Final Report. Mini Project TET Group nr 7 - Project nr 4. Students: Hans Lavoll Halvorson, NTNU

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1 Final Report Mini Project TET Optimal Distance for connection of offshore wind farm with AC cable with SVC or STATCOM To increase the distance compensation technologies such as SVC and STATCOM can be used. Comparison of the 2 technologies and with these technologies what will be the maximum cable length. Group nr 7 - Project nr 4 Students: Hans Lavoll Halvorson, NTNU hanslavo@stud.ntnu.no Kalle Teearu, NTNU teearu@stud.ntnu.no Raghbendra Tiwari, NTNU raghbend@stud.ntnu.no Contact person: Kamran Sharifabadi, Statkraft kamran.sharifabadi@statkraft.com Professor: Tore Marvin Undeland tore.undeland@elkraft.ntnu.no 1

2 Content Abstract... 3 Introduction... 3 SVC presentation of technology... 3 STATCOM presentation of technology... 4 Subsea Cable... 6 Comparing SVC and STATCOM... 7 Simulation results and discussion... 9 Conclusions REFERENCES

3 Abstract This report is the study of the behaviour of STATCOM and SVC in connection of an offshore wind farm to an onshore grid substation with an HVAC cable. The study is based on analytical and simulation analysis. The simulation is the expansion of the pre-defined models of STATCOM and SVC in EMTDC/PSCAD and the results have been studied for the improvement of voltage in steady state and the change in the reactive power in the system with different lengths of HVAC cable. The report also depicts the principle of operation of STATCOM and SVC and concludes for the combination of compensating device and the length of the cable. Index Terms SVC, STATCOM, HVAC Cable, wind farm, EMTDC/PSCAD Introduction Many companies are looking into the possibilities of establishing large offshore wind parks, and connecting them to onshore grids. FACTS units together with HVAC subsea cables make an interesting alternative to HVDC. This paper looks shortly at the maximum distance of a subsea cable and the two compensation technologies SVC and STATCOM. The goal is to transmit power from a 700MW wind park, on a 400kV cable. Results show that a 100km cable is possible. 700MVA offshore wind farm Subsea cable Power grid FACTS SVC presentation of technology This part speaks briefly about one of the reactive power compensation methods, called Static Var Compensator, which by definition is a static var generator whose output is varied so as to maintain or control specific parameters of electric power system. [2] First option is variable impedance type static var generators which as the name states change their impedance as a result of changing system properties. To control reactive power flow through the cable and avoid too high voltages, it is needed to use different variations of reactive elements in different configuration. Shunt compensation is used to increase the maximum power that can be transmitted through a cable. In the case of a wind farm, it is needed to have the area of regulating the reactive power. That means the reactive components should be controllable. In theory four types of static var generators are used. First of these is the Thyristor Controlled Reactor (TCR). The current in the reactor can be controlled by the firing delay angle of the thyristor bank. When the angle α is equal to zero then the compensator is called Thyristor Switched Reactor. TSR just switches after every half period. 3

4 Fig. 1 Schematic diagram of SVC [4] As an opposite of that the Thyristor Switched Capacitors are used. Although the capacitor is the main part of the SVC, there is also inductor (reactor) in parallel to limit the surge current. Because of the switching transients TSC can be switched only when current is 0. Therefore firing delay angle is not used here. Hence TSC is used as an admittance which is either connecter or disconnected from the power system. Also Fixed Capacitor, Thyristor-Controlled Reactor type var generators are used. This is basically capacitor bank in parallel with TSR. By changing the angle α of the thyristor bank on the reactor side we change the total amount of reactive power flow to network. Since capacitor has constant output at all times it is the reactor side that controls the total amount of reactive output to the network. To decrease the capacitive output, the current in the reactor is increased by decreasing delay angle α. From the point where α is equal to zero going on, it appears that the bank is going to have larger inductive current than capacitive current. This means inductive var output to the network. Thyristor-Switched Capacitor, Thyristor-Controlled Reactor (TSC-TCR) banks are basically number of capacitors in parallel, one of these parallel elements is reactor which is used to smooth the reactive current output. This works as the previous (FC-TCR), but for smaller losses on the standby mode it has the possibility of turning off some capacitance. STATCOM presentation of technology The Static Synchronous Compensator or STATCOM is a device of generating controllable reactive power directly, without the use of AC capacitors or reactors unlike to Static VAR Compensators (SVC), by various switching power converters assembly. This FACTS device was disclosed by Gyugyi in These converters (dc to ac or ac to ac) are operated as a voltage and current sources and they produce reactive power without reactive energy storage components (capacitor or inductor) by circulating alternating current among the phases of the ac system. 4

