EE201800025A - A device for avoiding overvoltages due to the direction change of the energy flow in the nanogrid - Google Patents

Abstract

A device for avoiding overvoltages due to the direction change of the energy flow in the nanogrid is a frequency converter that connects nanogrid with the distribution grid. The frequency converter consists of bidirectional input and output inverters, a dc-link, energy storage and a voltage damping circuit. The input connects to the distribution grid while the output connects to the nanogrid. The voltage damping circuit can be a resistor matrix, a fixed voltage source or a dc-dc converter controlled load that consists of a passive resistor or energy storage.

Description

Technical field

The present invention belongs to the field of power electronics devices. More specifically, the invention relates to suppression circuits for suppressing overvoltages that occur when changing the direction of energy flow in a nanowire.

State of the art

The invention relates to the bidirectional control of energy flows in a nanogrid. A nanogrid is a small (up to 100 kW) low-voltage electrical system that can operate both in connection with the distribution network (on-grid) and autonomously (off-grid). The transition from one mode to another must occur automatically and in such a way that the voltage quality of the nanogrid does not suffer. A nanogrid consists of energy sources, storage devices and consumers. A nanogrid can be small buildings, apartment buildings, private houses and other small structures.

Storage devices and renewable energy sources used in today's buildings usually operate as voltage followers, i.e. they require a stable sinusoidal grid voltage to output energy. It is important that the grid voltage meets the quality required by the standards. In general, a deviation of 10% is enough for devices operating as voltage followers to switch themselves off. However, the nanogrid is decoupled from the distribution grid, which is why the corresponding sinusoidal voltage must be generated by a frequency converter located at the connection point. The remaining sources in the nanogrid simply operate as voltage followers.

Energy flows in the nanowire in two directions: from the frequency converter to the nanowire (C2N) and from the nanowire to the frequency converter (N2C). In the case of C2N, the frequency converter acts as a sinusoidal voltage source, and in the case of N2C, it acts as a controlled load consuming a sinusoidal voltage. The N2C mode is used when there is a surplus of energy in the nanowire that is needed

to the distribution grid or energy storage. The most critical moment is when the direction of energy flow changes. At this moment, the load on the frequency converter briefly drops to zero, i.e. it runs idle. In such a situation, a conventional frequency converter cannot stabilize its output voltage and a dangerous overvoltage is generated in the nanogrid.

An autonomous uninterruptible power supply (US4673826A) is known, which ensures a stable energy supply at its output. However, the disadvantage of the device is its unidirectional

output inverter. A bidirectional inverter cannot be implemented because there is no way to suppress the above-mentioned overvoltages in the dc bus and also in the nanogrid.

A three-phase ac-ac (alternating current-alternating current) series resonant power converter (US5010471A) is known, which allows bidirectional power control. The disadvantage of the device is that there is no DC link with a stable voltage that would act as an energy storage. Therefore, the device is not suitable for a nanogrid power converter.

A power supply unit (KR100964631B1) designed for electric railways that stabilizes the line voltage is known, which directs the excess energy generated in the train's contact line back to the network. This system is intended for powering rail transport with a grid voltage and is not suitable for bidirectional control of alternating current nanogrid energies.

A grid-connected energy management system is known, which also includes an energy storage device (US20110115295A1). The energy management system is able to control energy flows bidirectionally between sources and the grid. The invention notes that the DC bus between the storage device and the renewable energy source may require an additional voltage stabilizer. However, it is not specified what such a device should look like.

The closest to the invention in terms of technical nature is a special frequency converter (R. Majumder, “A hybrid microgrid with dc connection at back to back converters,” IEEE Trans. Smart Grid, vol.5, no.1, pp.251-259, 2014). As a rule, the nanogrid is connected to the distribution grid at a single connection point using a special frequency converter (see Figure 1). The task of such a frequency converter is to ensure the stability of the nanogrid voltage and frequency in all operating modes, i.e. on-grid, off-grid and transition mode from one to another. This frequency converter has a DC voltage intermediate link, which acts as an energy storage device, to which electrical energy storage devices (batteries, capacitors, etc.) can be connected. In this case, the frequency converter starts to work as an uninterruptible power supply (e.g. UPS), ensuring the energy stability and independence of the nanogrid from the distribution grid. The disadvantage of this solution is that when the direction of the energy flow changes, the load briefly drops to zero, i.e. it runs idle.

The essence of the invention

The aim of the invention is to develop a device that prevents the occurrence of overvoltages in a nanowire due to a change in the direction of energy flow.

