A more effective approach for developing AC-AC automatic voltage regulators




Development of AVRs has moved to digitally controlled systems built with 8-bit microcontrollers, but programmable mixed-signal ASICs offer advantages in reducing cost and size — particularly important for small portable AVRs that are very popular in some regions.

Automatic Voltage Regulators (AVRs) are used to regulate a supplied voltage level by compensating for any fluctuations in the incoming voltages. AVRs are also commonly known as voltage stabilizers and have applications in many industrial and residential applications. For example, AVRs are used in generator sets on ships, in emergency power supplies, and on oil rigs to stabilize voltage levels during fluctuations in power demand.

For power utility companies, voltage regulation in the distribution network is a key responsibility determining the power quality delivered to end consumers. To this end, utility companies must ensure proper short- and long-term planning, maintenance of the power equipment, and deployment of regulators on distribution lines. However, this may be a challenging task, especially in some regions of the world. In many South Asian countries, including Pakistan, India, and Bangladesh, the power distribution system is fragile due to issues of power theft and a dearth of power generation, which can lead to periods of load shedding and other disruptions. As a result, end-users can face issues of power line voltage fluctuations. Therefore, to ensure the safety and proper functionality of expensive devices like air conditioners, refrigerators, and televisions, the use of small portable AVRs is very popular. AVRs are easy to use devices that generally operate in a predefined range of voltage levels (e.g. 150 V – 240 V or 90 V – 280 V).

Functionally, AVRs generally use a tapped autotransformer to maintain the AC output in an acceptable range. A feedback mechanism is employed to control the position of the taps by switching appropriate relays to regulate the output voltage. This usually consists of two units: a sensing unit and a regulating unit. The job of the sensing unit is to determine the input and output voltage levels of the stabilizer, whereas the regulating unit keeps the output voltage in acceptable predetermined limits.

Traditionally, op amp ICs are used in conjunction with analog comparators for control in relay-based AVR designs. More recently, the use of 8-bit microcontrollers (MCUs) has significantly increased in digitally controlled commercially available AVRs. However, similar functionality and features can be achieved using low-cost GreenPAK™ programmable mixed-signal ASICs (application-specific integrated circuits) from Dialog Semiconductor. This replacement can be advantageous in terms of reduced cost and space requirements, and no need to explicitly program an MCU.

In this article, we explain how developers can use programmable ASICs such as a GreenPAK SLG46537V IC to develop an AVR. The overall system design and the GreenPAK design will be thoroughly described. To validate the feasibility and operability of this AVR, we also present the experimental results obtained from a prototype.

System Design


Figure 1: Block diagram. (Source: BarqEE)

The functional block diagram of the proposed AVR design is shown in Figure 1. The system is essentially based on a feedback mechanism. The AC voltage at the output of the AVR is conditioned to reduce it to the functional DC limits of the SLG46537V IC. Depending on the sensed voltage, appropriate relays are driven by the IC to select the suitable tap windings on the autotransformer.

The specifications of an AVR depend on the particular application. In this article, our AVR has the following specifications:

  • The input voltage ranges from 125 V to 240 V.
  • The output voltage is regulated between 200 V and 240 V.
  • Undervoltage and overvoltage protection features are provided. When the AVR output voltage falls below 180 V (undervoltage) or rises above 255 V (overvoltage), the output supply is disconnected.
  • Four electromechanical relays are used in the AVR design.
  • An autotransformer is used for voltage step-up which has a 0 V neutral connection and four additional taps at 135 V, 174 V, 196 V, and 220 V.
  • The output waveform and frequency are unchanged from the input.
  • The AVR (controller) design is inexpensive.
  • LED indicators are used to signal normal, overvoltage, or undervoltage conditions.

Note that these specifications are arbitrary. The given specifications can easily be adjusted in the configuration of the GreenPAK IC depending on the actual application.

Functional Design

click for full size image

Figure 2: Proposed AVR design. (Source: BarqEE)

Figure 2 shows the proposed functional design of the AVR using the SLG46537V IC.

Power Conditioning

The power conditioning block powers the GreenPAK IC. It takes the live AC as input and steps it down to 12 V, which is further converted to 5 VDC using a suitable voltage regulator IC.

AC Voltage Sensing

For voltage sensing, the output AC voltage (Live_out) is stepped down and rectified to obtain a low voltage DC level using a diode and resistive divider network. Subsequently, an output filter (electrolytic capacitor) is used to minimize the ripple and obtain a constant smooth DC voltage. A bypass capacitor is also employed to filter out the transients. Hence, a filtered DC voltage (Vsense) is obtained. To ensure the DC voltage levels are compatible with the IC, a step-down factor of (approximately) 0.01 was used (i.e. 200 VAC Û 2 VDC).

