Introduction
The electrical power system of a ship functions like a city’s electrical grid from the power generation to medium voltage distribution to the low voltage networks located in various sections of the ship’s structure. It only makes sense to treat the power system like a city’s power system, after all larger vessels include services that a city would provide and a population that would rival a city, including a military base and an airport.
This article limits the scope to the low voltage power quality standard released in September 2018, where a medium voltage standard (MIL-STD-1399-300-2) was brought forth to deal with the greater than 1 kV (typical 4 kV) portions of the power system. Note that MIL-STD-1399C is a ships interface standard with many sections published as individual documents and this review deals with section 300 Part 1 for low voltage AC power. Generally, voltages less than 1 kV are considered low voltage.
Type I is the primary low voltage power defined as 440 or 115 Vrms, 60 Hz. The 440 Vrms interfaces are three-phase or single phase either ungrounded or high-resistance grounded. The 115 Vrms interfaces are three-phase or single phase either ungrounded or solidly grounded. The Type I power is used unless a deviation is approved.
Type II power is 440 or 115 Vrms, 400 Hz ungrounded for limited applications.
Type III power is 440 or115 Vrms, 400 Hz ungrounded with tighter tolerances than Type II.
Special power classifications are provided for selected applications such as 115/200 Vrms, 60 or 400 Hz for aircraft service and operations or hotel services and 230 Vrms, 60 Hz single phase for NATO load equipment.
Power sources may be low voltage generators or medium voltage sources that are distributed to the low voltage transformer and distribution panels. Conversion equipment is used to create the required voltage and frequencies for other than Type I power.
Electromagnetic Compatibility (EMC) is not directly covered by the standard but MIL-STD-461 CE101 testing can be used for the measurement of distortion associated with the input current waveform requirements. Also solving EMC issues with line-to-ground capacitors is limited to ensure that human body ground current does not exceed the personnel safety limits.
Background
MIL-STD-1399 Section 300 was introduced in August 1978 with updates by revision “A” in October 1987, revision A1 in March 1992 and revision “B” in April 2008. In September 2018 the Section 300 was broken out into low (300-1) and medium (300-2) voltage parts to deal with the ship power system that had been using a medium voltage source for some time.
The need for medium voltage distribution became evident as the demands for capacity increased, especially for the large vessels. The power demand required that distribution feeders from the prime movers support very high current loads requiring large cross-sectional area wiring. The necessary long cable runs produced significant voltage drops associated with I2R losses by the wiring. Distribution using a higher voltage reduced the current on the main feeder lines reduced the wire gauge because the current was lower for the required power. Conversion to the low voltage could be accomplished near to the user equipment for individual circuits with more efficiency.
Data Item Descriptions (DIDs)
MIL-STD-1399-300-01 states that DI-EMCS-80201 (current revision) be used for test procedure content and DI-EMCS-80200 (current revision) be used for test report (standard states procedure but the DID is associated with reports). The test paragraphs cited for the DI-EMCS-80200 document are typically associated with calculated results based on the test data. Both DIDs are associated with MIL-STD-461 so instructions are provided to change “EMI” to “Interface” and cite MIL-STD-1399-300-1 to convert the documentation for the power quality test and evaluation.
Requirements
As I go through the requirements, I will try to expand on the content in the standard with some information on the “how to” confirm conformance. It is not my intention to repeat the detailed information presented in the standard, but to supplement that information for clarification.
Many of the tests require that the test article be operated until thermal stability is reached prior to the test. Thermal stability is reached when successive temperature measurements taken at 30-minute intervals vary by less than 1°C. The measurement point is normally near the top inside the EUT because of the normal heat rise. When determining the temperature variance make sure that the facility ambient conditions are not contributing to the variance.
