Electric power generation projects in the coverage area of the Electric Reliability Council of Texas (ERCOT) must comply with specific interconnection study requirements. Proposed wind power projects in the ERCOT area connecting to transmission service providers’ electric grid systems must be evaluated for the possibility of subsynchronous resonance (SSR), a grid event that occurs due to the interaction of series-capacitor-compensated transmission lines and wind turbine generators (WTGs).
The phenomenon manifests itself as grid voltage and current oscillations below the fundamental 60 Hz frequency of the North American grid. Simply put, as a wind power plant (WPP) becomes connected electrically closer to series-compensated transmission lines, the possibility for SSR-related issues becomes greater. Wind turbines exposed to SSR may be damaged if they are susceptible to two types of interactions with this resonance, namely subsynchronous control interactions (SSCIs) and subsynchronous torsional interactions (SSTIs).
In October 2009, an SSR event in the Zorillo area of southeast Texas created significant damage to two WPPs with Type 3 WTGs that became radially connected to a series-compensated transmission line after clearing of a grid system fault. The event only lasted for 1.5 seconds before the series-connected caps were bypassed and initiated the event’s conclusion. WTG crowbar (generator/converter over-voltage protection) circuits were compromised as the WPP equipment saw system voltages at about two times above normal. Because electrical equipment is designed for only 110% normal voltage levels for continuous operation and protected for up to about 25% higher than nominal voltages by time-dependent relays, this event quickly harmed the WTGs.
Prior to this event, conventional wisdom held that wind turbine technology was immune to SSR issues based on historical operation. Because the wind plants near Zorillo were the first to experience damage from SSCI and the fact that there are already more than a dozen series-compensated lines scheduled for service by the end of 2013, ERCOT requires all newly proposed projects to provide studies of potential SSR-related interactions.
The application of series compensation is associated with long-distance transmission lines with series-connected capacitor banks that must transmit large amounts of electrical power from remote generating sites to large cities. As mentioned, there are several lines in the ERCOT area that already have series compensation. It is expected that as the Texas Competitive Renewable Energy Zone program becomes more active, there will be more such lines built.
ERCOT is by no means the only entity with SSR concerns, as series compensation is used in other existing grid systems. When transmission providers seek to increase the power-carrying potential of specific lines, this appears as an attractive solution. Mitigation techniques are either grid based or WPP/WTG based.
All wind turbines are susceptible to SSR. However, a Type 3 WTG (double-fed asynchronous generator) has shown to be the most likely to pose issues from SSCI/SSTI. It is also recognized that Type 1 (simple induction generators with squirrel-cage rotors), Type 2 (wound-rotor induction generators with controlled rotor resistors) and even Type 4 (full-scale converter connected to stator output terminals) WTG designs can have the potential for issues related to an SSR event. Therefore, developers must evaluate WTGs to confirm either no issues are evident or any identified issue can be mitigated to ERCOT’s satisfaction.
SSCI is a low-frequency voltage/current resonance interaction between the wind turbine’s internal power electronic controllers and a series-compensated system. The SSCI resonant frequency (see Figure 1) varies depending on the grid system impedances. As discussed earlier, “electrically closer” is when the grid’s meshed network can become aligned so as to place the wind project radially, or nearly radial (one other line in parallel), connected with the series-compensated line.
Under that grid alignment, if the apparent impedance of the grid (as seen by the WPP/WTG looking out into the grid) matches the characteristic impedance of the WPP/WTG with its internal control systems as implemented (seen from the grid into the WPP/WTG), then there is the possibility of resonance between the WTG and the grid. If, at the same time, the WTG’s apparent resistance is negative over a credible sweep of frequencies (indicating negative damping) – see Figure 2 – then oscillations of voltage and current become sustained and grow in magnitude very quickly, quite possibly to damaging levels as experienced at Zorillo.
