TESTING & COMMISSIONING ENGINEER INTERVIEW QUESTIONS

 

ANSWER'S UPDATING SOON...........

Transformer theory

Transformers are required throughout modern interconnected power systems. The size of these transformers ranges from as low as a few kVA to over a few hundred MVA, with replacement costs is more. Power transformers are usually very reliable, with a 20-35 year design life. In practice, the life of a transformer can be as long as 60 years with appropriate maintenance.

However, the in-service failure of a transformer is potentially dangerous to utility personnel through explosions and fire, potentially damaging to the environment through oil leakage, is costly to repair or replace, and may result in significant loss of revenue. In a large public power utility, the number of transformers in the sub-transmission and transmission network (excluding the lower-voltage distribution network) can be from a few hundred to over one thousand.

As transformers age, their internal condition degrades, which increases the risk of failure. Failures are usually triggered by severe conditions, such as lightning strikes, switching transients, short-circuits, or other incidents. When the transformer is new, it has sufficient electrical and mechanical strength to withstand unusual system conditions. As transformers age, their insulation strength can degrade to the point that they cannot withstand system events such as short-circuit faults or transient overvoltages.

To prevent these failures and to maintain transformers in good operating condition is a very important issue for utilities. Traditionally, routine preventative maintenance programs combined with regular testing were used. With deregulation, it has become increasingly necessary to reduce maintenance costs and equipment inventories. This has led to reductions in routine maintenance.

The need to reduce costs has also resulted in reductions in spare transformer capacity and increases in average loading.There is also a trend in the industry to move from traditional time-based maintenance programs to condition-based maintenance. These changes occur at a time when the average age of the transformers in service is increasing and approaching the end of nominal design life.

The change to condition-based maintenance has resulted in the reduction, or even elimination, of routine time-based maintenance. Instead of doing maintenance at a regular interval, maintenance is only carried out if the condition of the equipment requires it. 

Hence, there is an increasing need for better non-intrusive diagnostic and monitoring tools to assess the internal condition of the transformers. If there is a problem, the transformer can then be repaired or replaced before it fails.

Knee point voltage

How to calculate Knee point voltage for CT

Knee Point Voltage,

Vk = [(VA/Is^2) + Rct]*ALF*Is

VA = CT Burden,

Is    = Secondary Current,

Rct = CT Resistance with loop,

ALF = Constant (ie in 5P20 class ALF=20)

vector group

Why is vector group important?

• Because it determines the phase angle displacement between the primary (HV) and secondary (LV) windings.

If the vector-group is changed from Yd11 to Yd1, how will it affect generator synchronization?

  

• The Yd11 has an HV to LV phase angle displacement of plus 30°, while that of the Yd1 is minus 30°. Synchronization could be done if the HV and LV configurations were reversed, i.e., Yd11 and Dy1 in parallel. But, it is not possible if the Yd1 were to be paralleled with a Yd11 if the primary of both transformers were supplied from the same source.

Phase displacement:

• Phase rotation is always anti-clockwise. (international adopted convention) 
• Use the hour indicator as the indicating phase displacement angle. Because there are 12 hours on a clock, and a circle consists out of 360°, each hour represents 30°. 
• Thus 1 = 30°, 2 = 60°, 3 = 90°, 6 = 180° and 12 = 0° or 360°. 
• The minute hand is set on 12 o'clock and replaces the line to neutral voltage (sometimes imaginary) of the HV winding. This position is always the reference point. 
• Because rotation is anti-clockwise, 1 = 30° lagging (LV lags HV with 30°)and 11 = 330° lagging or 30° leading (LV leads HV with 30°)
  

What is apparent power?

Apparent power is the combined power value that is obtained by allowing for the different values of current and voltage. Essentially, the measure of apparent power is achieved by multiplying the applied voltage to the current generated. Most electrical systems operate by the use of apparent power. 

There are a few basic understandings about the nature of apparent power that help to define the nature of the power. Components of both reactive and real power must be present in order for apparent power to exist. That is, the load must be both reactive and resistant in order for the flow of apparent power to take place, and thus create a means for measuring the nature of the generated power. In addition to the presence of resistant and reactive loads, it is also important to determine the watts involved in the current as well. Just as in determining the rate of any complex power, knowing the watt value is key to understanding the nature of apparent power. 

