Power Systems Basics

Alternating Waveforms

Voltage

The voltage v(t) in an AC circuit can be described by the equation:

\( \Large v(t) = \Large V_{max} \sin(\omega t+\phi_{v} ) \)

Current

The voltage v(t) in an AC circuit can be described by the equation:

\( \Large i(t) = \Large I_{max} \sin(\omega t+ \phi_{i} ) \)

Where:

\( \Large{ V_{max} :} \) is the maximum (peak) voltage
\( \Large{ I_{max} :} \) is the maximum (peak) current

\( \Large{ \omega :} \) is the angular frequency, calculated as \(\Large 2 \pi f\)
\(\Large f\) is the frequency in hertz
\(\Large f = \frac{1}{T} = \frac{ \omega}{2 \pi} \)

\( \Large{T :} \) is the period of the sinusoid
\( \Large{t :} \) is the imaginary unit
\( \Large{\phi :} \) is the phase angle of the voltage waveform in radians.

Inductance

Voltage

The voltage v(t) across an inductor:

\( \Large V(t)_{L} = \Large V_{\normalsize source} \: e^{-t/\tau} \)

Current

The current I(t) through an inductor:

\( \Large I(t)_{L} = \Large I_{\normalsize source} \: e^{-t/\tau} \)

Inductance

Capacitive Resistance magnitude equation, in ohms:

\( \Large X_{C}= \Large 2 \pi fL \)

Where:

\( \Large{ V_{max} :} \) is the maximum (peak) voltage
\( \Large{ I_{max} :} \) is the maximum (peak) current

\( \Large{t :} \) Time
\( \Large{ \omega :} \) is the angular frequency, calculated as \(\Large 2 \pi f\)
\(\Large f\) is the frequency in hertz
\(\Large f = \frac{1}{T} = \frac{ \omega}{2 \pi} \)

\( \Large \tau \); Inductor resistance time constant
\(\Large \tau = \frac{L}{R} \)

Capacitance

Capacitance

\( \Large Q = CV \)

\( \Large C = \frac{\varepsilon A}{d} \)

Voltage

The voltage v(t) across an capacitor:

\( \Large V(t)_{c} = \Large V(0)_{c} + \frac{1}{C} \int_{0}^{t} i(\tau)_{c} \; d\tau \)

Current

The current I(t) through an capacitor:

\( \Large i(t)_{c}= C(\frac{dv_{c} }{dt}) \)

Impedance

Ideal Capacitance, in Farads (F):

\( \Large X_{C}= \Large \frac{1}{\Large 2 \pi fC } \)

Capacitance

\(\Large f\) is the frequency in hertz
\(\Large f = \frac{1}{T} = \frac{ \omega}{2 \pi} \)

\( \Large \tau \); Inductor resistance time constant
\(\Large \tau = \frac{L}{R} \)

Where:

\( \Large{ V_{max} :} \) is the maximum (peak) voltage
\( \Large{ I_{max} :} \) is the maximum (peak) current

\( \Large{t :} \) Time
\( \Large{ \omega :} \) is the angular frequency, calculated as \(\Large 2 \pi f\)

Capacitance

Voltage

The voltage v(t) across an inductor:

\( \Large V(t)_{L} = \Large V_{\normalsize source} \: e^{-t/\tau} \)

Current

The current I(t) through an inductor:

\( \Large I(t)_{L} = \Large I_{\normalsize source} \: e^{-t/\tau} \)

Inductance

Capacitive Resistance magnitude equation, in ohms:

\( \Large X_{C}= \Large \frac{1}{\Large 2 \pi fC } \)

Where:

\( \Large{ V_{max} :} \) is the maximum (peak) voltage
\( \Large{ I_{max} :} \) is the maximum (peak) current

\( \Large{t :} \) Time
\( \Large{ \omega :} \) is the angular frequency, calculated as \(\Large 2 \pi f\)
\(\Large f\) is the frequency in hertz
\(\Large f = \frac{1}{T} = \frac{ \omega}{2 \pi} \)

\( \Large \tau \); Inductor resistance time constant
\(\Large \tau = \frac{L}{R} \)

Impedance

Impedance (Z) is a comprehensive measure that combines resistance (R) and reactance (X), accounting for the total opposition a circuit presents to alternating current. It's expressed in ohms (Ω) and plays a crucial role in analyzing AC circuits, affecting voltage, current, and power factor.

\( \Large Z = \huge \frac{V}{I} \)

\( \Large Z = \Large R \times jX \)

\( \Large X = \Large X_{L} - X_{C} \)

\( \Large X_{L}= \Large 2 \pi fL \)

\( \Large X_{C}= \Large \frac{1}{\Large 2 \pi fC } \)

Where:

\( \Large{R} \): is the resistance, the real part of impedance, which opposes both direct current (DC) and alternating current (AC)
\( \Large{X} \): is the reactance, the imaginary part of impedance, which opposes only AC

\( \Large{j} \): is the square root of -1, representing the imaginary unit

\( \Large{X_{fL}} \): is the inductive reactance, with L being the inductance in henries (H) and f the frequency in hertz (Hz),
\( \Large{X_{fC}} \): s the capacitive reactance, with C being the capacitance in farads (F)

Complex power

Complex power in electrical engineering refers to the power flow in AC circuits, encompassing both real and reactive power components, which represent the actual power consumed by loads and the power stored in the system, respectively. It's crucial in understanding power system operations, including generation, transmission, and distribution, as well as in the design and analysis of AC electrical systems.

