Hybrid HVDC System for Multi-infeed Applications. Shilpa G. P G student. Department of Electrical and Electronics. M S Ramaiah Institute of Technology.
Hybrid HVDC System for Multi-infeed Applications Shilpa G
Dr. Premila Manohar
P G student Department of Electrical and Electronics M S Ramaiah Institute of Technology Bangalore-560054
Professor and Head of the Department Department of Electrical and Electronics M S Ramaiah Institute of Technology Bangalore - 560054
Abstract— The application of HVDC for stabilizing the large interconnected AC networks are resulting in several HVDC links inverting in the close electrical proximity. These multi-infeed HVDC system inverting into a weak AC network are of serious concern. One of the possible solution for better stability is using a VSC-HVDC as one of the infeed for the multi-infeed system. In this paper a multi-infeed HVDC system combining a conventional LCC-HVDC system and a Hybrid HVDC system consisting of LCC as the rectifier and VSC as the inverter is proposed for supplying a very weak AC network. The system performance during steady state and transient conditions is studied by simulation software PSCAD/EMTDC. Keywords — Multi-infeed HVDC system, LCC - HVDC, VSCHVDC, hybrid HVDC, weak AC system, commutation failure.
I.
Fig. 1. Multi-infeed HVDC system
INTRODUCTION
High Voltage Direct Current (HVDC) technology has been in operation for more than 60 years now. It is gaining in importance within the existing power grids due to its advantages compared to conventional AC transmission for bulk power transmission over large distances. There is a vast expansion of HVDC technology due to the increasing demand for electrical power and rapidly growing electrification. Hence a number of high capacity long distance HVDC systems are planned. It is becoming more common to have several HVDC transmission links connected to the same AC network with close electrical proximity. The integration of multiple HVDC links feeding power into different points in the same AC network area is called “Multi-infeed HVDC system” (MIHVDC) [1]. Multi-infeed converters either share a common AC bus or are connected to buses which are electrically close as shown in Fig. 1. There are many major issues of concern in a MI-HVDC system. Among these important ones are increased overvoltages, high risk of commutation failure, voltage and power instabilities and AC/DC fault recovery [2]. These issues are of serious concern when the MI-HVDC is inverting into a weak system. The AC system is characterised by an index called the short circuit ratio (SCR). The SCR is defined as the ratio of the AC system short circuit MVA (SCMVA) at the converter terminal AC bus to the rated DC terminal power (Pdc), as in Fig. 2. SCR
An SCR greater than 3 results in a strong system. If SCR is in between 2 and 3, the system is moderately strong or weak and if less than 2, the system is known to be very weak.
SCMVA P
V ⁄z P
Fig. 2. AC system at inverter
The simulation studies show that serious concerns to a MIHVDC system are largely unaffected in a strong system at the inverter side [3-4]. There is no risk or tendency towards dynamic instability in a strong system. The studies also show that in a strong system the overvoltages are within the specified limits and commutation failure is not repetitive. However, in a weak system, a small disturbance can cause large deviations in the voltages and other variables in the network. This can cause disastrous effect on the MI-HVDC system. All these problems are particularly relevant in the case of LCC-HVDC. Recently, VSCs are becoming an alternative technology for HVDC transmission [5]. The VSC-HVDC has many advantages like independent control of active and reactive power, possibility to invert into a weak or passive AC network,
reduced harmonics and good controllability [6]. A VSC-HVDC infeed in a MI-HVDC system are expected improves the steady state as well as transient behavior [7]. On the other hand, VSC-HVDC systems are expensive and have higher losses due to high frequency switching than compared to LCC-HVDC. Also the voltage and power rating of the VSC system are relatively less. One of the possible solutions to overcome the limitations of VSC-HVDC system is a Hybrid HVDC system [8]. A hybrid HVDC system combines the advantages of both LCC and VSC-HVDC systems. The LCC system reduces the cost and the VSC system reduces the problems of commutation failures. In this paper a multi-infeed HVDC system is considered which integrates the conventional LCC-HVDC and Hybrid HVDC inverting into a very weak AC system. The system performance of the MI-HVDC transmission during steady state and AC fault conditions are studied. The speed of recovery from faults are considered in the analysis. The simulations are carried out in the simulation software PSCAD/EMTDC. II.
