UPS

Engineering Encyclopedia Saudi Aramco DeskTop Standards

Introduction to DC/UPS Systems

Note: The source of the technical material in this volume is the Professional

Engineering Development Program (PEDP) of Engineering Services. Warning: The material contained in this document was developed for Saudi

Aramco and is intended for the exclusive use of Saudi Aramco’s employees. Any material contained in this document which is not already in the public domain may not be copied, reproduced, sold, given, or disclosed to third parties, or otherwise used in whole, or in part, without the written permission of the Vice President, Engineering Services, Saudi Aramco.

Chapter : Electrical File Reference: EEX21101

For additional information on this subject, contact W.A. Roussel on 874-1320

Engineering Encyclopedia

Electrical Introduction to DC/UPS Systems

CONTENTS

PAGES

DC/UPS DC/UPS SYSTEMS: SYSTEMS: PURPOSE PURPOSE AND APPLICATION APPLICATIONS.................................................... S.................................................... 1 DC/UPS SYSTEMS: MAJOR COMPONENTS COMPONENTS AND MAJOR COMPONENT COMPONENT FUNCTIONS FUNCTIONS .............................................................................................. 5 UPS SYSTEMS: SYSTEMS: CONFIGURAT CONFIGURATIONS IONS AND TRANSFER TRANSFER SCHEMES SCHEMES .............................15 GLOSSARY........................................................................................................................34

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DC/UPS SYSTEMS: SYSTEMS: PURPOSE AND APPLICATIONS

DC/UPS systems are classified as standby or emergency power systems. As this classification classification suggests, the purpose of standby or emergency power systems is to provide electrical power for operation of critical functions and equipment under the following conditions: When the quality of the normal power source becomes inadequate. When the normal power source completely fails. The quality of the normal power source can become inadequate due to disturbances that occur in the utility system system or that occur in the facility in which the DC/UPS system is installed. The following are the typical types of disturbances that can cause the quality of the normal power source to become inadequate and the causes of these disturbances: Microsecond disturbances in the form of transient undervoltages and transient overvoltages that are caused by lightning and switching surges in the utility system. Millisecond disturbances in the form of undervoltages of up to several cycles (50 ms) that are caused by capacitor switching and transformer energization in the utility system, by circuit breaker trips or blown fuses in the facility, and by cyclic facility loads such as welders. Second disturbances in the form of undervoltages that are caused by feeder switching in the utility system or by motor starting in the facility. Complete failure of normal power sources occurs less frequently than complete failure of the previously described disturbances. The following are the typical causes of complete failure of  normal power sources: Transmission or distribution line failures due to storms. Vehicle accidents that result in downed lines. Equipment failures in the utility system. Equipment failures in the facility. The frequency with which normal power system disturbances or failures occur at a given facility and the criticality of the equipment at a given facility are the key factors that are evaluated in a determination of the need for a DC/UPS system. The specific types of loads that require emergency or standby power are the key factors in a determination of whether the standby or emergency power system is a DC system, a UPS system, or a DC and a UPS system. The sections that follow provide information on the following topics that are pertinent to the purpose and applications of DC/UPS systems: Categories of Loads Served by DC/UPS Systems Economics: When to Use a UPS System and When Not to Use a UPS System System

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Categories of Loads Served by DC/UPS Systems

The exact loads that are supplied by DC/UPS systems are varied. The loads on DC/UPS systems are categorized in accordance with the potential problems that loss of these loads can cause. Knowledge of the exact loads that are served by a DC/UPS system is not as important as an understanding of the categories of loads that are served by DC/UPS systems. Such loads fall into three categories: Life Threatening Safety Critical Equipment Life Threatening

A load is categorized as life threatening if a loss of power to the load is likely to cause a loss of  life. Most of the loads that fall into the life threatening category are found in hospitals and other medical facilities. The following are typical examples of life threatening loads: Patient monitors Remote instrumentation Heart and lung machines Safety

A load is categorized as a safety load if, upon a loss of power to the load, there is a possibility of  personal injury or equipment damage. It is not necessary for a loss of power to a safety load to cause an injury, but the loss of power must increase the potential for injury. The following are typical examples of safety loads that can be found in Saudi Aramco facilities: Emergency lighting systems Communications systems Fire monitoring and alarm systems Portions of industrial process control systems Critical Equipment

A load is categorized as critical equipment if, upon a loss of power, there is a necessity to maintain the equipment in operation for a period of time to allow for the controlled shutdown of  another piece of equipment. The following are typical examples of loads that are categorized as critical equipment: Auxiliary oil pumps Breaker control power Indication and status lights Valve control power supplies

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Economics: When to Use a UPS System and When Not to Use a UPS System