\( \Large S = \Large P \times jQ \)

\( \large P = \large VI \cos(\varphi) = V^{2}I = I^{2}R \)

Real power P is the capacity of the circuit for performing work in a particular time. It is given by:

\( \large Q = \large VI \sin(\varphi)\)

Reactive power Q represents the energy that oscillates between the source and the reactive components (inductors and capacitors) in the system. It is given by:

Where:
\( \textbf{S :} \) is the complex power, measured in volt-amperes (VA)
\( \textbf{P :} \) is the real power, measured in watts (W)
\( \textbf{j :} \) is the imaginary unit
\( \textbf{Q :} \) is the reactive power, measured in volt-amperes reactive (VAR)
\( \textbf{V :} \) is the RMS voltage
\( \textbf{I :} \) is the RMS current

3 Phase Power

\( \Large S = \Large P \times jQ \)

\( \Large S = \Large 3V_{p} I_{p}^{*}\)

\( \large \hspace{1em}\: = \sqrt{3} V_{L} I_{L} ( \cos(\theta_{P}) +j \sin(\theta_{P} ) \)

\( \large |S| = \large 3V_{P}^{2} I_{P} = \sqrt{3} V_{L}^{2} I_{L}\)

DELTA

\( \Large S = \Large 3\frac{ V_{P}^{2}}{ Z_{DELTA}^{*}} \)

WYE

\( \Large S = \Large \frac{ V_{P}^{2}}{ Z_{WYE}^{*}}\)

Power Factor Power

Power factor (PF) is a critical measurement in AC electrical systems, representing the ratio of real power flowing to the load to the apparent power in the circuit. It is a dimensionless number ranging between -1 and 1, and it's a key indicator of electrical efficiency.

Real power P is the capacity of the circuit for performing work in a particular time. It is given by:

\( \large PF = \large \cos(\varphi)\)

where \(\Large \varphi\) is the phase angle between the voltage and the current waveforms. A power factor of 1 (or -1) means that all the power is real power, which is ideal because it means all the power supplied is being used for useful work. A power factor less than 1 indicates the presence of reactive power in the system, which does not perform work but creates additional load on the electricity supply.

There are two types of power factors: leading and lagging. A lagging power factor occurs in inductive loads, such as motors and transformers, where the current lags behind the voltage. A leading power factor occurs in capacitive loads, where the current leads the voltage.

Improving the power factor in a system can lead to several benefits, including reduced transmission losses, improved voltage levels, and lower electricity costs. Methods to improve power factor include:

  • Adding capacitors or synchronous condensers to the circuit to offset the lagging power factor caused by inductive loads.
  • Using power factor correction devices that automatically adjust the compensating reactive power as needed.
  • Optimizing equipment operation and maintenance to ensure it operates efficiently.

In the context of motor starting and electrical load monitoring, as discussed in the documents, power factor plays a crucial role. During motor starting, the inrush current can cause a significant drop in power factor, leading to higher demands on the electrical supply and potential voltage drops. Proper sizing of start-up equipment and protective devices is necessary to manage the effects of low power factor during start-up phases. Additionally, in electrical load monitoring, understanding and correcting power factor issues is essential for ensuring system reliability, optimizing performance, and reducing operational costs. Monitoring equipment not only tracks current and voltage but can also measure power factor to identify inefficiencies and areas for improvement in the electrical system.

Circuit Breakers
Purpose

Circuit breakers are protective devices, which perform two primary functions:

  1. Open and close electrical circuits

    2. Similar to switch, circuit breakers are the primary way to energize and de-energize the circuit. Specialized circuit breakers can also be opened or closed remotely.

  2. Current overload and short circuit protection of electrical equipment.

    3. Overloading of electrical equipment, such as cables, can deteriorate insulation due to thermal stress cause by heat.

    As current increases past the cables design rating, insulation will begin to deteriorate. Over an extended period of time, leakage current will increase, eventually causing it to fail.

Internal Breaker mechanism
Name Plate Rating Definitions
  1. Continuous Current Rating

    2. Continuous current is the maximum value of steady state amperes that the CB contacts and internal conductors are designed to carry.

  2. Rated Voltage

    3. Rated voltage is the maximum operating voltage for which the circuit breaker is designed. Voltage ratings are given in terms of threephase linetoline voltage.

  3. Rated Interrupting Current (AIC)

    4. Rated interrupting current is the maximum current that the CB is designed to interrupt at the time the contacts part.

  4. Impulse Withstand Voltage

    5. Rated voltage is the maximum operating voltage for which the circuit breaker is designed. Voltage ratings are given in terms of three-phase line-to-line voltage.

  5. Short Time Current

    6. The short time current rating is the maximum amount of current in amperes which the CB contacts and internal conductors can carry, without damage, for a short time period (typically, three seconds). This rating also accounts for permanent stress to insulation, heat, and electromagnetic effects.

