With the increasing demand of uninterrupted and high quality power for critical loads, high power (>500KVA) UPS systems are becoming more and more popular. In order to properly design the critical power system, the type of UPS and amount of redundancy must be matched to the nature of the load, the type of power distribution, the quality of local power, and the required reliability. This requires a general understanding of the performance of different UPS topologies and a guide to the trade-off analysis. In this two-part article, the first of which is presented here, the four popular UPS topologies for high power (>500KVA) applications are described and a comparative study of their response to grid-based PQ (power quality) events is provided. A thorough analysis of the system design, desired reliability, and various site constraints using the guidelines presented here will help ensure that the customer receives the correct UPS topology.

Introduction
During the late 1990s, the rise of internet-based companies - those heavily dependent on servers and other computer loads - resulted in an explosion in demand for electrical infrastructure capable of protecting delicate equipment from outages (as short as 1.5 cycles). While these particular companies have suffered a recent downturn, the need for conditioned power continues due to the increased dependence on computer-controlled equipment in fields such as medicine, bio-technology, and semiconductor manufacturing. Even old economy processes like coating, painting, and machining now have a single computer controlling the finishing operation on millions of dollars worth of finished goods (such as the coatings on gas turbine blades).

Uninterruptible Power Supplies (UPSs) are used to improve power source quality as well as protect these critical loads against disturbances, such as frequency shifts, voltage spikes and interruptions.¹

There are three major types of UPSs as specified by IEC standard 62040-3.
- Passive Standby (IEC 62040-3.2.20) DEG. Line Interactive (IEC 62040-3.2.18) - Double Conversion (IEC 62040-3.2.16)

These terms define how the UPS delivers power to the critical load. There are two popular styles of Line Interactive UPS: Static and Rotary. The static line interactive topology is popularly known as Delta Conversion topology and the rotary line interactive topology is termed as Rotary T topology in this document.

Figure 1. Block diagram of Double Conversion Topology

Double Conversion Topology
The most common type of large UPS today is the Double Conversion UPS, shown in Figure 1. In this topology, an inverter is connected in series between the AC input and the load, with power for the load flowing continuously through the inverter. Three operating modes are possible in this topology, namely "normal', "stored energy' and "bypass'. Under normal mode, the load is continuously supplied by a rectifier-inverter combination, which carries out the double conversion (AC-DC, then DC-AC).

The UPS goes into stored energy mode when the AC input to the system fails or goes out of the specified tolerance range. The inverter and the battery continue to support the load under this mode.

The UPS runs in stored energy mode until the stored energy is exhausted or until AC input returns to the specified tolerance level.

This type of UPS is generally equipped with a static bypass switch, allowing instantaneous transfer of the load to the bypass AC input. This switch is used in the event of a UPS internal malfunction, load current transients (in-rush, or fault clearing), prolonged overloads, or at the end of battery back-up (autonomy time). However, the presence of a bypass implies that the input and output frequencies and phase must be identical, and the voltage coordinated. The UPS is synchronized with the source of AC bypass supply to allow a transfer to bypass without any interruption.

At the input of this UPS there is a rectifier, which performs the AC-DC conversion. A 6-pulse rectifier with a fifth harmonic filter reflects less than 7% input current distortion. 12-pulse rectifiers or IGBTs (insulated-gate bipolqr transistor) reflect even less, although at reduced levels of efficiency, simplicity and reliability. While lower distortion is desired, it is rarely needed and 7% THD is sufficient to prevent any disruption to adjacent loads or to the incoming utility feed for most applications.