Except in cases where the use of a UPS system is mandated by regulations, the decision of when to use a UPS system and when not to use a UPS system normally is based on economic reasons. Where use of a UPS system is not mandated, the Electrical Engineer must economically justify the need for a UPS system in a given facility. The economics of installation of a UPS system involves two categories of costs: the total average annual cost of the UPS systems and the total average annual cost of the power outages that installation of a UPS systems will prevent. If the total average annual cost of the UPS system is proven to be less than the total average annual cost of the power outages that installation of the UPS system will prevent, the need for the UPS system is economically justified. An economic  justification for a given installation can be made through performance of a cost/benefit analysis. The first step in the performance of a cost/benefit analysis involves the estimation of the number of times per year that a power outage will occur at a given facility and the average duration of  each power outage. The statistical data that concerns the frequency and the duration of power outages at a given facility should be available from the local utility. If the local utility does not have such data, or if the normal supply of power for the facility does not come from the utility, statistical data from a similar installation can be used. The next step in the performance of a cost/benefit analysis is to estimate the average cost/hour of  each of the following items: The salaries of the employees that are not involved in productive work during the outage. The salaries of the employees that must re-perform work that was lost due to the outage. The cost of the system startup after the power is restored. The cost of any lost or damaged product. The cost of any equipment that may be damaged as a result of the power outage and the cost of the time that is necessary to repair the damaged equipment.

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After the cost/hour of each of the individual losses has been determined, these values are summed to determine the total cost/hour of a power outage. The total cost/hour of a power outage then is multiplied by the average duration of a power outage to determine the total cost of a power outage. In turn, the average annual cost of power outages is determined through multiplication of  the total cost of a power outage by the average number of power outages that occur per year. The next step in the cost/benefit analysis is to determine the average annual cost of a UPS system that would alleviate the losses that result from a power outage. The Electrical Engineer must consider each of the following factors to determine the average annual cost of a UPS system: The annual capital cost of the UPS system (i.e., the initial purchase and installation cost of the UPS system divided by the useful life of the UPS system). The added annual cost of electricity due to the operational efficiency of the UPS system. The annual cost of additional floor space. The annual cost of the maintenance that must be performed on the UPS system. The individual annual costs of the UPS system then are summed to determine the total annual cost of the UPS system. The final step in the cost/benefit analysis is to compare the average annual cost of power outages to the annual cost of a UPS system. If the annual cost of the UPS is less than the average annual costs of power outages, the UPS system is economically justified. If the annual cost of the UPS system is more than the average annual cost of power outages, the UPS system is not economically justified. If the outcome of the cost/benefit analysis shows that the use of a UPS system is economically justified, a UPS system should be installed.

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DC/UPS SYSTEMS: MAJOR COMPONENTS AND MAJOR COMPONENT FUNCTIONS

DC systems and UPS systems are both standby or emergency power sources and share many of  the same major components. Because of the similarities between these systems, the majority of  this course will discuss DC systems and UPS systems together. This section of the Module, however, will discuss the functions and the components of each system separately and will state the differences between the two systems to ensure that these differences are understood. DC Systems

A DC system is designed to continuously provide direct current (DC) power to the connected loads for normal operation and, upon a loss of the normal source of power, to provide DC power to the connected loads for a specified period of time. This section of the Module will provide information on the following topics that are pertinent to DC systems: Major Components Component Functions Major Components

Figure 1 is a block diagram of a simple DC system that consists of the following major components: The AC power source The battery charger The battery The DC distribution panels The DC loads The AC power source is the ultimate source of power for the DC system that is shown in Figure 1. The AC power sources for Saudi Aramco DC systems can be single-phase or three-phase power sources that are able to supply 60Hz AC power at one of the following voltages: 120 volts 208 volts 240 volts 480 volts