Breaker name plate
Molded Case Circuit Breakers (MCCB)

MCCB are the most widely used type of circuit breakers. They are available in a wide range of ratings and are generally used for low-current, low-energy power circuits. They can be found in residential, commercial, and industrial facilities.

MCCB have two protective elements built in to them.

  1. Thermal Bimetallic Element

    2. provides an inverse time–current characteristics for over-current protection

  2. Mechanical Magnetic Trip Element

    3. provides short circuit current protection

Neta Table 100.1
thermal-magnetic breaker
Insulated-case circuit breakers

Insulated-case circuit breakers are a type of molded-case breaker constructed with glass reinforced insulating material for increased dielectric strength. These breakers can have Electromechanical trip units which was discussed above, or an Electronic trip units offer capabilities such as programming monitoring diagnostics communications system coordination and testing that are not available on thermal magnetic trip units.

Neta Table 100.1
Draw-Out Power Circuit Breakers

Generally, these breakers have draw-out features whereby individual breakers can be put into test and fully de-energized position for testing and maintenance purposes.

Generally, these breakers have draw-out features whereby individual breakers can be put into test and fully de-energized position for testing and maintenance purposes.

Neta Table 100.1
Motor Circuit Protector (MCP)

Magnetic-trip-only breakers have no thermal element. Such breakers are principally only used for isolating the circuit and short-circuit protection.

Molded-case breakers with magnetic only trips find their application in motor circuit protection. MCP's can be found inside
Motor Control Center (MCC). .

They are typically placed inside a cubical or enclosure, along with motor control elements and a motor over-current device; commonly knows as a heater.

  1. Operating Handle Mechanism
  2. Overload Reset Button and Reset Rod Extension Kit
  3. Unit Draw-out Top Rail
  4. Terminal Blocks
  5. Control Transformers Primary/Secondary Fuse Holder Kit
  6. Device Panel/Pivot Tube Fusible Disconnect Block Kit
Neta Table 100.1
Electric Motors

Name Plate Data

Motor nameplate terminology refers to the standardized information provided on the nameplate of an electric motor. Here's an overview of common terms and data typically found on a motor nameplate:

  • Full Load Amps (FLA):
    Indicates the current the motor is expected to draw under full-load conditions. It's a crucial parameter for selecting overload protection devices .
  • Voltage Rating:
    The voltage at which the motor is designed to operate. Motors may have multiple voltage ratings, allowing for different wiring configurations (e.g., dual-voltage motors).
  • Speed (RPM):
    The rated speed of the motor in revolutions per minute (RPM) at full load. It's a function of the motor's design and the frequency of the electrical power supply.
  • Inrush Current:
    Refers to the initial surge of current when the motor starts. This is significantly higher than the FLA and can be 20 times higher than the motor’s normal full load current initially, subsiding to 4-8 times the normal current for several seconds .
  • Frame Size:
    A designation that refers to the physical dimensions of the motor's mounting face, shaft, and bolt holes, ensuring compatibility with equipment.
  • Rated Power:
    The output power the motor is designed to produce on a continuous basis at full load and a specified temperature.
  • Service Factor (SF):
    A multiplier that indicates how much over the rated load a motor can handle for short periods without damage. For instance, a motor with a 1.15 SF can operate at 115% of its rated load without overheating .
  • Enclosure Type:
    Describes the motor casing's construction and its ability to protect against environmental conditions. Examples include Open Drip Proof (ODP) and Totally Enclosed Fan Cooled (TEFC).
  • Starting Method:
    Some motors might specify the recommended or integrated starting method, such as direct-on-line (DOL), star-delta, or soft starter, affecting the inrush current and the method of control.

Induction Motors

Also known as asynchronous motors, these are the most common types of AC motors. Induction motors operate on the principle of electromagnetic induction, where the rotating magnetic field of the stator induces a current in the rotor. This category can be further divided into:

  • Squirrel Cage Induction Motors:
    These have a simple, rugged construction with a rotor resembling a squirrel cage. They are known for their durability and efficiency in constant-speed applications.
  • Wound Rotor (Slip Ring) Induction Motors:
    These motors have a rotor with windings connected to slip rings. They offer advantages in applications requiring speed control and high starting torque.

Synchronous Motors

Synchronous motors operate at a constant speed, regardless of the load, synchronizing with the frequency of the supply current. The rotor speed is directly proportional to the frequency of the supply current. Types include:

  • Non-Excited Motors:
    These include permanent magnet motors, where the rotor is a permanent magnet, and reluctance motors, which operate based on the reluctance principle.
  • Excited Motors:
    These motors have an external DC source connected to the rotor for creating a magnetic field. They are used in applications requiring precise speed and position control.

Single-Phase Motors

Designed for single-phase power supply, these motors are typically used in domestic appliances and small machinery. They include:

  • Split Phase Motors:
    Utilize a secondary winding for starting, providing moderate starting torque.
  • Capacitor Start Motors:
    Have a capacitor in series with the starter winding, offering high starting torque.
  • Shaded Pole Motors:
    The simplest form of single-phase motors, using a shading coil to create a rotating magnetic field, suitable for low-power applications.

