Company of State Gri Jilin Electric power Co., Lt., Jilin 0, Jilin, China *Corresponing author Abstract. With the evelopment of AC / DC hybri transmission technology, converter transformer as noes of AC an DC conversion of HVDC transmission technology, its reliable safe an stable operation plays an important role in the DC transmission. As a common problem of DC transmission, commutation failure poses a serious threat to the safe an stable operation of power gri. Accoring to the commutation relation between the AC bus voltage of converter station an the output DC voltage of converter, the generalize transformation ratio is efine, an a new metho of converter transformer protection base on generalize transformation ratio is put forwar. The metho uses generalize ratio to realize the on-line monitoring of the fault or abnormal commutation components, an the use of valve sie of converter transformer bushing CT current characteristics of converter transformer fault accurately, an is not influence by the presence of commutation failure. Through the fault analysis an EMTDC/PSCAD simulation, the protection can be operate correctly uner the conition of various faults of the converter.. Introuction With the "west to East, North South mutual supply, full implementation of the evelopment strategy of national network of power gri, the DC transmission moe, the working mechanism, operation moe, fault characteristics an the traitional AC power systems have great ifferences []. In AC / DC hybri system, converter transformer an converter are noes that can realize switching between AC an DC in power system, an its reliable an safe operation is the basis of safe an stable operation of HVDC system. At present, converter transformer protection is current ifferential protection. Because the converter is a nonlinear element, for the AC sie, harmonic current is the main source of the converter transformer ifferential protection uner normal conitions subjecte to harmonic current is large, resulting in great influence on the inrush current an TA transformer saturation criterion []. At the same time, AC system fault has le to a ecreasing voltage an a imbalancing current, which easily trigger DC protection misoperation, resulting in DC locking an other accients []. After commutation failure, the short circuit current equivalent AC current an pure AC system into the AC sie of HVDC system has ifferent Content from this work may be use uner the terms of the Creative Commons Attribution.0 licence. Any further istribution of this work must maintain attribution to the author(s) an the title of the work, journal citation an DOI. Publishe uner licence by Lt

3 characteristics, namely, the amplitue an phase fluctuations, the istortion of the above fault characteristics impact on ifferential protection, may cause internal fault system momentum larger than movement, causing misaction. At the same time, ue to the failure of the AC system on both sies of the system, the ifferential protection of the converter an the ifferential protection of the converter only meet the time limit, but there is no coorination between the setting values, When AC an DC transient intrusions occur, may cause uncertainty between converter protection an converter transformer protection [4]. It is necessary to carry out the overall protection an performance analysis of the two components. Accoring to the commutation relation between the AC bus voltage of converter station an the output DC voltage of converter, efines the generalize ratio an propeses a new metho of converter transformer protection base on generalize ratio. The metho uses online monitoring base on generalize ratio for the abnormal or fault of converter element, an accurate fault iagnosis of converter transformer is carrie out by utilizing the CT current characteristic of converter transformer valve sie bushing, it also solves the problem that the ifferential protection of converter transformer has been affecte by commutation failure for a long time, resulting in refusal or malfunction.. The Mechanism of Commutation Failure An Its Influence On Differential Protection.. The Mechanism of Commutation Failure In the multi-fee DC transmission system stable operation, if the AC system short circuit failure, it may cause the fault point near the inverter station commutation failure; the commutation failure of the DC system is explaine by the six-leg commutation circuit. After the commutation of the upper an lower arms is commute, the internal current of the converter valve whose en is turne on is not extinguishe to 0 or when the converter valve is subjecte to the reverse voltage If the commutation process is not complete at the en of the commutation process, if the applie voltage is positive, the converter valve is switche on again an the phase change occurs. At this time, the commutation circuit is internally short-circuite so that the converter valve Off, commutation process faile to complete, this phenomenon is calle commutation failure... The effect of commutation failure on traitional protection DC system protection is esigne base on a security constraint (evice attribute) of a safety evice that protects a evice, an its principles, logic an value of the power gri operation of the complexity an coorination with the lack of consieration. After commutation failure, the equivalent frequency current of the DC system into the AC sie has ifferent characteristics from the short circuit current of the pure AC system, that is, the amplitue ecreases an the phase angle fluctuates, the istortion of these fault characteristics has an impact on ifferential protection, the system may cause more than the amount of action uring the zone failure, triggere protection against motion.. A New Metho for Converter Transformer Protection Three phase brige rectifier circuit base on pulse thyristor thyristor element, the relationship between the DC voltage of converter outlet an the valve sie voltage of converter transformer can be obtaine, Such as formula (), formula () an formula (). * 6 sin t( t) *.4 cos a a () * 6 sin t ( t) *.4 cos b b ()

4 * 6 sin t( t) *.4 cos c c () Where α is the converter conuction angle Define the commutator area for the converter transformer an the converter as a whole. In the case of non-internal fault, the ratio of the primary voltage of the converter transformer to its seconary voltage (valve sie voltage) is equal to the transformer ratio T, as shown in equations (4), (5) an (6). a Ta a (4) Tb Tc b b (5) c c (6) In the case of non-internal failure of the commutation area,by formula () ~ (), type (4) ~ (6) can be obtaine, the relationship between the primary voltage of the converter transformer an the DC voltage of the converter outlet is shown in formula (7), formula (8), an (9). a Ta *.4*cos (7) b Tb *.4*cos (8) c TC (9) *.4*cos Orer =/, is efine as generalize ratio. When an external fault occurs in the converter zone, consiering the actual operation of HVDC transmission system a=0, the generalize transformation ratio is constant, expresse as c, as shown in equation (0). c (0) Similarly, the generalize transformation ratio of low voltage briges can be obtaine a () b ()

