Electronic Thesis and Dissertation Repository

Thesis Format

Monograph

Degree

Doctor of Philosophy

Program

Electrical and Computer Engineering

Supervisor

Varma, Rajiv K.

Abstract

Transmission lines across the world are extensively compensated by series capacitors to increase their power transfer capacity. However, series compensation can potentially cause subsynchronous resonance (SSR) which can lead to significant damage in shafts of synchronous generators and wind farms. Dynamic reactive power compensators such as STATCOM and SVCs are typically used for mitigation of SSR, but these are quite expensive.

Large scale PV solar farms are growing at a very rapid rate across the world. It is quite likely that they may get installed near synchronous generators or wind farms which are directly or indirectly interfaced with series compensated lines.

For the first time, novel SSR damping controls for the patented technology of PV solar farm controlled as STATCOM (PV-STATCOM) are presented in this thesis for mitigating SSR both during night and day in synchronous generators connected to series compensated lines. The PV-STATCOM successfully mitigates SSR whether the solar farm is located at generator terminals or remotely from it.

New SSR alleviation controls are also developed for PV-STATCOM for mitigating SSR in wind farms connected to series compensated networks, during night and day, irrespective of PV-STATCOM location. The SSR damping controls are based on wind farm generator speed and line current, both of which are locally available signals.

As soon as a system disturbance is detected, the solar farm discontinues its real power generation and makes available its entire inverter capacity for dynamic reactive power exchange as STATCOM, for SSR mitigation. Once subsynchronous oscillations are damped the solar farm utilizes a novel ramp-up technique according to which SSR damping function is kept activated with remaining inverter capacity during power ramp-up. Such a fast ramp-up technique is not envisaged in Grid Codes. The entire process of SSR damping for all cases studied takes less than half a minute.

A novel coordinated control of PV-STATCOM and DFIG based Type 3 wind farms is developed for faster and effective SSR damping, when both are connected to series compensated lines.

MATLAB based eigenvalue analysis validated by nonlinear simulations through SIMULINK and PSCAD software confirm the effectiveness of proposed SSR damping controls.

Summary for Lay Audience

Because of environmental pollution, energy security, and economic concerns, many large-scale renewable energy sources including PV solar farms and wind farms are being increasingly employed worldwide to satisfy the high-growing demand of world energy consumption. As there are usually considerable distances between large renewable energy systems and load centers, they need to be connected to transmission lines to transfer the generated power to load centers. Therefore, the power transfer capacity of transmission lines needs to be increased in some cases to evacuate these large amounts of power to the grid.

As an alternative to the construction of new and expensive transmission lines, series capacitive compensation is a simple and economic technology to increase power transmission limit of existing transmission lines. Nevertheless, Subsynchronous Resonance (SSR) is a preventing concern for using series capacitive compensation extensively. SSR is an unstable electrical power system condition which resulted in generator shaft failure, power outage, and costly damages to the electricity grid.

Several countermeasures have been investigated and utilized to alleviate SSR since its first occurrence, including flexible AC transmission system (FACTS) devices such as STATCOM and SVC. But these countermeasures are much expensive besides having many installation difficulties.

Nowadays, large-scale PV solar farms with rating in excess of 100 MW are already commissioned and increasingly being developed worldwide which are comparable to transmission level shunt connected FACTS devices such as, Static Var Compensators and STATCOMs.

This thesis presents a novel concept of utilizing PV solar inverter as a FACTS devices STATCOM, termed PV-STATCOM, to alleviate SSR in the series compensated power system on a 24/7 basis. The efficacy of PV-STATCOM SSR alleviation in synchronous generator and wind farms (IG-based and DFIG-based) is demonstrated through various simulations and studies. Also, SSR alleviation with utilizing PV-STATCOM and DFIG-based wind farm, simultaneously, is presented in this thesis.

The PV-STATCOM involves embedding additional controls with measurement circuitry on the inverters and the electrical/civil infrastructure (buswork, breakers, transformers) of an existing PV solar farm to transform it into a STATCOM. This makes PV-STATCOM at least 50-100 times cheaper than an equivalent size STATCOM to achieve the same goal.

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