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The battery charger for Saudi Aramco DC systems must be a constant potential, semi-conductor type of static charger that is designed to simultaneously supply DC power to a float-type battery and to the connected DC loads. The battery charger should be equipped with controls for the adjustment of the output voltage. The battery charger also must be equipped with a filter to reduce the AC ripple in the output voltage under the following conditions: The battery charger is rated at 10kW or below and is connected to a battery that has an ampere-hour rating that is four times the current rating of the battery charger. The battery charger is rated at more than 10kW. The battery charger is installed in a DC system that supplies a UPS system. Battery chargers that are installed in DC systems that only supply power to emergency lighting do not require filters. The batteries for Saudi Aramco DC systems can be nickel-cadmium alkaline types that use potassium hydroxide for the electrolyte, or they can be lead-acid types that use sulfuric acid for the electrolyte. The size of the battery should be sufficient to supply power to the connected loads for the required period of time after a loss of the normal AC power source. The DC distribution panels contain the wiring, circuit breakers, and fuses that are required to connect the DC loads to the DC power supply (battery charger or battery). The DC loads that are shown in Figure 1 are categorized as follows: continuous loads, essential loads, and momentary loads. A continuous DC load is a load that requires power for at least 12 to 24 hours after a loss of the normal AC power source. The only continuous load that is shown in Figure 1 is the facility alarms. An essential DC load is a load that requires power for at least three hours after a loss of the normal AC power source. The essential loads that are shown in Figure 1 are the emergency lube oil pump, the emergency shutdown system, and the emergency lighting. A momentary DC load is a load that requires power for one-minute durations at random periods of time that follow a loss of the normal AC power source. The momentary loads that are shown in Figure 1 are the circuit breaker control and the motor-operated ESD valves.

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Block Diagram of a Simple DC System Figure 1

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Component Functions

Each of the major components of a DC system has a distinct function that will be described below. AC Power Source -

The function of the AC power source is to supply the input power to the battery charger at the battery charger's rated voltage and frequency. The AC power source is the normal source of power for a Saudi Aramco DC system. - The battery charger is a device that receives AC input power from the AC power source and that converts (rectifies) this AC input to a regulated DC output. The regulated DC output power is used for the following purposes: Battery Charger

To supply power to the DC loads during normal system operation. To maintain a float charge on the battery during normal system operation. To recharge the battery after a loss and the subsequent return of the normal AC power source. - The function of the battery is to convert electrical energy into chemical energy during the charge cycle and to convert the stored chemical energy into electrical energy during the discharge cycle. After a loss of the normal AC power source, the battery will convert its stored chemical energy into electrical energy to supply power to the connected DC loads. Battery

- The DC distribution panels provide a convenient means to distribute power to groups of DC loads. The DC distribution panels also provide a means to enclose the power supply terminations, circuit breakers, and fuses. DC Distribution Panels

- The DC loads can be anything that operates on DC power. The functions of  the DC loads are dependent on the types of loads that are installed. The functions of the DC loads that were shown in Figure 1 are as follows: DC Loads

The emergency lube oil pump maintains the pressure in the lube oil system after a loss of the normal lube oil pump. The emergency shutdown system supplies power to the circuits that are required to allow a safe and orderly shutdown of the facility.

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The circuit breaker control power supplies the necessary power to trip and to close circuit breakers. The facility alarms warn personnel of problems that require attention. The emergency lighting provides the illumination that personnel require to perform a safe and orderly shutdown of the facility and to locate the exits. The motor-operated ESD valves operate to control the flow of fluids through the process as necessary to allow a safe and orderly shutdown of the facility.

UPS Systems

A UPS system is designed to provide a continuous, transient-free supply of electrical power to critical loads to protect these loads from outages, surges, or dips in the normal AC power that is supplied from the utility or from an engine generator. The output power that is provided by a UPS system is AC power, which is one of the key differences between UPS systems and DC systems. The other key differences between UPS systems and DC systems are associated with the additional major components of the UPS system. This section of the Module will provide information on the following topics that are pertinent to UPS systems: Major Components Component Functions Major Components

Figure 2 is a block diagram of a simple UPS system that consists of the following major components: AC power source Battery charger Battery Inverter Static switch Bypass source Manual bypass switch Critical AC loads

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The AC power source also is the ultimate source of power for the UPS system that is shown in Figure 2. The AC power sources for Saudi Aramco UPS systems can be single-phase or threephase power sources that are able to supply 60Hz AC power at one of the following voltages: 120 volts 208 volts 240 volts 480 volts The battery chargers for Saudi Aramco UPS systems must be constant potential, semi-conductor type static chargers that are designed to simultaneously supply the DC current that is required for float-charging the UPS battery and that is required for the inverter. The battery charger should be equipped with controls for the adjustment of the output voltage. The battery charger also must be equipped with a filter to reduce the AC ripple in the output voltage. The batteries for Saudi Aramco UPS systems can be nickel-cadmium alkaline types that use potassium hydroxide for the electrolyte, or these batteries can be lead-acid types that use sulfuric acid for the electrolyte. UPS system batteries should be dedicated to the UPS inverter and should not be connected to supply other DC loads. In cases where other essential DC loads cannot be connected to a separate battery, these DC loads can be connected to the UPS battery as long as the load does not exceed 10% of the ampere-hour capacity of the UPS battery. The size of the UPS battery should be sufficient to supply the inverter with the necessary input power to keep the critical AC loads energized for the required length of time. The inverters for Saudi Aramco UPS systems must be static inverters as opposed to rotating generators. A static inverter contains silicon-controlled rectifiers (SCR's) and other solid state components that enable the inverter to perform its design function. Most Saudi Aramco UPS installations use pulse-width modulated inverters. The static switches that are used in Saudi Aramco UPS systems contain thyristors that are arranged in back-to-back pairs to form an electronic single-pole, double-throw switch. Static switches are used instead of mechanical switches because static switches have a faster speed of  operation; static switches can open or can close in less than 4ms whereas mechanical switches require at least 30ms to open or to close. The bypass source is a separate source of AC power that is connected between the AC power source and the static switch. This power source is also connected to a contact of the manual bypass switch.

