Satellites, hybrid electric vehicles (HEV), uninterruptible power supplies (UPS), green energy, and high-power battery systems rely on bidirectional, renewable energy systems and devices to store energy and provide continuous power when needed. These systems and devices include: N6900 Series Advanced Power Sources Satellites, hybrid electric vehicles (HEV), uninterruptible power supplies (UPS), green energy, and high-power battery systems rely on bidirectional, renewable energy systems and devices to store energy and provide continuous power when needed. These systems and devices include:
‾ Rechargeable battery pack
‾ Supercapacitor
‾ Motor-generator system
‾ Bidirectional DC/DC converter
‾ Battery management system (BMS)
‾ Braking energy recovery system
These systems and devices usually operate in the power range of several kilowatts. When conducting tests in the R&D and production processes, a higher power source and load are required to provide power input to these DUTs and absorb the energy released by them. The power can be as high as several kilowatts or even higher. This is an extremely difficult challenge for test engineers. The most common method is to use a separate power supply and then use a load to absorb the energy released by the DUT. However, this method has great defects. The main problem is that this method cannot achieve continuous conversion of power supply and load functions, which is very different from the actual working conditions of the system; moreover, high-power switches, relays, etc. must be used in the system, and the system is very complex, and reliability and repeatability often cannot meet the requirements. Therefore, these defects can only be overcome by fully integrating the functions of power output and power absorption into a single instrument or system, and realizing seamless conversion of source and load functions.
Comparison of two-quadrant operation and four-quadrant operation
When testing such bidirectional, renewable energy systems and devices, test engineers sometimes have some misunderstandings about what kind of power supply and absorption load are needed. They often think that bidirectional means bipolar, so they need to use a bipolar power supply for testing. This is actually irrelevant. As shown in Figure 1, in this four-quadrant, the current and voltage operating areas of the unipolar, bidirectional power supply are in quadrants I and II in the figure. It can output and absorb current, and the voltage must be positive. It can be used as both a DC power supply and an electronic load, and is a two-quadrant DC power supply.
In contrast, a bipolar power supply can transition across zero voltage and operate at either positive or negative voltages. It can source power in quadrants I and III and sink power in quadrants II and IV.
Since these bidirectional, renewable energy devices and systems need to be powered on one hand and absorb the current output on the other hand, their working state is obviously unipolar voltage, and their testing requires a unipolar dual-quadrant DC power supply to complete, in fact, this is also the best choice. When the dual-quadrant DC power supply is continuously operated in the II quadrant as an electronic load, it must be in a fully controllable, continuous working mode. Using only the power supply's down-programming capability to absorb current is not enough to fully test these bidirectional energy systems and devices.
Figure 1. IV diagram of the four quadrants
When using a DC power supply and an electronic negative method, there will be a voltage dead zone, which will affect the output/absorption characteristics.
Currently, it is difficult to find dual-quadrant DC power supplies with kilowatt-level power on the market. Engineers often use a separate DC power supply to provide the required power, and an electronic load to absorb the output power of the device under test for testing their bidirectional renewable energy systems and devices. Individually, the DC power supply can continuously output power, while the electronic load can continuously absorb it, and both have excellent DC accuracy, stability and fast dynamic response, regardless of the device under test. This performance is necessary during testing because the device under test is active and dynamic, switching between output power and absorption power depending on its state and operating conditions.
A battery simulator system (BSS) shown in Figure 2 combines a DC power supply and an electronic load for both power supply and absorption.
Figure 2. Common DC power supply and electronic load test solution, integrated battery simulator system (BSS)
The battery simulation system is a typical voltage system; both the DC power supply and the electronic load operate in constant voltage (CV) mode. The voltage setting of the electronic load is slightly higher than that of the DC power supply. There is a voltage difference between them, which will cause a dead zone voltage, making the battery simulation system unable to operate in this area. BSS is used in the test of battery management system (BMS). Other DUTs that require unipolar dual-quadrant DC power supply testing can also be tested with this system. When the DUT absorbs current, the voltage is maintained by the power supply voltage. When the DUT outputs current, its voltage rises, the output of the DC power supply is cut off, and the electronic load enters the CV working mode and clamps the voltage at a slightly higher level. Usually, a blocking diode is added to the output of the DC power supply to prevent reverse current from flowing back into the power supply when the DUT starts to output power. In this configuration, the DC power supply directly reads the current value, and the electronic load directly reads the absorbed current. However, there are some unavoidable drawbacks when using this test scheme:
Remote sensing cannot be used with DC power supplies because if the remote sensing line is at the diode end, the blocking diode will cause the DC power supply to be unstable.
