Improve generator networks with modern consumers through the use of an AHF (Active Harmonic Filter).
Case study: AHF in generator networks
Our new case study shows how generator networks with modern consumers can be improved by using an AHF (Active Harmonic Filter).
Voltage quality in generator networks – always a challenge!
Mobile generators ensure the electrical power supply in the film and event industry, as well as in emergency situations. Today, these generators are increasingly connected to devices with non-linear power consumption. This leads to asymmetrical loads and severe voltage distortion. The unit, diesel engine, and generator begin to "pumping" and become overloaded. An active harmonic filter (AHF) provides relief and optimizes the load. A test scenario with real-world loads at Sparks Filmtechnik GmbH in Munich confirms the positive effects.
Task
Compared to standard networks with transformer feed-in, generator networks have an internal resistance that is approximately three times higher. As a result, the operation of consumers with non-linear load characteristics leads to significantly higher voltage distortions at the generator terminals. Figure 1 shows the waveforms of the voltages and currents in a generator network. The generator has a nominal output of 100 kVA. It is loaded by electronic dimmers and switch-mode power supplies with a total output of 40 kVA. The uneven connection of the mostly single-phase devices on the three phases results in a highly asymmetrical load on the generator. This is shown in the right-hand part of the "RMS current" diagram in Figure 2. The asymmetry results in a neutral conductor current (blue) that loads the cables and the generator star point.
The clearly visible voltage distortions increase the risk of damage to sensitive loads, resulting in mechanical stress on the generator and thermal overload of the stator windings. These negative effects are often counteracted by deliberately selecting an oversized generator.
Figure 1: Waveform of voltage (top) and current (bottom) without AHF
The grid analysis showed that the current consumption of the lights differed significantly between normal operation and battery operation. While the actual load current was shown in normal operation, a massive overlay of the resonance current was added in inverter operation. The supply voltage was so distorted that the inverter was not operating stably. As the Power Quality Measurement As shown, the resonance point shifts as each light is connected. As soon as eight lights are powered by the battery network, a significant harmonic current flows, which heats up the circuit breakers and thus promotes premature tripping.
Figure 2: Effective voltage (top) and effective current (bottom)
Solution
In the past, passive harmonic filters were primarily available for filtering voltage distortion. These so-called filter circuits are constructed as a series circuit of chokes and capacitors. To achieve sufficient filtering efficiency, a significant proportion of capacitive reactive power is accepted. However, generator networks, in particular, should not be operated in the capacitive range. The reason is illustrated in Figure 3. With a cos φ = 0.6 capacitive, for example, only 40 % of the generator's rated power can be queried. At higher loads, the maximum possible torque decreases, while the terminal voltage increases dangerously.
Figure 3: Maximum permissible power of a generator depending on the cos φ
Today, power electronics offer active filters that do not burden the generator with capacitive reactive power, but rather can fully compensate for the load's reactive power. These active filters are connected in parallel to the devices and eliminate their feedback with a response time of less than one millisecond. Active filters are universal troubleshooters in the power grid and can:
- Compensate inductive and capacitive reactive power
- Reduce harmonics
- Eliminate voltage distortions
- Balancing asymmetric loads
These electronic "load optimizers" are offered by PQ ENGINEERING with various filter currents from 15 A to 300 A and in a wide variety of designs: as wall-mounted units (AHF-W), as plug-in modules (AHF-R) for rack mounting, or as floor-standing units (AHF-C) in outdoor enclosures. Outdoor installation, in particular, simplifies use for temporary installations and events.
Figure 4: Test scenario with AHF-C-100 in outdoor housing
Advantages
By using an active filter (AHF), the frequently chosen oversizing of the unit can be avoided. The AHF's network cleaning offers the following advantages:
- Lower fuel consumption due to reduced counter torque on the generator
- Longer service life of the diesel engine and generator
- No damage to electronic devices
- Greater security of supply for connected consumers
- Optimal grid conditions with adjustable cos φ
- Symmetrical voltage
- Balanced phase currents with reduced neutral conductor current
And this is what the same generator network looks like when operated with an AHF: A significantly cleaner voltage compared to Figure 1 with symmetrical, sinusoidal current consumption.
Figure 5: Waveform of voltage (top) and current (bottom) with AHF
Figure 6: Active filter AHF-W for wall mounting and AHF-R for rack mounting
PQ ENGINEERING would like to thank the following for providing the test location and equipment:
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"By using AHF, we offer our customers added value while simultaneously protecting our systems from overload. A true win-win situation!" Ralf Hauschild Managing Director Sparks Filmtechnik GmbH |
