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Power Quality Issues in Renewable Energy Systems

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The term power quality is a measure of the purity of the AC power, including the presence of unwanted harmonics and noise on the power line and the frequency.

harmonic is a frequency that is a whole-number multiple of another basic frequency. Harmonics on the power line are multiples of the power line frequency (50 Hz or 60 Hz).

Under certain conditions, the electrical equipment can be damaged by harmonics. Harmonics are created both at the source and in nonlinear loads and because of hysteresis in transformers or motors.

In addition to harmonics, nonlinear loads at a user’s site can inject noise on the line that is transmitted to other users. Nonlinear loads include motor variable frequency drives, welders, arc furnaces, electronic ballasts, and more.

Harmonics can affect the operation of equipment, particularly sensitive controllers and computers, and can increase losses in transmission. Their magnitude diminishes with frequency, and harmonics higher than the thirteenth harmonic (780 Hz for a 60 Hz fundamental frequency) are not included in measuring harmonic content.

One way to reduce harmonic content in power lines is to use a pulse width modulation (PWM) converter at the user’s site or filters to remove frequencies other than the power line frequency. A PWM converter can generate a signal of any desired shape or frequency in high-power applications.

In renewable systems, noise from the electrical equipment at the source can put poor-quality AC on the grid. In the case of PV solar systems, the dc from the solar panels is converted to ac by an inverter.

Inverters are notorious for creating harmonic and radio frequency noise because of the high-speed switching commonly used. It can be made worse if the unit is not properly grounded. Filtering can help alleviate noise from inverters.

Noise can cause voltage transients and is difficult to deal with due to its random nature. It includes lighting and power interruptions.

In critical cases where a brownout, blackout, or noise can be a serious problem (a computer center or hospital, for example), the customer can install an uninterruptible power supply (UPS), which automatically switches to an auxiliary generator or batteries if utility power is lost or has other problems.

The ideal power quality exists when the voltage and current produced by the generating system has a perfectly clean sinusoidal waveform with no harmonic content or noise. The frequency must match the grid frequency exactly.

The voltage of the ideal power source stays constant regardless of changing conditions. Of course, the ideal source does not exist, but high-quality sources come close to this ideal. If parameters are too far from the ideal power, they need to be corrected before connection to the grid.

The major issues affecting power quality are frequency and voltage control, low-voltage ride through, and flicker. These issues are discussed in the next subsections.

Frequency and Voltage Control

Frequency and voltage on the grid are controlled by the individual power producers who put large amounts of electrical power into the grid.

The grid receives the voltage from large energy producers such as nuclear-powered generators, coal-fired generation systems, and hydroelectric generators, as well as from renewable energy systems.

Large energy systems can control voltage and frequency easily, but some renewable energy systems may have difficulty because of the energy from renewable sources, particularly wind and to some extent solar, may be variable.

Because most of the electrical energy on the grid has a constant voltage and constant frequency from the larger energy producers, the grid itself can help synchronize the frequency from renewable energy systems (for example by using doubly fed induction generators).

Low-Voltage Ride Through

Low-voltage problems in the grid may occur for several AC voltage cycles, or it may last for several hours. It may also be caused in one phase of a three-phase system or in all three phases.

Low-voltage is a condition that exists any time the voltage on the grid drops 10% or more below the rated voltage level.

When low-voltage occurs at the point where the renewable energy system is connected, the renewable energy system must have the capability to ride through this condition or disconnect from the grid.

It does not matter if the renewable energy system is causing the low voltage or if the low voltage is caused elsewhere on the grid. The safety system for this problem is called a low-voltage ride through (LVRT).

When the low-voltage fault occurs in the grid, instrumentation measures the severity and the duration of the problem and determines the best solution for correcting it.

If the renewable energy system is causing the low-voltage condition, it must be able to correct the condition quickly or disconnect from the grid.

The choice is to disconnect the source from the grid and automatically reconnect after the low-voltage condition has cleared or to remain connected and ride through the low-voltage condition.

Another possibility is to stay connected and help correct the problem by adding power from the renewable energy source’s capacitive banks, which can help the low-voltage condition.

 The method used by the renewable energy system may depend on the type of problem the grid is experiencing. Obviously, corrective action is useful only for large systems because smaller systems cannot affect the grid enough to change the condition.

Flicker

Flicker is defined as a short-lived variation in electrical power. It is sometimes seen as a momentary dropout, as when a light varies.

This problem is more noticeable when a renewable energy system is connected directly to a residential or commercial application as the sole source of power.

If the renewable energy system uses an inverter, it typically has the ability to control the increase or decrease in voltage so that the change in voltage is kept to a minimum in both duration and amount.

Review Questions

  1. What issues concerning power quality must renewable energy systems address when connecting to the grid?
  2. What is a UPS? Where is its use important?

Answer

  1. Power quality issues include unwanted signals such as harmonics and noise being injected in power lines. Renewable energy systems that are connected to the grid need a clean sine wave, so inverters need to be checked and any sources of noise eliminated.
  2. Uninterruptible power supply; it is important in critical cases where a brownout, blackout, or noise can be a serious problem (a computer center or hospital. for example).
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About Ahmad Faizan

Mr. Ahmed Faizan Sheikh, M.Sc. (USA), Research Fellow (USA), a member of IEEE & CIGRE, is a Fulbright Alumnus and earned his Master’s Degree in Electrical and Power Engineering from Kansas State University, USA.