ADVANCED CONTROLS FOR MITIGATION OF FLICKER USING DOUBLY-FED ASYNCHRONOUS WIND TURBINE-GENERATORS

Transcription

1 ADVANCED CONTROLS FOR MITIGATION OF FLICKER USING DOUBLY-FED ASYNCHRONOUS WIND TURBINE-GENERATORS R. A. Walling, K. Clark, N. W. Miller, J. J. Sanchez-Gasca GE Energy USA ABSTRACT Interconnection of wind generation with power distribution systems may result in rapid voltage variations, or flicker, much more readily than for HV transmission interconnections. Wind turbine generators employing doublyfed asynchronous generators provide a means to rapidly control reactive power, as well as to smooth variations in real power. This capability is exploited to minimize flicker, despite rapid variations of wind velocity. The principles of distribution voltage control using WTGs with doubly-fed technology are explained, and practical examples of performance are illustrated. Comparisons between and pitch-regulated induction generators are provided. INTRODUCTION In recent years, wind generation has expanded greatly, driven by decreasing costs for this form of generation and increasing demand for non-polluting renewable energy. Whereas once this form of generation was of limited importance and impact, the significant penetrations of wind generation in many places today have focused increased attention on its grid impacts. An inherent characteristic of wind generation is that the real power output is a function of the wind speed, and is thus prone to variations. The variations of the wind turbine generator (WTG) real and reactive power interact with the system and, depending on the strength of the network, cause voltage variations leading to objectionable lamp flicker. There are three significant sources of flicker in wind generation applications: 1) random power variations due to changes in the wind speed, particularly gusts, 2) cyclic pulsations of power due to the wind speed versus elevation gradient and the wind shadow effect of the tower, and 3) steps of real and reactive power when the WTG is cut in and out, or reactive compensation (capacitor banks) are switched. The flicker impact of wind generation has been recognized in the grid codes applied to wind generation. A standard [1] has been developed to specify performance of WTGs with respect to flicker. Flicker constraints can limit the amount of wind generation that can be interconnected at a particular point in the grid [2], require grid upgrades, or require application of expensive mitigation equipment such as static var compensators. Wind Integration with Distribution Systems Wind generation is presently integrated into power grids in one of two different ways: as wind farms, consisting of many individual wind turbine generators (WTGs) connected to a dedicated collection system and interconnected to the high voltage (HV) transmission grid through a dedicated substation, or as individual WTGs interconnected to the local medium voltage (MV) distribution grid. Wind farm interconnections to the transmission grid usually have a relatively stiff short-circuit strength, relative to the combined WTG power ratings, and are isolated from end-use customers by several levels of voltage transformation. Also, due to the geospatial diversity of the many individual WTGs in a wind farm, the coincident power output is less variable than that of any individual turbine [3]. Distribution interconnections, however, are challenged to a greater degree by flicker considerations than are wind farm interconnections to transmission systems. Distribution interconnections inherently result in closer coupling between the WTGs and customers that are sensitive to flicker. Due to reduced diversity, there is greater relative output variability. Small distribution-connected wind projects do not have the benefits of scale to justify extensive control systems dependent on remotely telemetered information. Often, the MV distribution system presents a weaker grid impedance relative to the aggregate WTG power rating. Finally, the ratio of reactance to resistance (X/R ratio) of MV systems is typically much less than for an HV transmission system. Unlike a high X/R HV transmission system, for which voltage magnitude is primarily affected by reactive currents, the voltage magnitude of low X/R MV systems are particularly sensitive to changes in the real component of current. WIND GENERATION TECHNOLOGIES There are several different wind generation technologies in common use; each having significantly different attributes with respect to production and mitigation of flicker. Because flicker issues are of particular importance to distributioninterconnected wind generation, the choice of generation technology is critical to the conceptual planning of a project it is not a design detail to be left until the design or procurement phase. Wind generation technologies minimizing flicker impact allow more wind generation to be interconnected, reduce requirements for grid upgrades or mitigation equipment installation, reduce customer complaints