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The manual bypass switch is a manually controlled switch that is used to transfer the critical AC loads from the primary power source (inverter output) to the alternate power source (AC bypass source). The manual bypass switch is used in the performance of maintenance and service functions or in the rare event of an inverter or static switch failure. Both the inverter output and the AC bypass source are connected to the contacts of the manual bypass switch. The transfer action of the switch is make-before-break; therefore, the critical AC loads can be transferred from the normal power source to the bypass power source with no interruption of power to the critical AC load. Generally, the manual bypass or "maintenance" bypass switches that are used in Saudi Aramco UPS systems have the following positions: Normal Bypass Test Position Full Bypass Position When the switch is in the normal position, the inverter output and the AC bypass source are connected to the critical AC loads through the static switch. When the switch is in the plain bypass test position, the critical AC loads are connected to the AC bypass source, and the inverter output and the AC bypass source are still connected to the static switch input. When the switch is in the plain full bypass position, the critical AC loads are connected to the AC bypass source, and the static switch is totally disconnected from the AC bypass source. In the plain full bypass position, the manual bypass switch provides total electrical isolation of the UPS system from the output for purposes of maintenance. The critical AC loads can be any loads that require uninterruptible source of AC power.

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Block Diagram of a Simple UPS System Figure 2

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Component Functions

Each of the major components of UPS system has a distinct function that will be described below: AC Power Source -

The function of the AC power source is to supply the input power to the battery charger of the UPS systems at the battery charger's rated voltage and frequency. The AC power source also must supply power, through a separate circuit breaker, to the bypass source at the required voltage and frequency. - The battery charger is a device that receives AC input power from the AC power source and that converts (rectifies) this AC input to a regulated DC output. The regulated DC output power is used for the following purposes: Battery Charger

To supply power to the inverter during normal system operation. To maintain a float-charge on the UPS battery during normal system operation. To recharge the UPS battery after a loss and the subsequent return of the normal AC power source. - The function of the battery is to convert electrical energy into chemical energy during the charge cycle and to convert the stored chemical energy into electrical energy during the discharge cycle. After a loss of the normal AC power source, the battery will convert its stored chemical energy into electrical energy to supply the inverter with the required DC input so that the inverter can continue to supply the critical AC loads. Battery

- The function of the inverter of the UPS system is to convert the DC input from the battery charger or the battery to an AC sinewave output of the correct voltage and frequency to supply the critical AC loads. Inverter

- The function of the static switch is to automatically connect the critical AC loads to the output of the inverter during normal operation and, if the inverter output becomes unsatisfactory, to automatically transfer the critical AC loads from the inverter output to the bypass source. Static Switch

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- The function of the bypass source is to provide an alternate source of AC power to the critical loads under the following conditions: Bypass Source

The inverter malfunctions and is unable to supply an output. The current demand of the critical AC loads exceeds the capabilities of the inverter. Load in-rush currents and faults are the typical causes of high current demands. - The function of the manual bypass switch is to provide a means to manually transfer the critical AC loads from the inverter to the bypass source. This switch is normally used to keep the critical AC loads energized while maintenance is being performed on the inverter. Manual Bypass Switch

- The functions of the critical AC loads are dependent on the types of  loads that are installed. The function of a critical AC load on a UPS system would be identical to the function of an AC load that is not connected to a UPS system. Critical AC Loads

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UPS SYSTEMS: CONFIGURATIONS AND TRANSFER SCHEMES

UPS systems are available with a variety of configurations and transfer schemes to meet the needs of each particular installation. The type of configuration and transfer scheme that is used in a UPS system has a direct effect on the reliability, the cost, and the operation of the UPS system. Configurations

The configuration of a UPS system is directly related to reliability and relative cost. Also, the factors of reliability and relative cost are directly related to each other. This section will discuss non-redundant and redundant configuration for UPS systems. This section will also discuss the reliability and the relative cost of these system configurations: Non-Redundant Redundant Non-Redundant

