10 Questions You Should to Know about power factor tester

Re: questions on Power Factor

Here are some sample questions from our equivalent exam in Canada, the Interprovincial.

1. Calculate the power factor of a 10 HP, 460 volt, 14 amp, three phase motor at full load, if it's efficiency is 86%. ___________%.


2. Determine the full load current drawn by a 2 HP, 230 volt, single phase motor, operating with an efficiency of 80%, and a power factor of 67%. __________amps.


3. A 480 volt, three phase feeder is supplying a load of 38.4 kilowatts at 80% power factor. Calculate the reactive power. _________kilovars.

Ed

[ June 08, , 01:27 PM: Message edited by: Ed MacLaren ] Re: questions on Power Factor

Here are some more that I know have been seen on the EXPERIOR EXAM.

1. The power factor of an incandescent lighting circuit is:

a. 0 b. .5 c. .707 d. .866 e. 1.0

2. The phase relationship between voltage and current at a capacitor is the current:

a. leads the voltage by 45 degrees
b. is in phase with the voltage
c. leads the voltage by 90 degrees
d. lags the voltage by 90 degrees

3. Power factor is:

a. R/X
b. power/apparent power
c. the efficency
d. the ratio of current to in-phase current

Not all power factor calculations are mathematical in nature. Re: questions on Power Factor

Mike Holts exam prep has representative questions and has been highly recommended by those who have used it.
By the way, Washington State dropped Experior (they were bought out by Sylvan Learning Systems) as there were too many complaits, and the state electrical dept felt that Experior had no incentive for an applicant to pass the first time, retake=more money for Experior. Re: questions on Power Factor

Originally posted by tom baker:
By the way, Washington State dropped Experior (they were bought out by Sylvan Learning Systems) as there were too many complaits, and the state electrical dept felt that Experior had no incentive for an applicant to pass the first time, retake=more money for Experior.

It is my understandiong that there are 19 counties in the state of Florida that has dropped local examination when Experior took over Block and Associates. I agree that Experior is in the business to sell a test and not assist in applicant qualifying.

However, what testing agency is really testing the applicant on their electrical abilities and not their testing abilities?

Here is a quote from the State of Florida Ulimited Certified Contractors Exam Application Form:

A person who is not familiar with electrical contracting and cannot use the N.E.C. will find it hard to guess the correct answer for any question because they present the candidate with a choice of common faults, incorrect practices, or plausible nonsense.

Preparing for a power system technical interview can be stressful. Depending on the role you apply for, technical questions will be asked to assess your overall knowledge of power systems analysis, power devices, and system protection. In this article, I will discuss ten questions you will likely see in a power systems technical interview. Answering these questions correctly will show the interview panel your understanding of important power system concepts and may increase your chance of being hired. However, the key points in the sample answers are not exhaustive. And, you should NEVER remember the answers mechanically, but ALWAYS understand the concepts being asked and describe them in your own words. Your answers should be relevant and concise. Don't explain every detail (this is very important!), but you may ask the interviewer if your answer fully addresses their questions. Be ready to defend your answer and elaborate if needed.

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Question 1: What is the AC power flow? What is the "DC" power flow? What are the pros and cons?

While these are technically three questions, they are highly related. Power flow is arguably the most important concept of power system engineering. Before interviewing for a power engineering job, you should know what power flow is and be able to compare its variations.

Sample answer:

The objective of the AC power flow is to

• determine the voltage and current levels at each bus in a power system, and
• calculate the active and reactive power flow in each transmission line/transformer under a given set of operating conditions, such as the load and generation levels and the network topology.

The "DC" power flow is a simplified version of the AC power flow. The classic DC power flow assumes the following:

• Reactive power is negligible (Q = 0).
• The bus voltages are fixed at their nominal values, i.e., 1 p.u.
• The angle difference across any line is small.

Compared to the AC power flow, the "DC" version is faster, non-iterative, and guarantees convergence. On the other hand, the AC version is more accurate and a better representation of physical reality.

Question 2: What are the typical bus types in the AC power flow analysis? Explain them.

As a power system engineer who regularly performs power system analysis, it is important to understand different types of buses and their modeling assumptions to be able to interpret power flow results.

Sample answer:

Generally speaking, there are three types of buses in the power flow analysis. They are the PQ bus, PV bus, and slack/swing bus.

• PQ bus: These buses represent the loads in the system. For PQ buses, the real and reactive power drawn from the bus is known. The voltage magnitude and angle at these buses are solved in the power flow.
• PV bus: These buses represent the generators in the system. For PV buses, the real power injection (from the generator) and voltage magnitude at the bus are known. The reactive power output and the voltage angle at the bus are solved in the power flow.
• Slack or swing bus: This is the bus at which the system's real and reactive power balance is maintained. For the slack bus, the voltage magnitude and angle (normally set to 0 as the reference) are fixed.

The above answer should give you an A. But if you really want to impress the interviewer, you can talk about the PV-PQ bus conversion logic. In short, A PV bus will become a PQ bus if the reactive power regulation of the generator is exhausted.

Question 3: What are the critical inputs to solve a power flow?

