Harmonic issues, load matching issues, and heat generation issues that occur during the operation of frequency converters

Abstract: This paper analyzes the harmonic problems, load matching problems, and heat generation problems that exist in the operation of frequency converters, and proposes corresponding solutions.

Keywords: frequency converter; harmonics; load; heat generation

Abstract: This paper analyzes the problem of harmonic wave, matching of load and calorification for inverters in running, and made the relatively the measure.

Keywords: inverter harmonic wave loading calibration

1. Introduction

Since general-purpose frequency converters entered the Chinese market in the 1980s, they have been widely used in just over a decade. Currently, general-purpose frequency converters are increasingly favored for their advantages such as intelligence, digitalization, and networking. However, with the expansion of their application scope, more and more problems have emerged, mainly in the following aspects:

① Harmonic problems

② Inverter load matching problem

③ Fever problem

These issues have attracted the attention of relevant management departments and factories, leading to the formulation of related technical standards. For example, regarding harmonics, my country passed the "Interim Provisions on Harmonic Management of Power Systems" in 1984 and the GB/T-14549-93 standard in 1993 to limit harmonic pollution from power supply systems and electrical equipment. This paper analyzes the above problems and proposes solutions and countermeasures.

2. Harmonic Problems and Countermeasures

A general-purpose frequency converter typically consists of three main circuit components: a rectifier section, an inverter section, and a filter section. The rectifier section is a three-phase bridge uncontrolled rectifier, and the inverter section is an IGBT three-phase bridge inverter with a PWM waveform output. For a bipolar modulation frequency converter, its output voltage waveform is expanded as follows:
(1)
In the formula: n—the order of the harmonics, n=1,3,5……;
a1—Switching angle, i=1,2,3……N/2;
Ed—DC-side voltage of the frequency converter;
N—Carrier ratio.
As can be seen from equation (1), the amplitude of each harmonic is
(2)
Let n=1, then the fundamental amplitude of the inverter output voltage is:
(3)
As can be seen from equations (1), (2), and (3), the output voltage of a general-purpose frequency converter does indeed contain harmonics other than the fundamental frequency. Lower harmonics usually have a greater impact on the motor load, causing torque pulsation, while higher harmonics increase the leakage current of the frequency converter output cable, resulting in insufficient motor output. Therefore, both high and low order harmonics in the frequency converter output must be suppressed.

As mentioned earlier, because the rectifier section of a general-purpose frequency converter uses an uncontrolled diode bridge rectifier circuit, and the intermediate filtering section uses a large capacitor as a filter, the input current of the rectifier is actually the charging current of the capacitor, which presents a relatively steep pulse wave with significant harmonic components. To eliminate harmonics, the following countermeasures can be adopted:

① Increase the internal impedance of the inverter's power supply

Typically, the internal impedance of a power supply unit acts as a buffer against the reactive power of the inverter's DC filter capacitor. This internal impedance is the short-circuit impedance of the transformer. When the power supply capacity is smaller relative to the inverter capacity, the internal impedance is relatively larger, and the harmonic content is lower; conversely, when the power supply capacity is larger relative to the inverter capacity, the internal impedance is relatively larger, and the harmonic content is higher. For the Mitsubishi FR-F540 series inverter, a power supply internal impedance of 4% provides excellent harmonic suppression. Therefore, when selecting a power supply transformer for an inverter, it is best to choose a transformer with a high short-circuit impedance.

② Install reactors

Installing a reactor effectively increases the internal impedance of the inverter's power supply from the outside. An AC reactor can be installed on the AC side of the inverter, or a DC reactor on the DC side, or both, to suppress harmonic currents. Table 1 lists the contents of the Mitsubishi FR-A540 inverter with and without a reactor.

③ Multiphase operation of transformer

The rectifier section of a general-purpose frequency converter is a six-pulse rectifier, which generates relatively large harmonics. If multi-phase operation of transformers is applied, and the two transformers with a phase angle difference of 30°, such as a Y-Δ or Δ-Δ combination, are combined to form an effect equivalent to a 12-pulse rectifier, the low-order harmonic current can be reduced by 28%, achieving a very good harmonic suppression effect.