Figure 3 shows a single-path, non-redundant UPS system configuration that contains the following components: AC power source Battery charger Battery Inverter Critical AC loads In the system that is shown in Figure 3, the AC power from the AC power source is supplied to the input of the battery charger, where it is converted to DC power. The DC power output from the battery charger is used to maintain the battery in a float-charge condition and to provide the input power to the inverter. The inverter converts the input power from the battery charger back  to AC power. The AC power output from the inverter supplies the power that is needed to operate the critical AC loads. The single-path, non-redundant UPS system is the simplest and the least expensive UPS system configuration; however, because this system only has a single path through which power can be supplied to the critical AC loads, this system is also the least reliable. The low reliability is based on the fact that a failure of any one component in a single-path, non-redundant system will cause the entire to system to fail.

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Single-Path, Non-Redundant UPS System Configuration Figure 3

The reliability of the single-path, non-redundant UPS system configuration can be significantly improved through addition of a static switch and a bypass source, as shown in Figure 4. The normal flow of power through the system that is shown in Figure 4 is the same as the flow of  power through the system that was shown in Figure 3. The reliability of the system that is shown in Figure 4 is improved, however, because the static switch will transfer the critical AC loads to the bypass source if the inverter output fails or becomes unsatisfactory. As mentioned previously, the reliability of a UPS system is directly related to the cost of the UPS system. The reliability of the system that is shown in Figure 4 is eight to ten times higher than the system that was shown in Figure 3; however, the cost of the system that is shown in Figure 4 also is 10% more than the cost of the system that was shown in Figure 3.

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Non-Redundant UPS System Configuration with a Bypass Source Figure 4

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Redundant

Figure 5 shows a dual-path redundant UPS system that contains the following major components: AC power source Two battery chargers Battery Two inverters (primary and alternate) Three static switches Critical AC loads In the system that is shown in Figure 5, AC power from the AC power source is supplied to the input of the battery chargers, where it is converted to DC power. The DC power output from the battery chargers is used to maintain the battery in a float-charge condition and to provide the input power to the inverters. The inverters convert the input power from the battery chargers back to AC power. The AC power output from the inverters is supplied to the poles of static switches 1 and 2. During normal system operation, static switch 1 is closed, and the primary inverter supplies all of the AC power to the critical AC loads. If the primary inverter fails, or if its output becomes unsatisfactory, static switch 2 will close and static switch 1 will open to allow the alternate inverter to supply all of the power to the critical AC loads. In the event that both of the inverters fail, static switch 3 will close and static switch 2 will open to allow the bypass source to supply all of the power to the critical AC loads. This basic discussion of the operation of the dual-path redundant UPS system configuration demonstrates the increased reliability of this system over the previously discussed non-redundant systems. In actual practice, a redundant UPS system is two to four times more efficient than a non-redundant system. For a system to be considered redundant, it must have at least one more path (e.g., battery charger, inverter, and static switch) than is required to supply the needed power to the critical AC loads. In the system that is shown in Figure 5, each path is rated to supply all of the needed power to the critical AC loads. This system is redundant, because it has one more path than is required to supply the needed power to the critical AC loads. Duplication of the battery is not necessary because of the inherent reliability of batteries. Such a duplication would add cost to the system without an appreciable increase in the overall reliability of the system. The cost of a redundant UPS system relative to the cost of a non-redundant system can be estimated through use of the following formula: where: N = The least number of parallel paths that are required for a non-redundant system.

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For example, if a non-redundant UPS system for a given installation would require two parallel paths, and if the cost of the non-redundant UPS system is $75,000, the cost of installation of a redundant UPS system for the given installation can be estimated as follows: Relative Cost

=

(N + 1) N

Relative cost

=

(2 + 1) 2

Relative cost

=

3 2

Relative cost

=

1.5

_Cost of Redundant System =

(Relative Cost) (Cost of Non-Redundant System)

=

(1.5) (75,000)

=

$112,500

This example shows that the increased cost of installation of a redundant UPS system is $37,500. The increased cost of the redundant UPS system must be justified through performance of a cost/benefit analysis. Except for the following items, the cost/benefit analysis is performed in the same manner as previously described: The average number of times per year that an outage will occur is determined by the mean failure time of the UPS system rather than by the statistical data on the utility power. The duration of the power outage is determined by the average repeat time for common UPS system failures rather than by statistical data on the utility. If the outcome of the cost/benefit analysis shows that the use of a redundant UPS system is economically justified, a redundant UPS system should be installed.