    • Variable Frequency Drives (VFD) Motors

    Though not a separate category of motors, VFDs are crucial in controlling the speed of AC motors. By varying the frequency and voltage supplied to the motor, VFDs allow precise control of motor speed, enhancing efficiency and control in applications ranging from industrial machinery to HVAC systems.

Specialized AC Motors

This category includes motors designed for specific applications, such as:

  • Servo Motors:
    Used in precision positioning applications, featuring high efficiency and control.
  • Linear Motors:
    Produce motion in a straight line, as opposed to the rotational motion of conventional motors.

AC Motor Equations

The formula for an induction motor primarily relates to its basic operation, performance characteristics, and efficiency. Induction motors operate based on the principle of electromagnetic induction, where a rotating magnetic field is produced by the stator (stationary part), inducing a current in the rotor (moving part), which creates another magnetic field that interacts with the stator field to produce torque.

Synchronous speed

One key formula for an induction motor is the calculation of the synchronous speed \( \large N_{s} \), which is the speed of the rotating magnetic field in the stator:

Synchronous Speed:

\( \large \hspace{10 mm}\Large N_{s}= \frac{120f}{p} \)

Where:

\( \textbf{N}_{\textbf{s}}: \) is the synchronous speed in revolutions per minute (rpm)
\( \textbf{f :} \) is the frequency of the AC power supply in hertz (Hz)
\( \textbf{P :} \) is the number of poles the motor has.

Slip Equation

Another important set of equations relates to the slip (\( \Large\textbf{s} \) ), which is the difference between the synchronous speed and the actual rotor speed, expressed as a percentage of the synchronous speed:

\( \large \hspace{10 mm}\Large s = \Large\frac{N_{s}- N_{r} }{N_{s}} \)

Where:

\( \Large\textbf{s} \) : is slip
\( \textbf{N}_{\textbf{r}} \) : is the rotor speed in rpm

The actual speed of the rotor ( \( \textbf{N}_{\textbf{r}} \)) can be calculated as:

\( \large \hspace{10 mm}\Large N_{r} = N_{s} (1-\Large\frac{s}{100}) \)

Torque

The torque ( \( \textbf{T} \)) produced by an induction motor can be approximated by the formula:

\( \large \hspace{10 mm}\Large \textbf{T}=\Large \frac{9.55 \times P_{out}}{N_{r}} \)

Where:

\( \textbf{T}: \) is the torque in Newton-meters (Nm)
\( \textbf{f :} \) is the output power in watts (W)

Efficiency

Efficiency( \( \eta \)) of an induction motor is defined as the ratio of output power to input power, usually expressed as a percentage:

\( \large \hspace{10 mm}\Large \eta =\Large \frac{P_{out}}{P_{in}} \times 100 \)

Where:

\( \textbf{P}_{\textbf{in}} \) is the input power in watts.

Medium Voltage Equipment

Medium voltage equipment evaluation has two components: momentary and interrupting ratings. The momentary rating is the asymmetrical current seen ½ cycle after the fault occurs. The interrupting rating reflects the fault duties at the time when a protective device will operate to clear a fault (typically 2, 3, 5 or 8 cycles).

ANSI allows a simplified momentary rating calculation of 1.6 times the symmetrical fault duty. The actual value is calculated as follows.

\( I_{asym \frac{1}{2}cycle}= I_{rms-sym} x \sqrt{1+2e^{\frac{-2\pi} {\frac{X} {R}} \times C} } \)

\({C}\)\(\text{= is the first} \frac{1}{2}cycle\)

Equipment Rating Evaluation

The purpose of the equipment evaluation is to compare the maximum calculated short-circuit currents to the short-circuit ratings of protective devices or the withstand rating of an enclosure. The Device Evaluation Report, located in the appendix of the project report, provides a summary of fault duties. It compares these duties, factoring in ANSI multipliers, with equipment ratings for each location within the modeled system. This comparison aims to determine whether the device is capable of either interrupting or withstanding the fault currents present in the electrical system where it's applied.

LSIG Curve
Model layout performed on SKM software Interface

Bus Name
Device ID of the switchboard, panelboard, or device.

Status
Device Evaluation

Equipment Category
Equipment or device type. LV or MV stands for low voltage or medium voltage.

Calc Isc_kA
Calculated short-circuit duty. The highest value of all the fault calculations is reported.

Dev Isc_kA
Device short-circuit rating. The calculated duty has been adjusted according to system X/R ratio and device test X/R ratio.

Isc Rating%
percentage of the device short circuit rating divided by the Calculated short-circuit fault duty.

Series Rating
Device series rating. Only applies to device which have a series rating. If the device is fully rated, this value will be zero.

Calc Mom_kA
The momentary fault duty or the closing and latching duty is the current that flows through the medium- and high-voltage system at one half-cycle after the fault occurs.

Dev Mom_kA
The momentary rating of the device

Mom Rating%
percentage of the device momentary rating divided by the Calculated momentary fault duty

Power Transformers

Power Transformers

Electrical power transformers come in various types, each categorized based on its construction, operation, application, and cooling methods. Every type is designed to serve a unique purpose within the electrical power system, ranging from stepping voltage levels up or down, isolating circuits, to managing phase shifts. The following structured outline provides an overview of the main categories and specific types of transformers, highlighting their distinct functions and applications.

