Discussion about Power Quality Problems

Harmonics are signals whose frequency differs from the fundamental frequency of the power system. Moreover, the frequencies of the harmonic components are multiples of the fundamental frequency. In three-phase systems, characteristic harmonics of current/voltage have odd orders as (6n ± 1) (n = 1, 2, 3...).

In three-phase system, current harmonics are mainly generated by nonlinear loads, which are typically implemented from semiconductor devices [45]. Some common applications of nonlinear loads are adjustable speed drives, uninterruptible power sources (UPS) and AC/DC power supplies in domestic, industrial and commercial sectors [42], [46]–[48].

Numerous harmful effects caused by nonlinear loads are:

 Causing voltage distortions that adversely affect to other sensitive loads.

 Increasing of power loss and heat in transformers and line impedance.

 Increasing of reactive power and heat on capacitor.

 Production of pulsating and oscillating torques in turbines and generators.

 Increasing of stator’s and rotor’s losses in motors.

 False breaker tripping or fuse blowing.

 Interferences in communication circuits and other EMI-related problems.

Because of the above negative impacts, different international standards have been published to restrict the amount of harmonics. TABLE 2.1 shows the maximum permissible individual harmonic current component consumed by loads, according to the IEEE 519-1992 standard.

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According to TABLE 2.1, the total harmonic distortion (THD) of common bus voltage must be limited at 5%. To calculate common bus voltage THD_V, the following equation is derived:

2 2

1

100%

ACh h V

AC

V

THD V







(2.1)

where V_AC1 and V_ACh are respectively the root-mean-square (RMS) value of the fundamental and harmonic AC bus voltage, while h is the harmonic orders. In this dissertation, the IEEE 519-1992 harmonic standard is used as a reference for THD of the AC bus voltage so that the THD value of this voltage must be lower than the 5% limitation after compensation.

2.3.2. Voltage Sags in AC Microgrid

According to [41], voltage sag is defined as a decrease of the supply voltage between 0.1 and 0.9 per unit (pu) in RMS value at the power frequency for durations from 0.5 cycle to 1 minute. The term sag describes a short-duration voltage decrease. Generally, voltage sags relates to grid faults. However, energization of heavy loads or starting of large motors also cause voltage sags. Voltage sag in electrical system results in serious problem since they may generate high economic losses [49]–[52].

TABLE 2.1 Maximum Permissible Harmonic Current Distortion Allowed by IEEE 519-1992 Standard.

11

h 11 h 17 17 h 23 23 h 25 35h THD

4% 2% 1.5% 0.6% 0.3% 5%

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In fact, the tolerances of devices connected to the supply network against voltage sags are not similar. Because sensitivity of electrical devices against different voltage sags is not fixed, the tolerance of each device against voltage sag depends on the duration of the sag and the magnitude of the voltage during the sag. Some remarkable effects caused by voltage sags are listed as follows:

 Resetting or tripping of consumer electronics or domestic appliances.

 Shutting down or restarting of computer-controlled industry processes.

 Tripping of the adjustable speed drives due to the operation of their voltage protection circuits.

 Torque and speed variations in the motors.

2.3.3. Voltage Swells in AC Microgrid

The voltage swell can be defined as an increase to between 1.1 and 1.8 p.u. in RMS voltage at the power frequency for durations from 0.5 cycle to 1 minute. So that, voltage swell is quite opposite to voltage sag. Similar to voltage sag, voltage swell also relates to grid fault, de-energization of heavy loads or stopping of large motors. Voltage swells temporarily appear during the single line to ground fault. However, unlike voltage sag, voltage swells does not commonly occur. The negative effects caused by voltage swell are quite similar to the effects caused by voltage sag. Besides that, voltage swell also leads to different serious problems such as:

 Malfunction of sensitive electronic equipment.

 Data errors in large computer system.

 Interruption of equipment operation.

 Damage or lifespan reduction of electrical and electronic devices.

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2.3.4. Voltage Unbalance in AC Microgrid

A normal three phase supply has the three phases of same magnitude but with a phase shifted by 120◦. Any deviation (magnitude or phase) of one of the three signals means presentation of negative and/or zero sequence components. In order to evaluate the imbalance, the voltage unbalance factor (VUF) is defined [53]. VUF is expressed as the ratio between the negative sequence component V_AC^ and positive sequence components

VAC^ as below:

AC100%

AC

VUF V V



  (2.2)

Technically, voltage unbalance is caused by the imbalance in AC system. Imbalance includes unbalanced/single-phase load, as well as fault/unbalanced grid voltage.

Additionally, asymmetry in transformer winding impedance also costs voltage unbalance.

Based on the above reasons, voltage unbalance may occur and introduce some critical results as follows:

 Reduction of motor efficiency.

 High heat together with premature motor failure.

 Introduction of zero sequence current component on neutral line.

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Moreover, in order to clarify the negative effect voltage unbalance, Fig. 2.4 depicts the increase of motor heat/loss by VUF. From Fig. 2.4, despite of small VUF, motor heat/loss are significantly high.

2.3.5. Voltage Ripple in DC Microgrid

Ripple is the residual periodic variation of the DC voltage within a power supply which has been derived from an alternating current source. Ripple voltage originates as the output of a rectifier or from generation and commutation of DC power. The ripple component is often small in magnitude relative to the DC component, but in absolute terms, ripple (as in the case of HVDC transmission systems) may be thousands of volts. Ripple itself is a composite (non-sinusoidal) waveform consisting of harmonics of some fundamental frequency which is usually the original AC line frequency, but in the case of switched- mode power supplies, the fundamental frequency can be tens of kilohertz to megahertz.

Ripple is wasted power, and has many undesirable effects in a DC circuit:

 High heat and loss on components.

 Undesired noise and distortion.

 Improper operation of digital circuits.

Fig. 2.4 Increase in motor heating and losses due to voltage unbalance.

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2.4. Power Quality Conditioning Techniques in AC