The presented device is a special frequency converter for connecting the nanogrid to the distribution network and for suppressing short-term voltage peaks. The frequency converter includes bidirectional input and output inverters, a DC link, an energy storage device and a suppression circuit, with the suppression circuit being placed in the DC link. The input of the frequency converter is the distribution network side part and the output is the nanogrid side part. The suppression circuit can be a resistance matrix that includes active resistors, switching paths and an isolation switch, a fixed voltage source, which is a device that provides a constant voltage at its terminals for a short time and which includes a battery switch and an isolation switch, or a load that is continuously adjustable with a DC converter and an isolation switch, with the load being a passive resistor or energy storage device. Since a higher voltage in the DC link also causes an overvoltage in the nanogrid, a suppression circuit is switched on at the moment of no-load operation to either dissipate or store excess energy.

List of drawings

Figure 1 shows a prior art frequency converter for connecting a nanogrid to a distribution network.

Figure 2 shows a frequency converter for connecting a nanogrid to a distribution network.

Figure 3 shows a diagram of a device (frequency converter) according to the invention, which includes a damping circuit A3 based on a resistance matrix.

Figure 4 shows the damping circuit A3, which is based on a fixed voltage source.

Figure 5 shows the damping circuit A3, which is based on a continuously adjustable load with a DC-DC converter, where the load is a passive resistor (a) or an energy storage device (b).

Example of carrying out the invention

The presented device for preventing overvoltages in a nanowire due to a change in the direction of energy flow includes bidirectional inverters: input inverter A1, output inverter A4, DC link, energy storage A2 and damping circuit A3. Damping circuit A3 is placed in the DC link.

The nanogrid is connected to the distribution grid through a bidirectional frequency converter, which includes an input inverter A1, an output inverter A4, a DC link, and an energy storage device A2, where a damping circuit A3 is added to the DC link of the frequency converter, which prevents the occurrence of overvoltages at the output of the converter due to a change in the direction of the energy flow, both in the nanogrid and in the DC link of the frequency converter. The quality of the sinusoidal voltage in the nanogrid is ensured by the frequency converter. If there is a surplus of energy in the nanogrid, the frequency converter directs it either to its energy storage device or back to the distribution grid. The most critical moment is when the energy flow direction changes from the C2N mode to the N2C mode. At this moment, the load on the frequency converter briefly drops to zero and the voltage in the DC link of the frequency converter starts to increase. The higher voltage in the DC link also causes an overvoltage in the nanogrid. To prevent overvoltages, the damping circuit A3 is switched on during idling, which either dissipates or stores excess energy.

There are three options for implementing the damping circuit A3: a resistance matrix (Figure 3), a fixed voltage source (Figure 4), and a continuously adjustable load with a step-down voltage converter (Figure 5).

The damping circuit A3 based on the resistance matrix consists of 1...n active resistances R2,R3, Rn, switches S2...Sn and an isolation switch S1. The value and number of resistors must be selected according to the power and the size of the storage device. The control system decides, according to the size of the energy flow, how many resistors must be switched on at the same time. At the same time, using the isolation switch S1, the control system disconnects the output inverter A4 from the energy storage device A2.

The damping circuit A3 based on the fixed voltage source B1 can be a small battery, supercapacitor, flywheel, stabilized power supply, or similar device that provides a constant voltage at its terminals for a short time. Analogously to the resistance matrix, the control system turns on the isolation switch S1 and disconnects the output inverter A4 from the energy storage device A2. At the same time, the switch S2 is turned on, which connects B1 to the output inverter A4.

The damping circuit A3 based on a load controlled by a DC-DC converter includes a control system, a DC-DC converter A5 and a load, which can be a resistor R4 or an energy storage device B2. When an energy surplus occurs in the DC-DC link, the control system interrupts the energy flow to the DC-DC link by opening the switch S1 and simultaneously starting A5, which smoothly sets the load to such a high value that a stable DC-DC link voltage is ensured.

Claims (7)

1. A device for preventing overvoltages in a nanowire due to a change in the direction of energy flow, comprising bidirectional inverters (A1, A4), a DC voltage link and an energy storage device (A2), characterized in that a damping circuit (A3) is arranged in the DC voltage link. 2. Device according to claim 1, characterized in that the damping circuit (A3) is a resistance matrix comprising 1...n active resistances R2, R3, Rn, switch paths S2...Sn and an isolation switch S1. 3. The device according to claim 1, characterized in that the damping circuit (A3) is a fixed voltage source, which is a device that provides a constant voltage at its terminals for a short time and which includes a battery switch S2 and an isolation switch S1. 4. The device according to claim 3, characterized in that the device is a small battery, a supercapacitor, a flywheel or a stabilized power supply. 5. The device according to claim 1, characterized in that the damping circuit (A3) is a load continuously adjustable by a DC-DC converter, wherein the damping circuit (A3) includes a control system, a DC-DC converter (A5), a load and an isolation switch S1. 6. The device according to claim 5, characterized in that the load continuously adjustable by the DC-DC converter is a passive resistor. 7. The device according to claim 5, characterized in that the load continuously adjustable by the DC/DC converter is an energy storage device.