Application

Station Transformers

Definition:
Designed for high-voltage transmission networks to step up or step down voltage levels.

Application:
Primarily used in power generation stations and transmission substations.

Characteristics:
High efficiency, large size, and designed for continuous operation at high loads.

Distribution Transformers

Used to step down voltage for distribution to residential or commercial users.

Application:
Installed at distribution substations or on utility poles.

Characteristics:
Smaller size compared to power transformers, designed for optimal performance at lower voltage levels.

Instrument Transformers

Current Transformer (CT)
The Purpose of a current transformer(CT) to reduce high current levels to a lower, measurable value. They are used for metering and protection in high-voltage circuits.

Potential Transformers (PT)

Voltage Transformers (VT), or
Control Power Transformers (CPT)

These transformers are also used to step down high voltage to a safer, measurable level. They are utilized for metering and protection by providing an accurate voltage measurement.

Transfomer Tanks

Transformer tanks are constructed from high-quality steel, ensuring they're tough and durable. The tanks are welded to be leak-proof, and their bolted covers come with gaskets for an extra seal of protection. Essentially, these tanks serve as a sturdy container for the core and windings, immersing them in an oil bath. This setup is crucial as it optimizes the insulation and cooling properties of the materials used in the transformer's core and windings. Should you spot any oil on the outside of the transformer or on the ground, it's a red flag that something might be amiss and should be inspected.

Sealed Tanks

Purpose
Sealed tanks are designed to protect the transformer's internal components from external environmental conditions by being completely sealed from the atmosphere.

Description:
These tanks are hermetically sealed and contain a fixed volume of oil, with no contact with the outside air.

Specific Application:
Ideal for transformers installed in polluted or humid environments where exposure to the elements could degrade the oil or internal components.

Pros:
Reduces the risk of oil contamination and oxidation.

Cons:
Limited ability to dissipate heat compared to other designs.

Conservator Tanks

Purpose
Conservator tanks are designed to handle the expansion and contraction of insulating oil due to temperature changes.

Description:
Features a separate conservator or expansion tank connected by piping, allowing oil to flow between the main tank and the conservator.

Specific Application:
Suitable for high-capacity transformers in power transmission where temperature fluctuations are common.

Pros:
Accommodates oil volume changes without compromising tank integrity.

Cons:
Increased maintenance requirements and complexity.

Expansion Tank

Purpose
Expansion tanks manage oil expansion due to thermal changes similarly to conservator tanks but in a more integrated manner.

Description:
Incorporates an expansion space or compartment within the main tank itself.

Specific Application:
Used in systems where external conservator tanks may be impractical.

Tests
Regular checks on internal pressure levels and seals.

Pros:
Simpler design than conservator tanks with fewer external components.

Cons:
Limited expansion capacity compared to separate conservator systems.

Gas Sealed Tank

Purpose
Gas sealed transformers use a gas, such as nitrogen, above the oil to provide insulation and prevent oxidation.

Description:
The tank is sealed with a gas layer above the oil to prevent contact with atmospheric oxygen.

Specific Application:
Often used in transformers where minimal maintenance is desirable and in sensitive environments.

Tests
Gas pressure and quality tests to ensure integrity and insulation properties are maintained.

Pros:
Reduces oxidation of the oil, prolonging its life.

Cons:
Requires monitoring of gas pressure and integrity to prevent leaks.

Bushings

Transformer bushings serve as the interface through which the transformer's windings are connected to the outside electrical network. They are essential for maintaining electrical safety and integrity by insulating the high voltage parts from the earthed metal casing of the transformer. There are several types of bushings used in transformers, which are primarily categorized based on their construction and insulating medium:

Solid Porcelain

Made from solid porcelain, these bushings are highly resistant to electrical stresses and environmental factors. The porcelain acts as an excellent insulator and can withstand high voltages while protecting the connections from dust, moisture, and mechanical damage.

Pros:
Excellent electrical insulation properties.

Highly resistant to environmental factors like moisture and pollution.

Durable and long-lasting under normal operating conditions.

Cons:
Brittle and may break if mishandled or exposed to mechanical stress.

Heavier compared to other materials, potentially complicating installation.

Not suitable for very high voltage applications where other types of bushings might be better.

Oil-filled Bushings

These bushings are designed to manage larger currents and higher voltages, often found in power transformers and large electrical apparatus. Oil-filled bushings contain insulating oil that helps in cooling and provides additional electrical insulation. The bushing's design ensures that the oil remains in contact with the internal components, thus enhancing its cooling and insulating properties.

Pros:
Excellent cooling and insulating properties due to the oil.

Suitable for high voltage and high current applications.

Can reduce the overall thermal and electrical stress on the system.

Cons:
Risk of oil leaks, which can lead to environmental issues and maintenance challenges.

Requires regular monitoring and maintenance to ensure integrity and performance.

Generally more expensive than other types of bushings due to their complexity and material requirements.

