Comparison between active power filters and conventional solutions: [Part 3/5: Active load balancers (ALB)]

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.

Features

The most typical features of ALBs that can be found nowadays in the market can be classified into the following categories.

Benefits

The most typical benefits of ALBs that can be found nowadays in the market can be classified into the following categories.

Comparison with conventional solutions

There are several solutions competing with active load balancers that can be applied to a certain installation once that the power quality and energy efficiency problems and their magnitudes are identified. They provide different levels of problem mitigation depending on their technical complexity and the financial investment. The primary goals of most of these solutions are to ensure that three-phase generators and loads are not affected by unbalances, and that electrical equipment at the higher voltages are not affected by unbalance caused at the lower voltages.

The different mitigation solutions can take care of several problems:

• Current unbalance: Phase balancing, load balancing, static balancers and active load balancers.
• Voltage unbalance: Transformer winding configurations, load balancing transformers and automatic voltage regulators.
• Propagation of unbalance into the higher voltage system: Transformer winding configurations and load balancing transformers.

Comparison with network reconfiguration

Network reconfiguration can be done by phase balancing or by load balancing, using switches or rearranging the loads of the system. Regardless of the algorithm used for network reconfiguration, the solution found determines the statuses (on/off) of controllable static switches strategically located on the system.?

Network reconfiguration by phase balancing is a process usually done on the high voltage system. It involves reassigning the connection of single-phase transformers along high voltage feeders.

Network reconfiguration by load balancing is a process usually done on the low voltage system. It involves reassigning single-phase loads to the different phases along the low voltage feeders.

Comparison with transformer winding configurations

Three-phase and single-phase loads are supplied through distribution transformers with different winding configurations. The ratings and impedances of these transformers together with the winding configurations can result in unbalanced secondary voltages.

The use of different winding configurations on distribution transformers is taking care of:

• Limiting unbalanced secondary voltages as a result of supplying single-phase loads.
• Blocking the propagation of zero sequence current unbalance into the higher voltage system.

Electric power systems that use three-phase transformers with delta/star or star/delta winding configurations are well suited for the supply of unbalanced loads.

Comparison with automatic voltage regulators

Automatic voltage regulators (AVR for short) are typically used for the voltage regulation of all three phases of a electric power system simultaneously. They can however be configured to regulate individual phase to neutral voltages to reduce voltage unbalance.

The use of automatic voltage regulators on a high voltage system typically consist of three single-phase step-voltage regulators wired together in either star or closed delta configuration. Each step-voltage regulator consists of an autotransformer and an on-load tap changer (OLTC for short).

When used to reduce voltage unbalance, the three single-phase regulators are independently operated, each controlled by its own compensator circuit. The taps on each regulator change separately, regulating individual phase to neutral voltages. With different tap positions on each regulator, the device is capable of balancing phase to neutral voltages. They have a very limited improvement capability of current balance.

Comparison with load balancing transformers

Load balancing transformers (LBT for short) are distribution transformers with an unconventional winding configuration. The novelty in this unconventional winding configuration is that it achieves a better sharing of currents on the primary side of the transformer when serving unbalanced loads. This type of winding configuration can therefore:

• Improve the utilisation of transformer’s capacity.
• Mitigate the propagation of current unbalance from the low voltage to the high voltage system.

The secondary of each phase consists of three windings connected such that each load current draws two-thirds of its current through one winding and one-third from a set of two windings in series. In this way each of the secondary load currents are not drawn equally from the three primary phases. This results in a better sharing of currents on the primary side of the transformer.

Comparison with static balancers

Static balancers can improve both current and voltage unbalance in low voltage installations. Static balancers once connected to the electric power system, they draw some of the neutral current (because they have a low zero sequence impedance) and share it equally onto the three phases of the system. As a result of this redistribution:

• The phase to neutral voltage on the loaded phase is improved as the voltage drop on the line as well as on the neutral are decreased.
• The phase to neutral voltages on the unloaded phases are improved because the voltage drop, which now appears on them, move in the same direction as that on the neutral.

Summary

The conventional ways to solve phase unbalance problems in the electric power system are related to artificial phase modulation, intelligent phase adjustment, the use of static balancers, load balancing transformers or network reconfiguration done by phase balancing or load balancing. Some of the main disadvantages of these conventional solutions can be summarised as:

• They do not provide the needed real time response that modern electric power systems require.
• They cannot fulfil the increasing requirements for smart devices and wireless connectivity technology including their integration in IIoT software platforms.
• They can be extremely time consuming and costly solutions (for example, implementing the optimal rearrangement of loads for a building).

It is very important to notice that the total cost of ownership of active load balancers compared to the TCO of conventional solutions for a certain application it is very much dependent on the topology of the installation, the design of the whole system, the ratings of the device needed for taking care of the problems of the application and the requirements of the end user.

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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.