Medium Voltage—Autotransformer (10–13.8 kV). Trane Drives, CVHG, Starters, CVHF, Electrical Components, CVHE, CDHG, CDHF

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Medium-Voltage Starter Types (10,000–13,800 Volts)

Medium Voltage—Autotransformer (10–13.8 kV)

Autotransformer Starter (10,000–13,800 volts)

Starting sequence

Refer to the autotransformer starting sequence ( “Starting sequence,” p. 38 ).

Standard features

• 4 kVA control-power transformer

• Primary and secondary current transformers (CTs)

• Potential transformers (PTs)

• Grounding provisions

• Bolted line-power connections

• Bolted load-side connections for remote starters

• Standard motor protection

Dimensions

Typical dimensions for remote-mounted autotransformer starters are shown in Figure 37, p. 48 .

PFCCs, if required, are housed in a 36-inch wide auxiliary cabinet (not shown). Always consult the submittal drawings for as-built dimensions.

Environmental specification

• Designed, developed and tested in accordance with IEC 60470, 62271-200

• NEMA 1 enclosure as standard

• Operation from sea level to 6,000 ft (1,829 m)

• Operating ambient temperature range 32°F to 104°F (0°C to 40°C)

• Relative humidity, non-condensing 5 percent to 95 percent

• Non-operating ambient temperature range -40°F to 158°F (-40°C to 70°C)

• Voltage utilization range ±10 percent

Figure 37. Remote-mounted, autotransformer dimensions

MAIN BUS

MAIN BUS

MAIN BUS

INCOMING

LINE

LOW-VOLTAGE

SECTION

96"

36"

132"

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Electrical System—Ratings

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“Normal” and “overload” conditions … “fault current” … “interrupt” versus “short-circuit” ratings” … “current-limiting.” Knowing what these terms mean and applying them correctly are fundamental to designing safe, reliable electrical distribution systems. This is especially true in light of more stringent code enforcement and the current design trend to deliver energy savings by selecting low-impedance transformers: How does this influence safety? Lower transformer impedances result in higher short-circuit currents.

Simply choosing a circuit breaker with a high-interrupt rating won't assure adequate protection under short-circuit conditions. An “ounce of prevention” helps avoid the code official’s “red tag” at the next system startup.The following section reviews the meaning of terms, defines some of the issues related specifically to HVAC motor starter applications, and identifies practical effective solutions.

Normal operation

“Normal operation” describes the full-load (or rated) conditions of each system component. For motors, it includes the amps initially drawn at startup, i.e. inrush current, as well as the full- or ratedload amps drawn while running. The magnitude of inrush current for a particular application depends on the motor, voltage, and type of starter used.

Normal operating conditions determine wire and transformer sizing. They are also used in conjunction with “fault conditions” to select overcurrent protection devices such as circuit breakers and fuses. Rating factors are applied, based on the type and number of connected loads, to assure that the devices selected adequately protect the motor as it starts and while it is running.

The size of the interconnecting wires between the transformer and starter reflects the type and rated amperage draw of the load, i.e. the chiller motor. Sizing the wires on this basis assures that they can carry the inrush current at startup without overheating.

Available Fault Current (AFC)

Refer to Figure 39, p. 52 , for the reference points or label locations of each rating. AFC is the calculated potential short-circuit current at a point just upstream of the starter. It is calculated by the electrical engineer and is a function of the electrical distribution system—including the transformers.

Imagine a wrench inadvertently left in a starter after servicing. Touching two power phases, it completes the circuit between them when the panel is energized. This results in a potentially dangerous situation, or “fault condition,” caused by the low-impedance phase-to-phase or phaseto-ground connection … a “short circuit”.

Fault current, also called “short-circuit current” (I sc

), describes the amount of current flow during a short. It passes through all components in the affected circuit. Fault current is generally very large and, therefore, hazardous. Only the combined impedance of the object responsible for the short, the wiring, and the transformer limits the fault current.

One objective of electrical distribution system design is to minimize the effect of a fault, i.e. its extent and duration, on the uninterrupted part of the system. Coordinating the sizes of circuit breakers and fuses assures that these devices isolate only the affected circuits. Put simply, it prevents a short at one location from shutting down power to the entire building.

Calculating the magnitude of short-circuit current is a prerequisite to selecting the appropriate breakers and fuses. If the distance between the transformer and starter is short, the calculation can be simplified by ignoring the impedance of the interconnecting wiring… a simplification that errs on the side of safety. One can also assume that the source of the fault has zero impedance, i.e. a

“bolted” short. Given these assumptions, only the transformer impedance remains. (Impedance upstream of the transformer is usually negligible.)

Suppose a 1,500-kVA, 480-volt transformer has impedance of 5.75 percent. With this value, use the equation below to determine how much fault current a short circuit will produce.The resulting I sc shows that a short would force the wiring to carry more than 30,000 amps when it was designed to handle only 400 amps!

