Power Electronics in Renewable Energy Systems

Power Electronics in Power Systems

• Power electronics converters are necessary to interface with the utility for fuel cell systems and photovoltaic systems.

Micro-Turbines

• Micro-turbines use highly efficient aircraft engines with natural gas as the input fuel.
• To improve efficiency, the turbine speed should be allowed to vary based on loading.
• This causes the frequency of the generated output to vary, requiring a power-electronic interface.

Energy Storage Systems

• Energy storage systems offer benefits such as load-leveling and peak-shaving in power systems.
• Energy is stored, usually at night when the load demand is low, and supplied back during peak-load periods.
• Energy can be stored in lead-acid batteries, Super-conducting Magnetic Energy Storage (SMES) coils, and in the inertia of flywheels.

Power Quality Solutions

• Poor quality of power can imply distorted voltage waveforms, unbalances, swells and sags in voltage, power outages, and more.

Main Power System Components

• Transmission bottlenecks must be identified when operating electrical power systems, including:
  • Steady-State Power Transfer Limit
  • Voltage Stability Limit
  • Transient Stability Limit
  • Thermal Limit
  • Short-Circuit Current Limit

Conventional System Solutions

• Conventional systems enhance transmission capability by:
  • Series Capacitors (X)
  • Switched Shunt-Capacitor and Reactor (V drop solution)
  • Transformer Tap-changer LTC’s (V drop solution)
  • Phase Shifting Transformers (δ – power transmission)
  • Synchronous Condensers (V - drop solution)

Categorizing Power Electronic Systems

• Power electronic systems can be categorized as:
  • Solid-State Switches
  • By connecting two thyristors in anti-parallel (back-to-back) as shown in Figure, a solid-state switch can be realized that can conduct current in both directions and turn-on or off in an ac circuit with a delay less than one-half a cycle.

Photovoltaic (PV) Systems

• Photovoltaic systems are the ultimate in distributed generation and have a greater potential than wind-electric systems.
• PV arrays provide a voltage of 52-V to 90-V DC, which the power electronic system converts to 240 V/50 Hz sinusoidal voltage suitable for interfacing with the single-phase utility.

Fuel Cell Systems

• The input to the fuel cell can be hydrogen, natural gas, or gasoline, and the output is a dc voltage.
• The efficiency of fuel cell systems can be as high as 60%.