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For naval shipboard power systems, designers rely on MIL-STD-1399 and MIL-STD-461 for harmonic distortion and EMI requirements respectively. When selecting a power supply for single or three-phase AC input applications, it is important to understand the impact that circuit topology has on line harmonic distortion. According to MIL-STD-1399 (shipboard power), the operation of user equipment shall not cause single harmonic line currents to be generated that are greater than 3% of the unit’s full rated load fundamental current between the second and thirty-second harmonic.
Line current harmonic distortion causes a distortion of the line voltage. This line voltage distortion increases the power loss within the generators making the overall system less energy efficient. Minimizing line current harmonics allows the system generators to operate closer to their maximum capacity. Meanwhile, EMI issues emerge when the line voltage has been severely distorted due to the energy that can be coupled to the chassis via the various EMI filter components within a system.
With those issues in mind, engineers at ACT analyzed a circuit topology approach that meets the 3% harmonic distortion requirements of MIL-STD-1399 and the low frequency, CE101, requirements of MIL-STD-461. Harmonic data is presented from a 3 kW output AC/DC power supply being powered from a three-phase delta, 60 Hz, 115 Vrms input source. Data from this power supply shows that the highest level of measured harmonic distortion was 1.6%, which occurred at the 5th harmonic. This level is significantly below the 3% level required by MIL-STD-1399.
Greater Efficiency in Demand
In many military applications, demand for power supplies with high power factors— greater than 0.99—and low harmonic distortion are on the rise. They are increasing in order to maximize overall system performance and efficiency. Optimum EMI performance and harmonic distortion can be achieved with the proper choice of input circuitry. Most modern AC/DC power supplies utilize a Power Factor Corrected (PFC) circuit topology. This topology allows a power supply to approach the ideal goal of having a sinusoidal AC input current that is in phase (Power Factor = 1.0) with the input voltage. Under these ideal conditions the load to the input circuitry can be simply modeled as a resistor. However, even assuming an ideal PFC, significant line current harmonics can still be generated.
Single-Phase Input Power
A common full wave bridge input configuration with an ideal PFC circuit is illustrated in Figure 1. This configuration is popular because an active PFC allows a power supply to accommodate a wide range of AC and DC input voltages and frequencies. Many single-phase power supplies do not contain an active PFC front end but possess a feature that will automatically switch between a full wave bridge configuration and a voltage doubler configuration depending upon the input voltage applied. Figure 2 illustrates both of these configurations and both circuit topologies generate similar non-sinusoidal line current wave shapes.
The data in Table 1 shows the harmonic content of the current wave shape shown in Figure 2. From this data it can be seen that the desired 3% level is not achieved until well beyond the 25th harmonic. Though these types of power supplies generate significant amounts of line current harmonic distortion, these configurations are popular because they tend to be lower in cost and complexity.
Excellent low line current harmonic distortion can be achieved in single-phase input applications by using a well-designed PFC power supply that utilizes a full wave bridge configuration as shown in Figure 1. It is important to note that this performance is achieved with a cost. A PFC adds additional cost and reduces the overall efficiency of the power supply.
Three-Phase Input Power
When three-phase input power is utilized there are a number of circuit configurations that can be selected. That decision has a major impact on the amount of line current harmonic distortion that is generated. Figure 3 illustrates a full wave bridge front end that is a common three-phase configuration. Similar results are achieved when using either a wye or delta input configuration. Analysis showed that even when using an ideal load—a resistor—a significant amount of line current harmonic distortion can be generated.
The line current generated in Figure 3 has a very non-sinusoidal wave shape due to the inherit circuit performance of a three-phase full wave bridge circuit. Table 2 shows the harmonic content for this type of wave shape. This circuit approach generates a significant amount of harmonic energy. It is only above the 25th harmonic that the line current harmonics approach the desired 3% level. The 5th and 7th harmonic levels make it difficult to meet the EMI requirements of CE101 of MIL-STD-461. CE101 measures the conducted emissions from 30 Hz to 10 kHz.
Low Harmonic Distortion Topology
As discussed above, the input configuration selected for a three-phase system can have a significant impact on the line current harmonic content. Realizing that a single-phase full wave bridge circuit (Figure 1, again) provides an inherently low harmonic current solution, the ACT engineers investigated a unique three-phase circuit topology, as shown in Figure 4. A PFC module is individually connected to each phase of a three-phase delta input configuration. The isolated outputs of these three PFC modules are connected in parallel or in series to provide the desired DC output voltage and power. Care is taken to insure that the total power is shared equally between each phase. This configuration produces a sinusoidal line current.
Advanced Conversion Technology has developed a number of power supplies based on the three-phase configurations shown in Figures 3 and 4. By comparing the line current wave shapes it’s clear that the circuit topology shown in Figure 4 provides a much lower line current harmonic signature. Table 3 details the measured harmonic content of the line current.
Examining this test data shows that the 5th, 7th, 11th and 13th harmonics exceed the required 3% level. This power supply was tested per MIL-STD-461E, CE101, and the results of a single phase are shown in Figure 5. As anticipated based on the harmonic data the 5th and 7th harmonics exceeded the desired limits. In short, this design does not meet the 3% harmonic distortion requirements of MIL-STD-1399 nor did it pass the EMI requirements of MIL-STD-461, CE101. Comparing the measured data from Table 2 to the modeled data in Table 1 it can be seen that there is reasonable correlation between the data. This verifies that at the lower harmonics the full wave bridge configuration dominates the line current harmonics.
Low Harmonic Distortion Three-Phase Supply
An ACT power supply based on the circuit shown in Figure 4 supplies 3,000W of output power utilizing a three-phase, 115 Vrms, 60 Hz, delta input power configuration. Table 4 shows the measured line voltage and current wave shapes and documents the harmonic content produced by this current wave shape respectively. An examination of the scope plots showed that the line current has a sinusoidal wave shape.
The plot containing both wave shapes showed the excellent phase performance (Power Factor) of this power supply. The Power Factor for this unit is greater than 0.99. This data demonstrates that ACT’s low harmonic distortion three-phase input design can meet the 3% harmonic distortion requirements of MIL-STD-1399. Additionally, this unit was tested and successfully passed the EMI requirements of MIL-STD-461, CE101.
In single-phase applications a power supply utilizing a full wave bridge front end and a well-designed PFC can achieve low harmonic distortion. The results in this article prove that a three-phase power supply can be designed and built to meet the harmonic distortion requirements of MIL-STD-1399 and the low frequency requirements of MIL-STD-461, CE101, using the ideas presented.
Advanced Conversion Technology