FACTS FOR GRID INTEGRATION OF LARGE OFFSHORE WIND FARMS BY MEANS OF AC

Transcription

1 FACTS FOR GRID INTEGRATION OF LARGE OFFSHORE WIND FARMS BY MEANS OF AC Rolf Grünbaum, Per Halvarsson ABB Power Technologies AB Introduction After decades in the starting block, wind power is finally taking off as a serious and accepted source of energy, sustainable and environmentally friendly. World-wide, it is expected that by the end of 2007, wind power will have increased to well over 80 GW. An example: for Germany, ten years ago, wind power was but of marginal importance. Today, with more than MW in operation, the country is Europe s most important user of wind power. In Denmark, 3000 MW of wind power is now contributing more than 15% of the country s electric energy balance. In the European Union as a whole, the target for wind power capacity for 2010 was set to 40 GW. In reality, this goal was met already in In 2003, the goal for 2010 within the EU was increased to 75 GW. 1 Offshore wind generation: fast emerging Now emerging are sea based wind farms, where large amounts of wind power generation (typically tens of MW up to several hundred) is located out in the sea and where the power is landed through powerful underwater cables. The largest offshore projects now in operation are the 160 MW Horns Rev and the 160 MW Rødsand, both in Danish waters. AC is utilized in both cases for landing of the power. As a matter of fact, AC transmission will turn out to be an economically and technically attractive option in many cases, and dynamic reactive power compensation will then be a natural part of the scheme. Evidence suggests that the increased revenue from exploiting higher wind speeds farther offshore can outweigh the increased cable costs and electrical losses. Growth is likely to accelerate as offshore electricity prices approach those of conventional sources [1]. The dominating kind of wind power generator is asynchronous, this since it is robust and cost effective. Induction generators, however, do not contribute to regulation of grid voltage, and they are substantial absorbers of reactive power. Ideally, they need to be connected to very stiff grids in order not to influence power quality in a detrimental way. This is not the case in reality, however. Quite on the contrary, wind power is usually connected far out in the grid, on subtransmission or distribution levels, where the grid was not originally designed to transfer power from the system extremities back into the grid. The reactive power balance of asynchronous generators can be improved to a certain extent by use of the recently introduced doubly-fed rotor concept. To keep this technology within reasonable cost margins, however, rotor converter ratings must be kept limited to steady-state requirements only. During transient occurrencies in the grid, the performance of doubly-fed induction generators (DFIG) may well prove inadequate to safeguard primarily voltage stability of the grid. Here, dynamic reactive power compensation plays an important role in supporting DFIG. 2 Sea cables: a key issue Comprehensive sea cable networks add another dimension, calling for additional elaborate reactive power control. The overall scope of reactive power control should encompass the wind farm just as well as the sea cables, to bring about a well regulated reactive power balance of the whole system, answering to the same demands on reactive power regulation as any other medium to large generator serving the grid. The reactive power generation of a sea cable (Q C ) is a function of the cable voltage (U) and the capacitance of the cylindrical capacitor formed by the cable (C): Q C = 3 ωc U 2 (1) C = 2πε / ln (R/r) (2) Here, R is the radius from the centre of the cable to the cable shield, and r is the radius of the inner conductor. Limits of transmission capacity of sea cables should be calculated with compensation, so half of the charging current flows to each end at maximum load. An example of transmission capacity for XLPE cables at four different voltage levels and as a function of cable length is shown in Figure 1 [2]. In the example, the cables all have a cross-section of mm 2. It is seen that the power transmission capability exceeds 350 MW at a distance of 100 km and a voltage of 220 kv. At a distance of 250 km, the power transmission capacity is still 175 MW.

