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 facilities with fast changing reactive power demand, highly dynamic loads (power factor fluctuates rapidly or in big steps), 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

A 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.

 

Typical low voltage connection of a SVG

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.

Typical high voltage connection of a SVG

SVGs for office building loads

Office buildings are designed to ensure that business activities therein and occupants are supported for optimal productivity. Facility managers and building owners are constantly challenged to reduce energy costs and meet environmental regulations, while maintaining or improving comfort, productivity and operational efficiency.

The electric power system of modern office buildings has a large amount of generators and loads that can cause power quality and energy efficiency problems. If these problems are not mitigated correctly, the whole electric power system of the office building could suffer severe operational problems and the energy losses of the building could increase drastically.

Power quality and energy efficiency improvement in office buildings involves providing a given level of performance, cost, quality, availability and comfort at a minimal level of energy usage throughout the lifecycle of the building.

Requirements

Background

An office building owner wants to investigate ways to reduce the increasing costs of their electricity expenditure and secure the building operations in order to maintain the quality and integrity of the businesses.

Due to the size of the office building and the complexity of its various electrical loads and systems, power quality and energy efficiency play a central role in ensuring the lowest possible energy consumption and costs, as well as avoiding process downtimes and overheating and malfunction of electrical equipment.

System description

The single-phase and three-phase electrical loads of the office building are divided between two distribution boards. The most important electrical loads and their characteristics are:

 

• Lifts: The required reactive power compensation is very dynamic and changes very fast between capacitive and inductive during operation and when they feed regenerative power back into the system.
• Indoor and outdoor lighting: All lights are LED lamps and CFL (compact fluorescent lamps), which are used to save energy but contribute to an overall low power factor.
• Fans and air conditioners: They require power factor improvement.
• Computers and other IT equipment: They require a high quality, reliable and secure power supply to operate properly.

 

Solution

Analysis

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

The average power factor measured in the distribution boards was 0.85. Current unbalance was also detected as single-phase and three-phase loads were not properly balanced between the phases.

Proposed solution

Based on the analysis of the measurements, it is possible to dimension a solution for the office building that would comply with owner’s requirements. A SVG rated 415 V +/-200 kvar installed in parallel with the loads in each distribution board was recommended instead of conventional capacitor banks due to SVG’s instantaneous dynamic response, being immune to existing system harmonics and its ability to correct the building phase unbalances.

Capacitor banks are economical and suitable for installations having mainly linear and balanced loads and generators. However, in modern electric power systems, linear and balanced equipment are not easy to find. Due to the proliferation of nonlinear, non-balanced, variable and other challenging loads and generators, today’s systems experience complex problems and challenges.

In this case, some of the loads are being switched so fast that conventional capacitor banks would struggle to maintain an effective compensation set-point. Therefore, they would be continuously under or over-compensating the system. Moreover, capacitor banks are not able to mitigate unbalances.

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

The SVG offers instantaneous stepless compensation. Analyses the requirements of the loads and injects in real-time only the reactive power needed at that certain moment. This avoids any possible over or under compensation in the system. It also maintains a power factor of unity, reducing the electricity bills of the building. Moreover, the SVG lowers and balances the currents of the three-phase system.

Conclusions

With high energy costs and the requirements to convert existing buildings into green buildings, the owners and facility managers of office buildings are always looking for solutions to reduce their energy demand costs. Active power filters like SVGs provide an instantaneous and dynamic response to power quality and energy efficiency problems enabling longer equipment lifetime and reduced energy losses, complying with most demanding power quality standards and energy efficiency requirements.

The benefits of the installation of SVGs at office buildings can be summarised as:

 

• Excellent power factor correction performance, they can maintain a power factor of unity if required.
• Reduction of electricity bills.
• Correction of load unbalance.
• Operation not affected by network resonance.
• Can provide full output at low voltages.
• Can work with existing capacitor banks.