Electrical Power Studies Important for Wind Power Plants
By David Mueller, EnerNex
Wind plants require much engineering, and electric power studies are a fundamental part of that engineering effort. These studies ensure safe and reliable operation of the power grid with the wind plant connected. Their objectives are to assess the impact of the wind plant on the transmission grid and determine the rating and sizing for the wind plant equipment. Wind power plant (WPP) studies are classified under two main categories: system interconnection and the balance of plant operation.
System Interconnection Studies
System interconnection studies evaluate the ability of the wind plant to “be a good neighbor” on the transmission grid. These characteristics have become important because in some transmission regions wind power can account for up to 50 percent of the power being delivered to the system. Wind plants must not negatively affect the system’s power quality, and they must actively provide voltage and reactive power support to the grid. Dynamic studies of transmission systems must include electrical models of the wind turbines and power equipment.
Reactive Power Requirements
The wind plant collector system provides reactive power during still (no generation) conditions, because the miles of underground cable contribute charging capacitance to the electrical situation. In locations where “net zero” reactive power capability is required, correctly sized shunt inductors are installed in the substation to cancel this charging capacitance.
During increasing levels of generation, the wind plant collector capacitance becomes less important, but the turbines’ reactive power characteristics and the transformers’ and cables’ reactive losses become dominant. As part of the transmission operator’s large generator interconnection agreement (LGIA), the plant is required to provide (or consume) reactive power to support (or lower) grid voltage. A reactive power study verifies that the plant can provide the reactor power support that is required, or it determines what additional capacitor banks, shunt reactors, or dynamic voltage restorer (DVR) and distribution static synchronous compensator (D-STATCOM) dynamic support may be required.
The LGIA also requires compliance with the IEEE 519 “Recommended Practice and Requirements for Harmonic Control in Electric Power System.” This standard recommends limits for controlling harmonic voltages and currents on the power system. The goal is to maintain a clean waveform as shown in Figure 1.
|FIGURE 1: System Voltages in Normal Condition|
At most wind power plants, the harmonics created by the wind turbine converters are small-the current distortion is generally less than 5 percent at full output voltage source-and inverters are used. At a few plants, the inverter controls interact with the grid resonance, causing problems as shown in Figure 2. These problems are generally addressed by modifying the turbine controls to implement “active harmonic damping,” or other countermeasures that will restore the converter’s stability at the grid resonant frequencies.
The IEEE 519 Standards were not originally intended for wind power plants, therefore, the limits are too restrictive and the standards’ applications need some engineering consideration.
|FIGURE 2: System Voltages in Resonance Condition|
Voltage Fluctuation (Flicker) Evaluation
Electric utilities measure voltage flicker to evaluate voltage variations’ effect on light flicker. They are measured on two-hour (Plt) and 10-minute (Pst) evaluations. In practice, most utilities use the short term 10-minute timescale as their “go to” evaluation. It measures values of Pst < 1.0 pu over all the 10 minute intervals.
Turbines installed at large scale wind plants operate diversely in terms of when they cut-in (begin operation) and generally do not create flicker concerns on large transmission systems. Figure 3 is an example of flicker levels that are below the level of concern. Flicker can be an issue for individual turbines connected to a lower voltage distribution system.
|FIGURE 3: Wind Plant Voltage Flicker Measurement (Limit is Pst=1)|
Subsynchronous Resonance Screening
Subsynchronous resonance (SSR) occurs when a series of capacitor compensated transmission lines create a grid resonance that interacts with generator equipment. This potential problem of series compensated lines was initially postulated in a 1937 paper by J.W. Butler, and C. Concordia titled “Analysis of Series Capacitor Application Problems.” The potential was not realized, however, until the 1970s when two large generator shafts failed due to torsional interaction. Then in 2009, ERCOT reported a similar event involving a wind plant.
In the case of the wind plant, system contingencies (lines out) required the wind plant to be radially fed from a series compensated transmission line. The subharmonic (less than 60 Hz) resonances interacted with the wind turbine controls. This phenomenon has been described as subsynchronous control instability (SSCI).
Screening studies evaluate a wind plant’s potential to go into SSCI. The evaluation involves a contingency evaluation of the transmission system or the control loop stability of the wind turbine equipment or a combination of both. Detailed knowledge of the transmission system and the wind turbine control loops are essential for these studies. Figure 4 shows the results of a study in which a line outage caused the wind plant to be radially fed from a series compensated transmission line and when the controls went into unstable operation.
|FIGURE 4: Wind Plant Output During Subsynchronous Controls Instability (SSCI)|
Power System Model Validation
Power grid transmission operators request wind plant power flow and dynamic response computer models for their studies. They perform dynamic simulations to evaluate system response to faults, lines tripping, generator tripping and other power system transient events to evaluate what the system needs to remain stable and avoid wide-scale blackouts. These studies are usually done using either the Siemens PSS®E (generally eastern U.S. utilities) or GE PSLF (generally western U.S. utilities) software.
