Scheme of Integration of Wind Power to Electric Power Grid

Scheme of Integration of Wind Power to Electric Power Grid

Hiren C Pathak

P.hd scholar of NIMS University

Abstract

Integration of large wind farms into bulk power systems presents multiple challenges to system operation and security. Wind generators may have to be disconnected from the grid once the system has a disturbance. The presence of wind farms in such weak systems raises serious concerns about system stability, voltage regulation, and post-fault power swings. WFMS and LVRT technology for wind turbine generators can provide much improved system performance compared to more traditional wind generation equipment. This chapter presents dynamic performance of GE wind turbine-generators with LVRT and WFMS technology. The information presented includes dynamic simulation results from existing power systems with large wind farm interconnections, and actual field measurements from operating systems. This paper also presents control design philosophy, innovative control designs and relevant control diagrams.

Key words: LVRT-Low voltage ride through; WFMS- wind farm ma nagement system.

1. Introduction

   At present a vast majority of the generating power plants are thermal, hydro or nuclear power stations with large synchronous generators. These plants have a very controllable generation capability of both the active and reactive powers within their capability limits. Moreover, the power system network has evolved around these machines; hence they go toget her very well. The characteristics and capabilities of wind WPPs are very different from the conventional power plants. Their operational behavior, dynamics, controllability and capability are dependent upon the type of wind turbine generators used, farm control architecture as well as instantaneous wind availability. In the past, wind power penetration in the power grid net work was relatively small and grid operators treat ed them as negative load, rather than a power generation source. They were not expected to provide grid support. The

   conventional power houses were required to provide controlling power to make up for the lost wind power generation and sup port grid recovery. With increasing wind penetration, grid operators are now imposing grid code requirem ents to specify the steady and dynamic requirements that wind farms must comply with for getting connected to the grid. Wind farms need to participat e in the frequency and voltage regulation by continuously controlling their active and reactive power outputs. They are expected to exhibit low voltage fault ride through capability and support the grid recovery[1]. E.ON Netz Grid Code states that every generating plant with a rated c apacity of over 100MW must be capable of supplying the control power[ 2].

   These requirements are being addressed by the latest generation of wind turbine-generator (W TG) equipment. WFMS and LVRT technology for wind turbine generators can provide m uch improved system perform ance c ompared to m ore traditional wind generation equipment. LVRT technology is now able to eliminate m ost concerns about tripping during system voltage events and allows for the rapid and well -behaved recovery of the wind farm and the grid when system faults are removed. Wind farms cont rolled by WFMS can provide extrem ely fast initial response to system events and wind induc ed perturbations, and voltage and reactive power response similar to that of conventional synchronous generation.

2. Theory

   The distinction is that TWGs equipped with a solid-state AC excitation system. The A C excitation is supplied through an ac-dc -ac converter. The fundament al frequency electrical dynamic performance of the WTG is completely dominated by the field converter. In practice, the electrical behaviour of the generator and converter is that of a current-regulat ed voltage source invert er[6]. The converter will mak e the WTG behave like a voltage behind a reactance that results in the desired active and reactive

   current being delivered t o the device terminals. Conventional aspects of generator performance related to internal angle, excitation voltage, and synchronism are largely irrelevant. These characteristics have significant implications from the standpoint of power-swing performance and modal interactions[5]. This model was developed specifically for the GE 1. 5 and 3.6 MW WTGs. This model is not designed for, or intended to be used as, a general purpose WTG. There are substantial variations between m odels and manufacturers.

   The overall W TG model consists of four major elements, as shown in Figure. Generat or/Network Interface, Electrical Cont rol,Wind Turbine, and Wind Power Model.

