UPS

Question 1

What is a rectifier?

Answer 1

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.

Question 2

What is an inverter?

Answer 2

A power inverter is an electronic device or circuitry that changes direct current (DC) to alternating current (AC).

Question 3

What is a static switch?

Answer 3

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.

Question 4

What is an input breaker?

Answer 4

3 Pole breaker that provides main input to the UPS and inverters

Question 5

What is the purpose of the maintenance bypass breaker (MBB/MBP)?

Answer 5

3 Pole breaker that bypasses around the UPS to allow the UPS to be shut down without power interruption to the load.

Question 6

What is purpose of the maintenance isolation breaker (MIS/MIB)?

Answer 6

An interlock maintenance isolation breaker (MIS) isolates the UPS from the bypass feed and the load.

Question 7

What is the purpose of the backfeed breaker or back feed protection breaker (BFB)?

Answer 7

Backfeed Protection is implemented to prevent leakage current via the static switch snubbers and SCRs from creating upstream voltages when on static or MBP.

Question 8

What is the purpose of the battery cabinet disconnect?

Answer 8

To allow the batteries to be isolated during shut down and maintenance conditions.

Question 9

What is an SCCS?

Answer 9

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.

Question 10

Explain Internal Bypass

Answer 10

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.

Question 11

Explain maintenance bypass

Answer 11

External breaker which cuts around the UPS allowing the UPS to be shut down for maintenance / troubleshooting without affecting the load.

Question 12

When battery cabinet disconnect is open, are there any potential risks?

Answer 12

i) Yes, voltage is still present on the batteries terminals.

ii) Both sides of the breaker still have potential.

Question 13

Explain system parameters (voltage, frequency, amperage)

Answer 13

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.

Question 14

Explain the term “clean” power

Answer 14

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.

Question 15

Explain the term “dirty” power

Answer 15

Dirty Power – refers to power directly from any source where any perturbation from switching or environmental interference would be transferred directly to the load.

Question 16

Explain the term “inrush current”

Answer 16

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.)

Question 17

Explain the term “Back Feed” and what it means when a UPS is unintentionally back fed

Answer 17

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.

Question 18

Explain the differences between a single module and parallel system

Answer 18

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.

Question 19

Explain drop out voltage (end of battery discharge)

Answer 19

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.

Question 20

Explain a standby single conversion UPS

Answer 20

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.

Question 21

Explain an Online double conversion UPS

Answer 21

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.

Question 22

Explain a Delta conversion UPS

Answer 22

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.

Question 23

What are the two basic functions of a UPS?

Answer 23

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)

Question 24

Explain the redundancy levels:

N, N+1, N+2, 2N

Answer 24

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

Question 25

What is a Valve Regulated Lead Acid Battery (VRLA aka VRSLA)?

Answer 25

Valve Regulated Lead Acid (VRLA) Batteries are low maintenance sealed lead-acid batteries. They limit inflow and outflow of gas to the cell – thus the term “valve regulated”.

Question 26

State the safety hazards associated with a battery?

Answer 26

a) Hydrogen generation – lighter than air and can be explosive in the right concentrations
b) Sulfuric Acid – corrosive requires safety glasses, rubber gloves, face shield , and rubber apron.
c) Batteries unless dead are always energized so they are a sparking and shock hazard at all times. Use insulated tools.

Question 27

Float Charge

Answer 27

Float voltage is the voltage at which a battery is maintained after being fully charged to maintain that capacity by compensating for self-discharge of the battery.

Question 28

Thermal Runaway

Answer 28

Thermal runaway occurs in situations where an increase in temperature changes the conditions in a way that causes a further increase in temperature, often leading to a destructive result. It is a kind of uncontrolled positive feedback.

Question 29

Cell drop out voltage

Answer 29

Drop out voltage – is the lowest useful voltage potential that the Cell can sustain without damaging currents generating in the battery (P=I*V)

Question 30

String Batteries

Answer 30

Batteries connected in series to achieve the required voltage of the system. Strings can be paralleled to reach the overall power capacity needed to support the desired operation.

Question 31

Describe how to conduct a visual inspection of a battery string

Answer 31

Using an inspection light look into the battery cabinet inspecting the top for any moisture buildup (Acid), verify vents are intact, inspect terminals to ensure no buildup of corrosion materials, and inspect the case of the battery to ensure that no bulging or cracking has occurred.

Question 32

Explain the safety measures associated with batteries

Answer 32

a) Hydrogen generation – lighter than air and can be explosive in the right concentrations
b) Sulfuric Acid – corrosive requires safety glasses, rubber gloves, face shield , and rubber apron.
c) Batteries unless dead are always energized so they are a sparking and shock hazard at all times. Use insulated tools.

Question 33

Explain spill containment for batteries

Answer 33

a) First isolate the area.
b) Whenever dealing with battery spills, ensure proper PPE is worn and the area is well ventilated.
c) Contain the spill if this can be done safely as the battery can be part of a string of batteries meaning high voltage is more than likely still present.
d) Sprinkle acid neutralizer around the spill concentrating around the edges first to help contain the spill then working towards the middle of the spill.
e) Use the absorbent pads to absorb the acid if it exceeds the capacity of the neutralizer.
f) Place all spill materials used in a bag including PPE. Label the bag and store in a safe area until a haz. Chem. company can be contacted to retrieve the waste.

Question 34

What is the proper temperature requirements for battery storage

Answer 34

Rated temperature for a VRLA is between 70F-80F (~77F)