2 Fig. 1: Limit of power flow as a function of length for compensated cables type 3x1x1.000 mm 2. Requirements related to network connection All National grid codes, even though several differences can be found, discuss the importance of grid support from all installed power generating devices. Many regulatory authorities require that the generators should be able to vary their reactive power output dependent on the grid voltage level. This requirement is a result of the desire to maintain voltage stability and limit dynamic voltage variations. Wind farms have often been excluded from these demands. They cannot, however, expect to enjoy this favoured treatment forever. In the past wind power plants typically have had a small power rating, when compared to the strength of the connecting electrical network. The power system as such did not depend on the contribution from the renewable power sources. Under these circumstances the behaviour of the wind mills at faults in the network seemed to be non-critical and the manufacturers could design a simple control system that simply dropped out the wind mills whenever disturbed conditions did occur on the network. Looking at the big wind farms presently being planned this design philosophy becomes questionable. When a fault occurs in a power system the faulty part will be disconnected from the system. Thus if the fault occurred on the feeder to which the windmill is connected it will be disconnected. But, if the windmill is connected to the non-faulted part of the system it is desirable that the windmill stays connected during the fault. As soon as the faulty feeder has been disconnected the wind generator shall return into operation in order not to cause consequential loss of generation in addition to generating units connected through the faulty feeder. If consequential loss of generation would occur it may lead to a system collapse. Therefore the wind farm connection must be designed so that the wind farm is capable of continuous uninterrupted operation during events when the voltage is being depressed during the time required to disconnect a faulty feeder ( ride-through capability ). Typically the time required in a high-voltage system is in the range ms. In one specification defining the connection conditions it was specifically stated that there is no reason to exempt wind generation from this requirement. 3 Voltage control Reactive power control is necessary to meet the criteria mentioned in Section 0 above. With synchronous generators, reactive power control is achieved by means of the exciter system. However, this is not possible for induction generators. An SVC positioned at the grid connection point acts as a central exciter system but with the advantage that reactive power can be controlled even when no power is generated. The subtransmission, or even distribution, systems to which offshore wind farms will be connected are usually designed to distribute power from the main grid to remote customers. The system in most cases is very weak and a change in power flow direction will strongly affect the voltage level. Mechanically switched capacitor banks, MSC, are often used to deal with voltage level problems. However, power production, and thus reactive power consumption, in wind farms vary with wind speed. The resulting frequent switching of MSC deteriorates power quality and decreases the lifetime of the MSC. An SVC, with continuously variable susceptance, is often a cost efficient alternative to several small MSC. Several phenomena associated with power produced from wind introduce voltage flicker on the connecting node, i.e. generators starts and stops, wind speed variations, and tower shadow effects. This flicker has a deteriorating effect upon other components connected to the grid, causing complaints from power consumers. By connecting an SVC at the grid connection points, this flicker can be mitigated. 4 Induction generator behaviour at shortcircuit The reason for the attractiveness of the induction generator is its extremely simple basic design, making it small, robust and cost-effective. The rotor simply comprises a stack of iron plates with axial conductive bars, which are connected between short-circuiting endrings. The rotor is mounted in the stator, coaxial and with a small airgap as shown in Figure 2. rotor flux stator flux Fig. 2: Structure of the induction generator. The small airgap causes a strong magnetic coupling between any winding on the stator and the rotor structure. The main portion of the magnetic flux passing through the stator winding also must pass through the rotor. But the