④ Adjust the carrier ratio of the frequency converter

As can be seen from equations (1), (2), and (3), as long as the carrier ratio is large enough, lower harmonics can be effectively suppressed. In particular, when the reference amplitude and the carrier amplitude are less than 1, odd harmonics below the 13th order no longer appear.

⑤ Dedicated filter

This dedicated filter is used to detect the amplitude and phase of the harmonic current of the frequency converter and generate a current with the same amplitude but opposite phase as the harmonic current, which is then passed into the frequency converter, thus effectively absorbing the harmonic current.

3. Load matching problem and its solutions

Production machinery comes in a wide variety of types, with varying performance and process requirements, resulting in complex torque characteristics. Broadly speaking, it can be categorized into three types: constant torque loads, fan and pump loads, and constant power loads. Different types of frequency converters should be selected for different load types.

① Constant torque load

A constant torque load refers to a load whose torque is independent of its rotational speed; the torque remains constant at any rotational speed. Constant torque loads are further divided into frictional loads and potential energy loads.

For friction-type loads, the starting torque is generally required to be around 150% of the rated torque, and the braking torque is generally required to be around 100% of the rated torque. Therefore, frequency converters with constant torque characteristics, large starting and braking torques, long overload time, and large overload capacity should be selected. For example, the Mitsubishi FR-A540 series frequency converter.

Potential energy loads generally require high starting torque and energy feedback capabilities, and must be able to quickly achieve forward and reverse rotation. Therefore, a frequency converter with four-quadrant operation capability should be selected, such as the Mitsubishi FR-A241 series.

② Fan and pump loads

Fan and pump loads are the most commonly used equipment in industrial settings. Although pumps and fans have diverse characteristics, centrifugal pumps and centrifugal fans are the primary applications, and general-purpose frequency converters are most frequently used in these loads. Fan and pump loads are square torque loads, and their speed n is related to the flow rate Q, and their torque T is related to the pump's shaft power N by the following formulas:
(4)

For this type of load, the performance requirements of the frequency converter are not high; only economy and reliability are required. Therefore, a frequency converter with a U/f=const control mode is sufficient, such as the Mitsubishi FR-F540(L) series frequency converter. During actual operation of fan loads, due to the large moment of inertia, the acceleration and deceleration times of the frequency converter are very important issues and can be calculated using the following formula:
(5)
(6)
In the formula: tACC — acceleration time (s);
tDEC — Deceleration time (s);
GD2—Moment of inertia referred to the motor shaft (N·m2);
g—acceleration due to gravity, g = 9.81 (m/s²);
TM—Electromagnetic torque of the electric motor (Nm);
TL — Load torque (Nm);
nAS—Initial velocity (r/min) during system acceleration;
nAE—the final velocity of the system during acceleration (r/min);
nDS—Initial velocity (r/min) of the system during deceleration;
nDE—The final speed (r/min) of the system during deceleration.

As can be seen from the above formula, the calculation of the system rotational inertia of the wind turbine load is very important. In the specific design of the frequency converter, the calculation results should be appropriately modified according to the above formula, and the shortest time should be selected while ensuring that no overcurrent tripping occurs during frequency converter startup and no overvoltage tripping occurs during frequency converter deceleration.

Pump loads are prone to surge, pressure buildup, and water sag during actual operation. Therefore, when selecting a frequency converter, it is essential to choose one suitable for pump loads, and the frequency converter's function settings should be specifically configured to address these issues.
Surge: Measure the frequency points where surge is likely to occur, and avoid system resonance by setting the jump frequency points and width.
Pressure buildup: When pump loads operate at low speeds, pressure buildup in the system can cause the flow rate to drop to zero, resulting in pump burnout. By setting the inverter's minimum frequency, the minimum system speed at the critical point of pump flow is limited, thus preventing this phenomenon.