Digital static transfer switch:

The ever-increasing reliance on computers has raised expectations of power reliability in critical data processing and operations centers. Moreover, the desire to implement system configurations that protect against failures, whether caused by human error or equipment glitches, have resulted in a trend toward independent, fast and intelligent watchdog systems. These solutions detect, decide and react in real time against electrical failures.

The digital static transfer switch (DSTS) incorporates all these features into its design. For this reason, it has become a key element of the recent power reliability solutions introduced in the marketplace.

The DSTS enables electrical systems engineers to take the overall redundancy and reliability of carrier data centers one step further by promoting a dual independent power system or multiple independent power systems. In this scenario, each individual connected load can be fed continuously from either of the two sources at any given moment.

Starting from this basic premise, many uninterruptible power supply (UPS) and building system configurations become a possibility-from independent and isolated systems with only DSTSs allowing transfer of loads between systems to isolated redundant systems (also known as segmented redundant systems), which have bypasses fed from a standby UPS.

A DSTS monitors its two or three connected sources and automatically transfers loads to the qualified source when it senses the degradation or total failure of either source. If the primary source fails or falls below user-defined parameters, the DSTS will transfer to the alternate source. The DSTS will then switch back to the primary feed when it comes back into the qualification range.

DSTSs are high-speed, open-transition switches that can transfer electrical loads from one AC power source to another in a fraction of a single electrical cycle (Figure 1).

User-adjustable settings of voltage and phase angle differentials allow use of a DSTS in various applications where different load types require different transient thresholds. Most loads, including the sensitive computer loads, allow a wide range of voltages and phase angles. Therefore, transfers are always possible without any load interruptions.

Some static switches also offer a user-selectable manual transfer capability and a user-selectable automatic re-transfer capability.

Failure protection Existing electrical distribution systems in most commercial and industrial buildings cannot protect loads against outages that may be caused by failures in a series combination of the sub-systems.

Although the internal building elements are designed to overcome utility outages, if one of the series elements in the chain of sub-systems in the building were to open for any reason, current flow to the load would be interrupted.

UPSs are able to overcome failures upstream by delivering reliable power at their outputs. However, statistics have shown that even these sub-systems-and everything between them and the final load-are fallible. Even sites with no history of UPS-caused interruptions can experience outages due to breakers, cables, faulty power distribution units (PDUs) and human error (such as operational errors or improper maintenance procedures).

Power failures can be costly, and avoiding such failures is an obvious management concern. In many cases, a DSTS is a cost-effective technology that can be incorporated in a system upgrade to avoid substantial operational losses.

The DSTS overcomes the fallibility of conventional electrical system design by using redundant power paths. Applied properly, it can overcome individual component failures in the electrical system.

Mean time between failures of the available power to the load can exceed 1,000,000 hours if dual output breakers are used. The DSTS provides redundant sources of power and protects against an internal failure of one of its two power stacks. Redundant paths allow for complete electrical isolation of all upstream distribution switch gear, bus and cable runs without the need for an access shut down or interruption.

Therefore, establishing preventive maintenance without load interruption, as well as an emergency recovery capability if a failure has already occurred, reduces costs and enhances the benefits. If a switchboard unexpectedly fails, the DSTS automatically switches to the other switchboards without interrupting the load.

Data center applications In general, the protective capability of a static switch is maximized if it is applied closest to the load. If the DSTS is protecting against a loss upstream, and if more of the components of the system are upstream from the switch, then it can provide backup against more potential sources of failure.

Each data center has a unique set of parameters. For instance, there can be limitations in space. Also, particular configurations of the UPSs can have different characteristics-their downstream distribution boards, arrangements of the PDUs and availability of feeders may differ. Specifically, in public network data centers, carriers are trying to reduce the amount of space devoted to power-related equipment and other pieces of equipment that are not directly involved in transmission or network management functions. This is driven by the need to reduce overall costs while maintaining network performance and improving functionality.

Consequently, there are several ways to apply DSTS technology in a general data center environment. These applications can be classified from the closest (least upstream) to farthest (most upstream) from the load.

Rack-mount deployment is one significant application. These units are installed at the location of the load they serve and provide maximum protective coverage (Figure 2).

The units are fed from two separate PDUs or power panels, each of which is fed from a separate upstream source. Sources may be two UPSs of any type or a UPS and a maintenance switch gear, which is fed from the utility and generators.

This DSTS configuration enables maintenance of PDUs or UPS output switchgear, which would otherwise be impossible without turning off power to the load. These units, designed for protection at the point of use, can be installed to feed all major loads in computer or telecom rooms.

