Not Your Father's Switchgear
Medium-voltage metal-clad switchgear have long been an essential component of safe, reliable electric-power distribution systems for industrial facilities. Now, innovative technologies are opening up a new world of possibilities in switchgear design. Switchgear, of course, is an assembly of switching and interrupting devices—in combination with control, metering and protective equipment...
Medium-voltage metal-clad switchgear have long been an essential component of safe, reliable electric-power distribution systems for industrial facilities. Now, innovative technologies are opening up a new world of possibilities in switchgear design.
Switchgear, of course, is an assembly of switching and interrupting devices—in combination with control, metering and protective equipment—with associated interconnections and supporting structures. The Institute of Electrical and Electronics Engineers (IEEE) Standard C37.20.2 defines the basic characteristics of medium-voltage switchgear as follows:
The main switching and interrupting device is removable.
Major parts of the primary circuit are enclosed and separated by grounded metal barriers.
All live parts are enclosed within grounded metal compartments with automatic shutters to prevent exposure of the primary circuit elements when the removable element is disconnected.
The primary bus connectors and connections are insulated.
Mechanical interlocks are provided to ensure a proper and safe operating sequence.
Secondary devices and their wiring are isolated from all primary circuit elements.
The door to a circuit-interrupting device may serve as a control panel and for access to some secondary elements.
While the standard recognizes other switching and interrupting devices, circuit breakers usually do this work.
Two breaker-interrupter technologies—one using sulphur hexafluoride (SF6) gas and the other employing a vacuum bottle—were developed in the 1960s, when the air-interrupter circuit breaker was the dominant technology. Large space requirements, frequent maintenance intervals and slower operation put air breakers at a disadvantage.
Today, the air breaker has almost disappeared from medium-voltage systems. The vacuum breaker dominates in Japan and the United States, while SF6 is favored in Europe and the Middle East. Minimum oil breakers are still in use in China, India and Eastern Europe, but their use is in decline.
Interrupting the arc in a vacuum offers these advantages:
Smaller space requirements.
Low contact erosion.
High repetitive-switching capability.
Dielectric immunity to temperature, humidity and dust.
But vacuum-interrupter technology also has its disadvantages, such as the absence of a scheme to detect the possible loss of vacuum and the generation of higher transient switching voltages.
The SF6 interrupter offers all the benefits of the vacuum interrupter, with the additional advantages that the gas is inert, non-flammable and non-toxic, and low-pressure alarms are available to signal loss of gas pressure.
A circuit breaker's job is to carry and switch load current and, when required, to interrupt short-circuit current. Capabilities are circuit voltage, frequency, continuous current, short-circuit current and close-and-latch current. Each duty requirement is selected or calculated and compared to the corresponding breaker characteristics. Fast interruption of short-circuit current is the circuit breaker's primary safety function. (Consult ANSI Standard C37.010 for guidance on calculation of short-circuit duties.)
A breaker with short-circuit capability that exceeds the calculated value is required. Circuit breakers are designed to stay closed (close-and-latch) against a maximum first-cycle current 1.6 times the short-circuit current. The standard close-and-latch capability is usually adequate. On a large motor circuit, the close-and-latch rating may be the determining factor, in lieu of interrupting rating.
Continuous current ratings should be greater than the maximum demand load expected, including a provision for future growth. Circuit breakers have no continuous current overload rating. For applications with apparatus that have a longtime overload rating, such as a forced cooled rating, the switchgear must have a continuous current rating at least equal to the overload rating.
Circuit breakers may be operated in excess of the continuous-current rating for short intervals; this allows for motor-starting current and cold-load pickup. Supplemental fan cooling can increase the continuous-current rating by removing heat from the breaker. In using this option, a system designer may consider using redundant fan systems or alarms for loss of the cooling system.
Service and Sizing
Switchgear ratings are based on "usual" service conditions, as defined in the standard. Normal conditions include ambient temperature between -30°C and 40°C, altitude not above 1,000 meters and insignificant solar radiation. Operation outside the usual conditions would include:
Increased ambient temperature, which requires continuous current derating to maintain the switchgear assembly's maximum temperature of about 40°C.
Higher altitudes that require derating factors for rated current and rated voltage. Air is used as heat-transfer and dielectric media in switchgear. Thinner air at higher altitudes results in higher temperature rise and reduced dielectric withstand ability.
Significant radiation for outdoor switchgear means that cooling and ventilation are required to limit the temperature rise (see ANSI Standard C37.24).
Voltage selection in retrofit projects is dictated by available power sources. For new systems, it depends on the number and size of loads to be served—and on large motors.
For a 3,000-ampere breaker, a 2,400-volt system can serve approximately 12 mega-volt-amperes (MVA), while a 4,160-volt system can serve approximately 21 MVA. Loads in excess of 21 MVA require a 15-kV-class switchgear voltage; 13.8 kV is the most prevalent.