5 Fig. 2 Reactive Power generation by a voltage sourced switched converter [2] The basic operating principle of reactive power generation by a voltage sourced converter is similar to that of the conventional rotating synchronous machine shown is figure beside. For purely reactive power flow, the three-phase converted EMF V 0 of the converter after the capacitor unit are in phase with the system voltages v a, v b, and v c. The reactive current I drawn by the synchronous compensator depends on the magnitude of the system voltage V, that of the converter voltage V 0 and the total circuit reactance (transformer leakage reactance plus reactance of the coupling Transformer) X: V V I 0 X The corresponding reactive power Q exchange is expressed as: 0 1 V V 2 Q V X The three phase output voltage is generated by a voltage sourced dc to ac converter operated from an energy storage capacitor. The converter consists of either 6 pulse bridges or 12 pulse bridges. By varying the amplitude of the output voltages produced, the reactive power exchange between the converter and the ac system can be controlled in a manner similar to that of the rotating synchronous machine. That is, if the amplitude of the output voltage is increased above that of the ac system voltage, then the current flows through the tie reactance from the converter to the ac system and the converter generates reactive (capacitive) power for the ac system. If the amplitude of the output voltage is decreased below that of the ac system, then the reactive current flows from the ac system to the converter, and the converter absorbs reactive (inductive) power. If the amplitude of the output voltage is equal to that of the ac system voltage, the reactive power exchange is zero. STATCOM has its application in Power quality improvement, improving Transient Stability margin, Stabilizing grid voltages in wind farms and in other transmission networks after a disturbance that is after the load changes or short circuit faults. 5

6 Subsea Cable Laying very long HVAC cables at sea offer many difficulties, and a real life project would have to consider important details like maximum weight load for cable laying vessels, axial forces during laying, cable and jointing technology for the highest voltages, costs etc. Sheath and armor current play a considerable role in very long lengths of cable. Different companies have different methods of determining acceptable losses in a power transmission. An overall more detailed approach must be taken. This report has a rather theoretical approach focusing on the FACTS units, and the cable theory is just briefly commented. The cables in table 1 are found from ABB cable catalog [3]. To transmit 700MW of power, the 400kV single core 1000mm 2 cable is chosen for the simulations. It has a current rating of 1290A at given circumstances. Threecore Threecore Singlecore Singlecore Singlecore Crosssection CU [mm2] Un (Um) [kv] 220 (245) 275 (300) 220 (245) 330 (362) 400 (420) Capacitance [μf/km] Charging current [A/km] Inductance [mh/km] Resistance [Ω/km] Current rating [A] Table 1 Assuming 20 C in seabed, 1m laying depth, 1Km/w thermal resistivity, max 90 C conductor. Wide spacing single cores. ABB cable catalog [3] A cable s capacitance will produce a reactive current, I c, lagging the useful current I p by 90 o. Both I p and I c contribute to heating the cable when considering the cables maximum current rating, I n, which is limited by the cables maximum allowed temperature and surrounding conditions. [1] At critical length, the charging current is equal to the rated current of the cable. I c = I n. Brakelmann[5] provides that voltage swing should not exceed 10%, and phase variation should not exceed 30 o. Maximum power that can be transmitted to the receiving end is 87% of the nominal power, when the two angles in the sending and receiving end of the cable are equal and 30 o. Useful current Reactive current Reactive power U 2 I p In Ic Ic C Qc 3 C U 3 As we can see in the figure, reactive current will build up more and more along the length of the cable, making the current distribution along the cable uneven. If not compensated, it will be highest at the end of the cable. The optimal solution is obviously to place FACTS units in the middle of, or all along the cable length to even out the current distribution. The FACTS unit should stabilize voltage, and provide reactive power at different load levels. In this report, end-compensation is simulated. 6