There is high impedance in the voltage dead zone between power source and sink.
Voltage programming instructions need to be sent to the DC power supply and the electronic load respectively so that they can track each other when the BSS voltage changes.
In order to coordinate the working status of the DC power supply and the electronic load during the test, a more complex system configuration is usually required.
The electronic load has to switch between cutoff and CV operation modes, which affects its dynamic performance.
When the current and temperature change, the voltage drop of the blocking diode will change, which directly leads to the need to increase the dead voltage of several hundred millivolts between the DC power supply and the electronic load voltage.
In particular, the latter two factors limit the flexibility and accuracy of dual-quadrant operation and affect the static performance of the system. In order to compensate for the dead-zone voltage under static operation, the BSS voltage needs to be programmed and controlled so that it can be adjusted up and down as required to make the voltage value closer to the required voltage. However, due to the inherent dynamic transients of the dead-zone voltage, it will be intertwined with the CV mode transients of the electronic load. As shown in Figure 3.
Figure 3. Output of a single DC power supply and electronic load - sink mode transition with significant dead-zone voltage
Using DC power supply and electronic load, dual-quadrant operation mode is achieved through overlapping working ranges
You can avoid the problems described above due to non-overlapping operating ranges by using a method that completely overlaps the operating ranges. Figure 4 shows a DC power supply and electronic load configured for completely overlapping operating ranges. Now, the electronic load operates in CC mode instead of CV mode. The current of the electronic load is set to a fixed value, which is greater than the maximum current that the device under test can provide. This way, the electronic load always remains in CC mode, absorbing a fixed level of current and power. The electronic load no longer has to deal with any mode crossing issues. The DC power supply always remains in CV mode and always sources current. Therefore, the diode is no longer required. As a result, this BSS configuration is always in CV mode throughout the entire source and sink range, without the electronic load mode crossing and quiet zone voltage transients that affect the BSS configuration with non-overlapping operation. However, it also has some disadvantages:
– The DC source needs to be very large to be able to supply both the maximum current and power required by the DUT, as well as the full current continuously drawn by the electronic load. For example, to achieve 100% current draw, the DC source needs to be more than twice as large.
– The electronic load often draws full power, and this must be taken into account for large systems.
– The measurement requires reading the current of the DC source and the electronic load and taking the difference, which is often a small value. This affects the measurement accuracy.
Integrated dual-quadrant solution
如果将输出和吸收功率的功能整合到单一仪器中,可以减少使用单独的直流电源和电子负载来配置功率输出和和吸收解决方案的缺点。这些功能进行整合之后,可以在闭环控制下工作,在输出和吸收电流和功率之间提供完美、无瞬态现象的切换。也无需经常消耗大量功率。直流精度和动态性能将得到提升而不是降低。使用单一测量系统测量所有电流,可以显著提高测量性能。主要挑战在于市场上缺少适合的仪器能够充分满足当前双象限和再生能源系统和器件的测试需求,使工程师除了使用单独的直流电源和电子负载之外别无选择。
Figure 4. DC power supply and electronic load for a battery simulator system (BSS) operating in overlap.
Keysight Technologies Advanced Power System (APS) N6900/N7900 DC Power Supplies with Integrated Source and Sink Features
The N6900A/N7900A DC power supply (Figure 5) is tailored to meet the testing needs of today's two-quadrant and renewable energy systems and devices. Key features include:
‾ Energy-efficient 1U high 1 KW models and 2U high 2 KW models provide high power in the smallest space.
‾ Built-in combined current and power sink capability equivalent to 10% of output capability. Easily increase power sink to 100% of output capability with optional N7909A power dissipation unit.
‾ A wide range of output voltages are available to meet the needs of today's wide range of DUTs and applications. ‾ Voltage and current priority operation provides greater flexibility for sourcing and sinking tests without being limited by DUT characteristics.
‾ Two-quadrant measurement system for precise voltage, current, power, charge and energy measurements.
‾ APS N7900 Dynamic DC Power Supply's advanced sourcing and measurement capabilities can be used to create dynamic output events, make transient measurements, continuously record voltage, current and power, and more.
‾ Advanced trigger signal routing with configurable logic can be used to create application-specific control, trigger and protection features to help easily deal with particularly challenging test problems.
‾ Unique modular architecture allows easy expansion of the integrated source-sink system up to 10 kW for testing higher power DUTs.
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