Capacitor Bushings:

Used in high-voltage applications, capacitor bushings help reduce electrical stress at the entry points of conductors into a transformer.These bushings feature layers of capacitive grading, which are materials designed to distribute the electrical field evenly across the bushing. This technology significantly reduces the risk of electrical breakdowns due to stress concentrations at the interface between high voltage conductors and grounded transformer cases.

Pros:
Highly effective in managing electrical stress and enhancing the lifespan of the transformer.

Suitable for very high voltage applications.

Helps in reducing the size of the bushing for a given voltage level due to capacitive grading.

Cons:
More complex and expensive to manufacture and maintain.

Potential for capacitive imbalances which require careful design and testing.

Not as robust in physical terms as solid porcelain, may require careful handling and installation.

Transformer Equations

One key formula for an induction motor is the calculation of the synchronous speed \( \large N_{s} \), which is the speed of the rotating magnetic field in the stator:

\( \Large Turns Ratio= \huge \frac{N_{1}}{N_{2}} \)

\( \huge \frac{N_{1}}{N_{2}} = \frac{V_{L1}}{V_{L2}} \)

\( \huge\frac{N_{1}}{N_{2}} = \frac{I_{L2}}{I_{L1}} \)

Where:

\( \Large V_{L1}, V_{L2} : \) are the line-to-line voltages on the primary and secondary sides, respectively.
\( \Large I_{L1}, I_{L2} : \) are the line currents on the primary and secondary sides, respectively.

Full load Amps (3 phase)

\( \Large FLA_{pri} = \huge \frac{VA}{ \sqrt{3} \times V_{L1} } \)

Full load Amps (Single Phase)

\( \Large FLA_{pri} = \huge \frac{VA}{ V_{L1} } \)

\( \Large VA\) : is the apparent power rating of the transformer in VA or kVA.

Transformer Short Circuit Current
(3 phase)

\( \Large FLA_{pri} = \huge \frac{VA}{ \sqrt{3} \times V_{L1} \times \% Z } \)

\( \Large \% Z \) : Nameplate Impedance value, in decimal form.

Name Plate Data

Power Rating:
Specified in kVA(Kilovolt-Ampere) or MVA (Mega Volt-Amperes). This ratings indicate the transformer's capability to handle different loads without overheating and damaging the insulation. They can also have multiple power ratings depending on their design and cooling methods.

Rated Voltages:
Includes High Voltage (HV), Low Voltage (LV), and sometimes Tertiary Voltage (TV) ratings, critical for matching the transformer with the system's voltage levels.

Impedance:
The transformer impedance value, typically given as a percentage, represents the inherent electrical resistance of the transformer to the flow of alternating current. This percentage represents the voltage drop across the transformer as a proportion of the rated voltage when the transformer is delivering full-load current.

Vector Configurations:
The vector configuration of a transformer describes the physical connections and phase angle difference between the primary and secondary windings.Common vector groups include Yy (wye-wye), Dy (delta-wye), Yd (wye-delta), and Dd (delta-delta).

Taps:
Transformer taps adjust the transformer's voltage ratio slightly to compensate for voltage variations and maintain the output within a desired range.

BIL:
Basic impulse insulation level (BIL) is the ability of the transformer insulation to withstand a transient overvoltage condition such as lightning or switching surges. Usually the BIL for the primary and secondary insulations are different and are listed separately.

General:

  • Rated Frequency
  • Year of Manufacture
  • Serial Number

Core and Windings:
Details about the core material, winding arrangements (like concentric or interleaved), and types of cores used, affecting the transformer's efficiency and performance.

Temperature Rise:
Indicates the maximum temperature rise above ambient temperature under full-load conditions, typically given in degrees Celsius.

Cooling:
Describes the cooling method used (like ONAN—Oil Natural Air Natural, ONAF—Oil Natural Air Forced, etc.), which affects the transformer's ability to dissipate heat generated during operation.

Type of Oil:
The specific type of insulating oil used, which can affect the transformer's thermal performance and dielectric strength. Common types include mineral oil, silicone, and less-flammable hydrocarbon fluids.

Oil Volume:
The total quantity of oil contained within the transformer, usually measured in liters or gallons. This information is essential for maintenance activities such as oil top-ups or replacements.

PCB Content:
If applicable, information about the presence of polychlorinated biphenyls (PCBs) in the oil, substances that are hazardous and subject to strict regulatory control.

Tap Changers

Tap changers are critical components in transformers, enabling voltage regulation by adjusting the transformer's turn ratio while it is in operation or de-energized. Understanding the different types of tap changers, their applications, advantages, and limitations is essential for optimizing transformer performance. Here's a detailed overview of various types of tap changers:

No-load Tap Changer (NLTC) / De-energized Tap Changer (DETC)

Purpose:
Adjusts transformer voltage ratios only when the transformer is de-energized, ensuring safe and non-disruptive maintenance.

Description:
DETCs are basic mechanical devices that require a manual operation to change taps. This operation must be performed with the transformer shut down, which ensures there are no electrical hazards or operational disruptions.

Specific Application:
Often used in applications where power can be disconnected without significant impact, such as in small industrial, residential, or remote installations.

How Taps Are Changed Dry-Type Transformers:
In Dry-Type Transformers, this involves loosening bolts which secure cables connectors (Image 1) between winding taps. In oil filled transformers, the process involves manually operating a switch (Image 2) or dial, which can be located on the transformer's interior or exterior.