7 Fig. 3 Current distribution of an 100km long 245kV three-phase submarine cable, conductor 1200mm 2. Full lines: compensation at both ends. Dotted lines: compensation only in the onshore-station. Thermal current rating: 1323A [5] Comparing SVC and STATCOM Both a STATCOM and SVC can be used to mitigate reactive power and stabilize voltage in a long subsea cable. The two FACTS units have been shown to have different technologies, and they work in different ways. The effects of these differences are shown and compared in this chapter. As the two following figures show, the STATCOM is superior to the SVC regarding the range of operation with different system voltages and reactive powers. V-I and V-Q characteristics are shown. Fig. 4 V-I characteristic of the STATCOM (a) and the SVC (b) [2] 7

8 Fig. 5 V-Q characteristic of the STATCOM (a) and of the SVC (b) [2] The STATCOM has a greater ability to mitigate transients than the SVC, as explained in figures below. This is connected to the fact that STATCOM s attainable response time is significantly better than SVC, with time lag in the control systems of μs compared to 2.5-5ms. Fig. 6 Transmitted power versus transmission angle of a two-machine system with a midpoint STATCOM (a) and a midpoint SVC (b) obtained with different var ratings [2] Fig. 7 Improvement of transient stability obtained with a midpoint STATCOM (a) and a midpoint SVC (b) of a given var rating. [2] 8

9 STATCOM can easily exchange electric energy onto any suitable energy carrier through its DC side. An SVC does not have such capabilities. When considering loss versus VAR output, STATCOM has more losses than SVC. Future development of better semiconductors may reduce and change this relationship. A STATCOM is smaller than an SVC, in view physical size and installation. No large capacitor or reactor banks are needed. Simulation results and discussion PSCAD has been used to simulate a STATCOM and subsea cable. Subsea cable is simply modeled as a PI-equivalent with half the cable capacitance in each end of the cable. Wind farm is modeled as an automatic power controlled voltage source. The receiving grid is modeled as voltage source. STATCOM model is found in the PSCAD example library statcom_6pls_pwm.psc. There is a breaker between the grid and STATCOM that closes 1.5s into the simulation. Fig. 8 PSCAD model for subsea cable and STATCOM compensation Three different cases are simulated. Case MW with STATCOM after 1.5s. 50km cable Case MW with STATCOM after 1.5s. 100km cable Case MW with STATCOM after 1.5s. 150km cable Generation: 700MW, 400kV, 60Hz. Subsea single phase cable: 1000mm 2 CU. Per unit base parameters: 700MVA, 400kV. STATCOM transformer: 100MVA, 25/400KV 9

10 Fig. 9 Reactive power of transmission line The STATCOM is turned on, and needs some time to stabilize. The stabilizing time is linked to the cable-length. Shorter cables have a bigger peak in reactive effect and less stabilizing time as the STATCOM turns on. The delay is due to the longer time constant of the reflected voltage through the increasing cable length. Reactive power generation in the cable is reduced due to the lower voltage after switching on the STATCOM. This is the reason why the amount of reactive power before and after is not constant. Fig. 10 Reactive power output from the STATCOM Longer cable means more reactive power must be consumed by the STATCOM. As previously mentioned, the stabilizing time is linked to the cable length. 10