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Dual-Path Redundant UPS System Configuration Figure 5

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Transfer Schemes

Transfer schemes are incorporated in UPS systems that are able to supply power to the critical AC loads from more than one source (e.g., from the output of the inverter, from the bypass line, or from the output of multiple inverters). The transfer scheme monitors the condition (e.g., phase synchronization, voltage and frequency) of the various sources of power and, through use of a transfer logic control circuit, automatically connects the critical AC loads to the proper source of  power. This section of the Module will discuss the following types of transfer schemes: Reverse Transfer for a Non-Redundant UPS System Forward Transfer for a Non-Redundant UPS System Reverse Transfer for a Redundant UPS System Reverse Transfer for a Non-Redundant UPS System

The name that is given to a transfer scheme (reverse or forward) refers to the source of power that is designated as the preferred source and the source that is designated as the alternate source. In a reverse transfer scheme, the inverter is the preferred source of power for the critical AC loads, and the AC bypass line is the alternate source of power for the critical AC loads. All Saudi Aramco UPS systems must use a reverse transfer scheme. Figure 6 shows a basic diagram of a non-redundant UPS system with a reverse transfer scheme. The UPS system that is shown in Figure 6 has a reverse transfer scheme because the preferred source of power for this system is the inverter output. The UPS system that is shown in Figure 6 operates as follows: When AC power is available, the AC power source supplies AC power to the input of the battery charger and to the AC bypass line. The battery charger converts this AC power into DC power. The DC power from the output of the battery charger then is supplied to the input of the inverter and to the battery. The inverter converts the DC power into AC power. The AC power from the output of the inverter is supplied to pole 1 of the static switch. If the voltage and the frequency on the output of the inverter are within the normal range, the static switch transfer logic control circuit will supply a gate signal to the SCR's that make up pole 1 of the static switch.

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The SCR's that make up pole 1 of the static switch start to conduct and supply AC power from the inverter to the critical AC loads. If the static switch transfer logic control circuit senses a problem with the output of the inverter, and if the AC bypass source is available, the circuit will remove the gate signal from the SCR's that make up pole 1 of the static switch and supply a gate signal to the SCR's that make up pole 2 of the static switch. The SCR's that make up pole 1 of the static switch will commutate the next time that the inverter output sinewave passes through zero, and the SCR's that make up pole 2 of the static switch will start to conduct. The critical AC loads now will be supplied with power from the AC bypass line (alternate source of power). When the static switch transfer logic control circuit senses that the inverter output has returned to normal, the circuit will remove the gate signal from the SCR's that make up pole 2 of the static switch and will supply a gate signal to the SCR's that make up pole 1 of the static switch. The SCR's that make up pole 2 of the static switch will commutate the next time that the AC power source sinewave passes through zero, and the SCR's that make up pole 1 of the static switch will start to conduct. When the SCR's that make up pole 1 of the static switch start to conduct, the critical AC loads will once again be supplied with power from the output of the inverter.

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Basic Non-Redundant UPS System with a Reverse Transfer Scheme Figure 6

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The diagram that is shown in Figure 6 is adequate to explain the basic operation of the reverse transfer scheme, but a more detailed diagram of the static switch transfer logic control circuit (see Figure 7) is needed to explain all of the various conditions that must be met prior to the occurrence of an actual transfer from one source of power to another source of power. The circuit that is shown in Figure 7 operates to initiate or to block the transfer of power from one source to another source as follows: When the UPS system is in automatic and the preferred source of power (output of  the inverter for a reverse transfer scheme) is available, the static switch gate drive circuit will always provide a gate signal to the SCR's that make up pole 1 of the static switch, and the inverter output will be connected to supply power to the critical AC loads. If any of the following conditions occur, the preferred source-sensing circuit will supply a transfer signal to the transfer/return circuit after the applicable time delay has timed out: The inverter output voltage goes outside of the normal range (e.g., _10% of the nominal voltage). The inverter output frequency goes outside of the normal range (e.g., _5% of the nominal frequency). An external transfer signal, such as the inverter in the current limit mode of  operation, is present. If there are no transfer inhibit signals present at the transfer/return circuit, the transfer/return circuit will send a transfer signal to the static switch gate drive circuit. If a transfer inhibit signal is present at the transfer/return circuit, the transfer signal to the static switch gate drive circuit will be blocked. The following are examples of conditions that will result in a transfer inhibit signal: The alternate source voltage is outside of the normal range (e.g., _10% of the nominal voltage). The alternate source frequency is outside of the normal range (e.g., _5% of the nominal frequency).

