Designing for power quality
- Understand how power quality can impact electrical system reliability.
- Gain the ability to identify applications where improvements are recommended.
- Learn engineering methods to protect equipment and optimize performance.
Devices using power electronics can produce distortion in electrical distribution systems, and it’s up to the electrical engineer to apply effective solutions to mitigate this added distortion. These solutions will ensure high-quality power is maintained within a nonresidential building, especially one with sensitive equipment or unique applications.
“Dirty power” is a phrase that is often used to explain unexpected behavior in the context of an electrical system. A noisy electrical system is painfully unpleasant for facility managers, owners, operators and customers. These difficulties are commonly communicated back to the electrical engineers who supported the design.
For many, the path of least resistance is to assign blame to the power utility company. In many cases, this snap judgment is inaccurate or neglectful. Careful analysis of the electrical system may identify opportunities to improve power quality at the facility.
Transient risks to power quality
A few suspects promptly come to mind when considering how a facility’s power source could be destabilized: lightning strikes, switching transients, starting large motors and failing equipment. Surge protection devices are designed to redirect transient overvoltages caused by lightning strikes and switching transients. Controllers for large motors are commonly designed and specified to minimize voltage sags. Protective devices and relays are designed to monitor system parameters and isolate components from faults due failing equipment.
Each of these transient conditions merits substantial individual engineering attention. This discussion, however, will focus on an expanding area of concern that is far more common and much less transient in nature.
Nonlinear load risks
Modern technology has changed the landscape of power distribution and control. We have all witnessed the market growth of battery–powered vehicles, adjustable speed drives and data centers. Battery chargers recharge the batteries that power electric cars. Variable frequency drives play a critical role in modern facilities and energy conservation measures. Today, servers and digital storage are components of nearly all modern buildings. These devices share a common thread — the power electronic devices with nonlinear load profiles.
The ideal sinusoidal waveform we all know so well is effectively mutilated by nonlinear load. Unlike a linear load, the impedance of a nonlinear load varies without a proportional relationship to the applied voltage. This unique impedance results in nonsinusoidal current. The choppy current waveform interacts with overall system impedance, which impacts voltage. As these currents make their way through the distribution network, the risk of operation beyond design constraints increases.
Sensitive equipment, such as computers, may not operate as intended or fail when supplied with a distorted source of power. Transformers, motors, generators, capacitors, conductors and computer equipment are all susceptible to harmonic distortion. Transformers, generators and conductors can overheat. Capacitors are vulnerable to harmonic resonance. Motor torque can oscillate, causing excessive vibration and strain on the motor shaft.
Fourier series analysis allows engineers to deconstruct the nonsinusoidal waveform into individual sinusoidal components or harmonics. The choppy complicated waveform can now be described as a series of sinusoidal waveforms at multiples of the fundamental frequency. For example, the first order harmonic component of a 60-hertz system is 60 hertz, the second order harmonic component is 120 hertz and the third order harmonic component is 180 hertz.
Harmonic components are grouped by rotation of phasors with respect to the fundamental frequency. Third, sixth and ninth harmonics rotate in-phase with fundamental frequency and are described as zero-sequence harmonics. Second and fifth harmonics rotate in the opposite direction of fundamental frequency and are described as negative sequence. Fourth and seventh harmonics rotate in the same direction as fundamental frequency and are described as positive sequence
Power system analysis software allows engineers to analyze harmonics during design and during the construction phase after equipment is procured and installed. Studying harmonics during the design phase provides engineers with an opportunity to incorporate harmonic mitigation equipment and strategies into the design.
Harmonic mitigation equipment requires space for installation. Studying harmonics during the construction phase allows engineers to model as-built conditions and check harmonic distortion before operation. Performance testing during construction allows for verification that harmonic distortion is within the specified tolerances.
Minimizing harmful effects of nonlinear loads
IEEE Standard 519 provides goals for electrical engineers to minimize the harmful effects introduced by nonlinear loads. Analysis of a power distribution system with respect to Standard 519 is becoming increasingly important in the modern marketplace where nonlinear loads currently are present or proposed.
Limitations for individual harmonics, total harmonic distortion, total demand distortion and current distortion are identified within the standard. Criteria for measuring harmonic distortion are also provided within the standard. It is important to review and clarify the definitions used in IEEE standard 519 because the concepts and criteria are technical in nature.