In another application, switches can be placed at the PDU secondary side (208 V) of the transformer (Figure 3). These units, primarily rated at 400 to -600 A, are installed on the raised floor adjacent to their respective PDU or power panel.

Purchasing this DSTS as an integrated unit is more economical. An integrated unit should contain two shielded isolation transformers (each protected by a main input breaker), the DSTS itself, output distribution panels and DSTS monitoring features.

It is not recommended to hang panels on the transformer (not fed by the DSTS) because dual cord loads fed from those panels will be limited to only one source per cord, whereas if fed from a panel that is fed from the DSTS, they will have access to both supplies. This is important because there is some controversy relative to dual cord loads-that with one cord off, they are not able to run at 100% capability.

Switches also can be deployed at the PDU primary side (480 V) of the transformer (Figure 4). These units, primarily rated at 200 to 400 A, are installed on the raised floor adjacent to their respective PDU or power panel. The PDU is fed from two possible sources providing its output loads with the protection of dual source redundancy. If one source is affected, the unit transfers instantly to the standby source without disruption.

Integrated design of a combined PDU/DSTS can also be furnished, although this option would be cost-effective only for a new installation. Otherwise, existing PDUs should be used. Because the cost of the DSTS is directly proportional to its amperage rating (as is the case with circuit breakers), it is more cost-effective to switch at 480 V than at 208 V.

Another key application focuses on load transfers between UPSs, adjacent UPS systems lacking the cross-connection but having instant emergency transfer capability.

This option can be provided with either a two-source or three-source DSTS, with power fed from two or three UPSs (Figure 5).

As in adjacent UPS system configurations, the total of connected loads must not exceed the rating of any UPS. This configuration provides a superior alternative to paralleling tie systems because it will not spread a fault from one system to the other-the two load segments are from redundant sources always available.

In addition, the DSTSs are independent of each other and the UPS controls. No logic or control connections exist between the UPSs. This means the systems can be of any type or manufacture, thereby opening a wide array of flexibility to the user.

This application also can involve three DSTSs. However, a single load segment can also be protected by one DSTS and two UPSs, with one as a backup to the others.

In another scenario, the DSTS is an active maintenance bypass for UPS systems. The use of redundant UPS systems provides a substantial investment. Although these are redundant UPS modules, they share common system logic and common output, and bypass circuit breakers. The commonality of these elements implies that their failure paralyzes the redundant UPS system.For true backup redundancy of system breakers and logic con trols, as well as the performance of the UPS as a complete system, installing a DSTS at the output as an active maintenance bypass is a viable solution. This provides redundancy for the modules and system elements.

In this configuration, the DSTS performs as an independent watchdog that prevents load failure if the UPS is unable to transfer to its bypass properly or if its output is opened by accident.

Most UPS static bypass switch fuses will not allow clearing of a fault in any one of the branch breakers in the output distribution (as is seen by comparing breaker operation curves with the fuse melting curve). Consequently, a faulted distribution feeder can lower the bus voltage before its breaker over-current elements trip (fed from a higher-impedance UPS output) and can result in a UPS transfer that causes the bypass fuses to clear (now fed from a lower-impedance utility). This causes all other feeders to be dropped.

Most people overcome this concern by specifying only non-fused, continuous-duty UPS static bypass switches that can continue conducting even if the bypass breaker does not close.

In this application, the DSTS provides the benefits of the continuous-duty switch, transfers if the UPS bypass fuses clear and provides backup to the UPS system controls-something a continuous-duty bypass switch alone cannot provide. DSTSs used for this application also allow transfer under overload conditions.

Three-source DSTS deployments provide another application case. Critical data centers are increasingly using three-source DSTSs to increase the redundancy of power to the critical load. The three-source DSTS ranges from 100 A through 4000 A.

Most data centers use two UPSs and a generator set as the three sources. In this configuration, the three-source DSTS signals the generators to start when the first source is lost. The generators remain on until the first source has recovered. Optionally, as a precaution for storm-related outages, the loads are transferred to the generators until both utility sources have recovered.

The main advantage of a three-source switch is that, even with one source lost, two redundant sources still remain to protect the load. A two-source switch has no redundancy when one source is out.

The most traditional application of the DSTS is a building entrance deployment. Because of the low probability of simultaneous failures of utility distribution lines, DSTSs provide UPS capability without the UPS-usually at a fraction of its purchase cost, installation and space requirements.

DSTSs are designed for a wide range of data center configurations and applications. Depending on need, DSTSs can be used to harden existing sites, add redundant sources of power to connected loads, overcome failures on the electrical distribution system, transfer loads between dissimilar sources of power, tie together dissimilar UPS systems and eliminate single points of failure in the electrical system.

Used in such applications, DSTS units can help make power management a less daunting issue for carriers and their customers.