2 and minimize duty requirements of utility voltage regulation equipment such as step voltage regulators and capacitor bank switchgear. The types of wind generation in use are: fixed speed technologies, based on various types of induction generators; and variable speed technologies, which include doubly-fed asynchronous generators and full-conversion interconnection of a synchronous or induction generator through an electronic power converter. There are variations of the induction generator approach, such as one where the rotor resistance is controlled via power electronics to allow a small degree of speed variation. The most commonly used technologies are the induction generator and the doubly-fed asynchronous generator. Presently, there are only a few types of WTGs using full-conversion technology, and these technologies are not considered further in this paper. Generators An induction generator consumes reactive power according to a fixed relationship to the generated real power. Figure 1 provides a typical reactive versus real power characteristic for an induction generator used in a wind power application. Typically, an induction generator will have a capacitor bank that compensates for the no-load reactive power demand of the WTG. In some cases, additional capacitors are also provided which are switched in steps to provide a semicontinuous compensation of the WTGs reactive demand to yield net performance in a narrow band near unity power factor. The switched capacitors associated with an induction WTG can be used to control the grid voltage changes caused by changes in WTG real power output. However, this voltage or power factor control is non-ideal. The compensation is switched in discrete steps, and the capacitors may need to be discharged between successive energizations. Cycle duty on the capacitor switches can be very high. Thiringer, et al, [2] reports that switched capacitors with induction WTGs are unsuitable for dynamic compensation of flicker and Smith, et al, [3] describes how flicker can actually be increased by switched capacitor banks. Reactive Power (p.u.) Step Compensation No-Load Compensation Uncompensated Real Power (p.u.) Figure 1 Typical induction WTG reactive power vs. real power characteristic, uncompensated, with no-load compensation, and step compensation. Fixed-speed induction WTGs are particularly vulnerable to the cyclic power variations caused by the tower shadow and wind gradient effects. Gerdes [4] reports that these effects can cause up to 2% modulation of the power output, at a frequency equal to the number of turbine blades times the turbine rotational speed. This effect can be the dominant cause of flicker from single-speed (induction generator) WTGs [5]. Some induction generator WTG applications use SVCs or STATCOMs to mitigate flicker. In distribution wind applications, this approach can be cost-prohibitive. Doubly-Fed Asynchronous Generators WTGs employing doubly-fed asynchronous generators, in addition to providing a widely variable speed, have the inherent capability of providing a smoothly and rapidly controllable reactive power output or absorption over a substantial range. This provides the capability to tightly regulate voltage or power factor without the need for additional reactive compensation equipment. A doubly-fed asynchronous generator () uses ac excitation to allow variable speed. An ac-dc-ac electronic power converter is used to apply a three-phase ac current to the rotor windings, as illustrated in Figure 2. The frequency of this excitation is such that it creates an apparent rotation of the rotor s magnetic field such that the summation of the rotor s mechanical rotation and the rotor electrical phase rotation is equal to the synchronous speed. If it is desired for the rotor mechanical speed to be less than the synchronous speed, the phase rotation of the applied excitation is positive sequence. For faster than synchronous rotation of the rotor, a negative sequence excitation is applied such that the mechanical speed minus the rotation speed of the rotor field, with respect to the rotor, is equal to the synchronous speed. VSI Converter + - Wound-Rotor Machine Figure 2 Doubly-fed asynchronous generator This type of generator is often called a doubly-fed induction generator. However, in the opinions of the authors, this is a poor description. Unlike an induction machine, in which rotor currents are induced by the stator, a is separately excited similar to a synchronous generator, with the exception that the excitation is ac instead of dc. Physically, the electrical machine, exclusive of the power converter replacing the rotor resistor, is the same as a wound-rotor induction machine. Conceptually, the machine could even be considered to be a variable speed synchronous machine. If a rotor excitation of constant frequency were applied, the