THE TEST SYSTEM
This paper considers three cases of a multi- infeed HVDC system. Case 1. Both infeeds are being LCC–HVDC, as in Fig.3 Case 2. One infeed as LCC-HVDC and the other infeed as VSC-HVDC, as in Fig. 4. Case 3. One infeed as LCC-HVDC and the other infeed as Hybrid HVDC with LCC at the rectifier side and VSC at the inverter side, as in Fig.6 In all the cases the common conventional LCC- HVDC infeed is of 1000MW with a DC voltage and current rating of 500kV and 2kA respectively. The other parameters are identical with the CIGRE HVDC Benchmark model. The second infeed is a 300MW HVDC system. This system can be LCC, VSC or Hybrid. In all the cases the SCR of the combined system of 1300MW is 1.92 which corresponds to a very weak AC network. III.
LCC-LCC MI-HVDC SYSTEM
In this case, the infeeds of the MI-HVDC system are both LCC-HVDC as shown in Fig.3. The rectifiers of both the system operates in constant current control mode and both the inverter operate in constant extinction angle (γ) control mode with γ = 15°. The 300MW LCC-HVDC is rated at 500kV and 0.6kA. The various simulation results are shown in Fig. 5. A. Steady state operation From the simulation results, the whole system reaches steady state at 0.5s. The DC voltage and current at the inverter side of 1000MW LCC-HVDC reaches the reference value of
1.0p.u. as shown in Fig. 5(a). In Fig. 5(a). The DC voltage and current at the inverter side of 300MW LCC-HVDC reach the reference value of 1.0 pu. and 0.3 pu. respectively. The rectifier alpha order of 1000MW and 300MW LCC-HVDC system is respectively 23° and 34°. The extinction angle of both the LCC-HVDC systems reaches the final steady state value of 15.4°. B. Single-Phase-to-Ground Faults Single-phase-to-ground fault at the inverter side of the MIHVDC is applied at 2.1s for 0.05s. Simulation results of singlephase-to-ground faults at inverter side of the MI-HVDC are shown in Fig. 5(b). During the fault there is repeated commutation failure in both the systems. The DC voltage drops and the DC current increases for both the systems. The active power output of both the systems increases temporarily. The whole system achieves stable state within about 0.55s after the fault is cleared. C. Three-Phase-to-Ground Faults Three-phase-to-ground fault at the inverter side of the MIHVDC is applied at 2.1s for 0.05s. The simulation results are shown in Fig. 5(c). It can be seen that when the fault occurs, there is a second commutation failure in both the systems, at rectifier and inverter side. The DC voltage drips and the DC current increases for both the systems. The rectifier alpha order of the systems increases to a maximum value of 102° and 150° for 1000MW and 300MW LCC system respectively. The inverter extinction angle of both the systems decreases. The active power output of both the systems increases temporarily. The whole system recovers within 0.85s after the fault is cleared.
IV.
LCC-VSC MI-HVDC SYSTEM
In this case 1000MW system is of LCC and 300MW system is of VSC as shown in Fig. 4. The rectifier of LCC system operates in constant current control mode and the inverter operates in constant extinction angle (γ) control mode with γ = 15°. The 300MW LCC-HVDC is rated at 220kV and 1.36kA. The direct control modes of VSC system are as follows: the rectifier regulates the real power and its reactive power. The inverter regulates the DC side voltage and its AC voltage on the bus bar. A. Steady state operation The simulation results show that the MI-HVDC system reaches the steady state at 0.5s. The DC voltage and current at the inverter side of 1000MW LCC-HVDC reaches the reference value of 1.0p.u. as shown in Fig. 6(a). The rectifier alpha order of 1000MW LCC system is 13°.
Fig. 3. LCC-LCC MI-HVDC system
Fig.4. LCC-VSC MI-HVDC system
(a) Steady state
(b) Single-phase-to-ground fault at BUS 1
(c) Three-phase-to-ground fault at BUS 1 Fig. 5. System performance for LCC-LCC MI-HVDC system
(a) Steady state
(b) Single-phase-to-ground fault at BUS 1
(c) Three-phase-to-ground fault at BUS 1 Fig. 6. System performance for LCC-VSC MI-HVDC system
Fig.7. LCC-HYBRID MI-HVDC system
(a) Steady state
(b) Single-phase-to-ground fault at BUS 1
(c) Three-phase-to-ground fault at BUS 1 Fig. 8. System performance for LCC-HYBRID MI-HVDC system
The extinction angle of the LCC-HVDC system reaches the final steady state value of 15°. The DC voltage and current at the inverter side of 300MW VSC-HVDC reaches the value of 222.9kV and 1.36kA respectively as seen from the Fig. 6(a). B. Single-Phase-to-Ground Faults The performance of the system during AC faults was investigated by creating a single-phase-to-ground faults at the inverter side of the MI-HVDC. The faults are applied at 2.1s for 0.05s. These results are shown in Fig. 6(b). As seen from Fig. 6(b), when the fault occurs, there is a commutation fault in both the systems. But there is no second commutation failure as in case 1, for single-phase-to-ground fault. The system reaches the steady state within 0.45s after the fault is cleared.