Figure 2. Block diagram of Delta Conversion Topology

Delta Conversion Topology
The static line-interactive topology is termed Delta Conversion, and shown in Figure 2. This is a line interactive UPS topology with active series-parallel power conditioning capability. The sinusoidal output voltage regulation capability results in low input current and output voltage THD, in back up as well as in standby mode. The series converter (mains side) can be operated as a current source and the parallel converter (Load side) as voltage source. In this case the battery charging is done through the series converter. Alternatively the parallel converter can be operated as a current source and the series converter as a voltage source under standby mode. In stored energy mode, the parallel converter changes its operating mode from current source to voltage source. The static switch is switched off during stored energy mode to prevent any power flow from the battery to the mains. An advantage of this line interactive topology over the double conversion topology is its increased overall efficiency.

The input power factor and the line current harmonics can be controlled in Delta Conversion.

The power factor is typically controlled to be close to unity and the reflected line current distortion is controlled to near 1% in many instances.

Figure 3. Block diagram of Rotary T UPS Topology

Rotary T UPS topology
The most common line-interactive topology is the Rotary T, illustrated in Figure 3. Under normal operating conditions (i.e. when the mains voltage is within the accepted range), the synchronous motor draws a compensating current so as to maintain a constant load voltage level. Also during normal operation, kinetic energy is stored in the flywheel (or induction coupling). When the mains fail, the stored flywheel energy is extracted to support the load.

Flywheel inertia (or induction coupling speed range) is sized to support the full load operation for a specific period (typically a few seconds), during which time the backup diesel generators are started.

Special construction of the synchronous motor allows this topology to provide good harmonic compensation of the line current. Bi-directional power converters are used to control power flow between the dual winding synchronous motor/generator and the flywheel synchronous motor/ generator.

The buffer inductor consists of two inductively coupled windings. The impedance of the isolation (mains side) is high (~49%) while the coupling winding has a low impedance (~1%). The mutual inductance between the two windings is equal to the sub-transient reactance of the generator. The isolation winding provides high impedance de-coupling between the input and output of the UPS while the generator regulates the voltage. The mutual inductance between the windings offsets any voltage variations at the output of the generator.

The main advantage of this topology is the ability to clear large faults or start motors without transfer to bypass. It is also favored for its compact size and relatively high efficiency.

Figure 4. Block diagram of Passive Standby UPS Topology

Passive Stand-By
In this topology, the system monitoring and control unit continuously monitors the utility voltage and frequency to determine if they are within the specified limits. If the utility source parameters are within +/-10%, the load is directly fed from the utility mains through the fast acting static switch. During this condition (standby mode) the power inverter remains energized but does not supply any power to the load. The battery charger maintains the voltage level of the batteries during this time.

In the event of any power failure at the mains or if the mains power parameters go out of the specified limit, the system control turns off the fast acting static switch and turns on the inverter. Under this condition (stored energy mode) the inverter supplies the load power by converting energy from the battery.

Operation of the UPS under stored energy mode is the same as that of a double conversion topology.

The major drawback of this topology is that it does not condition the utility power in any way during standby mode operation. Thus, in this topology the UPS transfers to stored energy mode very often.

However, a major advantage of this topology is its simplicity of control, high efficiency and low cost.

Response to Power Quality Events
The four UPS topologies described in the previous section were simulated² in Saber, a commercially available computer modeling tool. The models did not include all components. However, they were suitable to simulate the macro behavior of the UPS in response to typical gridbased power quality events with sufficient detail to make comparative performance assessments.

Line voltage sag and swell. Line voltage sags and swells happen during the application or removal of large loads to the grid system. These are also caused by fault conditions at various points in the AC distribution system. The typical duration of voltage sags and swells at the grid system is 0.5 sec.

In Double Conversion and Rotary T topologies, the output is very well isolated from the input side voltage sags and swells. For the Delta Conversion topology with a voltage swell >15%, the load voltage experiences the swell until the static switch is commutated off. Natural commutation at current zero crossing can lead to maximum commutation time of � cycle. In the Passive Standby topology input sag/swell directly gets passed to the load for short durations (~15ms).