GreenPAK

Using Vsense and based on GreenPAK logic (Section 2), the IC drives the required relays (through BJTs) for actuation. Digital outputs from the IC are also used to toggle LED indicators to inform the user about the normal and over/undervoltage conditions of the AVR. The schematics of the IC, showing the IO connections, have been provided for reference.

Actuation

Three electromechanical relays (RL1, RL2, and RL3) are employed to switch the input AC voltage (Live_in) connection between the 135 V, 174 V, 196 V, and 220 V taps of the autotransformer. A fourth electromechanical relay (RL4) is used to disconnect the AVR output in case of undervoltage or overvoltage conditions, thus preventing any damage to the attached load at the AVR output.

GreenPAK Logic

The complete design file created in the GreenPAK Designer software (available for free) can be found here.

click for full size image

Figure 3: GreenPAK Design Schematic. (Source: BarqEE)

Figure 3 shows the schematic of the GreenPAK design. Vsense is fed to different comparators using Pin 6. Analog comparators ACMP0 and ACMP1 are used for regulation in the normal working range of the AVR, whereas ACMP2 and ACMP3 are used for overvoltage and undervoltage detection. Since the maximum internal reference of the comparators can be set no greater than 1.2 V, a gain of 0.33 is used to ensure the output voltage can be compared and correctly categorized in different ranges. The references of the comparators are set to meet the specifications outlined in section 1.2. An asynchronous state machine (ASM) block is employed to set up a finite state machine for voltage regulation.

click for full size image

Figure 4: Finite State Machine. (Source: BarqEE)

Figure 4 depicts the five states that are used. In each state relays 1, 2, and 3 are actuated using the ASM outputs OUT3, OUT2, and OUT1 respectively. This enables the selection of the relevant autotransformer taps and consequently the autotransformer turn ratio. Moving from State 0 to 4 causes a stepwise decrease in the autotransformer turns ratio. Table 1 shows the correspondence of each state to the turns ratios.

Table 1: AT turn ratios corresponding to each state (Source: BarqEE)

State 0 1 2 3 4
AT turn ratio 220/135 ≈ 1.63 196/135 ≈ 1.45 220/174 ≈ 1.26 196/174 ≈ 1.13 220/220 = 1

Voltage regulation is achieved using state transition which occurs if Live_out is greater than the upper limit (≈ 240 VAC, set by reference of ACMP1) or less than the lower limit (≈ 200 VAC, set by reference of ACMP0). If any state does not produce the desired regulated output voltage level (200 V < Live_out < 240 V), a state change (autotransformer turns ratio) takes place. In particular, if Live_out is greater than the upper limit, a transition to a higher state takes place. Transitions to a higher state (lowering of autotransformer turns ratio) continue until the desired voltage level is achieved. Similarly, for Live_out less than the lower limit, a transition to a lower state occurs.

To ensure that electromechanical relays function properly, abrupt state transitions are controlled by using delays in the feedback of the ASM block. To this end, outputs of the ASM block OUT3, OUT4, OUT5, OUT6, and OUT7 are fed to the delay blocks DLY2, DLY3, DLY4, DLY5, and DLY6, respectively. Figure 5 depicts the configuration of the RAM block of the ASM, where the status of each of the binary outputs OUT0 – OUT7 is shown. 


Figure 5: RAM Block. (Source: BarqEE)

States are retained for a predefined timed period tp (≈ 0.5 s) set in the delays. State transitions occur only if Live_out remains out of the desired range for at least tp. Outputs of the delays are fed back to different LUTs (and AND blocks) along with outputs of ACMP0 and ACMP1, as shown in Figure 4. This ensures that state transitions only occur once tp elapses and Live_out is out of the desired range. The particular state transition depends on the outputs of ACMP0 and ACMP1. For example, if State 1 is retained for tp, no transition is possible to State 0 and State 2. If the desired voltage level has been reached, State 1 is maintained. Otherwise, a transition to State 0 and State 2 occurs depending on whether Live_out is greater than the upper limit or less than the lower limit.

Another important feature of the proposed GreenPAK design is protection during overvoltage and undervoltage conditions. Comparators ACMP2 and ACMP3 are used for overvoltage and undervoltage conditions respectively. The output of the ACMP2 and the inverted output of ACMP3 are passed to the delay blocks DLY0 and DLY1 to ensure that overvoltage and undervoltage conditions are not detected for any transients. Subsequently, the outputs of DLY0 and DLY1 are fed to a LUT block which decides if it is a normal, overvoltage, or undervoltage condition. Under normal conditions, RLY4 is kept energized and the AVR regulates the voltage. Otherwise, no regulation is possible and RLY4 trips. Indications for the normal, overvoltage, and undervoltage conditions are also provided for the user.