3 rotor cage is a highly conductive structure, which at any time opposes every change of the magnetic flux in the rotor. It serves like a magnetic screen that prevents external fields from penetrating into the rotor. Forced, fast changes of the magnetic flux in the rotor can only be achieved at the expense of large stator (and rotor) current. Due to their close linkage the rotor flux follows the stator flux with a first order delay of about 100 ms. 4.1 General principles of operation When the machine is connected to a strong network, the applied voltage determines the magnetic stator flux, because the derivative of the stator flux must equal the applied voltage. The network voltage creates a magnetic stator flux, which rotates with constant speed in the airgap. The rotor flux derivative, and consequently the stator current, can be low only if the rotor rotates almost synchronously with the stator flux. This is obvious in the upper graph in Figure 3, which depicts the stator current versus the rotor s speed deviation from synchronous speed at constant stator voltage. It is characteristic that the induction machine draws a lot of current at start and at high overspeed. to the involved mechanical time-constants in the wind turbine blade control system it is not possible to reduce the turbine torque during the short circuit. The average speed increases as long as the short circuit is applied. Thus the negative slip frequency increases during the short circuit. It has already been pointed out in Fig. 3 that the induction generator current increases rapidly, when the rotor frequency deviates from the synchronous speed. 5 Recovery from a fault The line, where the short circuit arose, automatically will be disconnected from the rest of the transmission system with little delay, typically within ms. If the isolated, faulty part does not involve the wind farm, it is anticipated that the latter restores its steady state generating operation, when the network voltage returns after fault clearance. However, as it has been described above, the machine has been demagnetised during the short circuit and its rotor speed has increased so that the negative slip has increased. Both these factors impact on the process of recovery. is (pu) Tel (pu) Stator current (pu) Electrical torque (pu) rotor freq (Hz) Fig. 3: Typical induction generator characteristics versus rotor frequency (f network f mech ) at fixed stator flux: stator current (upper) and electrical torque (lower). Small deviations from synchronous speed, on the other hand, excite rotor currents that produce mechanical torque. This is shown in the lower graph of Fig. 3. It can be seen that the torques having the highest magnitudes, the pullout torques, are produced at quite small speed deviations from synchronous speed. The normal operation range for the induction generator is on the slope between the positive and negative pullout torque. The nominal operating point is marked by a small ring in Figure Mechanical effects at short-circuit A short-circuit on the transmission system, somewhere between the generator and the receiving network, prevents any electrical power from passing through that point as the voltage becomes zero. The loss of the electrical output from the generator causes a mismatch between decelerating electro-dynamical torque and the (initially unchanged) accelerating mechanical torque on the generator shaft from the wind turbine. Thus the generator will accelerate during the short circuit. Due The induction generator can recover successfully from the short circuit fault only if it becomes magnetised sufficiently fast so that it can produce torque and reduce the generator rotor over-speed. If the over-speed becomes too large the generator will pass over the pullout frequency and then it will consume large amounts of reactive power. If the network is weak this situation will cause a voltage collapse to occur in the transmission system. 6 SVC to improve recovery from grid faults It was shown in the preceding section that it is crucial that the induction generator is magnetised very rapidly, when the network voltage returns following a network fault. If this cannot be achieved sufficiently fast the generator will speed up and then the slip frequency can not be brought down close to zero by the electrical network. The speed reduction then must be achieved using the mechanical turbine blade angle control system. The time constants then become much larger than the typical short circuit duration and the windmill generator will require a restart in order to regain its pre-fault power generation operation. When the electrical network is weak the behaviour of the windmill park at network faults will be strongly improved by reactive power support at the connection point. A Static Var Compensator, SVC, can be provided as a reactive power source located close to the machine. This approach brings about some advantages: one device serves the whole windmill park the voltage in the connecting point is stabilised by a voltage controller flicker due to tower shadow effect and variations in wind speed will be decreased.

