Microprocessors have given today’s power systems the potential to be more reliable, easier to maintain, and simpler to diagnose than ever before.
While technology is rushing forward, the underlying implementation of an appropriate power system design is often lagging, especially when it comes to supplying a reliable source of control power. While these “smart” devices may be considered the brains of the power system, the control power used to sustain them is certainly the heart.
Types of control power
To begin with, most manufacturers’ low-voltage circuit breaker devices, whether thermal-magnetic or electronic in design, are self-powered from current flowing through the breaker. That is, they need no external control power for proper and safe protective operation. They can sense the fault, trip their operating mechanism and clear the fault on their own.
Now add ground fault protection to that mix. As long as ground fault is part of the basic breaker’s trip unit, it will also be self-powered and require no external control power. That holds true even when remote current sensors are added for 3-phase, 4-wire systems, and when ground fault protection is built into a high-pressure contact (HPC) switch (see Fig. 1).
Fig. 1: High-pressure contact switch.
On the other hand, low-voltage breakers and fused switches do sometimes need external control power. Some electronic trip units on low-voltage breakers may require external control power to activate their display panels and for communication with higher level power monitoring and control systems.
Illumination of the display panel may be considered desirable but is not crucial. Breakers also have a large variety of accessories available such as shunt trips and motor operators which can be used in control circuitry; the application will determine their level of importance. The HPC switch only requires separate control power to test the integral ground fault function or its remote alarming, not its operation; this is often considered desirable but not a critical control power application. However, HPC and other fuse applications often incorporate blown fuse detection accessories as well as loss of phase protection relays. These devices may require external control power to operate properly and, when not available, could result in downstream equipment damage. For such applications it is important to specify a reliable source of control power (see Fig. 2).
Fig. 2: Circuit for external control power.
Reliability
Applications where molded case breakers are group mounted in panelboards or switchboards rarely require separate control power. Accessorisation for these applications warrants further consideration and should be investigated when they arise. Devices or accessories in this category include multi-function meters and the alarm elements of TVSSs. Neither of these are typically considered critical but they do require that determination be made by the design engineer.
Large mains in switchboards and all breakers in low-voltage switchgear applications may include functions such as remote electrical open/close operation and will therefore require a source of control power. If the remote operation capability is only for convenience or safe remote operator location, then the level of control power reliability is a matter of design evaluation.
When the system design progresses to multi-sourced switchboards, PLCs with embedded control logic, applications requiring co-ordinated or synchronised operation, timely communication of data or when protective relays operate separate breakers, then there is a need for an uninterruptible source of control power. The engineer must make an informed decision on how best to fill the need.
Applications
For the classic switchgear application, reliable control power is usually supplied from a battery and charger system at 48, 125 or 250 V DC. For applications where the battery bank can be located close to the gear and the control power for charging and closing the circuit breakers is derived independently, 48 V DC is usually selected (see Fig. 3).
Fig. 3: Control power circuit with battery and charger.
If the DC system supplies both tripping and closing power, 125 V DC is commonly the choice with a few applications such as high control power needs and long conductor runs requiring 250 V DC. One battery bank can service an entire location. The charger normally provides power to all of the control circuit continuous loads plus a small amount of current to “float” or “trickle” charge the battery. If the incoming AC supply fails, the battery takes over the duty of providing DC power.
Battery systems are sized to supply the maximum conservatively estimated continuous load requirement of the application for eight hours and then provide one minute of inrush loads at the end of the eight-hour period. This design is time-proven and is considered extremely reliable. It can, however, occupy significant floor space, requires special ventilation considerations, needs regular maintenance and monitoring, and can be costly in applications where only a few critical control elements need to be served. Battery systems are available as lead-acid units, which may require ventilation, depending on their design but are lower cost or nickel-cadmium which are sealed but considered hazardous waste requiring strict government regulatory compliance.
For systems which only require a reliable source for circuit breaker tripping and/or lockout relay operation, capacitor trip units (cap trips) are used. The typical cap trip unit consists of a rectified AC control power source with circuitry to charge and maintain the voltage level on a capacitor. These units are not designed as a continuous source of control power. The module stores just enough energy in a capacitor to operate one circuit breaker shunt trip coil or lockout relay. The remaining control circuits are usually powered from an AC control power source.
This approach relegates the need for batteries to only critical control circuits which are continuous, such as PLCs. The result can be a much smaller battery system which will permit its replacement with a small UPS. Some applications warrant the use of a special cap trip unit which incorporates a small rechargeable battery with a charging circuit built in. These units maintain the charge on the capacitor for up to 72 hours. When used in combination with the appropriate protective relay devices, battery back-up cap trip units can increase the reliability of remote and unmonitored power distribution systems where the loss of control power may not be detected for up to three days.
The optimum means of providing a clean source of power for critical needs has evolved into the uninterruptible power supply (UPS). This technology is ideal for small applications with a limited need for continuous critical control power, about 1 to 3 kVA for a period of 15 minutes or so. Such a small UPS control source can be embedded in the standard switchboard or switchgear structure without increasing required floor-space, and with reduced costs. The reduced capacity, however, is a limitation and must therefore be considered carefully.
When there is a large energy demand for tripping an ANSI breaker (1000 – 1300 VA), most switchgear systems without station control batteries rely on individual cap trip units for supplying tripping and lockout relay power, and a central UPS for microprocessor-based devices. In other cases such as the HPC switch example where tripping power requirements are limited (200 VA or less), a single small UPS may meet all of the control power requirements. The UPS will still require periodic maintenance and monitoring, but many UPS manufactures incorporate alarms and monitoring ports which serve as excellent tools to aid in these tasks.
The electronic capabilities built into today’s protective, metering and control equipment require power to function properly, especially under extreme conditions. The design engineer must evaluate the application and nature of the control power needed by the system elements being applied and determine when a supplemental source of back-up control power is appropriate.
Contact Gavin de Haas, GE Energy, Tel 011 237-0162, gavin.dehaas@ge.com
