Overcoming Power Challenges With Power Distribution Units

May 31, 2012
PDUs offer a wide array of options to meet the various power specifications of consumer, commercial and industrial systems. By understanding the four pillars of power distribution, better decisions can be made to best solve power distribution challenges that before were problematic.

As consumer, commercial and industrial electronic systems become more advanced, they require more precise and reliable power sources to function at optimum levels. System integrators and designers are turning to more sophisticated power distribution systems (PDUs) with customized specifications to better tackle power hurdles.

Today’s advanced PDUs can implement multiple functions into a single package to help reduce space, weight, and cost. Understanding the key pillars of power management is vital to identifying the common attributes of power distribution management: Power Conditioning, Power Conversion, Power Control, and Power Monitoring.

Voltage spikes (Fig. 1), brownouts and electromagnetic interference (EMI) can wreak havoc with sensitive electronic systems. Any imperfections in alternate current (AC) can adversely affect electrical equipment causing poor performance, incorrect functionality, or even damage to sensitive circuitry.

EMI is a common cause in disrupting the signal from the desired perfect sine wave form. EMI is a broad term covering multiple causes where the net effect is that electromagnetic waves of a wide spectrum of frequencies cause interference with the power signal.

EMI Noise

EMI can be classified in several ways. One way is to identify how interference gets into the power line which includes conduction, capacitive coupling, and induction. Another way is to address the frequencies being added to the signal. RFI, or radio frequency interference, is a name to identify a subset of frequencies common to communications and other equipment. A third distinction is identifying common mode and differential mode noise. Common mode noise (Fig. 2), manifests identically on multiple power conductors where the noise signal flows in the same direction, in phase, and typically returns through the ground. Differential mode (Fig. 2) noise occurs when the noise on each power line is not identical. Here, the noise signal flows through one power line and returns through another.

The primary defense for reducing induced noise is shielding. This is commonly, but not exclusively, done to mitigate RFI. The remaining noise types can be reduced through EMI filters and isolation transformers. Filters use a combination of capacitors and inductors to block or divert frequencies outside the 50 or 60 Hz of the power signal. Isolation transformers provide good reduction of common mode noise, and allow an isolated ground which can offer significant benefits in noise reduction on downstream power lines connected to the PDU.

Since signal problems are introduced to power lines throughout a facility’s wiring, it is beneficial to add power conditioning at points throughout the facility. An effective place to do this is at power distribution points near the end use equipment with a PDU.

Power Conversion

PDUs are most often used to take a single facility power connection and fan it out in a controlled and convenient manner to the various branch circuits needed to support a given application. However, in many cases one or more downstream devices require power in a different configuration than what is provided by the facility. In these cases, it is advantageous to use a PDU with power conversion features built-in.

Conversion types and methods include (Fig. 3):

  • AC to AC: Typically this involves converting the available facility voltage to a different voltage as required by the application. A transformer is used for this type of conversion. A transformer can also convert the power configuration to suit the application (e.g., converting facility 208VAC delta configuration to a 120/208VAC wye configuration).
  • AC to DC: This is accomplished by incorporating a power supply into the PDU. It allows a PDU to provide a DC output when only an AC facility power source is available.
  • DC to DC: This type of conversion changes the available DC facility voltage to one or more outputs at a different voltage level.
  • DC to AC: This conversion is accomplished by incorporating an inverter into the PDU. It allows a PDU to provide an AC output when a DC facility power source is available.

Power Control

Control capabilities in PDUs offer a range of options including no controls, switches, breakers, remote signals across Ethernet, and even automated switching between power sources (Fig. 4). Whether for safety, redundancy, or convenience, power control features integrated into the PDU can help reduce the complexity, cost, and packaging size of a power distribution application.

Local control is where the switching device is housed in, or mounted directly on, the PDU. Circuit breakers will almost always be implemented locally on the PDU. They can be used to combine the functionality of circuit protection and manual switching at the main power input, circuit branches, or even outlets. Some industries or safety policies may not allow circuit breakers to be used as switches as frequent toggling can degrade their performance.

Switches of push button, toggle, or rocker types mounted on the PDU can be used to control power on and off. This can be done directly on low-current power lines, or indirectly with high current power lines.

Power Switching

With direct power switching, the power line being controlled is routed directly through the switch providing the on/off control. This is common for most DC power sources, and for AC power sources of 120 volts or less and currents up to 15 amps.

