Impact of Reactive Power in Power Evacuation from Wind Turbines ()
1. Introduction
Wind is one of the most important resources found in nature’s bounty totally free of cost and without any hazardous effects. Perhaps that is why, in this hour of energy crisis,the entire human race has diverted its attention to wind energy as a suitable alternative to the conventional sources of energy we have been using for more than a century.
Having an insight into the statistics concerning installed wind power capacity,we see that the total capacity as of today is 93,849MW (2008). India stands fourth in wind power commissioning with an installed capacity of 7844.5MW (2008). But Germany with a capacity of 22,247MW (2008) shows that the distance to be covered is still huge. Of course, it instills a hope that the future will be bright with no reliance on conventional sources of energy and wind energy forming a large chunk of the installed capacity.
There are two types of utility-scale wind turbines, fixedand variable-speed. Fixed-speed wind turbines operate at a near constant rotor speed at all times and are directly connected to the power grid [1]. Fixed speed wind turbine which is used here for analysis operates within a very small range (around 5% of the nominal value) and in general they use a fixed shunt capacitor to provide reactive power compensation [2].
1.1 Islanding
A phenomenon which generally occurs in a network with Wind Generation in which a portion of the distribution network becomes Electrically Isolated from utility grid due to transmission system events & after disconnection, wind generator maintains supply to local loads. Islanding can be categorized as Intentional Island and Unintentional Island. Islanding is a condition in which local Distribution Generation systems continue to supply stable real power and reactive power to the local loads at a sustained voltage and frequency while the main Energy system is de-energized. An islanding condition creates a safety hazard and may cause damage to power generation and power supply facilities as a result of unsynchronized re-closure [3]. In general, after loss of the main source, the DG has to take charge of the remaining network and the connected loads; therefore, the loading condition of the DG is suddenly changed after islanding. Since the distribution networks generally include single-phase loads, it will be highly possible that the islanding changes the load balance of DG [4].
1.2 Intentional Island
Planned islanding is often called Intentional Islanding. This condition arises when a portion of the network is separated from the rest of the network for the sake of system reliability & maintaining the safety of the network. Utility personnel monitor the islanded system continuously & it poses no problem for the utility.
During the occurrence of a system outage, when several groups of generators can go out of-step with each other, it may be desirable to have selected islands where there is a minimal mismatch between load and generation. Switching operations carried out to split the system into such self-sufficient regions is called intentional islanding.
1.3 Unintentional Island
Sudden and unpredicted islanding is termed as unintentional islanding which mainly results due to the failure of anti-islanding techniques resulting in electrical isolation of an energized portion [5]. The energized portion will continue to supply power to the loads present in the island affecting the safety concerns for the utility personnel.
A section of the grid having balanced load and generation may become isolated from the rest of the grid by a sudden opening of a switch or a circuit breaker that causes the disconnection of the feeder from the grid. A common form of disconnection would typically arise from a momentary ground fault on the feeder, which is detected by the feeder ground relay and results in tripping of the feeder circuit breaker. Such an event is described in Figure 1. The single line to ground fault shown in the Figure1 is assumed to be transient in nature [6]. This fault could cause operation of utility breaker shown as a disconnection point suggested by the given Figure 1.
However, the transient fault may be undetectable by the DG and cause to exist after opening of the utility breaker. In this scenario, an unintentional island is formed after opening of the utility breaker. After disconnection from the grid, this island may be initially deficient in active and reactive power. The deficiency of active power is balanced by the release of kinetic energy from the rotating machinery connected to the system, and hence reduction in system frequency [6]. The deficiency of reactive power is mainly balanced by the export of reactive power from the embedded synchronous generator .An islanding event, such as described in this section, can be conveniently illustrated by a frequency and voltage versus time graph of Figure 2.
1.4 Implications of Inadvertent Islanding
Inadvertent islanding presents a number of safety, commercial, power quality, and system integrity problems [7]. In summary, the major issues are:
Figure 2. Voltage and frequency response
1) Line worker safety can be threatened by DG sources feeding a system after primary sources have been opened and tagged out.
2) Public safety can be compromised as the utility does not have the capability of de-energizing downed lines.
3) The voltage and frequency provided to other customers connected to the island are out of the utility’s control, yet the utility remains responsible to those customers.
4) Protection systems on the island are likely to be uncoordinated, due to the drastic change in short circuit current availability.
5) The islanded system may be inadequately grounded by the DG interconnection.
6) Utility breakers or circuit reclosers are likely to reconnect the island to the greater utility system when out of phase.
2. System Configuration
The parameters used for the simulation of the above model of an Induction Generator based wind turbine are as follows:
2.1 Induction Generator
A 3-phase squirrel cage induction generator with a nominal power of 843KVA, 690V (φ-φ), 50 HZ is used for the above system with the parameters shown in Table 1.
Table 1. Induction generator parameters
2.2 Three Phase Transformer
A Yg/ ∆ (D1) configuration of three phase (2-winding) transformer is used with a nominal power of 1MVA with following primary/Secondary Windings given in Table 2. Parameters used for the simulation of three phase PI section Line was given in Table 3.
2.3 Grid
A three-phase source with internal R-L impedance is used to implement a grid which is connected to the wind Generator through a T-Line & Transformer. The three phase Short-circuit Level at base voltage of 33KV is 25MVA with X/R ratio of 10.
2.4 Load
A 3-Phase resistive load of 675KW is used which is connected at the terminals of wind turbine. Simple wind farm based on fixed speed wind turbines is connected to a grid through a T-Line at Point of Common Connection (PCC).
Table 3 Parameters for PI section transmission line
Figure 3. System configuration in SIMULINK
Figure 4. Outline of grid connected wind farm
Figure 5. Wind farm connected to grid using SIMULINK
The second module of the paper deals with the Implementation of a Wind Farm & its Impact on Grid during Islanding which can be categorized as:
1) Wind Farm Configuration in SIMULINK.
2) Impact on Grid during Disconnection of a Wind Generator.
2.5 Wind Farm Configuration
A Wind Farm with three Identical Wind Turbines connected together to form a Wind farm of capacity 2.25MW is shown in the Figure 5.
In implementing a wind farm connected to a grid through a PCC needs a separate step-up Transformer at the PCC which is finally connected to the grid via Pi section T-Line. The difference between a single wind Turbine connected to a grid & a wind farm connected to grid can be seen by an additional Transformer’s & Individual short transmission lines connecting each Generator to PCC with their parameters mentioned below.
2.6 Transformer
With nominal power 10MVA a 33/66KV transformer with same parameters specified before.
2.7 T-Line
Short transmission lines connecting Individual Wind generators are of length 1 Km each with same parameters specified before.
The Islanding of a Wind turbine model is shown in Figure 6 using SIMULINK. The Impact of Disconnection of one of WG from the rest of the network is implemented here by Inserting a 3-phase breaker in series with the same WG & PCC in which switching of all the three phases from closed to open condition takes place after 1.5 Sec of operation of WG.
Figure 6. Islanding of a simulated wind turbine model
Figure 7. Network layout used for simulation [8]
The parameters for the Breaker are Transition Time: 1.5 Sec, Breaker Resistance: 0.001 Ω, Snubber Resistance: 0.000001 Ω, Snubber Capacitance: ∞.
Figure 7 is implemented in SIMULINK to Simulate Unintentional Islanding Condition.