UPS Systems Flashcards
1
Q
What is a Critical Load?
A
- An electrical or electronic device that is considered essential to human activity
- Whenever there is a power outage, a critical load must continue to operate for a limited period of time without any interruption
2
Q
What are some examples of Critical Loads?
A
- Electronic data storage and processing
- Boiler flame controls in industrial plants where interruption of processing and heat must be avoided
- Emergency motorized valves in processing plants for safety or processing control
- Equipment in surgery rooms where human life is the prime concern
- Equipment used in air traffic control or similiar control rooms where safety is a priority
3
Q
What kind of supply can a critical load operate on?
A
- A clean and conditioned power supply
- Which means the power must have:
- A voltage waveform that is purely sinusoidal
- The voltage is within a narrow acceptable range with no significant over or under voltage
- The frequency is within a narrow acceptable range with no significant over or under frequency
- In three-phase systems, all line and phase voltages are balanced
4
Q
What are the three key componets in a basic UPS system?
A
- The Rectifier - An electronic device capable of converting the power supply from single or three-phase AC to DC
- The DC Storage Battery - An electrochemical device that can store DC energy received from a DC charging source like a rectifier. And when the DC supply source is unavailable (OFF), the battery can become the supply source to another circuit by discharging
- An Inverter - An electronic device capable of changing the power supply from DC to single or three-phase AC
5
Q
What is the DC Link of a UPS system?
A
- The term for the DC connection between the rectifier output and the inverter input
- The utility supply is the AC input to the rectifier, the DC output of the rectifier feeds the DC link which is the connection between the rectifier output and the input of the inverter, the output of the inverter supplies the critical loads with clean AC power
6
Q
How does a UPS system with a bypass switch operate in the event of a power outage?
A
- The UPS system operates on batteries during the outage
- Near the end of the limited on-battery operation, the UPS system initiates a shutdown to itself
- Note - You must arrange for a safe shutdown or arrange for a start-up of an emergency generator before this happens
- When the UPS cannot operate any further due to high temperature, overloading, or a failed inverter, it requires immediate shutdown
- The UPS system shuts down and instantly closes the bypass switch and connects the critical loads to the utility supply power
- There is no significant interruption of power to the critical loads
7
Q
What is the difference between a basic UPS system and a UPS system with a bypass switch?
A
- A basic UPS system can maintain the power supply to critical loads for a limited time during a power outage
- A UPS system with a bypass switch can maintain the power supply during an overload or following a UPS failure
8
Q
How does the synchronizing circuit inside a UPS system with a bypass switch work?
A
- It controls the phase relationship between the inverter output and the bypass source
- The inverter output and the utility bypass supply are always in-phase during a bypass transfer
- The AC power from the bypass ource is no longer condition and may not be good quality power, so once the UPS failure or overload condition has been cleared, the UPS system automatically returns to the normal mode of operation
9
Q
What is a way of providing higher reliability in supplying power to critical loads?
A
- Installing a Redundant UPS System
- Basically two independent UPS units are used where one UPS unit is actively supplying the loads at any time and the other UPS unit operates as a stanby
- The second unit is the alternate power supply to the critical loads when the first unit has an internal fault so there is always conditioned power available to the critical loads
10
Q
Can the two independent UPS units in a redundant UPS system share a common storage battery?
A
- Yes they can
11
Q
What is a disadvantage of using a common storage battery arrangement in a redundant UPS system?
A
- During any maintenance or failure on the common battery bank, no stored DC energy is available for the two UPS units
12
Q
What are the preferred type of switches used for the bypass witch in a UPS system?
A
Static/Solid-state switches
- Most critical loads supplied by a UPS system have sensitive electronic componets that require a clean and constant power, and there must not be an significant interruption of this supply
- Electromechanical relays cannot operate at high speeds, where solid-state switches operate in microseconds and can provide that high speed switching
13
Q
Describe the process of operations in a UPS system with a static switch when the conditions for a load transfer present
A
- The condition for a load transfer to the bypass source is present
- The UPS operation logic board sends a to-bypass signal to the bypass gate control
- The bypass gate control sends a firing signals to the gates of the SCR’s in the static switch
- The SCR is gated and conducts, so the static switch is now conducting and the bypass source is supplying power to the critical loads
- Once the UPS failure is corrected and is operational again, it becomes the preferred power supply to the critical loads
- When the UPS operation logic board detects this condition, it sends the to-inverter signal to the bypass gate control board
- The static switch stops conducting
- The UPS output supplies the critical loads again
14
Q
What is an inverter?
A
- An electronic device that is capable of transforming a DC input into an AC output
15
Q
What are some applications for inverters?
A
- In a UPS system to convert the DC voltage back into 60hz AC for a continuous supply to the critical load
- In Variable Frequency Drives (VFD’s) used for AC motor speed control. A VFD has a variable frequency and voltage output. A rectifier converts the AC to the VFD into DC, and then the inverter converts the DC back into a variable AC voltage with a variable frequency which is how it controls the speed of the motor
- In DC to DC conversion. Unlike an AC voltage, a transformer cannot change the voltage of a DC source to a different value. The practical way of doing this is to invert the DC voltage to AC, run it through a transformer and transform it into a desired value, and then rectify the AC back into DC