Standard 519 definitions
Point of common coupling: “Point on a public power supply system, electrically nearest to a particular load, at which other loads are or could be, connected. The PCC is a point located upstream of the considered installation.”
The limitations identified for total harmonic distortion are applicable to the PCC. The limitations identified for TDD are a function of short-circuit ratio at the PCC. The first step of a harmonic analysis must identify the location of the PCC and clarify the decision to choose that location. The PCC sets the stage for establishing the interface between the user and power supplier.
A rule-of-thumb for commercial systems with a utility service transformer is to establish the PCC on the secondary side of the service transformer. A rule-of-thumb for industrial facilities with campus-style distribution is to establish the PCC on the primary side of the distribution transformer or upstream distribution switchgear serving the facility.
Total harmonic distortion: “The ratio of the root mean square of the harmonic content, considering harmonic components up to the 50th order and specifically excluding interharmonics, expressed as a percent of the fundamental. Harmonic components of order greater than 50 may be included when necessary.”
THD limitations identified under Table 1 of IEEE 1547 vary based on the voltage applied to the PCC. The tightest requirements for THD are for low–voltage systems.
Total demand distortion: “The ratio of the root mean square of the harmonic content, considering harmonic components up to the 50th order and specifically excluding interharmonics, expressed as a percent of the maximum demand current. Harmonic components of order greater than 50 may be included when necessary.”
TDD limitations vary based on short-circuit ratio. Goals for TDD are arguably as important as THD. The goals for harmonic current distortion are critical to review when specifying sources of standby power.
Short-circuit ratio (ISC/IL): “At a particular location the ratio of the available short-circuit current, in amperes, to the load current, in amperes.” Short-circuit current must be calculated to analyze TDD.
Maximum demand load current (IL): “This current value is established at the point of common coupling and should be taken as the sum of the currents corresponding to the maximum demand during each of the previous months divided by 12.” IEEE 519 Table 2 footnote C further clarifies maximum demand load current as current at the PCC during normal load operating conditions. Load calculations considering diversity must be developed to evaluate TDD.
Table 1 of IEEE 519 identifies recommendations for voltage distortion limits. Individual harmonic distortion limits and total harmonic distortion limits are identified for a range of voltages at the point of common coupling.
Table 2 of IEEE 519 identifies recommendations for current distortion limits. Distortion limits for groups of individual harmonic orders are identified (for example, third through 11th harmonic). Total demand distortion limits also are identified. The goals for current distortion are grouped by short-circuit ratio. Note that power generation equipment, such as standby generators, is subject to the most stringent current distortion limitations.
Part 4 of IEEE 519 identifies criteria for measuring harmonics. An assessment of real-life harmonic distortion against calculated values is recommended before project closeout. Instruments used to measure harmonics should comply with IEC 61000-4-7 and IEC 61000-4-30. Specify submittal requirements for instruments and procedures used to measure harmonic levels to allow for engineering review and approval.
Always bear in mind that the criteria established to limit harmonic distortion is based on steady-state worst-case conditions. Transient conditions that exceed the limitations are entirely possible. The limitations identified by these standards are recommendations only. IEEE 519 does not cover means and methods to protect motors driven by variable frequency drives from damage.
Inverter duty motors per National Electrical Manufacturers Association MG1 (Part 31), VFD cables, insulated bearings and additional load side filtering devices may be recommended to prevent circulating currents, premature bearing failure or stator insulation failure. Installation guidelines from VFD manufacturers often recommend VFD cable. It is important to consult with the manufacturer for the VFD, particularly for large motor applications, to determine additional needs before completing an electrical design.
Generator selection and specification must consider the nature of the loads being served. Inductive loads such as constant speed motors and transformers may be impacted when operating within an environment with harmonic distortion. Changing the impedance of a system may exacerbate harmonic distortion that was previously not obvious. Calculations may not reflect actual conditions. Performance testing provides a means to verify operation is within specified guidelines.
Goals for harmonic distortion are not legal requirements. However, contractual requirements to limit harmonic distortion may apply to an individual installation. From an efficiency perspective, it is easy to dismiss harmonic analysis when designing an upgrade that does not add nonlinear load. Due diligence, however, must be exercised to prevent unforeseen issues. Engineers can minimize exposure to project risk by analyzing harmonics and specifying methods to mitigate unacceptable distortion.