Pros:
Simplicity, cost-effectiveness, and minimal complexity. It does not require sophisticated mechanical or electrical systems to operate.

Cons:
Lack of flexibility as it cannot respond to real-time changes in voltage demand. Requires downtime, which may not be feasible in critical applications

Image 1: Dry-type transformer TAP connector

Image 2:Pad Mount Transformer. TAP selector shown in red

Image 3: Oil-filled transformer TAP selector

On-Load Tap Changer (OLTC)

Purpose:
To adjust the transformer's voltage ratio while it is energized and carrying load, thus maintaining a consistent output voltage despite fluctuations in input voltage or load conditions.

Description:
OLTCs are equipped to handle tap changes without interrupting the transformer's operation, using either mechanical, electronic, or thyristor-based switching mechanisms to seamlessly transition between taps. This capability is crucial for managing voltage regulation in real-time, especially in large, interconnected power systems.

Specific Application:
Essential for utility and industrial power transformers where continuous service is critical. These are commonly used in high-voltage transformers in grid distribution and generation plants where voltage stability is paramount.

How Taps Are Changed:
Most OLTCs use a combination of diverter switches and selector switches that operate in a coordinated manner to change taps. The operation is automated and can be remotely controlled. Mechanical OLTCs briefly divert the current through a resistive element to minimize arcing and ensure smooth transitions between taps.

Pros:
Provides real-time voltage adjustment, enhancing system reliability and efficiency. It is critical for maintaining voltage levels within prescribed limits under varying load conditions.

Cons:
High complexity and cost, increased maintenance needs due to moving parts and electrical stress, and potential for oil degradation if not properly maintained.

Image 4: OLTC interior

Transformer Insulation

Transformers utilize various types of insulation to ensure effective separation of electrical components and to prevent electrical faults and failures. The choice of insulation is crucial because it directly impacts the transformer's efficiency, safety, longevity, and performance. Here are the common types of insulation used in transformers, along with their purposes and descriptions:

Air Insulation

Purpose
Air serves as a natural insulator in air-cooled or dry-type transformers.

Description:
In dry-type transformers, air is used to insulate and cool the transformer. Air insulation involves circulating air through the transformer’s enclosure to remove heat and maintain effective insulation between electrical components. This type of insulation is simpler and less hazardous than oil or gas insulation but typically offers lower thermal performance and is more suited to lower voltage applications.

Paper Insulation

Purpose
Paper insulation is used to provide additional electrical insulation and thermal barrier within the windings of a transformer.

Description:
Insulating paper, commonly made from cellulose fibers, is used extensively in transformers to wrap the conductors of coils. This type of insulation is impregnated with insulating oil to enhance its insulating properties and thermal conductivity. Over time, the quality of paper insulation can degrade due to heat and oxidation, which is a common reason for transformer aging.

Pressboard Insulation

Purpose
Pressboard is used in transformers to provide structure and additional insulation between the large components such as windings and between the windings and the earthed metal parts.

Description:
Made from cellulose, pressboard insulation is thicker and more rigid than paper. It is used for constructing spacers, barriers, and supports within the transformer. Pressboard can be molded into various shapes to fit complex configurations and is also oil-impregnated to improve its dielectric and mechanical properties.

Oil Insulation

Purpose
Oil insulation primarily serves two functions: it acts as a coolant to remove heat from the transformer's core and winding, and it provides electrical insulation to enhance dielectric strength between internal components

Description:
Transformer oil, typically a highly-refined mineral oil, is used in most power transformers. It is prized for its excellent insulating properties and its ability to dissipate the heat generated by the transformer during operation. The oil surrounds the transformer's coils and core, increasing the dielectric strength of the assembly and helping to prevent arcing and overheating.

Resin Insulation

Purpose
Resin insulation is used to encapsulate and protect transformer windings from moisture, dust, and other environmental factors.

Description:
In dry-type transformers, the coils are often encapsulated in epoxy resin, which provides excellent insulation and protection against environmental and mechanical stress. This type of insulation is particularly favored for transformers installed in harsh or moisture-prone environments as it prevents the ingress of contaminants that can lead to electrical faults.

Gas Insulation

Purpose
Gas insulation, typically using sulfur hexafluoride (SF6), is used in gas-insulated transformers to provide excellent dielectric properties and reduce the physical footprint of the transformer.

Description:
Gas-insulated transformers are sealed in a metal enclosure that is filled with SF6 gas. This type of insulation is especially useful in high-voltage transformers and in places where space is limited, such as in urban substations. SF6 is an inert gas that has high electrical insulation and thermal conductivity properties.

Insulation Fluids

Insulation fluids and gases are not just fillers; they play a crucial role in keeping our transformers optimally cool and well-insulated, ensuring seamless and efficient operations. Each type of fluid and gas brings its own unique advantages and, occasionally, some drawbacks. Let's delve into the diverse world of these essential substances, discover how they illuminate our lives, and remember to explore the recommended tests that help maintain their peak performance!

Mineral Oil

Description:
Derived from crude oil distillation, mineral oil is the most traditional and widely used transformer oil for cooling and insulating purposes.