This condition should lead to the WTG protection tripping itself offline, but it is desirable for overall system stability – especially in weak grid conditions – that the WTG continue to operate during and after the event.
Various levels of capacitor compensation provide different grid impedances and, thus, resonant points. All of the compensated lines in the ERCOT area are 50%. However, this 50% can be added as lower percentage steps and could total a much higher percentage, shifting the resonant point. Also, every variation of circuit connection lineups during normal and abnormal operations will change this value as seen by the WTG.
If the frequency scans indicate suspicion of SSCI, then further detailed analyses using electromagnetic transient-based software, such as PSCAD, will provide a full understanding of the nature of the issue for the WPP and whether additional mitigation is necessary.
Since SSCI is completely an electrical controls issue, mitigation usually entails proper tuning of the WPP/WTG as a system.
SSTI is a long-recognized mechanical interaction of power electronic controllers with the mass dynamics of the WTG’s drivetrain. In the case of SSTI, since this is more of an issue conditional on the drivetrain’s characteristics, electrical torque damping analyses provide data for identification of potential negative damping (see Figure 3) – and ultimate mitigation, if necessary.
To prepare to perform a project-specific SSR analysis, either the WTG manufacturer or project developer must initiate a request to ERCOT specifying the “what, where and when” intentions of the project.
ERCOT will perform load-flow simulations using PSS/E software on credible grid disturbance cases, determining circuit lineups of interest, and then convert these to create the PSCAD model(s) for transient study.
The transient models will include the point of interconnection (POI) of the project to the grid, a detailed model of the ERCOT grid from the POI to the series-capacitor banks and the other data essential to do a meaningful study while the rest of ERCOT’s system is simplified to equivalent parameters. The WPP generation can be considered as a single representative, aggregated WTG. Sensitivity studies, focused on WPP design, indicate that the WPP’s balance of plant could be considered as negligible and yet still obtain meaningful results. ERCOT will then send the load-flow cases, the transient model, a simplified one-line diagram and a list of the contingencies it wants to be tested. The SSR study is then performed, with the developer providing the results to ERCOT for review and approval.
Wind turbines in Texas already use Type 1 through 4 WTGs. Vestas has analyzed its WTG designs and found that other than the Type 3 offerings, its designs are negligibly susceptible to SSCI issues. In the case of Type 2, it can easily modify the rotor current control software to make it immune.
However, Type 3 WTGs have the potential for SSCI due to the nature of their control design. SSCI is primarily caused by the fast actions of the rotor-side current controller, although other internal converter control loops may also play a role. Type 3 WTG designs from any manufacturer are susceptible to SSCI issues.
Basically, Type 3 WTGs can create “negative resistance” operating points. This means that instead of providing positive damping that would dissipate the energy driving the voltage and current oscillations, negative damping allows the oscillations to be sustained and increase in magnitude.
Vestas has reviewed its Type 3 WTGs and determined that most issues can be mitigated at the WTG with changes to standard default control parameters. If the required transient model studies identify an issue beyond the simple control parameter changes described, then a more detailed customized solution has already been developed by research and development and can be readily provided.
Since Type 1, 2 and 3 turbines have their generator stators directly connected to the grid, potential for SSTI must be considered. Type 4 WTGs have the generator drivetrain electrically isolated from the grid conditions via the full-scale converter and, thus, appear immune to this issue.
All proposed wind power projects, regardless of WTG type used, must be assessed for SSR issues to the satisfaction of ERCOT. The Type 3 WTG has design characteristics that make it more likely to experience two particular SSR issues, namely SSCI and SSTI. A problem identified in transient model-based studies can be corrected by WTG controls to ensure acceptable turbine operation during SSR events. w
Industry At Large: Interconnection
Understanding And Assessing Subsynchronous Resonance
By Steven W. Saylors
Subsynchronous resonance, which occurs due to the interaction between series capacitors and nearby wind turbine generators, has caused significant damage to wind power plants.
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