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Management/Planning

There are two distinct parties that are involved with GC compliance assessment:

l A developer who selects the equipment and designs the RPP. The developer may utilise a consultant to assess and optimise the equipment in the design, and report on the offered performance against the GC framework.

l An assessor. In South Afr ica this is the renewable technical evaluation committee (RETEC) which is responsible for reviewing the plant and determining whether the performance offered by the RPP is compliant to the GC.

T h e c o m p l i a n c e a s s e s s m e n t i s undertaken via analysis of type tests, onsite measurement and power system simulation studies. Typically, standardised type testing documents are provided by the manufacturers of the plant equipment, ind icat ing the per fo rmance of the equipment under specific test conditions, e.g. response of wind turbine generators operating in low voltage conditions. The onsi te measurements are per formed to test compliance while the plant is in operation. The simulation studies serve to confirm compliance of the RPP for all of the expected electrical operating conditions. This art icle focuses on the simulation considerations of grid code compliance.

The GC power system simulation assessment is undertaken in the design phase in order to prevent rework and avoid expensive remedial actions. There are cost optimisation opportunities that exist during the design phase of the RPP. Some examples include:

l Optimising the number of generators required to satisfy the reactive power capability requirements.

l Optimising the reactive current capability of the generators to minimise or eliminate the additional cost of installing reactive compensation such as capacitor banks or STATCOMs.

Enhancing grid code compliance assessment of renewable power plants by William Yuill and Clinton Carter-Brown, Aurecon

South Africa is procuring electricity from independent power producers (IPPs) in accordance with the integrated resource plan (IRP). These IPPs must be compliant with the grid code (GC). The grid connection code for renewable power plants (RPPs) specifies the technical requirements with which the RPP must comply for connection to the interconnected power system (transmission or distribution network) [1]. The GC specifies the requirements that an RPP must comply with in order to connect to the grid and enter commercial operation. Compliance to the GC ensures a minimum standard of performance and support to the grid, thereby supporting the safe and stable operation of the grid.

l Optimising the design of filter banks to limit the amount of harmonic current emissions injected or absorbed by the RPP.

During GC compliance assessment the adequacy of the base design is also checked, including cable and transformer thermal loadings, voltage levels and power losses.

Simulation compliance assessment

Simulation studies of the utility network are per formed by the network service provider. In South Africa this is typically Eskom. The utility assesses stability issues, fault levels and thermal constraints in the utility grid. The simulation studies performed by the developer analyse RPP performance criteria such as the real and reactive power

capability, fault ride through ability, and power quality at the RPP’s point of connection (POC) to the grid. The developer requires a validated software model of the generator and the design of the plant, which includes engineering models of:

l The point where the plant connects to the utility network, typically represented as an equivalent Thevenin source with a three phase fault level and X/R ratio.

l HV/MV transformers (if applicable).

l MV cables.

l Generator MV/LV transformers.

A representative simulation model of the plant is a prerequisite for any performance assessment studies. A simplified PowerFactory

Fig. 1: Representative PowerFactory single line model of a renewable energy power plant.

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Management/Planning

single line model of a renewable energy power plant, indicating the aforementioned main components, is shown in Fig. 1.

S imulat ion s tud ies a re executed in accordance wi th the methodology, “Recommended practice for performing Grid Code Compliance assessment in South Africa” [2]. The simulation studies can be grouped under the following main categories:

l Reactive capability: Absorb or generate reactive power within the reactive power and voltage capability limits as is required for reactive power and voltage control.

l Fault ride through (FRT): Absorb or generate reactive current as is required to assist in stabilising the voltage during abnormal operating conditions.

l Power quality: RPP emissions limited to within

contracted values so that power quality

levels for the RPP and other customers are

maintained within national limits.

In each of the simulated studies, the RPP must satisfy specific GC criteria in order to achieve GC compliance. The compliance assessment is iterative in nature, as shown in Fig. 2.

For each main category, multiple tests are executed. The analysis is repeated for multiple simulation cases testing all of the operational states for which GC compliance is required. If the criteria a re sa t i s f ied then compl iance has been achieved. If the criteria are not satisfied then the parameters of specific equipment are adjusted where possible. The adjustment of equipment parameters to improve operating performance in one area can negatively impact performance in another area due to the parameter in te rdependency. For example, the adjustment of the MV/LV off- load tap changers to lower the operating voltage which increases the react ive power generation capability to satisfy GC PQ criteria can result in low generator voltages when supplying reactive power during conditions associated with low voltage ride through. Therefore, the equipment parameter adjustments are performed iteratively until compliance is achieved.

