Practical applications of active power filters (I) [Part 1/5: Static var generators (SVG) for resistance spot welding machines]

Static var generators

Static var generators (SVG for short), also called active power factor compensators or correctors (APFC for short) or instantaneous reactive power compensators (IRPC for short), have been around since the 1980s. Description of their topology and operating principle can be found as far back as 1984. They were developed as a customised design of shunt active power filters (APF for short) to take care of the problems in the electric power system created by fast changing reactive power demand or by highly dynamic loads that conventional passive solutions like mechanically switched capacitor banks (MSC for short) and mechanically switched reactors (MSR for short) or conventional active solutions like thyristor switched capacitor banks (TSC for short) and thyristor switched reactors (TSR for short) could not handle.

SVGs can be applied to small, medium or large installations in a wide range of segments. They have many low and high voltage potential applications where their use offers many benefits including equipment or facilities where reactive power and/or power factor fluctuate rapidly or in big steps, welding machines, solar inverters, wind turbine generators, and loads with low power factor, to name a few.

Functions

SVGs deliver in real-time exactly the right amount of inductive and capacitive reactive current that the application demands, providing accurate power factor correction, mitigating flicker, reducing voltage variations and reducing unbalances in the system without the drawbacks of conventional solutions.?

Modern SVGs can take care of several power quality problems and support the development of clean energy by combining different control functions in a single device.

Connection

An SVG is a power electronics-based shunt compensation device connected in parallel with the equipment generating the power quality problems or that has issues to comply with grid code and energy efficiency requirements. The SVG behaves as a controlled current source providing any kind of current waveform (in terms of phase, amplitude and frequency) in real time (typical reaction time is under 50 microseconds and typical overall response time is under 100 microseconds).

SVGs can be connected to the electric power system as 3-wire or 4-wire devices:

• 3-wire SVGs are typically used for industrial and generation applications.
• 4-wire SVGs are typically used for applications in buildings.

The most common operating voltage range for SVGs is 200 V up to 690 V as they are built using low voltage IGBT switches. It is possible to connect them to higher voltages using a suitable step-up transformer.

SVGs for resistance spot welding machines

Resistance spot welding machines, such as those found in many manufacturing facilities, have several unique operating characteristics that, if not addressed properly, can create several power quality problems in the installation. These problems can decrease productivity, damage other loads of the facility or decrease quality of the welds affecting the quality of the finished product. Facilities should be aware of these potential power quality problems and how to identify them. Doing so early on can save considerable money and time in the long run.

Among the main issues associated with the resistance welding process is a sudden inrush current demand. Welding machines draw high levels of inrush current during their operating cycle, which is often only few seconds in duration. These high cycle-to-cycle currents cause the flux (magnetizing current) of the upstream transformer to saturate. Flux saturation causes the transformer output voltage to drop and results in failure or poor performance of the welding machine.

Additionally, when the transformer output voltage drops, the source sees that drop and attempts to provide the needed current to maintain the faulting transformer voltage, thus creating an additional component to the current surge within the electric power system. This current surge accentuates the voltage drop of the source on an intermittent basis. If the voltage cycling is repetitive, it is defined as voltage fluctuation or flicker.

Requirements

Background

Manufacturing plants producing metallic structures usually have resistance spot welding machines. The welding cycle length of these machines is typically 50 to 200 ms. The sudden high inrush current demand for such a short period of time typically results in the appearance of voltage fluctuations (flicker) and high load unbalance levels in these installations.

Along with the annoyance, reduced task performance, and visual fatigue for employees at the site, flicker can also disturb the operation of other equipment within the plant and damage sensitive equipment such as PLCs, VSDs, computers, measurement devices and protection relays.

System description

The customer in this case has one welding machine in the plant. It is fed by a distribution board where other equipment is also connected. The welding machine is connected between two phases in a three-phase electric power system.

Solution

Analysis

The main target of these projects is to improve the operation of the manufacturing plants by reducing the amount of flicker and balancing the loads of the installations.

To be able to dimension a solution it is necessary to collect power quality measurement data from the welding machine over a period of time by using a power quality analyser. Based on the data from measurements it is clear that a solution with real time response is required to fix the power quality problems.?

The measurements show that the currents are unbalanced, the power factor of the installation is low and the short-term flicker index (Pst) is too high.

• Peak current values of each phase: L1 = 56 A, L2 = 588 A and L3 = 670 A.?
• RMS current values of each phase: L1 = 38 A, L2 = 325 A and L3 = 350 A.?
• Maximum Pst values of each phase: L1 = 0.4, L2 = 2.7 and L3 = 3.9.

Proposed solution

Based on the analysis of the measurements, it is possible to dimension a solution for the manufacturing plant that would comply with customer’s requirements. The solution is an static var generator (SVG) installed in parallel with the welding machine creating the power quality problems.

Conventional solutions like capacitor banks (even equipped with thyristor switch modules) are often too slow to respond to the fast and instant reactive power compensation requirements of unbalanced fast changing cyclic loads such as resistance spot welding machines. Conventional solutions also do not avoid possible resonances that might occur in the system.

Based on the values monitored, the following functions are proposed for the SVG.

Conclusions

Taking care of the power quality problems caused by welding machines is a very demanding application. Active power filters like SVGs provide a quick and effective response to power system disturbances enabling longer equipment life, higher process reliability and reduced energy losses, complying with most demanding power quality standards and energy efficiency requirements.

SVGs are designed to inject inductive and capacitive reactive current to support the current requirements of the load to reduce demand upon the upstream electric power system. They can also maintain voltage levels, reduce flicker and improve power factor thereby improving the quality of the welding process and the overall efficiency of these manufacturing plants, and reducing the amount of apparent power purchased from the electric utility.

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About the author:

Pedro Esteban is a versatile, multicultural and highly accomplished marketing, communications, sales and business development leader who holds since 2002 a broad global experience in sustainable energy transition including renewable energy, energy efficiency and energy storage. Author of over a hundred technical publications, he delivers numerous presentations each year at major international trade shows and conferences. He has been a leading expert at several management positions at General Electric, Alstom Grid and Areva T&D, and he is currently working at Merus Power Plc.