4 Offshore Platform Sea cable SVC Fault To Grid Voltage [p.u.] Time [s] Fig. 6: Simulation results with SVC. Fig. 4: Setup of generic study of recovery from network fault. Figure 4 shows the setup of a generic simulation of the recovery from a network fault. It is assumed that the network has a short circuit strength of ten times the rated power of the windmill generators. Further transformers are inserted between the high-voltage transmission system and an intermediate voltage level (20-36 kv) and further from this intermediate voltage level down to the lowvoltage internally used in the windmill towers. It is assumed that the no-load reactive power consumption is compensated internally in the windmills. Figure 5 shows the simulation results when a 200 ms fault is applied at the on shore high-voltage bus close to the grid connection point and when the SVC is not enabled. It is assumed that the turbine power is constant. Voltage [p.u.] Time [s] Fig. 5: Simulation results without SVC. The diagram shows the voltage at the grid connection point. The simulation indicates that the windmill park will be unable to recover from this fault, the network will thus suffer a voltage collapse situation. Figure 6 shows the simulation result for the same event as in Figure 5 when an SVC connected to the intermediate voltage level and having the same rating in Mvar as the generators have in MVA is used. The stability clearly has been improved a lot and the generators recover to the pre-fault generation within two seconds. The critical point occurs a few hundred ms after the fault clearance and at that time it is determined whether the recovery will be successful or not. This simulation gives a general indication that the conditions at fault recovery should be looked at if the (post-fault) network strength is lower than ten times the rating of the generators. 7 DFIG: a comment A key issue of concern regarding wind mill performance is that of fault ride-through. Network faults produce rapid voltage dips in any of the three phases connecting to a generator unit. The standard Dual Fed Induction Generator (DFIG) system is sensitive to such severe dips, inducing large transient currents in the stator and rotor circuits and risking over-current damage to the power electronic devices in the converters. To protect these devices, the rotor circuit is typically shorted or crow-barred, resulting in a considerable demand of reactive power from the grid, exacerbating the voltage problem. The transient currents produced from the voltage dip may force disconnection from the network and risk cascade disconnection of other generation sources [3]. With a crowbar applied, the rotor converter is disconnected, and the rotor windings are short-circuited. The generator then becomes a Singly-Fed Induction Generator (SFIG) with no power electronic control. The situation is worsened by the slip, which at rated speed for the DFIG is very high for a SFIG. Very large rotor currents are produced. At non-zero voltage, a large reactive power demand is produced, which in a power system will exacerbate the voltage dip. The loss of reactive power control will worsen the voltage depression on a weak network [3]. The conclusion must be that also with DFIG, support from FACTS devices will be called upon to comply with transmission system operator regulations. 8 FACTS implementation SVC and STATCOM (SVC Light ) are both members of the FACTS family. An SVC is based on Thyristor Controlled Reactors (TCR), Thyristor Switched Capacitors (TSC), and/or

5 Harmonic Filters. Two common design types are shown in Figure 7a and 7b. Fig. 7a: TCR / FC configuration. Fig.7b: TCR / TSC configuration. Basic diagrams of one phase of a TCR and a TSC are shown in Figure 8a and 8b. A TCR consists of a fixed reactor in series with a bi-directional thyristor valve. TCR reactors are as a rule of air core type, glass fibre insulated, epoxy resin impregnated. A TSC consists of a capacitor bank in series with a bidirectional thyristor valve and a damping reactor which also serves to de-tune the circuit to avoid parallel resonance with the network. The thyristor switch acts to connect or disconnect the capacitor bank for an integral number of half-cycles of the applied voltage. The TSC is not phase controlled, which means it does not generate any harmonic distortion. Fig. 8a: Operating principle of TCR. Fig. 8b: Operating principle of TSC. A complete SVC based on TCR and TSC may be designed in a variety of ways, to satisfy a number of criteria and requirements in its operation in the grid. The fast var capabilities of SVC make it highly suitable for fulfilling the following functions: Steady-state as well as dynamic voltage stabilisation, meaning power transfer capability increases and reduced voltage variations. Synchronous stability improvements, meaning increased transient stability and improved power system damping. Dynamic balancing of un-symmetric loads, a feature most useful for protecting wind generators from damage caused by grid un-symmetry. 8.1 Thyristor valves The thyristor valves consist of single-phase assemblies (Figure 9). Each valve comprises two stacks of antiparallel connected thyristors. The thyristors are electrically fired. The energy for firing is taken from snubber circuits, also being part of the valve assembly. The order for firing the thyristors is communicated via optical light guides from the valve control unit located at ground potential. Between thyristors, heat sinks are located. The heat sinks are connected to a water piping system. The cooling media is a low conductivity mixture of water and glycol. The TCR and TSC valves each comprise a number of thyristors in series, to obtain the voltage blocking capability needed for the valves. One thyristor is redundant, allowing the SVC to maintain operation with one thyristor level shortened.

