Electronic Thesis and Dissertation Repository

Thesis Format

Monograph

Degree

Doctor of Philosophy

Program

Electrical and Computer Engineering

Supervisor

Varma, Rajiv K.

Abstract

Frequency stability and low-frequency power oscillations are two major concerns in modern power systems. PV-STATCOM is a patented concept which enables PV inverters to provide STATCOM functions, during day and night, as well as real power modulation during daytime. This thesis aims to utilize PV-STATCOM capability effectively for enhancing frequency stability and power oscillations damping.

A novel, simultaneous real power-based Fast Frequency Response (FFR) and reactive power-based Power Oscillation Damping (POD) control is proposed for PV-STATCOMs. This control not only significantly reduces system under- and over-frequency deviations, but also uses the unutilized capacity of PV inverters to enhance damping of critical modes.

A novel night and day Reactive power-based Frequency Control (RFC) is proposed for PV-STATCOMs, that deploys the unutilized reactive power capacity of PV-STATCOM for frequency stability improvement. RFC modulates the system voltage, via PV-STATCOM voltage control loop, to control the power of voltage-sensitive loads and reduce the generation-demand imbalance. Sensitivity studies show that the load type and its composition, and location of RFC-equipped PV-STATCOM play a significant role in the efficacy of proposed controls. RFC not only provides a 24/7 complementary frequency support service but potentially obviates the impact of system inertia loss due to replacement of conventional synchronous generators by inverter-based generators.

A new combined RFC and POD controller is also proposed for PV-STATCOM utilizing unused reactive power capacity of PV inverters. The studies show that depending on PV-STATCOM location, the proposed combined RFC+POD controller can effectively enhance system frequency stability and power oscillations damping.

This thesis also proposes a fast power-frequency droop for PV generators and an enhanced synthetic inertial response for wind generators. These two controls operate in a harmonized manner to provide improved frequency support while reducing the stresses on wind generators.

The proposed PV-STATCOM grid support functionalities can potentially open up new revenue streams for solar farms. The mechanisms of such financial compensations are expected to develop in near future with the unprecedented growth of solar farms globally.

MATLAB based simulation studies are performed on two-area-four-machine and 12-bus study systems using modified WECC generic dynamic models for PV plants, wind plants and loads.

Summary for Lay Audience

This thesis is about employing PV solar farms during day and night for purposes other than solar energy conversion. Solar PV generators of both residential and utility scales are rapidly growing in electric grids. A PV solar farm mainly consists of solar panels, PV inverters, and control systems. PV inverters are electronic apparatus that process the electricity generated by the PV panels to adapt it for the electrical grid use. However, the expensive infrastructure of a solar PV farms remains unused during night. Even during the day, the full capacity of PV inverters and other devices is not necessarily deployed, except noontime in a sunny day.

This thesis proposes several new controls for PV inverters to use them more effectively during day and night. After large disturbances, e.g., a generator trip, the speed of the synchronous generators, which determine the system frequency across the grid, deviates from its nominal value, i.e. 60 Hz in North America. Moreover, generators may oscillate against each other. These phenomena may grow and lead to system instability and eventually blackouts, if not corrected rapidly.

In this thesis, three new controls are proposed to enable PV inverters to mitigate frequency deviations and generator oscillations. These controls include:

i) Simultaneous Fast Frequency Control (FFR) and Power Oscillation Damping (POD) control;

ii) Reactive power-based Frequency control (RFC); and

iii) Combined RFC and POD control.

The feasibility and applicability of these controls are investigated through accurate simulations. The results show these new functions of PV inverters can enhance the performance of the electric grid significantly. Furthermore, a new control for wind turbines and PV generators is developed to work cooperatively and reduce system frequency deviations. The proposed control reduces the stress on wind turbines that prolongs their life. All the proposed controls in this thesis demonstrate the potential of PV inverters to enhance the electric grid operation with a minimum applicable cost.

The proposed PV-STATCOM grid support functionalities can potentially open up new revenue streams for solar farms. The mechanisms of such financial compensations are expected to develop in near future with the unprecedented growth of solar farms globally.

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