Fault Current Contributions from Wind Plants

by IEEE PES PSRC Joint Working Group

The  assignment of the working group was to characterize and quantify short circuit current contributions to faults from wind plants for the purposes of protective relaying and equipment rating, and to develop modeling and calculation guidelines for the same.
The safe, reliable operation of electrical power systems requires the ability to predict and model the sources of fault current, including contributions from wind powered generating plants, in order to select equipment properly rated for the required duty and to properly set protective relays for selective operation. Groups of wind turbine generators are clustered and networked to form wind plants of varying power delivery capability. Several characteristics are unique to wind plants, but the most significant characteristic to the topic of this report is the response of the wind turbine generators to faults on the power system. Wind turbine generators (WTG) must be able to tolerate rapid fluctuations in wind speed (turbulence and gusting), as well as optimize the blade tip speed ratio and thus maximize energy capture. The turbines must be capable of operating at variable speeds, rather than the traditional solid mechanically and electrically connected synchronous generator.

There are wind turbine generators of five basic types that can, in some cases with supplemental equipment, tolerate the fluctuations in the wind speed and deliver electrical power in the form that meets the requirements of the transmission system.

The report describes general consideration for wind plant design in section 2.  Both theoretical and experimental performance during faults of these five types of wind turbine generators as individual generators and as networked wind plants are described in section 3.  Section 4 covers issues for the specification of fault interrupting devices and guidelines for designing protective relay systems for wind plants are presented in section 5.  Data necessary for appropriate modeling of individual generators and wind plants is described in section 6.  Several actual performance examples are described in section 7.

Wind Power Plant Design
In today’s competitive wind energy environment, the designs of wind power plants (WPP) and interconnecting substations are not created equal.  As a result, each WPP, commonly referred to by the wind community as a collector system, is a complex engineered system.

Some of the factors that affect wind plant electrical design for interconnecting substation(s)/switchyard(s) and the associated collector system are: interconnection lines and facilities, environment, available equipment logistics and lead times, grounding and arc flash, reactive compensation, power quality, harmonics, surge protection, protection and control devices, communications, transformers, and fault ride through requirements, to mention a few. All of these factors greatly affect the resulting design of a WPP which will typically be developed through a fast paced process, while considering electric power system design standards such as NEC, NESC, IEEE, ANSI, NERC, IEC, etc.

Wind plants are typically interconnected to existing infrastructure via radial lines, three terminal taps, switchyards, and substations. The interconnection equipment decisions are justified depending on the existing voltage levels, electrical losses, rights of way and easements, environmental concerns, economics related to conductors and land, as well as dependability and security.
Typical configurations of switchyards for WPPs include single bus, sectionalized bus, double breaker bus, ring bus, and breaker-and-a-half bus as shown in Figure 1.
This section of the report covers the interconnection of utility and wind power plants, collector systems and the related studies.

Wind Turbine Generator Response to Faults
Wind Turbine Generators may be classified in five types. Type I is a squirrel cage induction generator. Type II is a wound rotor induction generator with an adjustable resistor in the rotor circuit.  Type III is a variable speed asynchronous wound rotor generator which has a three phase AC field applied to the rotor from an AC to DC to ac converter, the power source for which is the generated voltage. Type IV is an AC generator in which the stator windings are connected to the power system through AC to DC to AC converter.  Type V generators use a variable speed / variable ratio hydraulic fluid coupling transmission to use the variable speed input from the turbine blades to drive a traditional synchronous generator at fixed synchronous frequency.
This section in the report covers in detail the response to faults of all wind turbine generators listed above.

Some Fault Interrupting Equipment Issues
Power circuit breakers are used to interrupt power system faults. As such they must be rated with sufficient interrupting capacity to safely interrupt the added fault duty resulting from the interconnection of wind plants to the electrical system. Additionally, in order to bring the added power output from the wind plants, new transmission lines may be added to the network. These transmission lines are usually EHV lines, which have relatively low impedance.  The overall result is that the fault duty on the system may increase.

As such studies need to be done to ensure that the apparatus is capable of interrupting the additional fault current.  Modern analytical programs usually provide the capability to perform fault duty analysis to determine whether the apparatus in question have the interrupting capacity required.
The impact on AC fault levels and the transient offset to fault current are described.

Wind Plant Protective Relaying
This section discusses the typical aspects of system design and in particular protective relay systems that require accurate knowledge of the fault contribution from wind powered generating plants.

As described in section 2, wind plants are electric power generation sources that consist of a varying number of wind turbine generators networked together by a collector system, typically 34.5 kV (North America) and 33 kV (Europe) feeder lines into a collector substation. Each of the collector lines have on the order of a dozen wind turbine generators connected to it.  At the collector substation the collector feeders are bussed together and connected to one or more step-up transformers which boost the voltage to the level of the transmission providers’ system voltage. Interconnection to the transmission system may be made at the collector substation or there may be an intervening length of transmission line between the collector substation and the interconnection switching station.
This section covers the protection practices and considerations of the collector system, transmission or distribution provider.

Data Requirements
Knowledge of contributions of fault current from a wind plant to a transmission system is required to assess the impact of the wind plant interconnection on the short circuit protection of the transmission system.  Recognizing that it may be difficult to model the wind plant as a traditional constant voltage source behind Thevenin equivalent impedance, a key set of data identifying the contributions from the wind plant to the faulted transmission system will need to be provided.  The data will consist of a set of currents supplied from the wind plant under various specified conditions. All currents are typically provided as fundamental frequency phasor currents with respect to pre-fault

A phase voltage in the indicated time frames. Phasors for all three phase currents and all three phase to neutral voltages at the high voltage side of the collector substation step-up transformer and at the point of interconnection are required. This section discusses in detail the data requirements for system interconnection and studies, and for the collector system design.

Actual Examples
Voltages and currents recorded by transmission line relays applied to the transmission tie lines of four specific wind power plants were collected and analyzed to better understand how different wind plants perform during transmission system faults.

The full report can be downloaded from the IEEE PES Power System Relaying Committee web site at:
http://www.pes-psrc.org/Reports/Apublications_new_format.htm

Let?s start with organization in protection testing