Electric Load Monitoring

Electric load monitoring, crucial for electrical system efficiency and safety, involves these key aspects:

  • Verifies system capacity: Confirms the electrical system can handle the current and future loads.
  • Optimizes power usage:. Helps in minimizing wasteful power consumption.
  • Identifies overloads: Detects when the system is overloaded, preventing potential failures.
  • Detects issues such as harmonic disturbances and poor power factor: Identifies issues that could affect system performance and equipment longevity.
  • Facilitates Facility Management: Aids in expansions, renovations, and solving operational problems through measured data over time.

Process: Involves measuring electrical load characteristics over time with specialized equipment.

Power Quality

Power Quality Studies

Power quality refers to the degree to which the electrical power supplied to devices and systems meets their operational requirements. It encompasses various aspects such as voltage stability, frequency stability, and the purity of the sinusoidal waveforms.

Power quality is crucial for a facility for several interconnected reasons, directly impacting operational reliability, equipment lifespan, safety, and financial performance. The significance of maintaining high power quality encompasses:

Equipment Efficiency and Longevity:
Poor power quality can lead to increased heat generation in electrical devices, stressing components and potentially shortening their operational lifespan. High-quality power ensures that equipment runs efficiently and within its designed parameters, thereby extending its service life.

Safety:
Transients and surges can pose a risk of physical damage to equipment, which in turn could lead to fire hazards or endanger personnel. Maintaining power quality helps in mitigating these risks, ensuring a safer environment for both workers and equipment.

Energy Efficiency:
Poor power quality can lead to unnecessary energy wastage, resulting in higher electricity bills and a larger carbon footprint. Harmonics, for instance, contribute to non-productive power that still needs to be generated and managed by the facility, directly affecting energy efficiency.

Data Integrity:
In facilities like data centers, power quality is vital for ensuring the integrity of the data being processed and stored. Voltage sags, swells, or interruptions can lead to data corruption, loss, or processing errors, which could be catastrophic for businesses reliant on accurate and accessible data.

Operational Reliability:
Fluctuations in power quality can cause sensitive equipment to malfunction or shut down unexpectedly, leading to operational interruptions. Facilities that depend on precision machinery, such as manufacturing plants, data centers, or hospitals, require stable power to maintain continuous and reliable operations.

Compliance with Standards and Regulations:
Many industries are subject to regulations that stipulate acceptable levels of power quality. Non-compliance can lead to penalties, legal issues, or forced downtime to rectify problems, impacting a facility's reputation and financial health.

Cost Savings:
Addressing power quality issues proactively can lead to significant cost savings. It not only reduces the risk of equipment damage and associated repair or replacement costs but also minimizes the potential for production downtime, which can have a substantial financial impact on operations.

Enhanced Facility Performance:
Overall, maintaining optimal power quality is fundamental to achieving enhanced facility performance. It supports the smooth functioning of all operations, contributes to a sustainable environmental profile, and ensures that a facility can meet its productivity and quality targets.

Common Power Quality Problem:

Voltage Sags (Dips):
Short-duration decreases in voltage, often caused by the startup of large loads or fault conditions. These can lead to malfunctions in sensitive equipment.

Voltage Swells:
Short-term increases in voltage, which can occur due to sudden load reductions or disconnects. Swells can damage electrical devices by exceeding their voltage tolerance.

Mathematically, if \( \Large \ V_{nom} \) is the nominal voltage, a sag or swell can be represented as a deviation, where \( \Large \ V_{actual} \) is the actual voltage during the event:

\( \Large Deviation(\%) = \frac{V_{actual}-V_{nom}}{V_{nom}} \times 100\% \)

Voltage Imbalance:
Unequal voltage magnitudes or phase angle differences in a three-phase system, leading to reduced efficiency and overheating in motors and other three-phase equipment.

Harmonics:
Distortions in the electrical current or voltage waveform, typically caused by non-linear loads such as variable speed drives, computers, and LED lighting.This can lead to overheating in equipment and conductors, nuisance tripping of circuit breakers, and reduced efficiency.

Transients(Spikes/Surges):
Very short, high-amplitude increases in voltage. These can be caused by lightning strikes, power outages, or switching operations, potentially damaging electronic equipment.

Frequency Variations:
Deviations from the nominal system frequency (50/60 Hz), which can affect the operation of clocks, motors, and other frequency-sensitive devices..

Introduction to Harmonic Distortion

Before the 1960s, electrical systems rarely encountered issues with harmonic distortion. The advent of digital electronics, marked by the digital switching of signals such as voltage, brought harmonic distortion to the forefront. Devices like computers and printers, known as non-linear loads, disrupt electrical systems by producing harmonics.

Non-linear Loads

In a typical AC power system, current flows sinusoidally at a certain frequency, usually 50 or 60 Hz. Connection of linear devices to the system results in current draw at the voltage frequency. Non-linear devices, however, lead to the generation of non-sinusoidal waveforms. These are constructed by superimposing sine waves at different frequencies, known as "Harmonics."

Non-linear loads not only produce harmonics but also significantly alter the current waveform, leading to complex interactions within the system. As of 2020, 75% off all electrical loads in North America operate with non-linear loads.