Fault ride through simulations

The purpose of the fault r ide through simulations is to study the performance of the RPP under abnormal operating conditions, when the voltage at the POC is outside of

Step Manual time requirement Automated time requirement

A. Simulation setup 20 minutes 0: automated

B. Execute simulations 2 – 24 hours 2 – 24 hours

C. Analyse results 20 minutes 0: automated

D. Criteria analysis 20 minutes 0: automated

E. Parameter adjustment 20 minutes 20 minutes

F. Compliance reporting 20 minutes 0: automated

Total time (excluding B) 100 minutes 20 minutes

Table 1: Comparison between manual and automated methods.

Fig. 2: Simulation assessment methodology.

Fig. 3: Detailed simulation assessment methodology.

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the normal operating range of 0,9 p.u. to 1,10 p.u. The FRT clauses of the GC require the RPP to remain connected to the external network, and to support the network by absorbing or injecting a controlled amount of reactive current.

Compliance ensures that during FRT the RPP is able to remain connected and assist in stabilising the voltage at the POC, and is therefore a key requirement in the GC. The main category of FRT can be subdivided into low voltage ride through (LVRT) and high voltage ride through (HVRT). For LVRT, 28 combinations of fault types, durations and pre-fault conditions (voltage and power) are assessed. The specific methodology is outlined in [2].

Fig. 3 is the detailed representation of the LVRT simulation steps A – F from Fig. 2. Items 1 – 11 are specific to the PowerFactory software, and represent the simulation setup that is required for each LVRT case. Items 12 – 13 indicate the independent simulation of each LVRT case, until all 28 cases have been simulated. Items 14 – 17 indicate the analysis of the simulation outputs. Item 18 represents the adjustment of plant electrical parameters (where the results do not comply with the GC criteria). Item 19 represents the recording of the results and the compliance reporting on a case-by-case basis. All of the items in steps B – F need to be repeated each time the equipment parameter adjustments are required to satisfy.

Automation of compliance assessment

DIgSILENT programming language (DPL)

is utilised to automate the simulation

setup, results analysis and compliance

reporting. The script accepts user inputs

that are specific to the RPP under study,

e.g. maximum export capacity (MEC) and

equipment ratings, and simulation tuning

parameters such as integration step sizes

and tolerances used in the calculation

routine. When the script is executed,

the input variables are passed to the

various function calls to sequentially step

through the assessment process. The script

compares the results of the simulations

against the GC criteria. The results of the

simulations and criteria comparisons can

be displayed via the program command

line, stored in the script memory or written

to and stored in Microsoft Excel. The user

interprets the compliance reporting to

determine compliance.

LVRT simulation case study

The LVRT studies of an actual 134 MW wind energy farm are utilised to illustrate the automated method to reduce the time required to execute the compliance assessment simulation studies via DPL scripting. The required simulation time was recorded using the automated method, and compared with reasonable estimates for a manual method. Table 1 provides a comparison of the time requirements of the two methods for the steps A – F in Fig. 2 for one iteration.

Conclusion

The article has outlined GC compliance assessment for RPPs, with a particular focus on automating the power system simulations by implementing user-built scripting functions. In the case where no scripting functionality is available, the manual simulation assessment would require five times longer than the automated method. The compliance assessment requires multiple iterations. Therefore, the time savings increase proportionally with the number of simulation iterations that are performed. The automation serves as a tool that enables the developer to structure the analysis to reduce the errors that can occur from iterative analysis and focus available time/budget on optimising the design, ultimately reducing cost and risk. Furthermore, the automation facilitates the developer in optimising a GC compliant design during the initial design and equipment selection. This potential involvement in the early design stage provides the developer with insight into the best technology selection that optimises cost whilst ensuring compliance with the GC.

Acknowledgement

This article was presented at the 2015 Cigre Africa symposium Cape Town October 2015 and is reprinted with permission.

References [1] Grid Connection Code for Renewable Power

Plants (RPPs) Connected to the Electricity Transmission System (TS) or the Distribution System (DS) in South Africa, July 2014, version 2.8.

[2] “Recommended practice for performing Grid Code Compliance assessment in South Africa”, M.P.E., August 2014.

[3] "Power System Analysis and Engineering" DIgSILENT Germany, www.digsilent.de.

Contact William Yuill, Aurecon, william.yuill@aurecongroup.com