Uninterruptible Power Supplies and Power Quality

Comprehensive vocabulary flashcards covering uninterruptible power supply systems, power quality anomalies, surge protection, harmonics, noise mitigation, and NSAI installation rules.

Uninterruptible Power Supply (UPS)

A power protection system that provides near-instantaneous power interruption protection using a battery, supercapacitor, or flywheel, typically offering a runtime of about $5$ to $20\,\text{minutes}$ to allow safe shutdown or standby generator startup.

Rotary UPS System

A mechanical UPS system that supplies the load using an alternator driven by an electric motor, storing kinetic energy in a rotating flywheel to maintain rotation during a mains outage until an engine (such as a diesel engine) engages via an electromagnetic clutch.

Multiple Redundancy

A technique of dividing total generating requirements across a number of smaller generating sets while keeping at least one set in reserve, ensuring essential services remain powered under almost all circumstances.

Static UPS System

A solid-state uninterruptible power supply utilizing electronic components (rectifiers, inverters, and battery storage) without large mechanical rotating parts, ranging in size from $100\,\text{VA}$ up to purpose-built units as large as $2\,\text{MVA}$.

Offline UPS

A standby static UPS configuration where the load is normally powered directly by filtered mains AC, switching to an inverter and battery backup via a transfer switch only when a mains failure occurs, typically within $5$ to $10\,\text{milliseconds}$.

Online UPS

A double-conversion static UPS that continuously powers the connected load through an inverter while the mains supplies a rectifier to feed the inverter and charge the battery, providing zero transfer time upon mains loss.

Static Bypass Switch

An automatic electronic switch path in an online UPS system that connects the load directly to utility mains power in the event of an internal UPS component failure, overload, or during maintenance.

Voltage Sag (Brownout)

A short-term decrease in voltage level down to $90\%$ or less of nominal voltage lasting from half a supply cycle to a few seconds (usually less than $1\,\text{minute}$), commonly caused by heavy starting currents of large motors or power system faults.

Blackout

A total loss of mains power caused by excessive power grid demand, lightning storms, damaged power lines, or equipment failures, leading to data loss in RAM or corrupted file systems.

Micro Cut (Micro-Break)

A momentary loss of mains power lasting less than one full supply cycle (usually under $1\,\text{second}$), typically caused by switching large machinery or copiers, which can cause digital systems to lock or hard disk heads to make damaging contact.

Voltage Spike (Impulse)

An instantaneous, dramatic increase in supply voltage typically caused by a nearby lightning strike or sudden restoration of power after an outage, capable of destroying sensitive electronic components.

Voltage Dependent Resistor (VDR)

A protective component connected across mains terminals that maintains an extremely high resistance until its breakdown voltage is reached, at which point its resistance drops sharply to suppress spikes and transients.

Transient Over Voltage

A sudden rise in voltage or current lasting from microseconds to a few milliseconds that dissipates quickly, typically caused by lightning strikes or the switching of large inductive loads.

Voltage Surge

A short-term increase in voltage lasting at least half a supply cycle, resulting from the rapid switching off of heavy inductive loads (such as large motors or air conditioners) in the immediate vicinity.

Electrical Noise

Conducted or radiated electrical interference (such as EMI or RFI) that disrupts the smooth sine wave of the public mains supply, leading to erratic performance, glitches, and data corruption in electronic systems.

Electromagnetic Interference (EMI)

Interference caused by voltages induced in cables via electromagnetic fields from neighbouring current-carrying conductors or alternating magnetic flux leakage from transformers.

Radio Frequency Interference (RFI)

High-frequency radiated or conducted electrical noise generated by unscreened dielectric heating equipment, sparking DC motor commutators, or arcing switch contacts in fluorescent luminaires.

Normal Mode Noise

Alternating current ($\text{AC}$) or direct current ($\text{DC}$) electrical interference occurring directly across the power supply lines between live and neutral conductors.

Normal Mode Rejection Ratio (NMRR)

A performance metric quantifying the ability of an electrical device to suppress or reject unwanted normal mode noise present across power feed conductors.

Common Mode Noise

Alternating current ($\text{AC}$) or direct current ($\text{DC}$) electrical interference occurring between the power feed conductors (live or neutral) and the common earth ground.

Common Mode Rejection Ratio (CMRR)

A performance metric quantifying the ability of an electrical device to attenuate or reject common mode noise occurring between the power conductors and earth ground.

Harmonic

A sinusoidal voltage or current waveform whose frequency is an integer multiple of the fundamental supply frequency, produced by non-linear loads such as rectifiers, discharge lighting, and variable speed drives.

Third Harmonic

A specific integer multiple harmonic that causes a sharp increase in zero-sequence currents, significantly elevating current levels in neutral conductors and creating excessive core heating in motors.

Line Conditioner

A power conditioning device containing components such as chokes ($X_L = 2\pi f L$), isolating transformers, and surge arresters to eliminate spikes, attenuate harmonics, and tighten voltage supply bands.

Pulse Width Modulation (PWM)

A power conversion technique in which inverter switching elements (such as SCR pairs) switch on and off multiple times per half cycle at constant amplitude and variable widths to synthesize an output wave closely approximating a sine wave without output filters.

Surge Protection Device (SPD)

A protective device that triggers at a preset voltage (typically $3$ to $4$ times the nominal mains voltage) to divert transient surge currents to earth, rated according to the amount of energy in joules it can absorb.

Metal Oxide Varistor (MOV)

A non-linear voltage-dependent component providing high electrical resistance under normal conditions, which drops dramatically when voltage exceeds a breakdown threshold to clamp voltage spikes.

No-Break Supply

An automatic supply classification under NSAI IS 10101:2020 clause 35.2 that guarantees continuous power delivery within specified voltage and frequency limits without interruption during transition.

Very Short Break Supply

An automatic standby supply classification under NSAI IS 10101:2020 clause 35.2 that restores power availability within a change-over threshold of $0.15\,\text{s}$.

Short Break Supply

An automatic standby supply classification under NSAI IS 10101:2020 clause 35.2 that makes electrical power available within a change-over threshold of $0.5\,\text{s}$.

Average Break Supply

An automatic standby supply classification under NSAI IS 10101:2020 clause 35.2 that restores electrical power availability within a change-over threshold of $5\,\text{s}$.

Medium Break Supply

An automatic standby supply classification under NSAI IS 10101:2020 clause 35.2 that makes electrical supply available within a change-over threshold of $15\,\text{s}$.

Long Break Supply

An automatic standby supply classification under NSAI IS 10101:2020 clause 35.2 that makes electrical supply available in more than $15\,\text{s}$.

By-pass Conductor for Screen Reinforcement

An additional protective conductor routed in close proximity to a screened signal or data cable to divert power system fault current away from the cable screen and reduce loop area against Lightning Electromagnetic Pulses (LEMP).

Equipotential Bonding Braid

A low-impedance bonding strap specified in NSAI IS 10101 having a recommended width-to-thickness ratio of $5:1$ to achieve low inductive reactance and impedance per metre.

Inverter

A power electronics device within a UPS system that converts direct current ($\text{DC}$) supplied by a battery or rectifier into alternating current ($\text{AC}$) for the output load.

Rectifier

An electrical power conversion stage within a UPS that converts incoming alternating current ($\text{AC}$) from the mains into direct current ($\text{DC}$) to charge batteries and supply the inverter.