Experimental Results

Experimental Hardware

click for full size image

Figure 6: Experimental Setup. (Source: BarqEE)

Figure 6 shows the experimental setup for the prototype. A Variac is used to control the input AC voltage supplied to the AVR. The AVR contains an autotransformer and a PCB which contains the control circuitry. A GreenPAK development board is connected to the PCB to control the electromechanical relays. An oscilloscope is used to log the input and output voltages.

click for full size image

Figure 7: PCB Circuitry. (Source: BarqEE)

Figure 7 shows the PCB circuitry where the electromechanical relays, BJTs, and other auxiliary components are mounted.

AVR Performance Data

Performance data of the AVR is summarized as below:

  • Load range: 450 VA – 550 VA
  • Input voltage range: 125 V – 240
  • Output voltage: 200 V – 240
  • Frequency: 50 Hz – 60 Hz.
  • Insulation resistance: > 5 MΩ
  • Response time: 10 msec – 15 msec
  • Transformer temperature rise: 65 °C – 70 °C (at 1.2 times full rated load)
  • System Efficiency: > 95%
  • Ambient Temperature: 0 °C -40 °C

Oscilloscope Results

The following figures show the oscilloscope logs for the experimentation. The yellow and blue markers show the input and output voltages respectively.

click for full size image

Figure 8: Quantitative Experimental Summary. (Source: BarqEE)

Figure 8 depicts the quantitative summary of the experimental results for the normal functionality of the AVR. The input voltage is swept in a voltage range (low to high) and the corresponding output voltage is observed. The IC successfully drives the relays to change the autotransformer taps and hence the turns ratio from 1.63 to 1 for voltage regulation. 

click for full size image

Figure 9: Normal Functionality. (Source: BarqEE)

Figure 9 shows the normal functionality of the AVR where the tap for turns ratio of 1.63 is successfully determined and selected.

click for full size image

Figure 10: Approaching Overvoltage. (Source: BarqEE)

click for full size image

Figure 11: Overvoltage Condition. (Source: BarqEE)

Figure 10 depicts the input and output voltages waveforms when the overvoltage condition is approaching. Both have similar waveforms as the tap for turns ratio is 1.

Figure 11 shows the case of the over-voltage condition. It can be seen that the output voltage has collapsed as the AVR has successfully tripped RL4 for protection.

click for full size image

Figure 12: Approaching Undervoltage. (Source: BarqEE)

click for full size image

Figure 13: Undervoltage Condition. (Source: BarqEE)

Figure 12 depicts the input and output voltage waveforms when the undervoltage condition is approaching. The AVR selects the tap for maximum turns ratio (1.63) under this condition.

Figure 13 shows the case of the undervoltage condition. It can be observed that the output voltage falls as RL4 has been tripped for protection.

Note that there is no frequency change or phase shift of the input and output voltages when the AVR is regulating.

Conclusions

In this article, we described the use of programmable ASICs such as a GreenPAK SLG46537V IC as a controller for AVRs, which are popular in residential and industrial applications. The ASIC can replace discrete components and MCUs that are currently used in these applications. The role of the SLG46537V in the proposed AVR was illustrated and the GreenPAK design was explained thoroughly. Moreover, the details of experiments on a prototype AVR were presented to validate the proposed design.

We conclude that the circuit provides ample capabilities to serve as a controller, particularly in residential AVRs. Hence, control units for AVRs can be designed using the IC that are inexpensive and reduce the PCB footprint. More sophisticated controllers can be designed using other ASICs offering ASMs with more states.


Aamir Hussain Chughtai is currently a candidate of Ph.D. Electrical Engineering at LUMS, Lahore. His areas of work are related to Signal Processing, Machine Learning and IoT. He is a cofounder of Lahore-based IT startup BarqEE. Aamir can be contacted at chughtaiah@barqee.com.
Muhammad Saqib received the M.S degree in Electrical Engineering from NUCES, Lahore. His major areas of work include Power Electronics, Embedded Systems and Instrumentation. He is a cofounder of Lahore-based IT startup BarqEE. M. Saqib can be contacted at saqib.awan@barqee.com.

Related Contents:

For more Embedded, subscribe to Embedded’s weekly email newsletter.

The post A more effective approach for developing AC-AC automatic voltage regulators appeared first on Embedded.com.





Original article: A more effective approach for developing AC-AC automatic voltage regulators
Author: Aamir Hussain Chughtai and Muhammad Saqib