Flexible AC Transmission Systems (FACTS) controllers’ commissioning, compliance testing, and model validation tests

The commissioning tests include a range of tests, measurements and simulations to demonstrate the performance and behaviour of the installed plant. For a FACTS controller the purpose of commissioning and compliance testing is to:

• provide evidence that the plant can remain safely connected to the power system, and that it meets the technical performance requirements specified in the connection agreement and/or in the technical specification of the FACTS controller (commissioning and grid compliance tests). 
• validate power system simulation models and associated parameters, and ensure that FACTS controller’s model is representative of the installed system (model verification tests). 

This is done by a comparison of the actual recorded results with the simulated results, with the two expected to indicate a close correlation for the model to be considered validated. Accurate modelling of the FACTS controller is fundamental to ensuring that future power system studies adequately demonstrate network behaviour.

The commissioning tests are generally preceded by several factory tests, type tests, off-site tests and on-site equipment and subsystem tests. These tests must be completed prior to the final on-site commissioning of the FACTS controller, and were not covered in WG B4.83.

The commencement of commissioning generally requires completion of design and desktop studies including system impact studies using appropriate power system simulation models. Successful completion of commissioning tests is generally a pre-requisite for the plant to remain connected to the power system and operate unrestricted. 

The work completed by CIGRE WG B4.83 discusses the entire suite of commissioning tests conducted as part of commissioning a new or modified FACTS controller. It is an extended version of chapter 22 of CIGRE Green Book on FACTS.  lt presents best practices for commissioning and compliance testing of different types of FACTS controllers. The Technical Brochure provides several practical examples of commissioning tests for Static Var Compensators (SVCs), Static Synchronous Compensators (STATCOMs), Unified Power Flow Controllers (UPFCs), Thyristor Controlled Series Capacitors (TCSCs) and Static Synchronous Series Compensators (SSSCs). These cover a variety of approaches and requirements in different countries.

The commissioning philosophy for FACTS controllers is divided into the four main subsections shown below.  Each test subsection should be executed in sequence, with successful completion of each test stage before proceeding to the next group of tests. This will ensure that the integrated behaviour of various plant and control systems and their interactions is understood, captured and verified to be as expected.

• Equipment Tests (Site Acceptance Tests)
• Sub-System Tests (Pre-Commissioning Tests)
• System Commissioning Tests (Commissioning Tests)
• Grid Compliance Test (Performance Verification Tests)

Of the four stages discussed above, the focus of WG B4.83 was primarily on system commissioning and grid compliance tests noting that other aspects have been covered in detail in CIGRE Green Book on FACTS. All tests have been presented in the Technical Brochure in a modular fashion comprising the following sub-sections.

• Purpose
• Pre-test conditions
• Methodology and procedure
• Measured quantities
• Acceptance criteria

This makes them a convenient template for preparing commissioning test plans which typically needs to be agreed between the original equipment manufacturer, asset owner and system operator several weeks before commencing live commissioning tests.

System commissioning tests are the first stage where the FACTS controller as a complete system is evaluated to ensure it is in accordance with the owner’s/system operator’s specifications. These tests are generally the same across different types of FACTS controllers and generally include:

• System energisation tests          
• Environment, noise and interference immunity and emissions     
• Heat run tests
• Redundancy tests          
• Loss determination tests

Grid compliance tests are then conducted following successful completion of system commissioning tests. These tests generally are as follows (Note that not all tests are applicable to each type of FACTS controller or for all applications).

• Steady-state characteristic tests
• Simulated/staged fault tests (optional)
• Power quality tests        
• Network switching tests (external mechanically switched shunt bank control)
• Step response tests       
• Parallel operation tests (if applicable)
• Power Oscillation Damping (POD) on/off tests
• Automatic gain reduction tests
• Mechanically switched shunt bank control (when part of FACTS) 
• System interaction tests

This is to ensure that there is no adverse interaction between multiple FACTS controllers, or between a FACTS controller and other power electronic connected devices such as an HVDC link or inverter-based generation. 

The Technical Brochure then discusses commissioning tests for system-wide control and protection schemes where FACTS controllers are used to provide a positive contribution to the overall power system dynamic performance. Examples covered include sub-synchronous interaction tests and an assessment of FACTS controller’s contribution to damping of low-frequency electromechanical oscillations.

Finally, the Technical Brochure presents examples of model validation analysis by comparing measured and simulated responses of FACTS controllers. Various examples are presented for different FACTS controllers, and using different simulation tools including root mean square (RMS) (or sometimes referred to as phasor-domain models), offline electromagnetic transient (EMT), and real-time EMT based on hardware-in-the-loop (HIL) simulation. The Technical Brochure also presents a practical example on how to address mismatches between measured and simulated responses during the model validation process.  Note that model validation studies often require special simulation studies in which the state of the assumed power system is adjusted to match the actual power system state at the time the tests are performed in order to validate the modelling assumptions.