Current source and voltage source inverter

Current source and voltage source inverter are the two basic types of indirect frequency converters. Therefore, it might be very interesting to describe and compare both types. This article can also be seen as an amendment to the LCI vs VSI series [1] that already thoroughly described and compared high power solutions load-commutated inverter (LCI) and voltage source inverter (VSI).

Both kinds of frequency converters in subject can be compared using different criteria and aspects. Current source inverter (CSI) and voltage source inverter (VSI) are used in several applications such as power transmission (namely HVDC – High Voltage Direct Current transmission [2]), variable frequency drives [3] (VFD), grid interties [4, 5, 6], static compensation (STATCOM [7]), variable speed pumped hydro storage power plants [8], medium frequency transformers and many more. As our blog predominantly focuses on variable frequency drives, we will compare CSI and VSI technology from drive perspective.

Voltage source inverter (VSI) is a family of indirect frequency converters having monopolar capacitance in the dc link. This capacitor provides the inverter the voltage source character. Most common topologies within VSI group are:

– Two Level Inverter

– Neutral Point Clamped (NPC) Inverter

– Cascaded H-bridge (CHB) Inverter

– Flying Capacitor Inverter

– Modular Multilevel Converter (MMC, M2C, MMLC)

Two level inverter is dominant in low voltage drives (typically 120 to 690 V), but practically not used in medium voltage range due to its poor output quality.

Besides the two level inverter the topologies are commonly called “multi-level inverters”. Neutral Point Clamped (NPC) inverter is traditionally a 3-level inverter. However, in a hybrid combination of 3-level NPC modules in H-bridge connection a 5-level inverter is realized. Flying capacitor (FC) inverters are typically 5-level inverters. However, inverters with more levels, e.g. 7-level FC, are available on market. The complexity increases exponentially with number of levels (number of possible switching vectors, balancing of capacitors etc). Modular multilevel converter (MMC) is one of the latest developments. The cell based design is scalable in voltage and power. Number of levels can be quite high resulting in a very high quality of output waveform. The cell can be either low voltage based (low to medium power) or medium voltage based (high power for large drives, large STATCOMs, variable speed hydro applications, grid interties, HVDC etc).

Current source inverter (CSI) family is characterized by a reactor in dc link providing the inverter a current source behavior. This family is a bit smaller with two major representatives:

– PWM Current Source Inverter (PWM CSI)

– Load Commutated Inverter (LCI)

PWM CSI has self-commutated power semiconductors. The devices can be turned on and off. Therefore, a pulse width modulation is possible similar like in voltage source inverters. The semiconductors are switching with frequency defined by carrier frequency of PWM. However, for the commutation additional bipolar capacitors are needed on the AC side (between inverter and machine). For higher power the complete VFDs are hard paralleled.

Load commutated inverter (LCI) is based on classical thyristors. The thyristors are externally commutated. Therefore, the switching frequency is equal the load fundamental frequency. Load is a synchronous machine operating with capacitive power factor (overexcited). LCI does not need the additional bipolar capacitors. However, it only works together with a synchronous machine and is used mainly in high power range.

1. Type of power semiconductor

Voltage source inverters use self-commutated power semiconductors with anti-parallel diodes (either discrete diode or integrated in the same package). Such semiconductors are classified as asymmetric, i.e. reverse conducting. IGBT is dominant mainly in low and medium power range. High power medium voltage VFDs then use IGCT with superior power density and low losses.

Current source inverters require for their work semiconductors that are symmetrical, i.e. they can block voltage in both directions. For this purpose robust semiconductors of thyristor type.

PWM current source inverter drives use e.g. IGCT semiconductor. These semiconductors have higher current loadability (several kA) compared to IGBT, but the switching frequency is somewhat more limited (up to roughly 1 kHz).

Current source inverter of LCI type (see [1]) are thyristor based power converters. As the name says, the inverter is commutated by the load (external voltage) and the switching frequency is therefore equal fundamental frequency. When the motor operates at 35 Hz, the inverter side thyristors switch with 35 Hz frequency (i.e. modulation such as PWM is not possible) .

2. Power flow

Bi-directional power flow is possible for both current source and voltage source inverter. With a classical rectifier the realization of bi-directional CSI is significantly simpler than corresponding bi-directional VSI.

CSI reverses the voltage polarity in order to reverse the power flow. VSI needs to reverse the current as the dc voltage cannot be reversed (anti-parallel diodes in inverter section).

VFDs allowing bi-directional power flow are often called ‘regenerative’ as they allow to inject the energy from the load side into the supplying grid (e.g. during active braking).

3. Control characteristics

Voltage source inverters are from control perspective less problematic. In active front end voltage source inverter (AFE VSI) the active rectifier only controls (stabilizes) the voltage in the dc link. (Note: AFE can also control input power factor, but in most cases it is controlled to PF = 1.0).

Current source inverters for induction machines require quite demanding coordination of rectifier and inverter control loops. Current source inverter of LCI type is an exception having quite straight forward control.

4. Dynamics

Voltage source inverters can achieve high dynamic performance. The practical limitation is often given by the machine parameters. In case of output sine filter the dynamics is somewhat limited.

Current source inverters have a reactor in the dc link which negatively impacts dynamics. On the other hand, many applications do not require such high dynamics and CSI works satisfactorily.

5. Impact on the load (motor)

Voltage source inverters act on the machine with pulsed output voltage waveform that could lead to accelerated aging of insulation system. In case of active rectifier the same effect is seen on the grid side, e.g. on the input transformer. Therefore, the voltage rate of change is being limited by using a dv/dt filter or more costly sine filter. Modern multi-level VSI topologies minimize the impact on motor insulation.

6. Main filtering components

Voltage source inverters have as main filter and storage a monopolar capacitor, sometimes combined with a smaller reactor (especially in high power range).

Current source inverters with PWM modulation contain reactor in the dc link and bipolar capacitors on the AC side. Current source inverters of LCI type do not need the capacitors on AC side, but require an overexcited synchronous machine that provides the reactive power for commutation.

7. Power rating

Voltage source inverters are being manufactured from very small power (micro drives) up to medium/high power. Very high power can be reached with topologies using paralleling of inverter modules, cascaded connection or modern multi-level concept. The MMC platform can be scaled up to very high power which makes the topology suitable for applications such as large full converters for variable speed pumped hydro storage power plants, high power STATCOMs or HVDC transmission systems.

Current source inverters are suitable especially for medium/high (PWM CSI and LCI) and very high power (LCI). It is mainly due to their robust power semiconductors of thyristor type that are symmetrical from voltage blocking perspective. Besides that, those semiconductors allow high current loadability minimizing the parts count and maximizing the power density.