Power Quality, Harmonics, and Predictive Maintenance

Comprehensive practice questions covering harmonics, power factor calculation, transformer K-factors, VFD characteristics, and power quality troubleshooting as detailed in the lecture transcript.

What are harmonics in electrical power systems?

Voltage and current equations that represent sine waves that are multiples of a fundamental frequency.

How is Total Harmonic Distortion (THD) defined?

The ratio of the rms value of the total harmonic currents and the rms value of the fundamental current.

What is the formula for Crest Factor (CF)?

$$\text{Crest factor} = \frac{I_{\text{peak}}}{I_{\text{rms}}}$$

What is the Crest Factor (CF) for a purely linear sinusoidal waveform?

The square root of 2, which is approximately $1.414$.

How is the form factor defined?

The ratio of the rms value of a waveform to the rms value of the waveform’s fundamental.

What are the four types of nonlinear power electronic devices commonly used in commercial facilities?

Fluorescent lighting, ASDs (Adjustable Speed Drives), SMPS (Switched-Mode Power Supplies), and uninterruptible power supplies (UPS).

What range of third harmonic distortion (HD) do magnetic ballasts typically generate?

$13\% \text{–} 20\%$.

What is the ANSI recommendation for THD in electronic ballasts?

THD less than $32\%$.

What equation determines the characteristic harmonics (h) for a VFD or ASD?

$h = (n \times p) \pm 1$ where $n$ is an integer and $p$ is the number of pulses of the rectifier.

Name the characteristic harmonics created by a six-pulse rectifier.

5th, 7th, 11th, 13th, 17th, 19th, and so on.

What is the difference between VSI and CSI drives regarding their DC link components?

VSI drives employ a large capacitor in the DC link, while CSI drives consist of a large choke to keep the DC current constant.

What is Displacement Power Factor (DPF)?

The component of the Power Factor (PF) associated with the angle between the voltage and the current without distortion.

What is the definition of True Power Factor (PF)?

Real power divided by total apparent power.

What is the peak inverse voltage (PIV) rating for power semiconductor switches in many VFDs?

$1200\,V$.

Why do SMPS current waveforms exhibit high Crest Factors?

Because the internal capacitor only draws a pulse of current near the peak of each sine wave to maintain constant DC voltage.

How is Power Factor (PF) defined for linear loads?

$$PF = \frac{\text{kW}}{\text{kVA}} = \cos(\theta)$$

What is the upper bound for True PF in nonsinusoidal situations based on current THD?

$$\text{True PF} \le \frac{1}{\sqrt{1 + (\text{THD}_I/100)^2}}$$

What is the phase sequence of the first harmonic (fundamental)?

Positive sequence (a–b–c).

What is the phase sequence of the second harmonic?

Negative sequence (a–c–b).

What is the phase sequence of the third harmonic?

Zero sequence.

Why do line-to-line voltages in commercial buildings typically lack the third harmonic?

According to Kirchhoff’s voltage law, they always sum to zero and thus never have zero sequence components.

What are the three categories of effects caused by voltage distortion?

(1) Insulation stress, (2) thermal stress, and (3) load disruption.

What two components make up total iron losses in power equipment?

Hysteresis loss and eddy current loss.

What is the formula for hysteresis loss (Ph)?

$$P_h = a_h f B_m^v$$

How are capacitors affected by the presence of harmonics?

Increased dielectric losses (heating), resonance conditions, and overvoltages.

What does the 'K-factor' of a transformer indicate?

The transformer's ability to tolerate the additional heating caused by harmonics.

What is the K-factor value for a standard transformer serving linear loads?

$1$.

What is supersynchronous resonance in a rotating machine?

A condition where mechanical vibration frequency exists close to the frequency of electrical stimulus, often caused by 5th and 7th harmonics.

What is the maximum continuous reactive power limit for shunt capacitors according to IEEE 18-2002?

No greater than $135\%$ of its rated value.

What is voltage flicker?

Voltage changes that are cyclical in nature, occurring in the range of $0.5\text{–}30\,Hz$.

What safety rating category is required for meters used in PQ troubleshooting?

CAT IV-600 V according to IEC 61010.

What is the purpose of an Earth Ground Tester?

To measure the ground electrode impedance using the fall-of-potential method.

According to typical surveys, where do the majority of PQ problems originate?

Within the facility itself, with $75\%$ related to wiring and grounding problems.

What is voltage flat-topping?

A distortion where the voltage peak is dragged down, typically caused by SMPS loads demanding current only at the peak of the wave.

What are the NEC-recommended limits for total voltage drop including the feeder and branch circuit?

Less than $5\%$.

Why is N–G voltage a useful measurement for checking circuit loading?

It acts as a mirror of L–N voltage; high current increases the voltage drop on the neutral, resulting in higher N–G voltage.

What is the industry rule-of-thumb limit for N–G voltage at a receptacle?

$2\,V$ or less is considered okay; $5\,V$ is seen as the upper limit.

How do triplen harmonics behave in shared neutrals of three-phase circuits?

They add up arithmetically and return on the neutral, potentially causing overheating.

What is the K-factor rating typically used for transformers serving data processing equipment or ASDs?

K-20.

What is the difference between THD and Total Demand Distortion (TDD)?

THD is the ratio of harmonics to the instantaneous load, while TDD is the ratio of current harmonics to the maximum load current ($I_L$).

What does a negative sequence harmonic like the 5th do to a polyphase induction motor?

It creates counter-torque, which produces opposing torque and causes motor overheating.

How much can a $1\%$ voltage unbalance affect motor current unbalance?

It can cause current unbalance in the neighborhood of $8\%$ ($8:1$ ratio).

What is the definition of electrical noise?

Random electrical signals coupled into circuits that disrupt information-carrying signals.

What are the four basic mechanisms of noise coupling?

Capacitive, Inductive, Conducted, and RFI (Radio Frequency Interference).

What differentiates impulsive transients from oscillatory transients?

Impulsive transients are high-frequency 'spikes,' while oscillatory transients involve a 'ringing' waveform riding on the normal wave.

What is the function of a Signal Reference Grid (SRG)?

An effective high-frequency grounding technique that provides an equipotential ground plane.

What is the primary role of a TVSS device?

To protect sensitive equipment by clipping excess voltage using components like MOVs (Metal-Oxide Varistors).

What is an isolation transformer used for in PQ applications?

To establish a power ground reference close to the point of use and reduce common-mode noise.

What are the two major types of UPS systems defined in the text?

Static UPS and Rotary UPS (RUPS).

What is the purpose of a harmonic trap filter?

An LC network connected in parallel at the source, tuned to a frequency like $280\,Hz$ to sink the 5th and 7th harmonics.