The Effects of Harmonic Voltage Instability

With the rapid advancement of the technology era, power electronics technologies have become increasingly prevalent across various domains, including industrial control systems, security & surveillance, base stations, telecommunications, smart devices, and beyond. This widespread adoption has significantly amplified the severity of harmonic-related issues in modern electrical systems.

1、What Are Harmonics?

In power systems, harmonics are fundamentally generated by non-linear loads. When current flows through such loads, it does not maintain a linear relationship with the applied voltage, resulting in non-sinusoidal current waveforms and the introduction of harmonics into the circuit.

Harmonic frequencies are integer multiples of the fundamental frequency. As demonstrated by French mathematician M. Fourier’s analysis principle, any periodic waveform can be decomposed into a fundamental sine wave component plus a series of sinusoidal harmonic components at integer multiples of the fundamental frequency.

Each harmonic is a pure sine wave with its own distinct frequency, amplitude, and phase angle. Harmonics are classified as even-order (2nd, 4th, 6th, 8th, etc.) or odd-order (3rd, 5th, 7th, etc.). For a 50 Hz fundamental, the 2nd harmonic is 100 Hz, the 3rd is 150 Hz, and so forth.

2、How Are Reactive Power and Harmonics Generated?

In industrial and residential loads, inductive loads (RL loads) constitute a major portion. Typical examples include induction motors, transformers, fluorescent lighting, and similar equipment. Induction motors and transformers account for a substantial share of the reactive power demand in power systems. Reactors and overhead lines also consume some reactive power.

Inductive loads inherently require reactive power absorption for normal operation due to their physical characteristics.

Non-linear power electronic devices also consume reactive power, especially phase-controlled equipment such as thyristor-controlled rectifiers, AC phase-controlled power regulators, and cycloconverters. During operation, their fundamental current lags the grid voltage, leading to significant reactive power consumption. Moreover, these devices generate substantial harmonic currents, and all harmonic sources inherently consume a certain amount of reactive power.

In diode rectifier circuits, the fundamental current phase aligns closely with the grid voltage, resulting in minimal fundamental reactive power consumption. However, they still produce large harmonic currents and therefore consume some reactive power overall.

Impacts of Reactive Power and Hazards of Harmonics

Over the past three decades, the proliferation of power electronic devices has made them the dominant source of harmonics in power systems. Among these, rectifier circuits represent the largest proportion. Most common rectifier topologies employ thyristor phase-controlled rectifiers or diode rectifiers, with three-phase bridge and single-phase bridge configurations being the most prevalent.

Rectifier circuits with inductive loads are well-known for generating harmonic pollution and lagging power factor. Diode rectifiers with capacitive DC-side filtering also constitute severe harmonic pollution sources. While their fundamental current component maintains near-unity power factor (phase alignment with supply voltage), the high harmonic content severely pollutes the grid and reduces the overall total power factor. Additionally, phase-controlled AC power regulators and cycloconverters introduce significant harmonic currents on the input side.

1.Impacts of Reactive Power

(1) Increased reactive power demand raises current levels and apparent power (S), necessitating larger capacity for generators, transformers, other electrical equipment, and conductors. User-side starting and control equipment, along with metering instrumentation, must also be upsized accordingly.

(2) Higher reactive power directly increases total current, leading to elevated I²R losses in equipment and lines—an obvious efficiency penalty.

(3) Voltage drops across lines and transformers increase. With impact loads featuring fluctuating reactive power, severe voltage fluctuations occur, significantly degrading power supply quality.

2. Hazards of Harmonics

An ideal utility grid should deliver voltage at a single, stable frequency with specified amplitude. The presence of harmonic currents and resulting harmonic voltages constitutes pollution of the public grid, degrading the operating environment for electrical equipment and posing risks to surrounding power electronic devices.

Before the widespread adoption of power electronics, harmonics and their effects received limited attention. However, over the last 30–40 years, the explosive growth of power electronic equipment has dramatically worsened grid harmonic pollution. Harmonic-induced faults and incidents have become frequent, elevating awareness of their serious consequences.

Key hazards of harmonics on the public grid and other systems include:

(1) Harmonics cause additional losses in grid components, reducing efficiency of generation, transmission, and utilization equipment. Large 3rd harmonic currents flowing through neutral conductors can overheat lines and even trigger fires.

(2) Harmonics disrupt normal operation of electrical equipment. Beyond added losses in motors, they induce mechanical vibration, noise, and overvoltages, while causing localized severe overheating in transformers. Capacitors, cables, and other components suffer overheating, accelerated insulation aging, reduced lifespan, and eventual failure.

(3) Harmonics trigger local parallel resonance or series resonance in the grid, amplifying harmonic levels and greatly exacerbating the issues in (1) and (2), potentially leading to major incidents.

(4) Harmonics cause malfunction or misoperation of protective relays, automatic control devices, and inaccurate readings in electrical metering instruments

(5) Harmonics interfere with nearby communication systems, causing noise and degraded quality in mild cases, or data loss and complete communication failure in severe cases.

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