10 Questions You Must Prepare for a Power Systems Engineer Interview

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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.

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, relative to the DC power flow, the AC version is more accurate and a better representation of the 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?

It is unlikely that you will need to solve a power flow by hand in an interview. However, it is possible that you will 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 the Surge Impedance Loading (SIL) of a transmission line?

Surge Impedance Loading (SIL) is an important concept in transmission design and operations as 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: What can you do to 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 things to do 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.

• 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 give at least a couple of options and discuss their trade-offs if needed.

Question 6: What can you do to 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 things to do 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 appliable (study first)

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

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 into one. One of the important differences 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 know the 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 Area Control Error (ACE)?

If you don’t understand ACE, don’t work in the control room. 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, taking into account 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 what power factor is shows a candidate’s understanding of AC circuits and the power triangle. Each electric utility company has rules of power factor to ensure the efficient usage of the transmission system.

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 is, the more efficient the transmission system is utilized as it is usually 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 in case 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 you find the above questions and sample answers useful. I may put together a high-frequency question list for an energy market job interview if there is interest. Thanks for reading.