6 δ: Phase difference between the voltages X: Reactance of the coupling reactor. From equations (3) and (4) it can be seen that by choosing zero phase-shift between the bus voltage and the VSC voltage (δ = 0), the VSC will act as a purely reactive element. (In reality, a small phase shift is allowed, in order to make up for the VSC losses.) It is further seen that if U 2 U 1, the VSC will act as a generator of reactive power, i.e. it will have a capacitive character. If U 2 U 1, the VSC will act as an absorber of reactive power, i.e. it will have an inductive character. Fig. 9: TCR valve (one phase out of three). 8.2 SVC Light STATCOM, or SVC Light, makes use of a power electronic voltage source (VSC). The converter utilises semiconductors having turn-off capability. The converter can inject or consume reactive power to/from the bus where it is connected. This alternative has the benefits of a smaller footprint as the big air-cored inductors are not used. Another advantage stems from the fact that a smaller parallel capacitor bank can be used as the converter itself may contribute reactive power. 8.3 The converter valve A VSC of three-level configuration is built up as in Figure 11. One side of the VSC is connected to a capacitor bank, which acts as a DC voltage source. The converter produces a variable AC voltage at its output by connecting the positive pole, the neutral, or the negative pole of the capacitor bank directly to any of the converter outputs. a b c Fig. 11: 3-level VSC configuration. Fig. 10: VSC: a controllable voltage source. The function of a VSC is a fully controllable voltage source matching the system voltage in phase and frequency, and with an amplitude which can be continuously and rapidly controlled, so as to be used as the tool for reactive power control (Figure 10). In the system, the VSC is connected to the system bus via a small reactor. With the VSC voltage and the bus voltage denoted U 2 and U 1 respectively, it can be shown that the output of the VSC can be expressed as follows: U1U 2 P = sinδ X (3) U1U 2 Q = cosδ - U 2 1 X X (4) By use of Pulse Width Modulation (PWM), an AC voltage of nearly sinusoidal shape can be produced without any need for harmonic filtering. This contributes to the compactness of the design, as well as robustness from a harmonic interaction point of view. In the converter, there are four IGBT (Insulated Gate Bipolar Transistor) valves and two diode valves in each phase leg. The valves are built up by stacked devices with interposing coolers and an external pressure applied to each stack (Figure 12). By combining SVC and SVC Light, a cost-effective dynamic compensator can be achieved, rated for a high dynamic yield during a short time and a lower yield for steady-state operation. P: Active power of the VSC Q: Reactive power of the VSC U 1 : Bus voltage U 2 : VSC voltage

7 The grid voltage and the VSC current set the apparent power S VSC of the VSC. Note that the peak of the active power of the battery does not have to be equal to apparent power of the VSC. For instance, the apparent power of the SVC Light can be 50 MVA and the active power of the batteries can be equal to 10 MW. Fig. 12: SVC Light valve assembly. 9 Battery Energy Storage As stated earlier in this paper, power transmission and distribution systems are facing an increased amount of wind generation where the power generated varies depending on the available wind. This creates new challenges for the stable and reliable operation of the power system, requiring, in many instances, network reinforcement to maintain network reliability. Energy storage will increasingly play an important role in economically managing power networks to meet demand and maintain supply reliability during these generation and demand fluctuations. A VSC has an available DC side. For reactive power control purposes, the voltage source consists of a small DC capacitor. The time constant of this capacitor, which equals the total energy stored in the capacitor divided by the rated power, is theoretically very small, but due to requirements for load balancing and running continuously in unsymmetrical network conditions, the time constant has reached values of several tens of milliseconds, effectively providing some limited energy storage capability. If this capability has to be increased further, other energy storage devices offer better alternatives than increasing the size of the DC capacitor further. One alternative that probably will become viable in the future is to use fuel cells. However with today s technology and for the applications considered here, requiring not too