3 machine would lock in to the system like a synchronous generator, only rotating at a fixed speed which can differ from synchronous. With a zero-frequency, or dc, excitation, the machine would actually be a synchronous generator. Operationally, a responds most similarly to a power converter. The power converter providing excitation to the rotor is typically a voltage-source inverter having high bandwidth controllability of output current phase angle, frequency and magnitude. Because the rotor steel is of laminated construction, this machine s time constants are much shorter than a synchronous generator s. Thus, using feedback control of the measured stator output, the excitation of the can be controlled to provide fast and independent real and reactive power outputs of the system. Independent control of the real and reactive power output allows a machine to mitigate flicker in two different ways. Firstly, the impact of mechanical power input variations, due to the tower shadow, wind gradient, and wind gusts on power output is moderated by the ability to control real power output. When a wind gust occurs, the power output does not rise abruptly as would occur on a fixed-speed WTG. Some of the input wind energy is stored in the rotational inertia of the wind turbine. While mechanical power input and electrical power output must balance over the longer term, the ability to allow the WTG speed to vary provides a cushion for gust response, and gives time for the WTG blade pitch control to respond to reduce mechanical power input where appropriate. The cyclic power variations due to the tower shadow and gradient effects are greatly eliminated, as well [6] by the partial decoupling of the input mechanical and output electrical power of the WTG. Secondly, flicker can be greatly mitigated by the highbandwidth controllability of the WTG s reactive power output. This can be used to control the voltage at the WTG terminals, the MV side of the WTGs step-up transformer, or to maintain a desired power factor. The speed of response is comparable to that of an SVC or STATCOM. STEADY-STATE VOLTAGE ANALYSIS Steady-state voltage analysis is useful for defining how reactive power should vary with the inherent variations in real power, such that flicker is mitigated. While dynamic behaviors such as the mitigation of real power variations and the response time of reactive power are not considered in steady-state analysis, the impact of changes in WTG real power output on voltages at various locations of the distribution system can be clearly determined. As a first approximation, the voltage change caused by a change in real and reactive power is: where: P Q U R X (1) U U U = Change in voltage P = Change in real power Q = Change in reactive power R = Driving point resistance X = Driving point reactance U = Voltage Using this approximation, with the objective that voltage should not change with variation of real power ( U = ), then the approximate power factor which minimizes voltage flicker can be estimated as: pf X R 1 + R ( X ) 2 The above relationship is imperfect as it ignores phase shifts in the network. Also, the voltage invariance depends upon a uniform X/R ratio in the distribution system. This is not the practical case: the substation source will have a much higher X/R than the feeders, and often a distribution feeder will be stepped to smaller conductor sizes with increasing distance from the substation. A smaller conductor size has a lower X/R ratio than does a larger conductor. Despite these limitations, Equation 2 indicates that constant power factor regulation is, in concept, a useful means to minimize the flicker impact of WTG power output variations. The impact of power factor from a varying WTG source on practical rural distribution systems was evaluated using the circuit model described in Figure 3. This system has a relatively weak substation source and a 1 km feeder. Constant-current loads are evenly distributed over the feeder s length. The last 5 km has a smaller conductor size than the first 5 km, yielding a non-homogenous X/R ratio in the system. The short-circuit ratio for the 3 MW WTG installation is 1.5, measured at the point of common coupling (PCC) kv Nominal Z s V s = 1.25 p.u. Figure 3 Distribution feeder model..5 + j5.75% 3 MVA 3 MW WTG kvar Line Length =.5 km per section Line Impedance =.15+j.32 Ω/km; Sections 1 1 =.4 + j.42 Ω/km; Section 11-2 Load = 25 pf per location Z s =.4 + j1.4 Ω Full load voltage variations are compared for different power factors in Figure 4 as a function of location along the feeders. The voltage variation profile is minimized with a WTG output power factor of.92. Note that a unity power factor, which is often used as a WTG reactive compensation design objective, leads to particularly large voltage variation with real power change. With a driving-point X/R at the PCC (MV side of WTG stepup transformers) of 1.81, the power factor provided by (2)

