Electric Springs- Part 3

PRACTICAL EVALUATION

In order to practically evaluate the performance and operating modes of the proposed electric springs, 3 different experiments have been set up at the Maurice Hancock Smart Energy Laboratory at Imperial College. a) The first test is to power the electric spring with its series-connected electric load using a standard ac power source so that the performance of each operating mode can be examined. 

The electric spring voltage and current are measured under the three operating modes. b) The second test is to program the electric spring with power reduction function and test it in the setup of Fig. 4. An unstable power source is created in the form of a wind power simulator, which is formed by generating electric power by a power inverter following a pre-recorded wind speed profile and a base power profile of the ac generator. The purpose is to check the voltage support capability of the electric springs and also the relationship of the intermittent renewable power (from the wind power simulator) and the load consumptions in the noncritical and critical loads. c) The last test is to check the performance of the electric spring in a power system setting as shown in Fig. 7. In this case, both voltage boosting and voltage suppression operations are evaluated

4.1 Operation of an Electric Spring as a Novel Smart-Grid Device

Fig. 4.1.1 . The experimental setup for the electric spring (with control block diagram).

Fig. 4.1.1 shows the practical setup of the first test. Using the input voltage control method, the voltage error is fed to a compensation controller which generates the magnitude control signal for the sinusoidal PWM generator. Via a synchronization network, a phase control signal is also fed to the sinusoidal PWM generator, which in turn provides the gating signals for the power inverter. The PWM voltage output of the inverter is filtered by the low-pass LC filter so that the electric spring voltage is sinusoidal. The phase control signal ensures that the

electric spring current is either leading or lagging the electric spring voltage by 90 . The test conditions are V_{s =}220 V (50 Hz), R₁=51.4Ω.

Fig. 4.1.2. Measured steady-state electric spring waveforms under “neutral” mode

When the electric spring (ES) is operated near the neutral position, the measured waveforms of the mains voltage(v_s) , noncritical load voltage(v_o) , the ES voltage(v_a) , and the ES current (same as the noncritical load current) are recorded and shown in Fig.4.1.2. In this case,v_o is essentially equal to v_o as the v_a is only 4 V rms for a 220 V mains. 

Fig. 4.1.3 . Measured steady-state electric spring waveforms under “capacitive” mode.

Fig. 4.1.4 . Measured steady-state electric spring waveforms under “inductive” mode.

Fig.4.1.3 shows the corresponding waveforms when the ES is operated in the capacitive mode. It can be observed that the ES current leads ES voltage. Here negative reactive power is provided by the ES and is smaller than v_s .  Then the ES is operated in the inductive mode and the corresponding waveforms are shown in Fig. 4.1.4. It can be seen that the ES current can be controlled to lag the ES voltage. Under the inductive mode, the ES injects positive reactive power into the system to provide voltage support.

 4.2 Operation of an Electric Spring in an Unstable Power Grid fed by Intermittent Renewable Power (a Demonstration of Load Demand Following Power Generation and Voltage Support)

Fig. 4.2.1. A photograph of the experimental setup with a three-phase electric load (consisting of a combination of resistors and lighting loads) and three electric springs (one for each phase).

Fig. 4.2.1 shows the second practical setup for a three phase system. The per-phase schematic is illustrated in Fig. 4.2.2. The electric spring is programmed with the voltage support function. The intermittent renewable power source is created by the power inverter which generates power according to a pre-recorded intermittent wind profile and the base power profile of 1.2 kW. A pre-recorded wind profile of 30 minutes (1800 s) with the based power is fed to a power inverter to generate a weakly regulated ac mains voltage pattern in the bus bar. Both the smart load and the critical load are connected across the power lines. After a 5-min interval of programmed voltage at 220 V as a separation (from 1800 s to 2100 s), the same 30-min wind-driven voltage pattern was repeated from 2400 s. The electric spring of the smart load is deactivated in the first voltage pattern by closing the bypass switch S and then activated in the second pattern with S open.

Fig. 4.2.3 . Measured root-mean-square values of the mains voltage v_s , noncritical load voltage v_o and electric spring voltage v_a before and after the electric spring is activated

.

According to (7), the vector of v_s is equal to the vectorial sum of v_o and v_a . Fig 4.2.3 shows the measurements of the (scalar) rms values of the mains voltage , the noncritical load voltage and the voltage of the electric spring before and after the electric spring is activated. Before the electric spring takes action in the first half of the test, the mains voltage fluctuates in the region below the rated value of 220 V in this study. Because the bypass switch is closed when the electric spring is deactivated, the noncritical load voltage overlaps with the unstable mains voltage in the first voltage pattern generated by the wind power simulator. However, it can be seen that, when the electric spring is activated in the repeated voltage pattern in the second half of the test, the mains voltage can be successfully boosted or supported to 220 V.