Line frequency variation. For satisfactory operation of most of the computer equipment, frequency variation should be limited to +/-0.5Hz. UPS topologies other than Double Conversion cover mains frequency variations by tapping into the stored energy resource (flywheel or battery). Double Conversion topology can maintain the load frequency variation within +/-0.1%, without tapping the stored energy. In the line-interactive topologies as well as in the Passive Standby topology, the load experiences equal frequency variation as the mains. When this variation reaches unacceptable levels, the control circuit isolates the load from the mains through the input disconnect. This results in a discharge of stored energy until the utility source is back within tolerance. If this type of frequency variation is very common, the battery/ flywheel discharges frequently, resulting in possible early battery failure or a high level of nuisance starts on the gen-set. This may also result in a decreased amount of stored energy during an actual power interruption.

Input voltage unbalance and single phasing. The ANSI (American National Standards Institute) recommended limit of voltage unbalance in power distribution system is 3%. Approximately 98% of US distribution systems have unbalances less than or equal to 3%. Single phasing can be considered as a special case of voltage unbalance equal to 100%.

All the UPS topologies except passive standby protect the load from typical input voltage unbalance (<3%). For higher levels of unbalance, the losses in the Rotary T topology increase.

When there is single phasing at the incoming utility feed, all of the UPS topologies draw from the stored energy source. Double Conversion and Delta Conversion topologies transfer the load to stored energy instantaneously without any disturbance. In the Rotary T topology, the load voltage experiences a ~20% dip for ~200ms. In Passive Standby topology, the UPS load experiences the single phasing for a minimum of 4ms.

Voltage waveform distortion at input. In the double conversion topology the load side voltage is completely independent of the mains waveform distortion. In delta conversion topology the parallel converter is controlled as a voltage source and regulates the load voltage to achieve a good voltage THD on the order of 3%, irrespective of the input voltage THD. In case of Rotary T topology, the isolation inductor acts as a filter to the input voltage distortion. However, the isolation inductor does not filter any sub harmonics in input voltage and passes on the distortion to the load side. A passive standby UPS does not protect the load from utility voltage distortion.

Input switch commutation time. Among the four topologies discussed here, all but double conversion have a switch at the input which dictates the performance of the UPS.

Delta conversion topology has a line commutated thyristorized switch at the input, which is commutated naturally at the current zero crossing.

Thus the time delay for the switch to isolate the UPS from input side fault depends on the type and instant of occurrence of the fault. This static switch operates within 1ms for faults like input short circuit or input open circuit provided input power factor is maintained at unity. For other faults, the maximum time is � cycle. Rotary T topology has one motorized breaker at the input. Operation of this breaker is relatively infrequent as the UPS isolates the output from input side faults very well, including large sags and swells at the input. Typical time of operation of this switch is 200ms. Slow operation of the breaker does not affect the performance of the UPS, as the UPS can isolate load voltage from mains for short duration (about 1-sec). The breaker operation time varies depending upon the rms ac as well as the DC content of the current through it. Passive standby UPS has one forced commutated static switch at the input. Typical commutation time of this switch is 4ms (� cycle).

The total input disconnection time is the sum of switch commutation time and fault detection time. The fault detection time depends on the control circuit, and is typically on the order of � cycle.

References  1. Prasad P, et al., "Topologies of Uninterruptible Power Supplies used in Critical Power Systems,' GE Digital Energy Report No. 11, 2001. 2. Prasad P, et al , "Sensitivities of various UPS systems to grid-based power quality events,' GE Digital Energy Report No. 12, 2001.

About the author Edward R. Furlong currently serves as the General Manager of Technology for GE Digital Energy (GEDE) in Atlanta, Georgia. Dr. Furlong oversees technology development, engineering, and the development of the full suite of GEDE products including the GE-IMV line of uninterruptible power supplies and the fully integrated Advanced Power Systems. He holds a doctorate from Stanford University in Mechanical Engineering with an Electrical Engineering Minor, his field of research being advanced diagnostic systems for real time control applications. He received his master's from Stanford and his bachelor's from the University of Texas.