Large renewable generating facilities-especially wind and solar-have been full-fledged citizens of the bulk electric system (BES) in the U.S. for almost two decades. The process toward achieving this status has been steady but full of challenges. The initial hurdles of characterizing and then capturing the steady-state and dynamic behavior of renewable plants and then putting this information in computer models that are used to analyze the BES have mostly been overcome. While work in this area will be ongoing-as is the case for all BES equipment-the industry is at a point where renewable plant technology should no longer be a mystery to transmission system planners and engineers.
|Keenan II Wind Plant near Woodward, Oklahoma|
The emphasis now will shift to validating models for bulk renewable plants. The approval and forthcoming implementation of NERC Standard MOD-033-1: Steady-State and Dynamic System Model Validation will up the ante on this topic. Set to be implemented on July 1, 2017, the standard aims to “establish consistent validation requirements to facilitate the collection of accurate data and building of planning models to analyze the reliability of the interconnected transmission system.” NERC planning coordinators are required to implement a validation process for which reliability coordinators and transmission operators must provide data on system behavior. In effect, the critical need to compare dynamic model performance to reality will be formalized via standards.
So, what does this mean? Industry experts have recognized for some time that all models must be validated, and this has been done for most bulk system equipment since the early days of computer tools and planning and analysis methods. The standard mandates that planning coordinators create and administer the processes. The language is stronger and clearer than in the previous standards, recognizing that computer models could not adequately explain the realities of major system failures that occurred during some previous system events, such as the August 2003 blackout.
The processes’ shape, form and details as implemented by the planning coordinators are still to be determined. Despite these uncertainties, a couple of things are clear about bulk renewable plants. The first is that monitoring at the point of interconnection (POI) to the BES will become a de-facto requirement. Measurement data that documents the response of the plant to an event or disturbance on the transmission network is essential for model validation, and continuous monitoring is the most effective way to acquire such data.
Secondly, the scope of this effort is huge. There are probably 500 or more individual renewable plants connected to the BES in the U.S. Through the processes mandated by MOD-033-1, each of these plants requires model validation. While many plants may be of similar size and employ the same technology (i.e. wind turbine or power converter), each plant is unique to some degree and the MOD-033-1 standard’s language (as well as the other standards in the NERC MOD family) views plants as individual entities.
|FIGURE 5: Arc Flash Warning Label for a Wind Turbine Electrical Enclosure|
Balance of Plant Studies
Balance of plant studies include system loss evaluation, power flow, voltage and reactive power evaluations. Short circuit and arc flash evaluations study internal plant faults. Temporary and overvoltage evaluations consider ratings for arresters and grounding transformers.
System Loss Evaluation, Powerflow, Voltage and Reactive Power Evaluation
A total accounting of plant auxiliary loads and I2R (power losses) due to current flow is an important economic evaluation for the plant project. Losses may factor into decisions on conductor and transformer sizing. The steady state power flow analysis is performed to determine if the wind plant can be operated to meet the voltage and power factor requirements specified by the interconnect agreement, which is usually to design within voltage limits of 95 percent to 105 percent of the nominal value 0 and power factor limits of +/-95 percent at the POI of the wind plant to the transmission system. If the power flow study reveals that wind turbine compensation packages do not meet the voltage and power factor requirements, then reactive compensation equipment must be specified and installed to meet the stated interconnect requirements.
Short Circuit and Arc Flash Evaluations
The main purpose of short circuit analysis is to determine the available short circuit levels at the buses, the cables within the wind plant collection system and the interconnect switchyard. The short circuit study also is used to specify the equipment ratings such as ampacity withstand, protective relay settings and concentric neutral cable size. Arc flash analysis blends the short circuit analysis with the result of a protection time current curve to determine the incident energy against which an electrical worker must be protected. This study provides information for warning labels (see example in Figure 5) that inform electrical workers of their personal protective equipment (PPE) requirements.
|FIGURE 6: Simulated Temporary Overvoltage on a Feeder|
Transient and Temporary Overvoltage Studies
Transient and temporary overvoltage studies evaluate transient overvoltages caused from energizing capacitor banks, switching operations, fault initiation and clearing operations. The size of grounding transformers or the high speed grounding switches or both, which are used to reduce the temporary overvoltages, are evaluated. A transient simulation package, such as EMTP-RV or PSCAD-EMTDC (two widely used simulation tools), is used for these evaluations. Figure 6 shows an example output of a temporary overvoltage event on a feeder. The insulation coordination study verifies the ratings and locations of the surge arresters to ensure proper insulation coordination. The protective margins for the equipment also are evaluated to ensure safe protective levels against overvoltages during lightning strikes.
Electrical power studies ensure a wind plant’s efficiency and reliability. Interconnection studies and models allow the plant to effectively interconnect with the power grid, maintain power quality and support system voltage requirements. As wind plants have become a significant component of the modern electrical grid, these interconnection requirements have become more significant.
David R. Mueller is a professional engineer and director of energy systems studies at EnerNex. His group has extensive experience in performing power system studies for utility, commercial and industrial clients. They have performed electrical power studies for some of the largest wind plants in U.S., and have worked with many different wind plant developers. Recent projects include offshore projects in the U.S. and Europe. Mr. Mueller is an expert in power quality and a troubleshooter of power problems.