The bouncing action of the electric spring voltage can be seen from Fig. 4.2.3 . The electric spring acts like a “voltage suspension spring” to maintain a constant mains voltage. It is noted that when the noncritical load voltage reaches 220 V (i.e., no voltage support is needed), the electric spring voltage drops to zero. The noncritical load voltage is reduced when the electric spring generates positive voltage to support the mains voltage. The consequential variation of provides an automatic mechanism to shape the load demand to follow the dynamic changes of the wind power profile. This effect can be observed from the practical power measurements of the smart load unit in Fig. 4.2.4.

Fig. 4.2.4. Measured power of the critical load and noncritical loads [Electric spring is programmed for voltage boosting function only.]

After the electric spring is activated, the noncritical load demand P₁ varies with the wind power profile while the demand of other loads P₂ remains essentially the same. This result demonstrates the effectiveness of the electric spring in both voltage support and shaping the load demand to follow the wind power. These measurements confirm the scientific theory and the effectiveness of the electric spring in supporting the mains voltage of an unstable power system and in balancing the wind power and the load power dynamically.

4.3 Test of Electric Spring in a Power System With Intermittent Renewable Power Injection (a Demonstration of Dynamic Voltage Regulation via Reactive Power Compensation and Automatic Noncritical Load Shedding)

A smart load unit comprising a combination of resistors (representing water heaters) has been setup. Two power sources separated by a transmission network box are used in this test. The experimental setup is shown in Fig.4.3.1. An AC voltage source (provided by a 90 kVA sinusoidal PWM power inverter) and an intermittent renewable voltage source (provided by a 10 kVA power inverter) are used together to simulate the situation when intermittent renewable power becomes a substantial portion of the total power generation. In order to simulate the wind power generation, a recorded wind profile is used for the power inverter to generate the

wind power.

Since the electric spring is tested in the distribution network, the choice of the line impedance to resistance (X/R) ratio should reflect the value used for distribution cables. For distribution lines, the typical ratio of reactance and resistance (X/R) is typically in the range from 2 to 8. It should be noted that the cables under consideration are those used in the overhead cables linking houses from one to the other in streets (e.g in the distribution network of the residential area in Australia). For a modest 150 A (240 V) overhead distribution copper cable, a typical phase size of 500 should be chosen. Copper cable with a phase size of 500 has a line impedance X=0.1202 Ω and resistance R=0.0247 Ω. The X/R ratio is about 4.87 (which is within the typical range of 2 to 8 for a distribution cable). In this test, the two transmission network boxes have X/R ratios of 7.5 and 3.8 respectively. These ratios are within the typical range for distribution cables.

A pre-recorded wind profile of 12 min (720 s) is fed to a power inverter to generate a weakly regulated ac mains voltage pattern in the bus bar. Both the smart load and the critical load are connected across the power lines. The same 12-min wind-driven voltage pattern was repeated from 720 s to 1440 s. The electric spring of the smart load is deactivated in the first voltage pattern by closing the bypass switch and then activated in the second pattern with open.

Fig 4.3.2. Measured power of the critical load and smart load. [Electric spring is programmed for both voltage boosting and suppression functions.]

This effect can be observed from the practical power measurements of the smart load unit in Fig 4.3.2. After the electric spring is activated, the load demand of the noncritical load is shed and the reactive power is generated to follow the unstable mains voltage whilst the demand of critical loads remains essentially the same. This result demonstrates the effectiveness of the electric spring in both voltage regulation and shaping the load demand to follow the wind power. These measurements confirm the scientific theory and the effectiveness of the electric spring in regulating the mains voltage of an unstable power system and in balancing the wind power and the load power dynamically.

 CONCLUSION

The Hooke’s law on mechanical springs has been developed into an electric spring concept with new scientific applications for modern society. The scientific principles, operating modes and limits of the electric spring are explained. An electric spring has been practically tested for both voltage support and suppression, and for shaping load demand (of about 2.5 kW) to follow the fluctuating wind power profile in a 10 kVA power system fed by an AC power source and a wind power simulator. The electric springs can be incorporated into many existing noncritical electric loads such as water heaters and road lighting systems to form a new generation of smart loads that are adaptive to the power grid. If many noncritical loads are equipped with such electric springs and distributed over the power grid, these electric springs (similar to the spring array in Fig. 1) will provide a highly reliable and effective solution for distributed energy storage, voltage regulation and damping functions for future power systems. Such stability measures are also independent of information and communication technology (ICT).

This discovery based on the three-century-old Hooke’s law offers a practical solution to the new control paradigm that the load demand should follow the power generation in future power grid with substantial renewable energy sources. Unlike traditional reactive power compensation methods, electric springs offer both reactive power compensation and real power variation in the noncritical loads. With many countries determined to de-carbonize electric power generation for reducing global warming by increasing renewable energy up to 20% of the total electrical power output by 2020, electric spring is a novel concept that enables human society to use renewable energy as nature provides. The Hooke’s law developed in the 17th century has laid down the foundation for stability control of renewable power systems in the 21st century.

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