Nantel Electrical Engineering

Grid

commonly used in reference to the transmission system since multiple flow paths (or a grid) are used to handle the load flows while allowing for maintenance and outages of lines.

Ohm's Law

defines the relationship between voltage, current and resistance, i.e. V = IR.

Real Power

is measured in watts and represents the power consumed by the resistive elements in an AC circuit.

Energy

quantifies the amount of power used as a function of time, e.g. a kilowatt-hour indicates one kilowatt of power was provided for a duration of one hour.

Offsite Power

refers to power supplied from the grid and is typically referred to as the preferred power source, i.e. the desired source for emergency loads should a loss of coolant accident (LOCA) occur.

Degraded Grid Voltage

represents a condition where the offsite power source is not adequate to support the emergency power system needs in a nuclear power plant should a loss of coolant accident (LOCA) occur.

Reactive Power

represents the power transferred to or from capacitive and inductive elements in an AC circuit and is measured in volt-amperes reactive. Voltage and current are 90° out of phase so no net energy is transferred to the load.

PSB-1

required nuclear utilities to install degraded grid voltage relay schemes to preclude connecting safety related loads to the grid following an accident if grid voltage should be inadequate to support required operation of the connected loads.

GDC-17

requires, in part, that (1) electric power from the transmission network to the onsite distribution system shall be supplied by two physically independent circuits (not necessarily on separate rights of way) designed and located so as to minimize the probability of losing electric power from any of the remaining supplies as a result of, or coincident with, the loss of power generated by the nuclear unit, or the loss of power from the transmission network.

CFR

stands for Code of Federal Regulations. References are usually to a specific Part and Section or Appendix.

Kirchoff's Current Law

states the sum of currents at any node in a circuit equals zero. In other words, the sum of currents flowing into any node must equal the sum of the currents flowing out of the node. A simple circuit with one source and parallel loads illustrates how the current from the source is divided between the loads.

Kirchoff's Voltage Law

states the sum of the voltages across the different elements in any closed loop equals zero. In simpler terms, the source voltage is equal to the sum of the voltage drops across the elements of the circuit and the load.

Per unit Voltage

the actual voltage divided by the rated voltage.

Power

the product of voltage times current (times the square root of 3 for 3 phase circuits). Since the values are continuously changing in AC circuits, RMS values are typically used in calculating power.

VAR

the reactive power lost across an inductor or stored in a capacitor. It may be delivered from generators or capacitors to offset the losses associated with inductance.

Ampere

the unit of measure for electrical current. Current is usually expressed in amperes or amps for short. See RMS for AC circuits.

Hertz

the unit of measure for frequency in AC systems and represents the number of polarity changes or cycles per second.

Ohm

the unit of measure for resistance. If one volt is impressed across a resistance of one ohm, one ampere will flow.

Volt

the unit of measure for voltage or potential. See RMS for AC circuits.

Apparent Power

the vector sum of real and reactive power in an AC circuit and is measured in volt-amperes. Apparent power is the product of voltage and current using RMS values and is the quantity that should generally be used in sizing equipment.

Non-Segregated Phase Bus

typically used for medium voltage applications where the 3 phase conductors can be mounted within the same enclosure.

Voltage Drop

typically used to refer to the losses across the elements in a circuit other than the load and represents the difference between the source voltage and that available to the load.

Train

used in nuclear power plants to identify a group of systems and equipment which collectively can be used to perform the required safety functions, and for which there is a redundant counterpart. A non-safety electrical train is used to provide power to a safety related train.

Transformer

used to convert from one voltage to another and may be used to either step-up the voltage or step-down the voltage depending on whether the source is on the high voltage or low voltage side.

Safety Related

used to define the systems and equipment within a nuclear power plant that are required to maintain the unit in a safe condition during normal operations, shut down the unit, maintain the unit in shutdown, and mitigate a nuclear related accident should one occur.

PTs

used to provide low voltage circuits for use in metering, monitoring, and protective relaying applications. The turns ratio of the transformer determines the low voltage rating as a function of the system voltage, and is often selected to be 120 volts.

RMS

values are used in AC circuits for voltage and current values. An AC current of one amp of this would produce the same heat as 1 amp DC if applied to the same resistor.

If a 1200 watt hair dryer were connected to 120 volt source, how much current would it draw?

10 amps

Power Factor

= cosine of theta where the opposite side represents reactive power and the adjacent side represents real power (consumed by the resistive element).