49

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Electrical System—Ratings

50

I

SC

=

1,500 kVA x 1000/k

480 V x 1.73 x 0.0575

I

SC

= 31,400 amps

Short-circuit current is often two orders of magnitude greater than normal operating current.

Unless a circuit breaker or fuse successfully interrupts the fault, this enormous amperage rapidly heats components to very high temperatures that destroy insulation, melt metal, start fires … even cause an explosion if arcing occurs or components disintegrate.The inherent likelihood of severe equipment and property damage, as well as the risk of personal injury or death, underscores the importance of sufficient electrical-distribution system protection.

Ampere-Interrupt Current (AIC)

Determined under standard conditions, the “interrupt rating” specifies the maximum amount of current a protective device can cut off safely … i.e. without harm to personnel or resulting damage to equipment, the premises or the device itself. For example, a circuit breaker that trips “safely” successfully interrupts the fault, can be reset, and will function properly afterward.

A common misconception is, “An overcurrent protection device with a comparatively highinterrupt rating limits current to other components.” Not so, not unless it is also a true currentlimiting device. Even though the device successfully breaks the circuit, all components in the circuit will be exposed to the full magnitude of fault current (as well as the severe thermal and magnetic stresses that accompany it) for the small amount time it takes the device to respond.

Short-Circuit Current Rating (SCCR)

Sometimes referred to as the short-circuit withstand rating (SCWR), the short-circuit current rating

(SCCR) is probably the most critical rating for short-circuit protection that the electrical engineer must obtain from the starter supplier.The term “withstand” is no longer used by NEC and UL.The

entire assembled starter enclosure has an SCCR.

The short-circuit current rating is the maximum fault current that the starter withstood during testing by Underwriters Laboratories, Inc. UL 508 defines the short-circuit test methods and parameters for HVAC equipment, and therefore the SCCR is mainly a low-voltage issue. Essentially, the test simulates an actual fault current in the starter enclosure, e.g. 50,000 amps. If the doors blow open or if the starter emits debris, the enclosure fails the test. For those that pass, it is “acceptable,” and even probable, that the internal components will be damaged beyond repair. Given the destructiveness and expense of this test, it is not surprising that most manufacturers prefer not to pursue higher-than-normal short-circuit current ratings for their equipment unless there is a documented need.

Recall that when a fault occurs, all components in the circuit experience the brunt of the short circuit until it is interrupted. Therefore, it is important to assure that all components “at risk” can withstand a fault condition without causing personal injury or damaging the surroundings. The

National Electric Code (NEC) states this requirement in Section 110.10, “Circuit Impedance and

Other Characteristics”:

The overcurrent protective devices, the total impedance, the component short-circuit

[withstand] ratings, and other characteristics of the protected circuit shall be selected and coordinated to permit the used circuit protective devices to clear a fault without extensive damage to the circuit’s electrical components.This fault is assumed to be either between two or more of the circuit conductors, or between any circuit conductor and the grounding conductor or enclosing metal raceway. Listed products applied in accordance with their listing shall be considered to meet the requirements of this section.

Commentary in the 1996 National Electrical Code™ Handbook further explains Section 110.10:

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Electrical System—Ratings

Overcurrent protective devices (such as fuses and circuit breakers) should be selected to ensure that the short-circuit withstand rating of the system components will not be exceeded should a short circuit or high-level ground fault occur.

System components include wire, bus structures, switching, protection and disconnect devices, distribution equipment, etc., all of which have limited short-circuit current ratings and would be damaged or destroyed if these short-circuit current ratings are exceeded. Merely providing overcurrent protective devices with sufficient interrupting ratings will not ensure adequate short-circuit protection for the system components. When the available short-circuit current exceeds the withstand rating of an electrical component, the overcurrent protective device must limit the let-through energy to within the rating of that electrical component.

To comply with this section of NEC without additional current-limiting devices, most chiller-motor configurations will require a short-circuit current rating well above UL’s standard ratings.

Let-Through Current (LTC)

This is the fault current that passes through the circuit breaker before the circuit breaker trip element has time to respond, typically equal to or less than the available fault current and lasting three-quarter of an electrical cycle.

Current limiting

All components and wiring in an electrical distribution system offer some degree of resistance.

Under normal conditions, the heat produced when current flows against this resistance readily dissipates to the surroundings; however, the enormous current generated during a short circuit produces damaging heat at a much faster rate than can be safely dissipated. Interrupt the current and you stop adding heat to the system. As Figure 38 suggests, time is a critical determinant of the amount of heat (energy) added. An electrical short that lasts three cycles, for example, adds six times the energy of an electrical short lasting just one-half cycle. It is in this sense that all circuit breakers and fuses “limit” current.

Figure 38 also shows the effect of a current-limiting device. To be truly current limiting, the interrupting device must open the circuit within one-quarter cycle (1/240 second), i.e. before the fault current peaks.

Figure 38. Illustration of short-circuit current

100,000

Prospective available short-circuit current that would flow when a fuse is not used.

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Current

10,000

0 t c

Peak Fuse

“Let-Thru”

Current

Total Fuse Clearing Time

Time

51