UPS Flashcards
What is a rectifier?
A rectifier is an electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which flows in only one direction.
What is an inverter?
A power inverter is an electronic device or circuitry that changes direct current (DC) to alternating current (AC).
What is a static switch?
STS – Static Transfer Switch are automatic static switching equipment designed to transfer electric loads between two independent AC power sources without interruption. The static transfer switch (STS) is an electrical device that allows instantaneous transfer of power sources to the load.
What is an input breaker?
3 Pole breaker that provides main input to the UPS and inverters
What is the purpose of the maintenance bypass breaker (MBB/MBP)?
3 Pole breaker that bypasses around the UPS to allow the UPS to be shut down without power interruption to the load.
What is purpose of the maintenance isolation breaker (MIS/MIB)?
An interlock maintenance isolation breaker (MIS) isolates the UPS from the bypass feed and the load.
What is the purpose of the backfeed breaker or back feed protection breaker (BFB)?
Backfeed Protection is implemented to prevent leakage current via the static switch snubbers and SCRs from creating upstream voltages when on static or MBP.
What is the purpose of the battery cabinet disconnect?
To allow the batteries to be isolated during shut down and maintenance conditions.
What is an SCCS?
Short circuit coordination study – this is a review of the maximum fault current to determine safe working distance and required PPE based on breaker settings.
Explain Internal Bypass
UPS has an internal bypass that goes from the same input from the USB as the MIB. The output of the static switch bypasses around the double conversion for the purpose of circumventing a fault in either component or a short on the battery bus.
Explain maintenance bypass
External breaker which cuts around the UPS allowing the UPS to be shut down for maintenance / troubleshooting without affecting the load.
When battery cabinet disconnect is open, are there any potential risks?
i) Yes, voltage is still present on the batteries terminals.
ii) Both sides of the breaker still have potential.
Explain system parameters (voltage, frequency, amperage)
i) Voltage – electrical force in a circuit based on a potential difference.
ii) Frequency –the number of occurrences of a repeating event per unit of time. ie. amount of times power reverses in a second (cycle) Measured in Hz
iii) Amperage - the strength of an electric current in amperes. Advanced: 1 Coulombs worth of charge passing a point in 1 second. Equivalent to 6.24 x 1018 electrons / second.
Explain the term “clean” power
Clean Power – refers to power that has been “filtered” by a UPS system. By design, a UPS naturally filters power by use of the rectifier and inverter thus a short swell or sag felt at the supply from the utility will not make it to the load, and the load sees a relatively constant power supply.
Explain the term “dirty” power
Dirty Power – refers to power directly from any source where any perturbation from switching or environmental interference would be transferred directly to the load.
Explain the term “inrush current”
Inrush current – is the initial rush of current to currently de-energized components that is a result of building the initial magnetic fields. ( the maximal instantaneous input current drawn by an electrical device when first turned on.)
Explain the term “Back Feed” and what it means when a UPS is unintentionally back fed
Back feed - Back-feeding of power means that the power transfer within an electrical device is towards the input terminals. This is an issue as even though the input has been disconnected there could still be hazardous potential there.
Explain the differences between a single module and parallel system
i) Single module is a single UPS unit with one input and one output.
ii) Parallel system – two or more UPS each with their own input and output breakers which are paralleled to provide more capacity.
Explain drop out voltage (end of battery discharge)
i) Drop out voltage – is the lowest useful voltage potential that the battery can sustain without damaging currents generating in the battery (P=I*V)
ii) This is when the battery breakers will trip on undervoltage.
Explain a standby single conversion UPS
UPS charges a battery, when the normal source (Utility directly supplies the load) is lost the UPS switches on providing the load with power from the battery.
Explain an Online double conversion UPS
UPS receives power from the utility which then goes through a rectifier and converting AC to DC to charge the batteries and supply the inverter which is then converts DC back to AC to supply the load. So all power during normal operations goes through the UPS.
Explain a Delta conversion UPS
Delta conversion uses a tap that goes off the delta transformer that connects the input of the delta inverter to the output of the main inverter this allows a straight power path bypassing both inverters supplying the load without double conversion this allows for an increased efficiency. Extra:That being said due to the design there is still a filtering network as the delta transformer acts like a choke with a capacitor on the far side of the main inverter out put this allows the UPS to choke in rush currents and filter out voltage spikes.
What are the two basic functions of a UPS?
Filter power providing relatively “Clean” power to the load. Also to provide emergency power during an interruption of normal power to prevent an interruption to the load long enough for another source to be made available (ie. Generator, Mechanical catcher, Critical Catcher)
Explain the redundancy levels:
N, N+1, N+2, 2N
N= required number of equipment to supply service or load without redundancy
N+1= N being number of required + 1 extra pieces of equipment
N+2= N being the number of required + 2 extra pieces of equipment
2N+ If N is the number required 2N is 2x that number