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Electrical Introduction to DC/UPS Systems

An external transfer inhibit signal, such as a sync disconnect signal, is present. The static switch gate drive circuit will transfer the critical AC loads from the output of the inverter to the AC bypass source through removal of the gate signal from the pole 1 SCR's and through provision of a gate signal to the pole 2 SCR's. The AC bypass source will continue to supply power to the AC critical loads until the external transfer signal is removed from the preferred source sensing circuit or until the output of the inverter meets the following conditions: Inverter output voltage returns to _2% of the nominal voltage and remains at this value for at least 30 seconds. Inverter output frequency returns to _2% of the nominal frequency and remains at this value for at least 30 seconds. When the above conditions have been met, the preferred source-sensing circuit sends a return signal to the transfer/return circuit. If there are no return inhibit signals present at the transfer/return circuit, the transfer/return circuit will send a return signal to the static switch gate drive circuit. If a return inhibit signal is present at the transfer/return circuit, the return signal to the static switch gate drive circuit will be blocked. A return inhibit signal will be present at the transfer/return circuit when the phase and voltage unbalance circuit detects an unsatisfactory phase or voltage balance condition between the preferred source and the alternate source. The static switch gate drive circuit will return (re-transfer) the critical AC loads from the AC bypass source to the output of the inverter through removal of the gate signal from the pole 2 SCR's and through provision of a gate signal to the pole 1 SCR's.

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Electrical Introduction to DC/UPS Systems

Static Switch Transfer Logic Control Circuit Figure 7

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Electrical Introduction to DC/UPS Systems

Several other options can be incorporated into a reverse transfer scheme to meet a variety of UPS system applications. A manual reset switch can be inserted in the automatic return circuit to defeat the automatic return feature. If this feature is incorporated, an operator must manually depress the reset button to return the critical AC loads to the preferred source after a transfer. A manual bypass switch can be incorporated to bypass the static switch. This switch normally has three positions: Normal, Plain Bypass Test, and Plain Full Bypass. When the switch is in the Normal position, the static switches are connected in the circuit to the critical AC loads, and the operation of the static switches is controlled by the static switch transfer logic control circuit, as previously described. When the switch is in the Plain Bypass Test position, the critical AC loads are directly connected to the AC bypass source through a separate set of contacts, and the inverter and the bypass remain connected to the input of static switch. When the switch is in the Plain Full Bypass position, the critical AC loads are connected to the bypass source and the static switch is totally disconnected from the bypass source. This switch is normally included to facilitate the performance of maintenance. Forward Transfer for a Non-Redundant UPS System

In a forward transfer scheme for a non-redundant UPS system, the AC bypass line is the preferred source of power for the critical AC loads, and the inverter is the alternate source of power for the critical AC loads. Forward transfer schemes are not acceptable for use in Saudi Aramco UPS systems. Figure 8 shows a basic diagram of a non-redundant UPS system with a forward transfer scheme. The UPS system that is shown in Figure 8 has a forward transfer scheme because the preferred source of power for this system is the AC bypass source. The UPS system that is shown in Figure 8 operates as follows: When AC power is available, the AC power source supplies AC power to the input of the battery charger and to the AC bypass line. The battery charger converts this AC power into DC power. The DC power from the output of the battery charger then is supplied to the input of the inverter and to the battery.

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Engineering Encyclopedia

Electrical Introduction to DC/UPS Systems

The inverter converts the DC power into AC power. The AC power from the output of the inverter is supplied to pole 1 of the static switch. Because the inverter output is the alternate source of power for the critical AC loads in a UPS system that uses a forward transfer scheme, the static switch transfer logic control circuit does not supply a gate signal to the SCR's that make up pole 1 of the static switch; therefore, the output from the inverter is maintained in this standby condition and is not connected to the critical AC loads. If the voltage and the frequency of the AC bypass line are within the normal range, the static switch transfer logic control circuit will supply a gate signal to the SCR's that make up pole 2 of the static switch. The SCR's that make up pole 2 of the static switch start to conduct and supply AC power from the AC bypass line to the critical AC loads. If the static switch transfer logic control circuit senses a problem with the voltage or the frequency of the AC bypass line, the circuit will remove the gate signal from the SCR's that make up pole 2 of the static switch and will supply a gate signal to the SCR's that make up pole 1 of the static switch. The SCR's that make up pole 2 of the static switch will commutate the next time that the AC power source sinewave passes through zero, and the SCR's that make up pole 1 of the static switch will start to conduct. The critical AC loads now will be supplied with power from the output of the inverter (alternate source), which has been maintained in a standby condition. When the static switch transfer logic control circuit senses that the voltage and the frequency of the AC bypass line power source has returned to normal, the circuit will remove the gate signal from the SCR's that make up pole 1 of the static switch and will supply a gate signal to the SCR's that make up pole 2 of the static switch. The SCR's that make up pole 1 of the static switch will commutate the next time that the inverter output sinewave passes through zero, and the SCR's that make up pole 2 of the static switch will start to conduct. When the SCR's that make up pole 2 of the static switch start to conduct, the critical AC loads will once again be supplied with power from the AC bypass line (preferred source), and the output of the inverter (alternate source) will return to a standby condition.