In the actual calculation of apparent power, the number of volts is multiplied by the number of amps. Results of the calculation are presented in terms of volt-amperes reactive and real power. This simple multiplication of voltage and current makes it possible to ensure that the flow of apparent power is sufficient to operate devices designed for a given level of power, as well as ensure the flow of apparent power can be handled by the components within the device.

What is reactive power?

• Reactive power is an odd topic in AC (Alternating Current) power systems, and it's usually explained with vector mathematics or phase-shift sine-wave graphs. 
• However, a non-math verbal explanation is possible. Note that Reactive power only becomes important when an "electrical load" or a home appliance contains coils or capacitors. If the electrical load behaves purely as a resistor, (such as a heater or incandescent bulb for example,) then the device consumes "real power" only. Reactive power and "power factor" can be ignored, and it can be analyses using an AC version of Ohm's law.  
• Reactive power is simply this: when a coil or capacitor is connected to an AC power supply, the coil or capacitor stores electrical energy during one-fourth of an AC cycle. But then during the next quarter-cycle, the coil or capacitor dumps all the stored energy back into the distant AC power supply. Ideal coils and capacitors consume no electrical energy, yet they create a significant electric current. 
• This is very different from a resistor which genuinely consumes electrical energy, and where the electrical energy flows continuously in one direction; moving from source to load. In other words, if your electrical appliance contains inductance or capacitance, then electrical energy will periodically return to the power plant, and it will flow back and forth across the power lines. This leads to an extra current in the power lines, a current which heats the power lines, but which isn't used to provide energy to the appliance. The coil or capacitor causes electrical energy to begin "sloshing" back and forth between the appliance and the distant AC generator. 
• Electric companies must install heavier wires to tolerate the excess current, and they will charge extra for this "unused" energy. This undesired "energy sloshing" effect can be eliminated. If an electrical load contains both a coil and capacitor, and if their resonant frequency is adjusted to exactly 60Hz, then the coil and capacitor like magic will begin to behave like a pure resistor. 
• The "energy sloshing" still occurs, but now it's all happening between the coil and capacitor, and not in the AC power lines. So, if your appliance contains a large coil induction motor, you can make the motor behave as a pure resistor, and reduce the current in the power lines by connecting the right value of capacitance across the motor coil. Why is reactive power so confusing? Well, the math is daunting if not entirely obscure. 
• And the concept of "imaginary power" puts many people off. But this is not the only problem. Unfortunately most of us are taught in grade school that an electric current is a flow of energy, and that energy flows back and forth in AC power lines. 
• This is completely wrong. In fact the energy flows constantly forward, going from source to load. It's only the charges of the metal wires which flow back and forth. Imagine that we connect a battery to a light bulb. Electric charges already present inside the wires will begin to flow in the circle, and then electrical energy moves almost instantly to the light bulb. The charge flow is circular like a belt, but the energy flow is one-way. 
• Now imagine that we suddenly reverse the connections to the battery. The voltage and current will reverse... but the energy still flows in the same direction as before. It still goes from battery to bulb. If we keep reversing the battery connections over and over, we'd have an AC system. So, in an AC system, only the voltage and current are "alternating," while the electrical energy flows one-way, going from source to load. Where AC resistive loads are concerned, electrical energy does not "alternate." 
• To understand energy flow in AC systems, it's critically important that we understand the difference between charge flow (current, amperes) and energy flow (power, watts.) What is imaginary power? Simple: it's the unused power which flows backwards and forwards in the power lines, going back and forth between the load's coil or capacitor and the distant AC generator. 
• If your appliance was a pure capacitor or inductor, then it would consume no electrical energy at all, but instead all the flowing energy would take the form of "sloshing energy," and we'd call it "imaginary power." Of course it's not actually imaginary. Instead it's reflected by the load.

Why is the transformer rated in KVA?

kVA is the unit for apparent power. Apparent power consists of active and reactive power. Active power is the share of the apparent power which transmits energy from the source (generator) to the user. Reactive power is the share of the apparent power which represents a useless oscillation of energy from the source to the user and back again.

It occurs when on account of some inertia in the system there is a phase shift between voltage and current. This means that the current does not change polarity synchronous with the voltage. But the heat generated in a winding as well as the eddy current losses generated in a transformer core depend on the current only regardless of whether it aligns with the voltage or not.

Therefore the heat is always proportional to the square of the current amplitude irrespective of the phase angle (the shift between voltage and current). So a transformer has to be rated (and selected) by apparent power.