1. Grounding
Requirements for the power to be ungrounded unless the equipment is associated with a special voltage system. Operation of the ungrounded equipment is not affected if one of the power lines would become grounded. Filters should be line-to-line but if line-to-ground capacitance is necessary the capacitor value is limited and line-to-ground current at the fundamental power frequency is limited to 30 milliamperes per line. Exceptions are provided for certain equipment items.Figure 1 shows the basic test configuration where 100 kΩ resistors are connected to the ungrounded power lines with a bypass circuit breaker or fuse. The power source should be isolated to prevent grounding of the utility power lines. After establishing EUT operation, the circuit breakers are engaged one at a time to make a ground connection to the line under test. Allow the EUT to operate long enough to assess performance degradation with a requirement for at least five minutes per line tested. If the circuit breaker trips, a grounded neutral is indicated. Testing is repeated for each operational mode. Three-phase power is shown in the diagram, but testing also applies to singe phase
Figure 1: MIL-STD-1399-300-1 Ground Test Configuration
2. Power Profile: Type of Power
Information is reported based on the measurements obtained by the voltage/frequency tolerance test (see paragraph 17 below).
3. Power Profile: Number of Phases
Information is reported based on the measurements obtained by the voltage/frequency tolerance test (see paragraph 17 below).
4. Power Profile: Operating Frequency
Information is reported based on the measurements obtained by the voltage/frequency tolerance test (see paragraph 17 below).
5. Power Profile: Operating Voltage
Information is reported based on the measurements obtained by the voltage/frequency tolerance test (see paragraph 17 below).
6. Power Profile: Line Current Magnitude
Information is reported based on the current measurements obtained by the voltage/frequency transient tolerance test (see paragraph 18 below).
7. Power Profile: Power
Information is to be documented that confirms that the equipment operates properly with the designated power type based on the power consumed by the equipment. The power is calculated based on the operating voltage times the line current magnitude documented in paragraphs 5 and 6 above recorded as kilovolt amperes (kVA).
8. Power Profile: Power Factor
For equipment loads >1 kVA, the power factor is to be documented in the power profile. The power factor is normally measured with a power analyzer with the measurements recorded in the power profile.
9. Power Profile: Duty Cycle
Equipment duty cycle information is to be documented in the power profile.
10. Power Profile: Surge/Inrush Current
Surge/Inrush current measurements are made and recorded in the power profile. A current probe is placed on each line while the equipment is energized to measure the peak current with an oscilloscope at power on. Measurements are made when the power source sine wave passes through 0-degrees and through 90-degrees. The larger inrush current is to be recorded in the power profile. If the measurement synchronization requires multiple energization cycle, allow enough time for capacitors to fully discharge between cycles.
11. Power Profile: Current (Load) Unbalance
Using the line current measurements recorded in paragraph 6 above confirm that the phase current is within the unbalance tolerance.
12. Power Profile: Pulsed Loading
Discussion omitted for this review because of limited applications and detailed information is provided in the standard.
13. Power Profile: Ramp Loading
Discussion omitted for this review because of limited applications and detailed information is provided in the standard.
14. Power Profile: Spike Generation
Spikes produced by the equipment are measured and reported in the power profile. While measuring the voltage line-to-ground and line-to-line operate the equipment in a manner that may produce transients such as energizing/de-energizing, switching loads or bus transfers. Record transients that appear on the power with an oscilloscope and document the transient measurements in the power profile.
15. Power Profile: Line-to-Ground Capacitance
The line-to-ground capacitance is calculated using the line-to-ground current measurements obtained in paragraph 16 Derive the impedance (Xc) by dividing the line-to-ground voltage by the line-to-ground current then calculate the capacitance by:
Record in the power profile.
16. Power Profile: Line-to-Ground Current
Measure the line to ground current by isolating the EUT from the ground by placing it on an ungrounded surface and disconnecting the ground wire. Install a jumper wire between the equipment chassis and ground and install a current probe on the jumper wire. Energize the power source and measure the current in the jumper wire. Divide by two to obtain the line-to-ground current assuming that the lines are balanced. Record the measurements in the power profile. Additional steps are applicable for three-phase equipment.
17. Voltage and Frequency Tolerance
The requirement is for equipment to operate normally with the input voltage and frequency at the tolerance extremes in a steady state condition. The magnitude of the voltage and frequency extremes are specified in the standard for the various power types. The testing uses the four corners method for the evaluation by setting the voltage and frequency to the lower tolerance limits, setting the voltage and frequency to the upper tolerance limits setting the voltage at the upper tolerance limit with frequency at the lower tolerance limit and setting the voltage at the lower tolerance limit with the frequency at the upper tolerance limit. With settings at each of the four corners, the EUT is operated until thermal stability is attained plus 30-minutesFigure 2 provides a diagram of a test configuration with the addition if a current probe to support the voltage and frequency transient tolerance testing. Note that the oscilloscope is used to measure the voltage and frequency in this arrangement, so make sure the oscilloscope measurement provides for true RMS measurements, especially if the power waveform is distorted. Repeat testing for each line and each operational mode.
