High Performance Active Harmonic Filter

Base on the Current Injection Mode

  1. INTRODUCTION
     
  2. TRADITIONAL SOLUTIONS THAT MINIMIZE       HARMONIC CURRENTS EFFECTS
     
    -2.1 Oversizing or derating of the installation
     
    -2.2 Specially connected transformers
     
    -2.3 Series reactors
     
    -2.4 Tuned passive filter
     
  3. TOPOLOGIES OF ACTIVE HARMONIC FILTERS
     
    -3.1 Series filters
     
    -3.2 Parallel filters
     
    -3.3 Hybrid filters
     
  4. PARALLEL ACTIVE HARMONIC FILTER: SYSTEM DESCRIPTION
     
    -4.1 Operating principle

Abstract:

In just over ten years, electricity power quality has grown to become a major issue for both the electric utilities industry and their commercial and industrial customers. The electric utilities industry, also referred to as electricity power delivery industry, has increasingly been concerned with harmonic distortion being generated by non-linear loads. The effects of harmonic distortion are hard to measure while the end results are easy to understand - higher operating costs and lower reliability. In the United States, the utilities have adopted the limits proposed in the IEEE 519-1992. Also, the utilities are advising their customers to adhere to IEEE 519-1992. | BACK TO TOP |

Why the concern over harmonic distortion? The concern to the electricity power delivery industry is three fold; greater demand, lower reliability and limited resources. Greater demands are caused by ineffective utilization of the power that is delivered and losses caused by poor power factor. Lower reliability is caused by the greater demands placed on equipment to meet customer power requirements and the resulting generated heat. Finely, the de-regulation of the power industry is limiting the capital resources available to expand and offer greater amounts of power to the consumer. The concerns of the customers are higher operating costs, lower reliability and production line shutdown. Higher operating costs are caused by poor utilization of the power delivered and associated costs to repair or replace equipment. Lower reliability of facility equipment, includes the plant's low voltage AC electrical distribution system, drive motors and expense electronic-based equipment. Production line shutdown can be caused from the lock-up of the equipment requiring the restart of the equipment to out-right equipment failure.

Today, the situation on low voltage AC systems, less than 1,000VAC, has become a serious concern. The quality of electric power in commercial and industrial installations in undeniable decreasing.

In addition to external disturbances, such as outages, sags, spikes due to switching and atmospheric phenomena, there are inherent, internal causes specific to each site and resulting from the combined use of linear and non-linear loads.

Untimely tripping of protected devices, harmonic overloads, high levels of voltage and current distortion, temperature rise in conductors and generators all contribute to reducing the quality and the reliability of a low-voltage AC system.

The above disturbances are well understood and directly related tot he proliferation of loads consuming non-sinusoidal current, referred to as "non-linear loads". This type of load is used for the conversion, variation and regulation of electric power in commercial, industrial and residential installations.

The prospect of a rapid return to linear-load conditions is illusory. Recent studies show that the consumption of non-linear current will sharply increase in the years to come.

However, the remarkable progress made power electronic devices in the recent years, fast IGBT's makes it possible to design self adaptable harmonic suppressors called Active Harmonic Filters. Active Harmonic Filters are proving to be viable option for controlling harmonic levels in many applications.

    TRADITIONAL SOLUTIONS THAT MINIMIZE HARMONIC CURRENTS EFFECTS | BACK TO TOP |

Today, a variety of approaches are used to minimize harmonic distortion, but all present disadvantages. All solutions will exhibit higher utility costs because of continued poor power factors. These solutions are listed here after.

2.1 Oversizing or derating of the installation -| This solution does not attempt to eliminate the harmonic currents flowing in the low voltage (less than 1,000VAC) electrical distribution system but rather to "mask" the problem and avoid the consequences.

When designing a new installation, the plan is to oversize all elements likely to transmit harmonic currents, namely the transformers, cables, circuit breakers, engine generator sets and the distribution switchboards. The most widely implemented solution is oversizing of the neutral conductor.

In existing installation, the most common solution is to derate the electrical distribution equipment subjected to the harmonic currents. The consequence is an installation that cannot be used to its full potential.

Con: The result is a major increase in the cost of electrical distribution system.