4 Equation 2 is.875. This, however, is the power factor at the PCC. In addition, the approximation used to derive Equation 1 and the non-homogeneity of the X/R of this model further contribute to the difference between the power factors defined by Equation 2 and the optimum determined via detailed analysis. Full Range Voltage Change 5% 4% 3% 2% 1% % 1. pf.93 pf.92 pf.9 pf Distance (km) Figure 4-Voltage change between zero and 1 p.u. WTG output. Figure 5 compares voltage variation, as a function of WTG output, for an induction generator and a in constant power factor regulation. The typical reactive power characteristic shown in Figure 1 (without stepped compensation) is used for the induction generator, and the was regulated to a constant power factor of.92 at the WTG terminals. While the regulation capability of the can minimize flicker, it cannot eliminate flicker over the entire length of the distribution feeder. The performance of the regulated, however, is far superior than that provided by the induction generator. Voltage Change 2.5% 2.% 1.5% 1.%.5%.% WTG Power (p.u.) Figure 5 Voltage change at end of feeder, comparing WTG with constant.92 pf, and induction WTG. PREFERRED REGULATION MODE As stated previously, the can regulate power factor or voltage. Neither mode can provide flicker-free performance over the entire distribution system. The power factor mode is effectively open loop, from a control standpoint, and can be tuned to respond to WTG power output changes more quickly than a closed-loop voltage regulator, which has greater control stability limitations. Also, the power factor control does not respond to external changes, such as load changes, capacitor switching, feeder step-voltage regulator changes, etc. Thus, undesired interactions with other utility controls are minimized. In North America, IEEE Standard 1547 [7] is becoming widely adopted as a grid code for distribution interconnection of generation. This standard prohibits regulation of voltage by a non-utility generator. Thus, although voltage regulation may be preferable in certain applications, power factor regulation provides an effective and simple means to minimize WTG-caused flicker. DYNAMIC PERFORMANCE Dynamic simulations were performed to compare the induction WTG with a. The wind-velocity time series used for the simulations are shown in Figure 6. The induction WTG was represented as a pitch-regulated machine. These simulations, however, do not show the cyclic voltage variations caused by wind gradients or the tower shadow effects. Thus, the induction WTG flicker in reality may be substantially more severe than shown in these simulations. The control loops to regulate the power factor were modelled in detail, with a setpoint power factor of.92. Figure 7 shows the reactive versus real power performance of the. Performance is very close to the desired power factor setpoint. Wind Speed (m/s) Time - (Seconds) Figure 6 Typical wind velocity time series Reactive Power (MVAR) Dashed line indicates constant.92 power factor Real Power (MW) Figure 7 Reactive versus real power for

5 Figure 8 shows voltage deviation at the end of the feeder. Flicker is substantially less severe with the WTG than with the induction WTG. Using the flicker-meter algorithm specified in IEC [8], the short-term flicker values (P st ) were determined as shown in Figure 9. The flicker produced by the WTG is nearly an order of magnitude less than produced by the induction WTG. Note that the induction WTG evaluation does not include the flicker due to the cyclic tower shadow and wind gradient effects, which would tend to make the induction WTG flicker even greater. Voltage Deviation from Average.5%.% -.5% -1.% -1.5% Time (Seconds) Figure 8 Voltage deviation at end of 1 km feeder Flicker (Pst) Feeder Location Figure 9 Flicker evaluation using IEC flickermeter. CONCLUSIONS For distribution interconnection of wind generation, constant power factor operation can substantially reduce flicker due to WTG power variations. The optimum power factor setpoint must be determined from analysis of the distribution system. Doubly-fed asynchronous generation () technology provides a means to closely regulate WTG output power factor. In addition, this technology also allows use of the turbine inertia to smooth real power variations due to the tower shadow effect, wind gradients, and wind gusting. This smoothing effect greatly contributes to the favorable flicker performance of wind turbines, compared to fixedspeed designs. REFERENCES [1] IEC Standard , 21, Measurements and Assessment of Power Quality Characteristics of Grid Connected Wind Turbines [2] T. Thiringer, T. Petru, S. Lundberg, 24, Flicker Contribution From Wind Turbine Installations, IEEE Transactions on Energy Conversion, Vol. 19, No. 1, [3] J.W. Smith, D.L. Brooks, 21, Voltage Impacts of Distributed Wind Generation on Rural Distribution Feeders, IEEE Transmission and Distribution Conference and Exposition, Volume 1. [4] G. Gerdes, F. Santjer, Power quality of Wind Turbines and Their Interaction with the Grid, 1994, Proceedings European Wind Energy Conference, pp [5] A. Larsson, 22, Flicker Emission of Wind Turbines During Continuous Operation, IEEE Transactions on Energy Conversion, Vol. 17, No. 1. [6] Å. Larsson, P. Sörensen, F. Santjer, 1999, Grid Impact of Variable-Speed Wind Turbines, Proceedings European Wind Energy Conference, pp [7] IEEE Standard 1547, 23, Standard for Interconnecting Distributed Resources with Electric Power Systems [8] IEC Standard , 23, Electromagnetic Compatibility, Part 4 - Testing and Measurement Techniques, Section 15 Flickermeter Functional and Design Specifications.