Figure 2: MIL-STD-1399-300-1 Voltage/Frequency Tolerance Configuration
18. Voltage and Frequency Transient Tolerance
The voltage and frequency transient tolerance is used to assess the EUT performance when the power input varies at extremes greater than normal for a short duration. Figure 2 above is used as the basic test configuration incorporating the use of the current probe. The magnitude of the voltage and frequency are provided in the standard with the duration of the transient extremes.The test calls for thermal stabilization with normal conditions prior to applying the transients. Measure the voltage frequency and line current before application if the transient and then record those parameters before, during and after the transient. The EUT should operate normally during and after the transient to show compliance with the requirement.
19. Voltage Spike
The voltage spike test is used to verify that the EUT can withstand transients that may be conducted on the power lines. Operation before during and after application of the voltage spike is required for compliance.Figure 3 provides a basic sketch of the test configuration but note that various power types require some adjustments of the configuration. Depending on the spike generator planned for the testing, the configuration will vary, so make the configuration adjustments to support the generator to be used. Prior to test the waveform needs to be verified with the 12/50 μsec double exponential waveform into an open circuit. The amplitude specified is set and the setting recorded to use when configured for testing. Line-to-line and line-to-ground testing is applicable with synchronization to the line under test to ensure the spike is applied at the correct phase angles. Five positive spikes are applied at the 0 and 90-degree angles and then fie negative spikes are applied at the 0 and 270-degree angles.
Figure 3: MIL-STD-1399-300-1 Voltage Spike Configuration
20. Emergency Conditions: tr Power Interruption
The short power interruption (tr) is established as 70 msec unless otherwise defined by the ship power system. Equipment classified as mission critical should operate without interruption. Testing uses a configuration as shown in Figure 2 where the programmable power source is set to provide the specified power interruption while the EUT is monitored for performance. During test the voltage frequency and line current are recorded.
21. Emergency Conditions: ts Power Interruption
The extended power interruption (ts) is established as two minutes unless otherwise defined by the ship power system. Equipment classified as mission critical should automatically restore operation without operator intervention and not experience a loss of data.The test paragraph in the standard is somewhat ambiguous regarding operator intervention but the requirement paragraph requires no operator intervention. One should clearly state the acceptance criteria regarding restoration of power in the test procedure.Testing uses a configuration as shown in Figure 2 where the programmable power source is set to provide the specified power interruption while the EUT is monitored for performance. During test the voltage frequency and line current are recorded. Include the inrush current as part of the recorded data.
22. Emergency Conditions: Power Source Decay
Loss of the power plant prime mover may result in a gradual decay of the voltage and frequency provided to the equipment. Although the decay varies depending on the type of prime mover, the standard provides a set of curves based on a steam turbine driven generator set.Using a configuration as shown in Figure 2, the programmable power source is operated to provide power following the half-load curve in the standard Figure 9. The testing calls for extrapolating the curve until the frequency and voltage reach zero. However, the power source may not support very low frequencies, so provisions are included to lower the frequency to less than 50% and then de-energize the power source. Record the voltage, frequency and current during the testing.
23. Emergency Conditions: Positive Excursion
Power source excursions may occur during the emergency conditions where loads are removed from the power system allowing increases in voltage and frequency.As with the decay test above, the configuration shown in Figure 2 is used for the conformance testing. The programmable power source is used to provide a voltage 35% above the nominal voltage. After the test duration (normally two minutes) the voltage is returned to nominal. After the EUT is stable, the frequency is increased to 12% above the nominal frequency for the test duration. Return the frequency to nominal and then increase both the voltage and frequency to the test levels for the specified duration. Record the voltage, frequency and line current during the testing.