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Electrical Introduction to DC/UPS Systems

Basic Non-Redundant UPS System with a Forward Transfer Scheme Figure 8

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Electrical Introduction to DC/UPS Systems

The static switch transfer logic control circuit for a non-redundant UPS system with a forward transfer scheme is identical to the static switch transfer logic control circuit for the reverse transfer scheme that previously was discussed. The differences between the two circuits are that the AC bypass source is connected as the preferred source of power and that the output of the inverter is connected as the alternate source of power. Reverse Transfer for a Redundant UPS System

In a reverse transfer scheme for a redundant UPS system, the primary inverter is the preferred source, the redundant inverter is the alternate source, and the AC bypass line is available in the event that both inverters fail. Figure 9 shows a basic diagram of a redundant UPS system with a reverse transfer scheme. The UPS system that is shown in Figure 9 has a reverse transfer scheme because the preferred source of power for this system is the primary inverter output. The UPS system that is shown in Figure 9 operates as follows: When AC power is available, the AC power source supplies AC power to the input of the battery chargers and to the AC bypass line. The battery chargers convert this AC power into DC power. The DC power from the output of the battery chargers then is supplied to the input of the primary and alternate inverters, and to the battery. The inverters convert the DC power into AC power. The AC power from the output of each inverter is supplied to the respective pole of the static switch. If the voltage and the frequency on the output of the primary inverter are within the normal range, the static switch transfer logic control circuit will supply a gate signal to the SCR's that make up pole 1 of the static switch. The SCR's that make up pole 1 of the static switch start to conduct and supply AC power from the primary inverter to the critical AC loads. If the static switch transfer logic control circuit senses a problem with the output of the inverter, and if the AC bypass source is available, the circuit will remove the gate signal from the SCR's that make up pole 1 of the static switch and supply a gate signal to the SCR's that make up pole 3 of the static switch.

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Engineering Encyclopedia

Electrical Introduction to DC/UPS Systems

The SCR's that make up pole 1 of the static switch will commutate the next time that the inverter output sinewave passes through zero, and the SCR's that make up pole 3 of the static switch will start to conduct. The critical AC loads now will be supplied with power from the AC bypass line. The static switch transfer logic control circuit will check the status and condition of the alternate inverter. If the voltage and frequency on the output of the alternate inverter are within the normal range, the static switch transfer logic control circuit will remove the gate signal from the SCR's that make-up pole 3 of  the static switch and will supply a gate signal to the SCR's that make-up pole 2 of  the static switch. The SCR's that make up pole 3 of the static switch will commutate the next time that the AC power source sinewave passes through zero, and the SCR's that make up pole 2 of the static switch will start to conduct. The critical AC loads now will be supplied with power from the alternate inverter. If the static switch transfer logic control circuit senses a problem with the output from the alternate inverter and if the primary inverter has not returned to normal, the circuit will remove the gate signal from the SCR's that make up pole 2 of the static switch and will supply a gate signal to the SCR's that make up pole 3 of the static switch. The SCR's that make up pole 2 of the static switch will commutate the next time that the inverter output sinewave passes through zero, and the SCR's that make up pole 3 of the static switch will start to conduct. The critical AC loads now will be supplied with power from the AC bypass line. When the static switch transfer logic control circuit senses that the inverter (Preferred Source-Primary/Alternate Source-Secondary) output has returned to normal, the circuit will remove the gate signal from the SCR's that make up pole 3 of the static switch and will supply a gate signal to the SCR's that make up pole 1 of the static switch (pole 2 if the alternate inverter reaches normality in advance of  the primary inverter).

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Electrical Introduction to DC/UPS Systems

The SCR that make up pole 3 of the static switch will commutate the next time that the AC power source sinewave passes through zero, and the SCR's that make up pole 1 of the static switch will start to conduct. When the SCR's that make up pole 1 of the static switch start to conduct, the critical AC loads will once again be supplied with power from the output of the primary inverter. The static switch transfer logic control circuit for a redundant UPS system with a reverse transfer scheme is identical to the static switch transfer logic control circuit for the non-redundant reverse transfer scheme that previously was discussed. The differences between the two circuits are that the non-redundant system employs one inverter circuit in conjunction with an AC by-pass line and the redundant system employs two inverter circuits in conjunction with an AC by-pass line.

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Electrical Introduction to DC/UPS Systems

Basic Redundant UPS System with a Reverse Transfer Scheme Figure 9

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