Practical applications of active power filters (V) [Part 2/2: Active load balancers (ALB) for distributed energy resources]

Active load balancers

Active load balancers (ALB for short), also called active phase balancers or dynamic phase balancers, have been around since the beginning of the 2000s. Description of their topology and operating principle can be found as far back as 2004. 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 phase unbalances that conventional solutions like static balancers, load balancing transformers (LBT for short) or network reconfiguration done by phase or load balancing could not handle.

ALBs can be applied to small, medium or large installations in few specialised segments. They have some low and high voltage potential applications where their use offers many benefits including facilities and distribution lines with single-phase loads not well distributed in the three-phase system, welding machines, and single-phase solar inverters and wind turbine generators, to name a few.

Functions

ALBs provide mitigation of negative sequence currents (balancing three-phase currents) and zero sequence current mitigation (neutral current mitigation and unloading of neutral conductors). They can balance any unbalanced load from the supply system point of view using current control, converting them into symmetrical three-phase active power loads. They can also balance system voltages if the unbalance is caused by unbalanced active or reactive power of the load.

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

Connection

An ALB 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 ALB 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).

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

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

The most common operating voltage range for ALBs 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.

ALBs for distributed energy resources

A distributed energy resource (DER for short) is a small-scale unit of power generation that operates locally and is connected to a larger electrical grid at the distribution level. They are often used in the residential, commercial and industrial sectors by utility providers, businesses and individuals. An important characteristic of a DER is that the power it produces is often consumed close to the source.

Examples of distributed energy resources include roof top solar photovoltaic systems, wind turbine generators, energy storage systems, electric vehicles used to export power back to the grid, combined heat and power units and biomass generators.

Various technical and economic issues occur in the integration of these resources into the distribution system. Technical problems arise typically in the areas of power quality, voltage stability, reliability, protection and control.

Requirements

Background

Increasing penetration of solar power generation and electric vehicles has an influence on power quality and energy efficiency in distribution systems. Particularly, single-phase solar inverters and electric vehicle charging stations injecting power into three-phase distribution systems may exceed unbalance limits in the power supply voltage.

In this case, a distribution system needs to integrate distributed energy resources. Voltage variation can be observed in specific loads along the system due to phase unbalances, especially where large single phase loads are connected. An increase in voltage variation leads to overheating of loads as well as malfunctions of protection relays and voltage regulation devices.

The system operator is looking for a solution with the ability to dynamically rebalance phase-to-phase loading on a second-by-second basis.

System description

The distribution system has multiple loads and distributed energy resources connect along its distribution lines. The increased usage of solar energy generation, electric vehicle charging and heat pumps at households in the area is a fundamental challenge for this system.

Solution

Analysis

To be able to dimension a solution it is necessary to collect power quality measurement data from the system over a period of time by using a power quality analyser. The collected measurement data need to be studied to determine the power quality problems and energy efficiency requirements.

A series of comprehensive analytical power system studies including load flow analysis are needed to find out the magnitude of the problems and the ratings of the possible solutions.

From the measurements it can be noticed that single-phase solar inverters are causing problems on the loads of the distribution system and voltage unbalance. These unbalances change as fast as the solar generation output changes. It can also be noticed that electric vehicle charging stations create very sudden single-phase load growth contributing to unbalance conditions in the system.

Proposed solution

Based on the analysis of the measurements, it is possible to dimension a solution that would comply with the distribution system operator’s requirements. The solution is to use active load balancers rated 400 V 300 A installed along the feeders in parallel with the distribution lines. As they will be installed outdoors, these devices should be designed to endure harsh climate conditions.

This solution gives the distribution system operator a dynamic and dispatchable control of power from phase to phase in the three-phase system. This balancing can make all power downstream of the device appear as perfectly balanced three-phase power to the upstream system.

Alternatives like rearranging all the distributed energy resources and loads along the distribution lines or grid reinforcement are much more costly and time-consuming.

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

Conclusions

Electric utilities are faced with the challenge of providing a reliable electricity supply on a least-cost basis. This means considering reliability, resiliency, capital costs, operational costs, energy efficiency, and future-proofing the electrical grid for the integration of distributed energy resources like distributed solar energy generation and electric vehicles.

Active load balancers help to solve power quality issues in distribution systems with distributed energy resources. They can fix phase unbalances and maximise the existing voltage distribution capacity.

The installation of active load balancers with distributed energy resources brings several benefits including:

• Balancing of the electric power system in real time.
• Reduction of energy losses and voltage drop in conductors.
• Improved distribution system energy efficiency and energy savings.
• Increased renewable generation and electric vehicle hosting capacity.

If you would like to receive any of my publications on the topic or to explore how #ActivePowerFilters can benefit your application, feel free to reach me through LinkedIn.

--------------------

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.