24. Current Waveform
Current waveform measurements assess the harmonic current produced by the EUT. Individual harmonic frequency current must be less that the limit provided in the standard for various load currents and frequencies. Note that equipment with multiple power inputs from the same source the summation of the power inputs to determine the applicable limit curve.The test configuration uses a current probe as indicated in Figure 2 above without the voltage measurement port of the oscilloscope. The total input current is measured at the nominal voltage to obtain the fundamental current and the harmonics limit is the specified percentage of the fundamental current. The MIL-STD-461 CE101 test method may be used to obtain the measurements with the results compared to the current waveform limit line.
25. Voltage and Frequency Modulation
Voltage and frequency modulation is inherent in the power distribution resulting from load and prime mover variations. For example, adding a load may cause the prime mover speed to drop slightly lowering the frequency until regulation makes the adjustment to compensate for the load.Figure 2 provides a basic test configuration where the programmable power source is set to vary the voltage or frequency or both. With the frequency at nominal, the voltage is varied from minimum to maximum for periods of 50 msec, 500 msec, 1 sec, and 10 sec. This is repeated 10 consecutive times before moving to the next modulation period.The rate of change for the variation is not specified, so the test could be interpreted to change the voltage from minimum to maximum once over the modulation period and repeat that modulation period of 10 times. The test could also mean to vary the voltage from minimum to maximum of 10 times within each modulation period. When preparing the test procedure, your approach needs to clearly document your approach and be submitted for approval.The test is repeated with the voltage at nominal and the frequency varied from minimum to maximum for the same modulation periods repeating 10 consecutive times. After completing the frequency modulation, the test is repeated for each modulation period varying both the voltage and frequency simultaneously.
26. Simulated Human Body Ground Current
Line-to-ground capacitance presents a risk of current flowing through a person contacting metallic parts of an equipment enclosure if the ground connection is disrupted. The simulated human body ground current test evaluates the potential for this current to be hazardous from an electrical shock perspective. The requirement is less than 5 mA for frequencies below 700 Hz and 70 mA for frequencies between 700 Hz and 100 kHz. The frequency range of the current is determined by using two different measurement transducers. The schematics for the two transducers are shown in the standard.The standard provides diagrams of the test configuration for various power types including placement of the metering circuit. The voltage on the metering circuit is measured and the current is determined by dividing the voltage by the metering circuit resistance.Remember that a True RMS voltmeter is specified. Also, note that the voltmeter used should have the necessary bandwidth—the high frequency measurement range is up to 100 kHz, so using a 20 kHz voltmeter would yield incorrect measurements.
27. Equipment Voltage Withstand
The equipment voltage withstand provides for two separate tests: 1) Insulation Resistance and 2) Active Ground Detection (AGD). In either case a DC voltage is applied to the AC power and the equipment is evaluated for the ability to withstand this voltage.The insulation resistance test places a megohmmeter between the power input leads (input power is disconnected) and the equipment ground. The megohmmeter voltage is increased to the test level for 60 seconds. The EUT is required to tolerate the applied voltage without damage or arc-over and the required resistance is 10 megohms or more.AGD withstand testing evaluates the equipment tolerance for a worst-case line-to-ground voltage by adding a line-to-ground DC voltage to the line-to-line AC voltage. The DC test voltage is set to provide for an AC line-to-line that includes tolerance factors. The test levels calculated in the standard, include the tolerance factor test voltages. The DC voltage is applied for three minutes while monitoring the EUT for proper operation and operation is again verified after removal of the DC voltage. The programmable power source may provide the capability to set a DC offset that can be set at the test level to accomplish the testing.
Summary
EMC and MIL-STD-1399-300-1 are viewed as separate requirements but neither can fail to consider the other to assure ship functionality and absence of interference.
Many evaluations are specified in the standard but using a programmable power source, the tests can be automated. Configurations are relatively common, so sequencing the tests to complete all tests with each configuration can minimize time spent in setting up the test. The necessary time spent to establish and verify thermal stability tends to extend the test period—but you don’t need to watch it warm up— just record and check after a predictable elapsed time.
As with many tests, hazardous voltages are present, so make sure that you establish good safety practices especially in dealing with unknown ground current pending evaluation. Isolating the ground system can make the test equipment a shock hazard— so pay attention to the work site.
Hopefully you will find this information useful and I welcome questions. If you have a topic associated with EMC that you would like to have reviewed, let me know and I will try to place it in the queue for future articles.