Bus

A large electrical conductor typically supported by insulators (see IPB and Non-segregated Phase Bus). They are also used in switchgear, load-centers, etc., to distribute high currents to individual breaker compartments within the equipment.

Distribution Center

A low voltage assembly of circuit breakers and motor starters used in DC systems, comparable to a motor control center (MCC) used in AC systems.

Who is affected by these systems?

Almost everyone is impacted by these systems and your work will most likely impact one or more of these systems. It is very important to understand these impacts when performing operations or considering modifications.

Conductor

An electrical term generally related to the wire or strands of wire used to carry current. It is typically insulated and may be supplied as a single conductor or may be combined with other insulated conductors and supplied as a cable.

Battery

An energy storage device that converts chemical energy to electricity when power is needed. The reverse process is used when the battery is being charged.

which changes do you think might impact voltage?

Control logic, Additional Loads, Load Characteristics

Two trains of safety equipment, such as safety injection pumps, are provided in case of a single failure. Why are two complete independent electrical distribution power trains included as well?

Electrical distribution equipment could be the single equipment failure that occurs

Which modifications will most likely cause a reduction in the voltage margin for an AC electrical system?

Increasing the horsepower required from a pump motor that is powered from that system

In a DC System, which one of the following components is used to convert DC power to AC power?

Inverter

Which of the following statements is correct regarding the requirement for two trains of safety equipment at a nuclear power plant?

Multiple trains allow safety functions to still be met in the event of a single failure during an accident

During an accident condition when the unit is off-line and the switchyard power is below the acceptable voltage, which one of the following will be used to supply power to the nuclear safety-related switchgear on the affected unit?

The affected unit sametrain diesel generator

CTs

They are used to provide low current circuits for use in metering, monitoring, and protective relaying applications. The turns ratio determines the amount of current that will be available in the secondary as a function of the system current on the primary side. The secondary current is typically rated for 5 amperes or less.

Impedance

This of a circuit or device represents the vectoral sum of the resistive and reactive parts. Since capacitance cancels out inductance, the reactive component is the net of the inductance minus the capacitance. It causes voltage drop during normal operation but limits fault current when a fault occurs

Fault

a condition where one or more phases have come into contact with ground or other phase(s) of the circuit, or the insulation separating them has been degraded. The current flow is only limited by the impedance between the source(s) and the fault

IPB

a configuration used for very high current bus assemblies typically operating at generator voltage levels of 20 to 24 kV where each conductor is mounted within its own enclosure.

Governor

a device and/or control system used to control the speed of a generator. It is critical in matching the frequency output of a generator with the grid prior to closing the breaker to connect the generator. It also provides a critical response in maintaining rated frequency following major disturbances on the grid.

Generator

a machine used to convert mechanical energy to electrical energy. A generator can be used to produce or consume reactive power thus helping regulate voltage - especially useful for grid operations.

Insulation

a material with very low conductivity used to enclose conductors such that they can be routed in close proximity to one another or structures, etc. at ground potential.

Circuit Breaker

a mechanical switching device used to either enable or interrupt current flow in a circuit. They typically provides both overload and fault protection. A breaker is similar to a fuse; however, a circuit breaker can be reset and reused.

MVA

a unit of measure indicating one million volt-amperes and is often used in rating medium and high voltage equipment.

KVA

a unit of measure indicating one thousand volt-amperes and is often used in rating medium and low voltage equipment.

AC

a voltage source that sees a polarity change each cycle. The number of cycles per second (hertz is the electrical unit of measure) represents the frequency.

Switchgear

an assembly of circuit breakers typically used to supply power to large motors and medium or low voltage transformers within the auxiliary power system. Each breaker is contained in a separate cubicle along with associated current transformers, and control and protective devices.

Motor Control Center

an assembly of low voltage circuit breakers and/or starters used to supply individual loads of small to moderate sizes. Each feeder circuit is typically contained in a separate compartment or bucket.

Capacitor

an electrical device used to store or release reactive power and is often used to help regulate voltage. For example when the inductive load increases on a system, they can be switched on to compensate or offset the inductive load. A capacitor tends to oppose voltage changes.

Inductor

an electrical device used to store or release reactive power. They are used on the transmission system as needed to reduce voltage during lightly loaded periods. The windings in transformers and motors represent an inductive load and cause voltage drops during normal operation.

DC

associated with a voltage source that does not change polarity such as that delivered from a battery.