If no constraints on voltage selection exist, a 4,160-volt system is the optimum choice for systems with several motors above 200 horsepower (hp) and few loads that exceed 4,000 hp. Alternatively, a 2,400-volt system can be used for fewer motors less than 1,000 horsepower. A system with motors larger than 3,000 hp—and any motors larger than 6,000 hp—might require a 7,200-volt system. Motors larger than 10,000 hp may require a 13.8-kV system.
Where costs for a system voltage higher than 4,160 can't be justified, the use of reduced-voltage-starting, two-speed motors or variable-frequency drive motors can be considered. Electrical system design for large motor applications must be carefully studied to assure the system can maintain the motor voltage at a level that will produce sufficient torque for the motor to start its load.
No Trip Units
A key difference between medium- and low-voltage circuit breakers is that the former does not have integral sensors or trip units. The circuit breaker receives signals to trip or close from external relays and control circuits. A stored-energy spring powered operating mechanism opens and closes the breaker's contacts.
Protective relays, current transformers, potential transformers and other controls are integrated into the switchgear.
Current transformers produce a model of the primary scaled down to a value usable for relays, meters and instruments. The primary current rating should roughly equal 125% of the circuit's full-load current. Standard practice in the United States is to utilize current transformers with a 5-ampere secondary current rating; European practice involves a 1-ampere secondary rating.
Voltage transformers are used to model the system voltage, usually at 120 volts. These scale quantities are the inputs to the protective relays that stand guard over the electrical system and the meters that provide data on its operation.
The traditional electro-mechanical feeder relay scheme had four relays—one for each phase and ground—an ammeter and an ammeter switch. A basic microprocessor-based overcurrent relay incorporates all these functions in one device. As one might expect, the multifunction digital relay now reigns supreme. It offers expanded relay functions, higher accuracy, integral metering, diagnostics and a reduction in the amount of wiring and panel space required.
More sophisticated models include voltage and frequency functions, with programmable input/output. This allows use of control schemes such as synchrocheck, bus transfer and automatic load restoration. All include programmable characteristics for pickup levels for trip, alarm or control and a choice of ANSI/IEC or user-defined curve shapes and multipliers.
Some models include circuit-breaker-condition monitoring features for maintenance purposes. The relay calculates the wear on the circuit breaker contacts by integrating the arcing current as they are opening. An alarm can be operated when the user-specified maintenance threshold is exceeded.
With an ever-expanding variety of features, manual programming has been replaced by software, which allows programming via a personal computer. Menu-driven programs allow easy access to all local functions and download of relay trip set-point files.
A new generation of microprocessor-based meters has led to changes similar to those in the world of relays. A single device can provide metering for current, voltage, real and reactive power, energy use, cost of power, power factor and frequency. Revenue-grade accuracy is now available in these meters.
Programmable set-points and output relays allow implementation of specific control applications. These include load management with demand-control-based load shedding and capacitor bank power-factor control. Measured quantities can be set to alarm conditions with an event recorder to allow diagnosis of system events or trouble.
Communications ports turn the meter into a data-gathering device for plant control systems that integrate process and electrical requirements. Most systems include RS485 ports using Modbus or DNP 3.0 protocols, making it easy for the meter to communicate with distributed control (DCS) or supervisory control and data acquisition (SCADA) systems. Meters with modems allow dial up access to the meter—and the ability to send messages via e-mail or pager and data postings to the Internet.
MV Circuit Breaker Standards Revised
Major revisions—the first since 1964—were made in 1999 and 2000 to American National Standards Institute (ANSI) standards for medium-voltage circuit-breaker short-circuit interrupting rating structure (ANSI C37.04, C37.06, C37.09, C37.010). The new "Constant kA" rating structure more closely represents the actual physics of interruption in today's circuit breaker.
With these revisions, the standards catch up to the modern circuit breaker interrupter technology of vacuum and SF6. The 1964 rating structure embodied the prevalent circuit interruption technology (air magnetic) of that time. Circuit breakers were assigned a "Constant MVA" rating basis over a range of operating voltages.
In 1968, the standards introduced a voltage range factor—"K"—to adjust breaker interrupting rating as a function of applied voltage. The interrupting capability of air magnetic circuit breakers increased as the operating voltage was reduced from rated maximum.
However, vacuum and SF6 interrupter technologies do not increase interrupting ability significantly as operating voltage is decreased; these are actually closer to constant current interrupter devices. The more-recent standards revisions changes all circuit breaker "K" factors to unity; this results in all medium-voltage circuit breakers having a constant interrupting rating irrespective of applied voltage.
For example, under the 1968 standards, a 250-MVA breaker had a 29-kA interrupting rating if applied at 4.76 kV. The same breaker applied at 4.16 kV had a 33 kA rating. With the new standard, a breaker certified as 36 kA retains the same rating across the range of rated voltage.
From Pure Power, Winter 2001.