Harmonic and capacitor selection

Effects of Harmonics

Schneider Capacitor

Harmonics in electrical installation

The presence of harmonics in electrical systems means that current and voltage are distorted and deviate from sinusoidal wave forms.

Harmonic currents are currents circulating in the networks and which frequency is an integer multiple of the supply frequency.

Harmonic currents are caused by non-linear loads connected to the distribution system. A load is said to be non-linear when the current it draws does not have the same waveform as the supply voltage. The flow of harmonic currents through system impedances in turn creates voltage harmonics, which distort the supply voltage.

The most common non-linear loads generating harmonic currents are using power electronics, such as variable speed drives, rectifiers, inverters, etc…. Loads such as saturable reactors, welding equipment, arc furnaces, also generate harmonics. Other loads such as inductors, resistors and capacitors are linear loads and do not generate harmonics.

Influence of Harmonics in Capacitors

Capacitors are particularly sensitive to harmonic currents since their impedance decreases proportionally to the order of the harmonics present. This can result in a capacitor overload, shortening steadily its operating life. In some extreme situations, resonance can occur, resulting in an amplification of harmonic currents and a very high voltage distortion.

Amplification of Harmonic currents is very high when the natural resonance frequency of the capacitor and the network combined happens to be close to any of the harmonic frequencies present. This situation could result in severe over voltages and overloads which will lead to premature failure of capacitors To ensure a good and proper operation of the electrical installation, the harmonic level must be taken into account in the selection of the power factor correction equipment. A significant parameter is the cumulated power of the non-linear loads generating harmonic currents

Equipment	Effect of Harmonics
Motor	Over heating, production of non-uniform torque,increased vibration
Transformer	Over heating and insulation failure, noise
Switch gear and cables	Neutral link failure, increased losses due to skin effect and overheating of cables
Capacitors	Life reduces drastically due to harmonic overloading
Protective Relays	Malfunction and nuisance tripping
Power electronic equipment	Misfiring of Thyristors and failure of semiconductor devices
Control & instrumentation Electronic equipment	Erratic operation followed by nuisance tripping and breakdowns
Communication equipment / PC’s	Interference
Neutral cable	Higher Neutral current with 3rd harmonic frequency, Neutral over heating and or open neutral condition
Telecommunication	Telephonic interference, malfunction of sensitive electronics used,
equipment	failure of telecom hardware

Rated voltage and current of Capacitor

According to IEC 60831-1 standard, the rated voltage (UN) of a capacitor is defined as the continuously admissible operating voltage.

The rated current (IN) of a capacitor is the current flowing through the capacitor when the rated voltage (UN) is applied at its terminals, supposing a purely sinusoidal voltage and the exact value of reactive power (kvar) generated.

Capacitor units shall be suitable for continuous operation at an r.m.s. current of (1.3 x IN).

In order to accept system voltage fluctuations, capacitors are designed to sustain over-voltages of limited duration. For compliance to the standard, capacitors are for example requested to sustain over-voltages equal to 1.1 times UN, 8h per 24h.  Capacitors should be designed and tested extensively to operate safely on industrial networks.

The design margin allows operation on networks including voltage fluctuations and common disturbances. Capacitors can be selected with their rated voltage corresponding to the network voltage. For different levels of expected disturbances, different technologies are proposed, with larger design margin for capacitors adapted to the most stringent working conditions (HDuty & Energy)

Capacitor Selection Based on operating conditions

The operating conditions have a great influence on the life expectancy of capacitors. For this reason, different categories of capacitors, with different withstand levels, must be selected according to operating conditions.

Capacitors must be selected in function of the following parameters:

- Ambient Temperature (°C),

- Expected over-current, related to voltage disturbances, including maximum sustained over voltage,

- Maximum number of switching operations/year,

- Requested life expectancy.

Capacitors are particularly sensitive to harmonics. Depending on the magnitude of harmonics in the network, different configurations shall be adopted.

Different ranges with different levels of ruggedness are proposed:

SDuty: Standard duty capacitors for standard operating conditions, and when no significant non-linear loads are present.

HDuty: Heavy duty capacitors for difficult operating conditions, particularly voltage disturbances, or when a few non-linear loads are present. The rated current of capacitors must be increased in order to cope with the circulation of harmonic currents.

Energy: Specially designed capacitors, for harsh operating conditions, particularly high temperature.

Capacitors with detuned reactors: applicable when a significant number of non-linear loads are present.

Tuned filters: when non-linear loads are predominant, requesting harmonic mitigation. A special design is generally necessary, based on on-site measurements and computer simulations of the network. Since the harmonics are caused by non-linear loads, an indicator for the magnitude of harmonics is the ratio of the total power of non-linear loads to the supply transformer rating.

This ratio is noted NLL, and is also known as Gh/Sn:

Example:

Supply transformer rating: Sn = 630 kVA

Total power of non-linear loads: Gh = 150 kVA

NLL= (150/630) x 100 = 24%

Capacitor selection taking account of harmonics

The percentage of non-linear loads NLL is a first indicator for the magnitude of harmonics. The proposed selection of capacitors depending on the value of NLL is given in the diagram below.

A more detailed estimation of the magnitude of harmonics can be made with measurements. Significant indicators are current harmonic distortion THDi and voltage harmonic distortion THDu, measured at the transformer secondary, with no capacitors connected. According to the measured distortion, different technologies of capacitors shall be selected:

Note:

The capacitor technology has to be selected according to the most restrictive measurement. Example, a measurement is giving the following results:

- THDi = 15 % Harmonic solution.

- THDu = 3.5 % HDuty / Energy solution.

HDuty or Energy with Detuned Reactor has to be selected.

Solution	Description	Recommended use for	Max. condition
SDuty	Standard capacitor	●● Networks with non significant non-linear loads ●● Standard over-current ●● Standard operating temperature ●● Normal switching frequency ●● Standard life expectancy	NLL ≤ 10% 1.5 IN 55°C (class D) 5,000/year Up to 100,000 h*
HDuty	Heavy-duty capacitor	●● Few non-linear loads ●● Significant over-current ●● Standard operating temperature ●● Significant switching frequency ●● Long life expectancy	NLL ≤ 20% 1.8 IN 55°C (class D) 7,000/year Up to 130,000 h*
Energy	Capacitor for special conditions	●● Significant number of non-linear loads (up to 25%) ●● Significant over-current ●● Extreme temperature conditions ●● Very frequent switching ●● Extra long life expectancy	NLL ≤ 25% 1.5 IN 70°C 10,000/year Up to 160,000 h*
HDuty +Detuned Reactor	Heavy-duty, harmonic rated capacitor + detuned reactor	●● High level of non-linear loads (up to 30%) ●● Significant over-current ●● Standard operating temperature ●● Significant switching frequency ●● Long life expectancy	NLL ≤ 30% 1.8 IN 55°C (class D) 7,000/year Up to 130,000 h*
Energy + Detuned Reactor	Energy, harmonic rated capacitor + detuned reactor	●● High level of non-linear loads (up to 30%) ●● Significant over-current ●● Extreme temperature conditions ●● Very frequent switching ●● Extra long life expectancy	NLL ≤ 30% 2.5 IN 70°C (class D) 10,000/year Up to 160,000 h*

*The maximum life expectancy is given considering standard operating conditions:

Service voltage (UN), service current (IN), 35°C ambient temperature

Reference:  "Guide for the Design and Production of LV Power Factor Correction Cubicles" By Schneider Electric