2.2 Specially connected transformers  | BACK TO TOP |

This solution inhibits propagation of third-order harmonic currents and their multiples. It is a centralized solution for a set of single-phase loads. However, it produces no effect on other harmonic orders that are not multiples of three (H5, H7, etc). On the contrary, this solution limits the available power from the source and increases the line impedance.

Con: The consequence is an increase in the voltage distortion due to the other harmonic orders.

2.3 Series reactors

This solution, used for variable speed drives and three phase rectifiers, consists in connecting a reactor in series upstream of a non-linear load. A reactor is not expensive, but has limited effectiveness. One must be installed for each non-linear load. Current distortion is divided by a factor of approximately two.

Con: Harmonic current still greater than IEEE 519-1992.

2.4 Tuned passive filte   | BACK TO TOP |r

The idea is to "trap" the harmonic currents in L/C circuits tuned to the harmonic orders requiring filters. A filter therefore comprises a series of "stages", each corresponding to a harmonic order, Orders 5 and 7 are the most commonly filtered.

A filter may be installed for one load or a set of loads. Its design requires in-depth study of the AC system and collaboration with a consulting engineer. Sizing depends on the harmonic spectrum of the load and the impedance of the power source. Rating also must be coordinated with reactive power requirements of the loads, and it is often difficult to design the filter to avoid leading power factor operation for some load conditions. This solution is moderately effective and its design depends entirely on the given power source and the loads.

NOTE: When appropriately designed, this type of filter may also be used to eliminate harmonic distortion already present on the electrical network of the power distributor, provided a significant overrating for harmonic absorption from the power system.

Con: It is not flexible and is virtually impossible to upgrade. Must be re-tuned if circuit environment changes.

3.0 TOPOLOGIES OF ACTIVE HARMONIC FILTERS  
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BACK TO TOP |

The idea of Active Harmonic Filters is relatively old, however the lack of effective techniques at a competitive slowed its development for a number of years. Today, the wide-spread use of IGBT components and the availability of new digital signal processing (DSP) components are paving the way to a much brighter future for the Active Harmonic Filter.

The Active Harmonic Filter concept uses power electronics to introduce current components, which cancel the harmonic components of the non-linear loads. A number of different topologies are being proposed and are discussed below. Within each topologies there are issues of required components ratings and method of rating the overall filter for the loads to be compensated.

3.1 Series filters   | BACK TO TOP |

This type of filter is connected in series with the AC distribution network and compensates both the harmonic currents generated by the load and the voltage distortion already on the AC system. This solution is technically similar to line conditioners and must be sized for the total load rating.

3.2 Parallel filters   | BACK TO TOP |

Also called shunt filters, they are connected in parallel with the AC line and need to be sized only for the harmonic current drawn by the non-linear load(s). The parallel topology selected for TimesOne™ is in no way dependent on the load or electrical AC system characteristics. It is described in detail in section 4.

3.3 Hybrid filters

This solution, combining an active filter and a passive filter, and may be either of the series or parallel type. In certain cases, it may be a cost-effective solution. The passive filter carries out basic filtering (5th order, for example) and the active filter, through its precise and dynamic technique, covers the other harmonics.

4.0 PARALLEL ACTIVE HARMONIC FILTER: SYSTEM DESCRIPTION

4.1 Operating principle    | BACK TO TOP

The Active Filter is connected in parallel with the AC line, and constantly injects currents that precisely correspond to the harmonic components drawn by the load. The result is that the current supplied by the power source remains sinusoidal.

IMAGE

I load = I fundamental + I harmonic

I correction = I harmonic

I load = I source + I correction

Then, the source supplies the load with the fundamental component of the current only.

The normal power source provides the fundamental current, and the Active Harmonic Filter (AHF) supplies the harmonic currents required by the load. The entire low-frequency harmonic spectrum (2nd – 25th and can be extended to 50th) is injected. If the harmonic currents drawn by the load are greater than the rating of the Active Harmonic Filter, the filter automatically limits the injected current to its rated output current.

Easy to implement, an active filter may be installed at any point on a low voltage AC system to compensate the power drawn by one or several non-linear loads, thus avoiding the circulation of harmonic currents throughout the low-voltage AC system.  | BACK TO TOP |

 

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