You will unlikely need to solve a power flow by hand in an interview. However, you may be asked to describe the critical inputs to run a power flow to show your understanding of the concept.

Sample answer:

You need the following information to solve a classic AC power flow:

• The real and reactive power drawn from each PQ bus.
• The real power output of the generator(s) and the voltage magnitude at each PV bus.
• The voltage magnitude and the angle at the slack bus.
• Series resistance (R), reactance (X), and charging susceptance (B) of each transmission line, and reactance of each transformer.
• The network topology.

Question 4: What is a transmission line's Surge Impedance Loading (SIL)?

Surge Impedance Loading (SIL) is important in transmission design and operations. It helps power engineers and operators estimate the voltage profile and determine if reactive compensation is needed.

Sample answer:

• Surge Impedance Loading (SIL) is the real power flow level (in MW) that makes a transmission line act like a resistor – the net reactive power injection or consumption across the line is zero.
• When the active power flowing in the line is less than its SIL, the line produces reactive power.
• When the active power flowing in the line is greater than its SIL, the line consumes reactive power.

Question 5: How can you correct a low voltage problem in real-time system operations?

Low voltage could occur in real-time operations for various reasons. Low voltages, if not corrected appropriately, could lead to voltage collapse and uncontrolled loss of load. As a power engineer, you should know how to mitigate a voltage issue and discuss their trade-offs if needed.

Sample answer:

Various actions can be taken to correct a low-voltage problem.

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• Switch in shunt capacitors. (increase Q injection)
• Switch out shunt reactors. (reduce Q consumption)
• Stop pumping hydro. (reduce load)
• Turn on generators in the low-voltage area. (increase Q injection)
• Increase the terminal voltage set point of generators in the low voltage area. (increase Q injection if they still have Q regulation available)
• Switch in series capacitor if applicable. (reduce Q consumption across the line)
• Return outaged transmission lines if applicable. (lower overall system impedance and reduce Q consumption across the line)
• Load management, including shedding firm load (a last resort).

The interviewer will likely not expect you to answer all the options above, but you should at least give a couple of options and discuss their trade-offs.

Question 6: How can you correct a high voltage problem in real-time system operations?

Compared to low voltage issues, high voltage, if not handled appropriately, could be equally harmful, especially to lines and generators. As a power engineer, you should know how to mitigate a voltage issue and their trade-offs. You can pretty much take the options in the question above and reverse them to get the correct answer for this one.

Sample answer:

Various actions can be taken to correct a high-voltage problem.

• Switch out shunt capacitors.
• Switch in shunt reactors.
• Start pumping hydro and charging battery storage.
• Turn off generators in the high-voltage area. (only applicable in a "load pocket," why?)
• Decrease the terminal voltage set point of generators in the high voltage area.
• Switch out the series capacitor if applicable
• Switch out lightly loaded transmission lines if applicable (study first)

Again, the interviewer will likely not expect you to answer all the options above, but you should at least give a couple of options and discuss their trade-offs.

Question 7: What is the difference between phase voltage and line voltage? What is the relationship between the phase and line voltages in a balanced three-phase system?

These two questions are highly related, so I again lump them together. One important difference between the high-voltage power system and the residential electricity supply is that the former uses a three-phase system. As a power system engineer, it is critical to differentiate the two systems and understand their mathematical and physical relationships.

Sample answer:

• Line voltage is the voltage between two phases, e.g., phase A and B, or phase B and C, whereas the phase voltage is the voltage between the given phase and neutral (ground).
• In a balanced three-phase system, the line voltage is √3 times the phase voltage.

Question 8: What is an Area Control Error (ACE)?

You should never work in the control room if you don't understand ACE. You may not remember the exact ACE equation (although you should), but you must know the ACE components and understand why ACE is important in real-time operations.

Sample answer:

• Area Control Error (ACE) is the difference between the scheduled and actual interchange in MW, considering the frequency bias and meter error contribution.
• Holding the frequency component constant, ACE indicates whether the generation and load in the system are balanced.

Question 9: What is the power factor?

Accurately describing the power factor shows a candidate's understanding of AC circuits and the power triangle. Each electric utility company has minimum power factor criteria to ensure the transmission system's efficient usage.

Sample answer:

• In power systems, the power factor is the ratio of the real power and the apparent power drawn by the load at a bus.
• The power factor can range from 0 to 1. A power factor of 0 means no real power component, whereas a 1 power factor means no reactive power component.
• The higher the power factor, the more efficient the transmission system is utilized, as it is better to compensate for reactive power locally.

Question 10: Why do the three protection zones overlap?

System protection is a broad topic. As a power system engineer, you should know basic protection schemes and their design philosophy. The three-zone design is one of the fundamental protection designs you should know.

Sample answer:

• The main reason for overlapping protection zones is to provide redundancy so that secondary/tertiary relays can back up the primary ones if they fail to operate.
• For example, if the primary (Zone 1) relay fails to detect and clear the fault, Zone 2 relays will kick in with a delay and clear the fault.

I hope the above questions and sample answers are helpful to you. If you are interviewing for a position in the energy market, the following article summarizes 10 popular questions/concepts you know.

Additionally, for information about power systems engineering career options, please refer to my two other articles below:

Thanks for reading.

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