IEEE 1531 has outlined key harmonic filter design considerations for its proper selection. They are grouped into performance and rating criteria. The former relate to normal expected operating conditions. Meanwhile, the rating criteria refer to unusual conditions that may place a more severe duty on the equipment.
Installing a passive shunt harmonic filter near harmonic producing loads is one of the common methods of mitigating harmonic distortion. The purpose of the harmonic filter is to shunt some of the harmonic current from the load into the filter to reduce the amount that goes to the electrical power system.
Key Design Considerations
A. Performance Criteria
1. Reactive Power (kVAR) Requirements
It is essential to know the capacitive reactive power needed and its corresponding savings, in order to optimize the system costs. Also, the reactive power and voltage control requirements could dictate the necessity for the harmonic filter bank to be switched in steps or not. Subsequently, the total and step kVAR sizes are normally determined by the fundamental frequency load flow and the voltage control requirements.
2. Harmonic Limitations
These are defined in terms of the following:
a. System limitations
These are defined to guarantee that equipment will properly function and not fail due to excessive harmonic distortion. IEEE 519 listed the recommended voltage and current distortion limits at the point of common coupling to the utility.
b. Equipment withstand capabilities
Capacitors
· IEEE 18 states that capacitors are intended to be operated at or below their rated voltage.
· IEEE 1036 gives additional guidelines for shunt capacitors.
In addition, capacitor fuses should be rated for the voltage and current, including harmonics, in a harmonic filter application. Moreover, higher frequency capacitor equipment designs may be required (excluded from IEEE 1531) where significant harmonics above 1 kHz exist.
Transformers
· IEEE C57.12.00 and C57.12.01 state that when transformers are operating at rated load, the total harmonic current distortion should be limited to 5%.
· IEEE C57.110 defines the method for derating transformers when supplying nonsinusoidal loads.
· UL 1561 and 1562 define the transformer K-rating that is intended for use in harmonic-rich environments.
Motors
· IEEE Task Force indicates that derating of the motor occurs for voltage distortions greater than 5%.
· IEEE P519.1/D8b14 – Significant derating of the motor begins at about 8% THD.
3. Normal System Conditions
These are assessed to ensure that the harmonic filter design will meet specific reactive power and harmonic performance requirements for such conditions.
a. All harmonic voltages and currents
· Characteristic harmonics of all expected loads.
· Uncharacteristic harmonics - Frequencies that are not theoretically characteristic of a perfectly operating device may sometimes occur.
· Background and future harmonic loads.
b. System voltage variation
· Overvoltage: +5% for normal load and +10% for unloaded system conditions.
· Undervoltage: Not critical for harmonic filter design.
· Interruption: Filters should be disconnected from the system immediately.
c. System frequency variation
Affect the duty of the harmonic filter and can also have a profound impact on the overall harmonic performance of the system.
d. Power system configurations
Changes in the system may affect the filter.
e. Loading conditions
Variations in the system and the harmonic-producing loads.
f. System voltage unbalance
Leads to increased harmonic injections from distortion-producing equipment, particularly triplen harmonics.
4. Normal Harmonic Filter Conditions
Filters are seldom tuned to their exact calculated values. It is necessary to allow for the following parameter variations when evaluating the performance of the harmonic filters:
a. Component Tolerances
Manufacturing tolerances must be considered for the inductance, capacitance, and resistance.
b. Ambient Temperature Variations
Capacitance and resistance both vary with temperature.
c. Capacitor Element or Unit Failures
These will result in a change in the harmonic filter tuning and in overvoltage on some parts of the capacitor bank.
B. Rating Criteria
1. Contingency System Conditions
The contingency system operating conditions are generally evaluated to assure that the harmonic filter design will be rated adequately to handle these conditions although the normal system distortion limits may be exceeded:
a. Switching
The switching of harmonic filters or other system components may result in significant overvoltage duties for the harmonic filter components.
b. Application of filters tuned to the same frequency
When harmonic filters are installed at the same location and are tuned to the same frequency, care must be taken to ensure that there is adequate sharing of the harmonics among the filters.
c. System frequency variation
Frequency variations greater than that for normal system conditions are considered.
d. Power system configurations
Modifications in system configuration such as outage or connection of harmonic filters, result in shifts of harmonic resonant peaks, therefore affecting the design of multiple harmonic filter installations.
e. Characteristic and uncharacteristic harmonics
Higher values than the values used to evaluate performance are typically used for the rating of the equipment.
f. Unknown harmonic sources
It is advisable to add a factor to the calculated harmonic duties to account for unknown or future harmonic sources when rating the equipment.
2. Contingency Harmonic Filter Conditions
When multiple harmonic filters are applied at the same location, the outage of a complete harmonic filter is considered in rating the filter components. In some applications, the outage of a single harmonic filter may require that other harmonic filters be disconnected so that their ratings are not exceeded.
Reference:
IEEE 1531-2003. Guide for Application and Specification of Harmonic Filters
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