Phasor Measurement Unit Test and Applications for Small Signal Stability Assessment and Improvement of Power System

The power system is constantly changing and as new technologies are being developed, it is pushing forward towards a decrease in fossil-fuel need. As the conventional generation is being replaced by renewable energy sources (RES) such as wind and solar power, it is expected for the power system to be less predictable.

Therefore, the methods used for stability and security assessment will most likely use information from the wide-area measurements systems (WAMS). The work presented in this thesis deals on one hand with the development of test methods and validation of phasor measurement units (PMUs) which are considered to be one of the key technologies in WAMS, and on the other hand with the possibility of using PMU measurements together with large wind power plants (WPPs) to help improve the damping of inter area oscillations.

To validate the PMUs, a laboratory test setup is assembled. The hardware components are capable of generating, with the required accuracy, the test signals injected in the PMUs. The signals are created according to the requirements defined in the current IEEE C37.118.1-2011 standard, to test the steady-state and dynamic compliance of the PMUs.

The performance of the PMUs is evaluated according to the IEEE C37.118.1a2014 amendment to the standard which defines the allowed error limits for the units. It was found that the devices under test did not meet all the specifications of the IEEE C37.118.1a-2014, especially for the dynamic tests. Furthermore, the PMUs were tested under three scenarios that were not covered by the current standard.

It was found that two of the scenarios affected the measurement accuracy of the units, while the third did not have a significant impact on the PMU performance. A full scale converter based wind turbine (WT) model suitable for small-signal stability analysis was developed during the project. The model can be used in both dynamic simulations of the nonlinear system, and it can be linearized together with the entire power system model in order to study the eigenvalues of the system.

In this thesis, the WT model was used as an aggregated WPP with the active and reactive power outputs controlled by a Wind Plant Controller (WPC). The WPP was used to help improve the damping of inter-area oscillations. The WPP was equipped with a power oscillation damping (POD) controller which modulated the active power output of the WPP.

Two types of POD were considered in the investigation: a conventional power system stabiliser (PSS) type and a phasor POD. Remote PMU measurements were used as input signals for the PODs, and measurement latency was included for comparison. It was found that the PODs had similar performance when there was no latency in the input signals.

The phasor POD showed a clear advantage when latency was considered. The reason was that the phasor POD can easily and adaptively compensate for delays in the input signals, while the conventional PSS type uses the lead-lag block to achieve a fixed phase compensation which is chosen during the design stage.