Pros:
Cost-effective, readily available, and has good thermal conductivity and electrical insulating properties.

Cons:
Flammable, poses environmental risks if spilled, and can degrade over time.

Specific Applications:
Ideal for general-purpose transformers in interior settings where fire risk is controlled.

Synthetic Ester Fluids

Description:
Synthetic esters are man-made organic compounds known for their fire resistance and biodegradability.

Pros:
Fire-resistant, biodegradable, and have a higher moisture tolerance.

Cons:
More expensive than mineral oils, may require retrofitting, and perform poorly in cold climates.

Specific Applications:
Ideal for environmentally sensitive areas and regions with variable climates.

Silicone Fluid

Description:
Made from silicon polymers, silicone transformer oil is noted for its high fire resistance and excellent temperature stability.

Pros:
High fire resistance, less environmental hazard, and performs well across a range of temperatures.

Cons:
More expensive than mineral oil, can absorb moisture, and may have compatibility issues with some seals.

Specific Applications:
Suitable for high-risk areas such as mines and industrial settings due to its fire-resistant properties

Natural Ester Fluids

Description:
Derived from vegetable oils, natural esters provide excellent environmental safety and moisture tolerance.

Pros:
Biodegradable, non-toxic, and have high flash and fire points.

Cons:
Lower oxidation stability, may solidify in cold weather, and more expensive

Specific Applications:
Preferred in environmentally sensitive areas, suitable for colder climates with precautions against solidification.

Cooling Methods

Transformers require effective cooling to maintain efficiency and safety during operation. Overheating can accelerate aging, damage insulation, and lead to potentially severe failures. To manage the heat produced by transformers, various cooling systems are employed. These systems can be installed individually, concurrently, or in stages, depending on specific cooling needs. The capacity of a transformer is often limited by the capability of its cooling equipment. Here’s a quick rundown of the different cooling methods available for power transformers, along with their intended purposes and applications. Here's an overview of various transformer cooling methods, their purposes, and examples:

Air Natural (AN)

Purpose
To use air as the primary cooling medium in dry-type transformers.

Description:
In AN cooling systems, the heat generated by the transformer's operation is dissipated directly into the surrounding air through natural convection. This method is only suitable for transformers where the thermal energy generated is relatively low.

Example:
Small dry-type transformers in indoor installations, such as those used in commercial buildings or for localized power distribution, often rely on AN cooling.

Air Forced (AF)

Purpose
To enhance the cooling of dry-type transformers by using forced air circulation.

Description:
AF cooling involves using blowers or fans to force air across the transformer’s core and coil assembly to increase heat dissipation. This method is suitable for higher capacity dry-type transformers where natural air cooling is inadequate.

Example
This method uses fans to enhance the movement of air inside large dry-type transformers. Temperature gauges trigger these fans, automatically turning them on or off to adjust the cooling effect based on the transformer’s current temperature.

Oil Natural Air Forced (ONAF)

Purpose
To enhance the cooling capacity of natural oil cooling systems by using forced air over the transformer’s radiator.

Description:
In ONAF, fans are installed to blow air across the transformer's radiators. This increases the heat dissipation rate compared to natural air cooling. ONAF is used when the transformer's load capacity is expected to exceed the capabilities of ONAN cooling under peak load or high ambient temperature conditions.

Example:
This method uses fans to enhance the movement of air across the transformer's radiators, significantly boosting its cooling capacity. Temperature gauges trigger these fans, automatically turning them on or off to adjust the cooling effect based on the transformer’s current temperature.

Oil Natural Air Natural (ONAN)

Purpose
To dissipate heat using the natural convection and radiation properties of oil and air.

Description:
In ONAN cooling systems, oil circulates naturally within the transformer as it heats up and rises, and cools down as it sinks. The warm oil transfers its heat to the transformer's outer casing, which is then cooled by the surrounding air. This method is typically used in smaller or medium-sized transformers.

Example:
Transformers are self-cooled by circulating oil through the radiator. Radiators increase the surface area of the oil exposed to the tank wall and ultimately the air, causing heat to be removed from the oil. Cooled oil in the radiator sinks while the hot oil rises, causing a natural circulation to occur

Oil Forced Air Natural (OFAN)

Purpose
To force oil circulation within the transformer to improve heat transfer from the core and windings to the radiator.

Description:
OFAN uses pumps to circulate the oil more effectively than natural convection allows. The forced oil circulation facilitates a more rapid heat transfer to the transformer's radiators, where it is dissipated by natural air cooling.

Example:
Forced-oil-cooled transformers use pumps to increase circulation of oil through the radiator. Cooling groups comprised of fans and pumps are used to increase the transformer MVA rating. The two groups are placed in service at different transformer temperatures.

Water-cooled (W)

Purpose
To use water to remove heat from the transformer in applications where very high loads or limited space requires superior cooling efficiency.

Description:
Water-cooled systems involve passing water through cooling tubes or panels within the transformer to absorb heat directly from the core and coils. The heated water is then cooled in an external heat exchanger.

Example:
Water-cooled transformers are typically found in underground installations, densely packed substations, or inside buildings where space and heat dissipation are critical factors.