3. Modeling of WTG

   Figure 1. Gener ator ne twork inter face

   The wind turbine model provides a simplified representation of a very complex electro – mechanic al system. The turbine cont rol is designed to deliver power over a range of wind conditions, taking advantage of the variable speed capability of the m achine. The controller enforces the power speed relationship s hown in Figure 2. Above about 75% rated power, the power levels of primary interest for stability studies, the controller works in two distinct regions. When the available wind power is above the equipment rating, the blades are pitched to reduce the mec hanical power (Pmech) delivered to the shaft down to the equipm ent rating (1. 0 p.u.), thereby returning the m achine to the reference speed for full power operation, 120% of synchronous speed. When the available wind power is less than rated, the blades are fixed to maximize the m echanical power, and speed

   control is accomplished by adjusting the generator electrical power. The dynamics of the pitch control are m oderately fast, and can have significant impact on dy namic simulation res ults. The block diagram for the model is shown in Figure 2.

   Figure 2. Power and Speed control

   Figure 3.Wind Power Model

   The wind turbine model repres ents all of the relevant controls and mechanical dynamics of the wind turbine. The model accepts the machine terminal active power from the WTG Electrical Cont rol Model and the mechanical power calculated by the Wind Power Model. The turbine control model sends a power order to the electrical control for the converter to deliver the requested power to the grid. The electric power actually delivered t o the grid is returned to the turbine model, for use in t he calculation of rotor speed. The s peed cont roller does not differentiat e between shaft acceleration due to increase in wind speed or due to system faults. In either case,

   the response is appropriate and relatively slow compared to the electrical control. The turbine control acts so as to smoot h out electrical power fluctuations due variations in shaft power. By allowing the machine speed to vary around its rated value (120%), the inertia of the machine functions as a buffer t o mechanical power variations. The function of the wind power module is to compute the wind turbine mec hanical power (shaft power) from the energy contained in the wind. [8].

4. Simulation results

   Figure 4. shows the simulated response of a wind farm of 108 GE 1.5 MW wind turbine generators (W TGs) to ten minutes of highly variable wind near rated wind speed. The red traces show the system response with WFMS, and the black traces show the system response with conventional fix ed power factor control. The fixed power factor control is local to each individual WTG. The utility bus ( the point of interconnection), the system voltage with conventional power factor (pf) control exhibits unaccept ably high variation.

   The fixed power factor control is local to each individual WTG. The utility bus (the point of interconnection), the system voltage with conventional power factor (pf) control exhibits unaccept ably high variation. By comparison, the WFMS controlled system voltage exhibits very small variations. The voltage flicker index, Pst, is less than 0.02 for this high stress condition well within industry requirements.

   The results dem onstrate t he capability of LVRT to handle unbalanc ed faults and a high level of fidelity in the digital simulations and models. Voltage and reactive power control perform ed by WFMS minimizes voltage flicker, improves system stability, provides voltage regulation, reduces the risk of volt age collaps e, and minimizes the impact of system disruptions. WFMS provides tight closed loop control of utility system voltages. The impact of active power fluctuations from wind variation on the grid voltages are minimized and the fast and precise voltage control effectively strengthens the grid, improving the overall power systems resilience to large disruptions.

5. Conclusion

   Voltage and reactive power control perform ed by WFMS minimizes voltage flicker, improves system stability, provides voltage regulation, reduces the

   risk of volt age collaps e, and minimizes the impact of system disruptions.

   Figure 4. Effects of various variables

   WFMS provides tight closed loop control of utility system voltages. This provides two major benefits: First, the impact of active power fluctuations from wind variation on the grid voltages are minimized; second, the fast and precise voltage control effectively strengthens the grid, improving the overall power systems resilience to large disruptions.

   LVRT feature renders this over-sensitive response obsolete by improving generator and control system design. Before LVRT t echnology, wind t urbines would trip off-line on any voltage sag below 70%. Now they are designed to ride through severe grid disturbances. LVRT technology enables wind farms to continue operation during and after severe faults or voltage depressions on the power grid. Power -electronic conversion and control technology incorporated into the generating system enables variable speed operation, while eliminating electrom echanic al power swing interaction with the grid. The com bination of thes e features enable wind power plants to achieve stability performance that can exceed that of conventional synchronous generations of the same rating, installed at the same loc ations.

6. Referances

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