Indirect power switching (Fig. 5) is used for higher voltages or currents to keep the power line off the control panel. The power line is wired through a mechanical relay, contactor, or solid state relay. A separate, low-power signal is run through the control switch to activate the power relay. The switch may be local, remote, or even another relay connected to a digital controller. Regardless of the exact hardware involved, the control switch is separated from the power relay to indirectly connect and disconnect power to the outlet(s).

Remote control is when the switching device is not located on the PDU itself, but rather a signal is sent to the PDU from a control panel on another piece of equipment. Remote signals will use indirect switching of power, and can be implemented several ways.

Power Monitoring

A growing trend is to monitor all aspects of electrical power. In the vein of “you can’t manage what you don’t measure,” facility managers and engineers are looking to decrease their carbon footprint by utilizing energy efficiency initiatives that reduce costs without sacrificing reliability and system uptime. At the PDU level, it’s not only important to know how the PDU is performing, but it’s also useful to view specific parameters such as input voltage to be assured that the power for the application is within acceptable limits. In cases with sensitive downstream equipment or processes, it may be important to monitor a wide range of parameters regarding power quality and status. With some equipment designs, the PDU is not visible to the user, so a method of remotely monitoring this information is desired. Since the PDU is the focal point of power in a design, it is the most logical place to implement some kind of power monitoring.

The following measurement types tend to fulfill the majority of monitoring applications:

  • On/Off status can be used to monitor whether a power source is enabled, an outlet is powered or not, or the active choice status of an A/B switch selection. Typically there are only two values, though multiple choices are possible.
  • Time can be used to measure the cumulative time that the entire PDU or individual circuits have been powered. This is often used to coordinate maintenance scheduling of equipment connected to the PDU.
  • Voltage at a power outlet, or more commonly, the incoming power source can be measured to ensure facility power is meeting the requirement of downstream equipment.
  • Current can be measured at one or more points within the system to reveal the total load at the power input, on a branch circuit, or even at an individual outlet. Measuring capacity usage can be useful where downstream loads may vary, and drawing too much current will trip a circuit breaker.

Power is derived from a combination of voltage and current measurements from the same point at the same time. Calculated measurements can include watts, volt-amperes, power factor, and crest factor. Energy factors in time for values such as kilowatt-hours. All of these values are useful for analyzing consumption and efficiency.

Power quality is a developing area of power measurement where some differences in formulas and calculations may still exist among device manufacturers. Generally speaking, power quality is represented by a number of measured and calculated parameters such as flicker, dips, swells, transients and harmonics among others. These values can be used to define the quality of the power being provided by the facility power source. A power quality meter can be integrated into a PDU, but such equipment is typically a separate device which is temporarily installed for troubleshooting purposes.

In addition to the panel displays which can be mounted on local or remote panels, it is possible to convey measurements to separately housed monitoring equipment with a variety of remote signals. Where a control signal is used to send a command, a monitoring signal is used to indicate a data value. The signal itself may technically be identical in both cases.

Signal Types

Signal types pertain to both remote control and remote monitoring. It is important to understand their functions for optimum data and communication signal management:

Dry contact is the simplest form of a remote control and monitoring interface. A switch or relay at the remote station is used to communicate an on or off condition of the PDU. This is most commonly used for safety signals such as Emergency Power Off (EPO), but can also be used for conveying basic control commands as well. For power monitoring, this signal would be typically used with an external control system.

Discrete signal involves sending a fixed voltage or current signal which has only two states, such as 0VDC vs. 5VDC or 0mA vs. 20mA, from the remote panel to the PDU. The signal is received by a digital control circuit or the coil of a relay of an external control system. The design requirements of the sender and receiver must be understood to ensure the signal is compatible with both. This type of signal is usually used for command signals to power on or off control circuits, and rarely for EPO circuits.

Analog signal is a variable voltage or current, typically 0–10VDC or 4–20mA, used to represent a data value such as line voltage, main input current, or chassis temperature. The signal is wired directly to a panel meter for display, or several of them may be wired to a digital controller to display multiple measurements on a single display.

Communications interface provides a means to transmit multiple measurements using programmed digital controllers. Multiple sensors will be wired into a local controller in the PDU. One or more remote stations will house their own controller. Measurements at the PDU are converted into pre-defined “messages” and sent to remote controllers following a standardized protocol such as RS-232, Modbus, HTTP, SNMP, or others.

Figure 6

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