5 c () Base on CL, when the normal operation of the converter zone or external faults (except for the converter outlet fault), since formula (0) has always been estabishe, *, then δ is approximately zero. When the converter area outlet fails, Since is reuce to zero will lea to equation (0) oes not vali, But from the formula () ~ () available at this time δ=. Accoring to the above analysis, using the size of the integrate error δ can correctly etermine the internal an external failure of the commutation zone. Consiering the current transformer error, an when calculating converter transformer, the leakage reactance is neglecte an the voltage rop is generate, h=0.. As shown in figure, I is the CT current of the valve sie bushing of the converter transformer, when the converter is in normal operation, I is the sum of current of common cathoe an common anoe converter valve group. When the Y / Y converter transformer fails, I is only the common cathoe group converter valve current, that is half of the rate current; an when the converter transformer fails, I is several times or even ten times of the rate current. Therefore, the I size can be use to locate the fault components accurately, if it is satisfie equation (4) ~ (6), then the flow transformer failure, otherwise, there is a fault outsie the converter transformer area. H is usually taken as the rate current in engineering. The principle block iagram is shown in figure. I h a (4) I h b (5) I h c (6) Begin Input ata,,k T,I δ =(-k T*)/ N >δ>h Y Normal or out of zone faults in the converter area I<h N Fault occurre outsie the converter transformer Y Converter transformer failure Figure. Protection logic scheme. 4

6 4. Analysis of the Effect of Commutation Failure on the New Protection Algorithm of Converter Transformer Fig. is the schematic iagram of the AC / DC hybri system. When the AC-DC hybri system in the AC system that is the AC transmission line failure will lea to AC bus voltage rop, it is not possible to provie commutation voltage for the inverter in the inverter station an the inverter station will fail.when commutation fails will lea to lower DC voltage, DC current increases, DC system transmission power ecreases, however, there is no failure insie the converter station, the commutation relationship between the converter DC voltage an the converter-sie voltage is still establishe, the efinition of the generalize ratio is still fixe. So the resulting error is approximately zero, it is etermine that no failure has occurre insie the commutation zone. Figure. AC an DC hybri system schematics Inverter station is in the worst state of the converter commutation failure at the same time insie the converter station failure. Fig. is a schematic iagram of a failure occurring at the same time as a commutation failure. When the AC-DC hybri system in the inverter station commutation failure, while the inverter station also fails, the commutation relationship between the inverter DC voltage an the converter-sie voltage is not establishe an the generalize ratio is no longer fixe. The resulting error is between 0 an, it was etermine that a failure occurre insie the commutation zone. further, the current measure by the valve sie sleeve CT of the converter transformer is use for fault location. Therefore, when the AC system in the AC / DC hybri system fails cause the commutation failure, integrate protection algorithm for converter station can also be applie to inverter station. It can correctly istinguish whether the converter area is out of orer or the location of the fault, reliable removal of the fault an avoi amage to the converter equipment, at the same time improve the reliability of power supply. Figure. A schematic iagram of a failure occurring at the same time as a commutation failure 5. Simulation Verification sing PSCAD simulation software to buil the simulation moel of ± 800kV bipolar DC transmission system shown in Fig.4. The control system reference moel mainly inclues constant current control, constant voltage control an fixe power control. In this paper, the length of the winow is selecte as 5ms, which can ignore the influence of the control system [6]. The transmission power is 500MW, rate current is.5ka, the conveying istance is 000km, two pole DC lines are joine by the same pole, DC transmission lines using 6 split steel core aluminum wire, the rate cross sectional area is 60mm*mm, R = 4.97 ohm. Voltage rop calculation of transmission line: = In*R =.5* 4.97 = kv. The failure time is 0.76s. In this paper only gives the A-phase simulation results. 5

7 5.. Inverter station commutation failure but no failure When AC / DC hybri system commutation failure occurs at s, the DC voltage an the AC voltage amplitue of the inverter sie will ecrease, the generalize ratio k is about 0.6 an the combine error is approximately zero, current I is about 5A, it is etermine that the converter station has not faile. 5.. Commutation failures an faults occur simultaneously in the inverter station When the AC-DC hybri system in the AC line three-phase failure cause commutation failure, at the same time, the converter in the converter station also fails, as shown in Figure 4, converter current DC voltage (shown in the figure voltage curve otte line) an the converter transformer primary voltage (soli line shown), the integrate error δ an the converter transformer valve sie casing CT current I, the protection of the action output signal are given. From figure 5 we can see when the system commapses at.5s, the uration is 0.5s, which woul egrae the amplitue of the AC voltage an the DC voltage, The integrate error δ is between 0 an, I value is reuce to half of the original. The protection can be reliably operate after the fault has occurre.,/kv I/A eltt trip t/s Figure 4. DC transmission system commutation failure,/kv trip eltt t/s I/A Figure 5. Commutation failure while the converter transformer also fails 6. Conclusion In this paper, accoring to the commutation relationship between the exchange station AC bus voltage an inverter output DC voltage, the CT current characteristics of the valve sie casing of the converter transformer I have put forwar a new metho of integrate protection for converter stations base on generalize transformation ratio. This metho can be use for accurate positioning of faulty components. A large number of simulation results show that: the metho is simple an fault characteristics are obvious. In various operating conitions protection can be fast, sensitive, reliable action. It is possible to improve the power supply reliability of the special DC transmission system. 6

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