11 Fig. 11 Active power of transmission line During the transient at STATCOM turn on, we see a fluctuation in active power. It is due to the power drawn by the STATCOM and transformer inrush current. Due to the same reason active power at the wind farm has the same fluctuations. As the cable length increases, a power peak after some seconds is being delayed. Fig. 12 Grid voltage at STATCOM injection point For long cable the grid voltage is high prior to STATCOM turn on. STATCOM stabilizes the voltage by drawing the surplus reactive power in the system. If a very short cable is simulated, the voltage will be low prior to STATCOM turn on. The STATCOM will stabilize the voltage by compensating the deficit of reactive power. The simulation results show very much ripple in the current. The reason for this may be tuning issues with the STATCOM, or unfortunate relationships between capacitances in the STATCOM and cable. Further study of this is required. In PSCAD replacing the STATCOM 11

12 capacitance with bigger or smaller capacitances doesn t influence the result. Replacing it with PSCAD voltage source may be a possibility, but this needs further studies and tuning of the model. Fig. 13 Phase angles at sending and receiving end The phase angels are changed slightly, but not significantly. Simulation results SVC PSCAD model tcr_tsc.psc in the example library has been amended to include subsea cable as in the STATCOM model. Before turning on the SVC at t=1.5s, voltages and reactive effects in the cable are measured to be equal to the STATCOM model. As the results show, heavy oscillating is observed when the SVC is turned on. Different SVC parameters and settings were tried, both at low and high loads and voltages, but the results where not as expected. Further investigation needs to be done into the details of PSCAD s SVC model. Fig. 14 PSCAD model for subsea cable and SVC compensation 12

13 Fig. 15 Voltage for 100km cable - comparing SVC and STATCOM In the SVC simulation, steady state voltage starts oscillating around its previous state. In this figure it is compared to the voltage of STATCOM regulation. As STATCOM stabilizes the voltage at 1 pu, SVC is making it oscillate. Fig. 16 Reactive power in 100km cable The reactive power goes from a stable steady state prior to SVC turn-on, to quite strong oscillation after. 13

14 Fig. 17 Active power in 100km cable Also the active effect on both receiving and sending end of the cable suffers from heavy oscillating. Conclusions The steady state behaviour of STATCOM and SVC were studied in this report. When using the pre-designed models of STATCOM and SVC from PSCAD s library, it appeared that the results of STATCOM were much more relevant to the expected outcome. The STATCOM has been found to have quite good response to any system voltages which may be higher or lower due to the surplus reactive power in the system generated by the HVAC cables. It maintains the predefined grid voltage by generating or consuming the reactive power. The other noticeable characteristic is the settling time of voltage when STATCOM is switched on. The longer the cable, longer the time it takes to reach a steady state value of 1 pu. It also limits the length of the cable in between offshore wind farm and the onshore grid substation. The SVC showed no stabilizing effect, in fact it caused worse situation than in uncompensated state. Therefore we can say that the conclusion on SVC s impact on the subsea cable in our brief survey has no significant meaning and should not be taken as reliable information. STATCOM however fulfilled our expectance in terms of voltage stabilization, power transmission and angle theory. Therefore, in this study, 100 km cable with STATCOM at grid end has been concluded to be a best combination for a 700 MW, 400 kv off shore wind farm. And, since the characteristics of the generating units of the wind generators have not been considered here, further study has been proposed to make a better result and conclusion. 14

15 REFERENCES [1] Cigre 370. Integration of large scale wind generation using HVDC and power electronics. Working Group B4.39. February 2009 [2] Understanding FACTS. Concepts and technology of flexible AC transmission systems. Hingorani. Gyugyl. IEEE press ISBN [3] XLPE Submarine Cable Systems. Attachment to XLPE Land cable systems users guide. Rev 5. ABB [4] Benefits of SVC and STATCOM for electric utility application. Noroozian, Petersson, Thorvaldson, Nilsson, Taylor. ABB [5] Efficiency of HVAC power transmission from offshore-windmills to the grid. H. Brakelmann. CIGRE.VDE 15