Sources of Harmonics
  • Variable Frequency Drives
  • Universal Power Supply (UPS) Systems
  • DC Converters
  • Solid State Rectifiers
  • Arc Welders
  • Heat Units/Furnaces
  • Switch Mode Power Supplies (those used in computers
  • Electronic Lighting Ballasts
  • PLC Systems, SCADA Systems, Computer Systems
Effects of Harmonics
  • Increase of line current with in a power system
  • Overheating of transformers, motors, generators, capacitors, cables, etc
  • Mis-operation of circuit breakers and other protective devices
  • Malfunction of electronic equipment which relies on zero voltage crossing detection or is sensitive to wave form distortion
  • Incorrect reading on meters
  • Premature failures of power supplies
  • Low power factor, requiring an upsizing of a transformer's kVA or neutral conductors
  • resonating and harmonic heating of power factor correction capacitors, which can lead to failure.
Definition

Harmonics have frequencies that are integer multiples of a waveform's fundamental frequency. Harmonic distortion is the degree to which a waveform deviates from it’s pure sinusoidal values as a result of the summation of a harmonic elements.

Harmonics are represented by their order \( \Large n\) related to the fundamental frequency \( \Large f\) with their amplitude \(\Large A_{n}\) and phase \(\Large \phi_{n}\) , given by:

\(\Large V_{t} = V_{0} +\sum_{n=1}^{\infty } A_{n} \sin(2n\pi ft + \phi_{n})\)

  • For a 60Hz fundamental frequency wave form, the 2nd, 3rd, 4th, 5th harmonic components will be at 120Hz, 180Hz, 240Hz, and 300Hz, respectively.
  • The ideal sine wave has zero harmonic components.
  • Harmonics are a steady state phenomenon that repeats with every cycle. Not to be confused with transient dips, spikes, impulses, or oscillations.

Maximum Total Harmonic Distortion (THD)

IEEE 519, Recommended Practice and Requirements for Harmonic Control in Electrical Power Systems, establishes limits for harmonic distortion and provides guidelines for harmonic control in electrical power systems. It defines limits for Total Harmonic Distortion (THD) for voltage and current waveforms. These limits are defined at the Point of Common Coupling (PCC).

Point of Common Coupling (PCC)

PCC is defined as the point in the power system closest to the user where the system, owner or operator could offer service to another user. Frequently for service to industrial users ( i.e. manufacturing plants) via a dedicated service transformer, the PCC is at the high voltage side of the transformer.
For commercial users (office, parks, shopping, malls, etc.) supplied through a common service transformer, the PCC is commonly at the low voltage side of the transformer.

The PCC is basically recognized at the point where it any harmonics could migrate onto the utility system and cause problems for other customers. The intention of having a point of common coupling is to prevent high levels of harmonics generated by one customer from causing distortion to other customers on the power grid.

In considering the primary site of the transformer (that supplies, one customer), the transformer's impedance will decrease the short circuit ratio.

This result in an increase in the harmonic current limits. The voltage distortion will be higher at the secondary side of the transformer.

Total Harmonic Distortion

IEEE 519 recommends that the THD in the voltage waveform should not exceed 5%for general systems. It's important to note that specific limits can vary depending on the system voltage and the point of common coupling (PCC).

For Current THD:
The recommended maximum THD for current depends on the short-circuit to load \( \Large I_{short-circuit}/ I_{Load}\) ratio at the point of common coupling. IEEE 519 specifies detailed thresholds based on this ratio, with lower THD limits for systems where the \( \Large I_{short-circuit}/ I_{Load}\) ratio is higher, to prevent harmonics from significantly affecting the system. For example, for an \( \Large I_{short-circuit}/ I_{Load}\) ratio of < 20 ,the maximum THD for current can be as low as 5%, and for ratios > 1000 , up to 20%.

Harmonic Mitigation
  • Limit the amount of non-linear loads
  • Drive isolation transformers
  • Line reactors
  • Replace 6-pulse drives with higher pulse drives, such as 12 or 18 pulse drives
  • Harmonic Trap Filters (active or passive)
  • Resonance problem indicates removal or relocation of power factor correction capacitors (PFCCs)

3rd Order Harmonic

Most Harmonic problems are caused by the 3rd harmonic.

Each phase in a three-phase system is 120 degree apart.the third harmonic s between the phases add together, which leads to oscillating currents.

3rd Harmonic Effects

Increased Neutral Current: In three-phase systems, the 3rd and multiples of 3rd harmonics (like 9th, 15th) are in phase and add up in the neutral conductor. This can result in neutral current being significantly higher than the phase currents, potentially exceeding the capacity of the neutral conductor and leading to overheating.

Interference in Communication Lines:
Harmonic frequencies can interfere with communication lines that run parallel to power cables, causing noise and affecting the performance of sensitive equipment like computers and other electronic devices.

Overheating of Electrical Components:
The increased current due to 3rd harmonics causes additional heat in electrical cables, transformers, and motors. This excess heating can reduce the lifespan of equipment and increase the risk of failures.

Reduced Efficiency of Energy Conversion Devices:
Harmonics distort the sinusoidal waveform of the electrical power, which can lead to poor performance in devices like motors and generators. This inefficiency is due to increased losses in these devices, manifesting as heat.

Tripping of Circuit Breakers and Blown Fuses:
The non-linear characteristics of harmonic generating loads can cause unexpected tripping of circuit breakers and blowing of fuses, as protective devices may misinterpret the harmonic currents as fault conditions.

Adverse Effects on Power Factor:
Harmonics can also lead to a poor power factor, which is a measure of how effectively the power is being used. A poor power factor means more power is being drawn from the source for the same amount of work, leading to inefficiencies and higher energy costs.

Damage and Malfunction of Equipment:
Long-term exposure to high levels of harmonics can lead to the degradation of electrical insulation and even damage to electronic components. Additionally, sensitive electronic equipment may malfunction or fail due to the distorted waveforms.