long charge/discharge cycles, batteries have been found to be the best solution. The natural interface for connecting batteries is on the DC bus of the SVC Light [4]. Since SVC Light is designed for high power applications and series-connected IGBTs are used to adapt the voltage levels, the pole-to-pole voltage U dc is high. Therefore, a number of batteries must be connected in series to build up the required voltage level. Figure 13 displays a schematic layout of the dynamic energy storage device consisting of an SVC Light together with a number of series-connected batteries on the DC-side. Thus, the device can both inject reactive power, as an ordinary SVC Light, and active power due to the batteries. Fig. 13: Dynamic energy storage device consisting of SVC Light with series-connected batteries on DC-side Size of battery energy storage The size of the battery energy storage depends on the application. However, a simple reasoning is to assume that a certain active power P l is injected into the grid during the load time T l, thus discharging the battery (Figure 14). The total energy E l injected into the grid becomes equal to P l times T l. To recharge the battery, approximately the same amount of energy must be absorbed from the grid by the battery. When charging the battery with the power P c, the charging time becomes equal to T c so that E l =E c =E c T c. During a certain time the battery is in an idle state before the cycle is repeated again. Fig. 14: Battery load cycle. During the total cycle time T, the battery is loaded with power P l during time T l and charged with power P c during time T c. Remaining time T i is idle time. To support the grid during contingencies, it is enough to have the necessary amount of power available during a relatively short time. An energy storage system can then provide the necessary surplus of active power and then be recharged from the grid during normal conditions. A different situation arises for a power system with significant amount of renewable generation, where the availability of fuels such as wind, sun and waves is a new parameter to consider when controlling the system. Here, an energy storage system can give the possibility to store the energy produced by renewables when the source is abundant and demand is low, and release the power during peak periods. However, such applications require storing high amounts of power for long periods in the

8 order of several hours, which is not yet economical with today s energy storage technologies. It is definitely a coming thing, however. 10 Conclusion In this paper the voltage variations associated with an offshore wind farm being introduced in an existing power network have been discussed. The paper concludes that FACTS at the grid connection point can mitigate voltage problems. In addition to this, the problems that occur in a wind farm due to network faults and the recovery from such faults have been discussed. The risk of voltage collapse emerges from the nature of the induction generator, which consumes large amounts of reactive power when its speed deviates more than slightly from the synchronous speed. There is a time window when it is possible to catch the accelerating turbine after fault clearance, but it requires fast magnetisation of the generator. This process can be dynamically supported by an SVC, a STATCOM (SVC Light), or a combination of both, installed close to the connection point of the wind farm. It seems reasonable that considerations about the behaviour at network faults will get increasingly important as the ratings of planned wind farm installations become considerable when compared to the network strength in the connection point. By connecting an energy storage device of suitable size to the DC side of a STATCOM converter, active power stored during normal operating conditions can be injected back into the grid when necessary, to support the grid during contingencies, or, in a certain perspective, to help even out active power fluctuations from wind farms. 11 References [1] D. Milborrow: Offshore wind rises to the challenge, Windpower Monthly, April [2] F. Rudolfsen et al: Power transmission over long three core submarine AC cables, 3 rd international workshop on transmission networks for offshore wind farms, Royal Institute of Technology, Stockholm, [3] G. Pannell et al: DFIG control performance under fault conditions for offshore wind applications, CIRED, Turin, June [4] J. Svensson, P. Jones, P. Halvarsson: Improved power system stability and reliability using innovative energy storage technologies, IEE ACDC 2006 Conference, London, March 2006.