C. Three-Phase-to-Ground faults Three-phase-to-ground fault at the inverter side of the MIHVDC is applied at 2.1s for 0.05s and the results are shown in Fig. 6(c). There is no second commutation failure in both the systems compared to three-phase-to-ground fault in case 1. During the fault the DC voltage drops and the DC current increases for 1000MW LCC system and vice versa for 300MW VSC system. The rectifier alpha order of LCC system increases to a maximum value of 109°. The inverter extinction angle of LCC system decreases. The active power output of both the systems increases temporarily. The system achieves steady state within 0.55s after the fault is cleared.
V.
LCC- HYBRID MI-HVDC SYSTEM
In this case, 1000MW system is of LCC and 300MW system is of Hybrid HVDC as shown in Fig. 7. The 300MW Hybrid HVDC system has LCC at rectifier side and VSC at inverter side. The Hybrid system is rated at a DC voltage and DC current of 220 kV and 1.36 kA respectively. The rectifier of 1000MW LCC system operates in constant current control mode and the inverter operates in constant extinction angle (γ) control mode with γ = 15°. The LCC rectifier of Hybrid system has constant current control. The VSC inverter of Hybrid system regulates the DC voltage and its AC voltage on the busbar. A. Steady state operation The MI-HVDC system reaches the stable steady state at 0.5s. The DC voltage and current at inverter side of 1000MW LCC-HVDC reaches the reference value of 1.0p.u. as shown in Fig. 8(a). The rectifier alpha order and inverter extinction angle of LCC system is 15.3° and 15.1° respectively. The DC voltage and current at inverter side of the VSC system is 225.15 kV and 1.364 kA respectively. B. Single-Phase-to-Ground Faults The performance of the system during AC faults at the inverter side of the MI-HVDC is investigated by creating a single-phase-to ground fault at 2.1s for 0.05s. As seen from Fig. 8(b), during the fault the disturbance is restricted to a single commutation failure as in case 1 for single-phase-toground fault. Compared to case 2, the disturbances are less severe. During the fault, the voltage variation is 236kVas compared to a peak of 244kV in case 2. The current dip during fault is only upto 1.3kA as compared to 0.14kA in case 2. The entire system reaches the stable state within about 0.4s after the fault is cleared. The recovery is also fast when compared to case 2. C. Three-Phase-to-Ground faults The results of three-phase-to-ground fault at the inverter side of the MI-HVDC , are shown in Fig. 8(c). As can be seen from Fig. 8(c), when the fault occurs, there is a commutation failure in both the systems. During the fault, the transients of DC voltage and current of 300MW system has improved considerably when compared to case 2. During fault the voltage peak is 352kV as compared to a peak of 452kV in case 2. The current dip during fault is only up to 0kA as compared to -2.21kA in case 2. The entire system reaches the stable state within about 0.5s after the fault is cleared. The recovery is also fast when compared to case 2. VI.
CONCLUSION
A MI-HVDC system consisting of conventional HVDC and VSC-HVDC inverting into a very weak system was investigated. The steady state and transient conditions were compared with a multi-infeed system of two LCC-HVDC system. The simulation studies in PSCAD/EMTDC shows that VSC system makes conventional LCC system less susceptible to commutation failure. A multi-infeed hybrid HVDC system consisting of LCC at rectifier side and VSC at inverter side, and conventional LCCHVDC has been proposed for a very weak AC network. The
system performance under steady state and AC fault conditions were studied. The simulation results show that the hybrid system improved the transient performance of the MI-HVDC system under AC fault conditions. The hybrid system uses the advantages of both LCC and VSC system and can be a good solution for MI-HVDC system inverting into a weak AC network.
VII. REFERENCES [1]
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