EP4335012A1

EP4335012A1 - System for damping mains frequency variations and/or mains voltage variations in an electricity transmission network

Info

Publication number: EP4335012A1
Application number: EP22727432.1A
Authority: EP
Inventors: Francesco PALONE; Gianluigi GEMELLI; Luca BUONO; Roberto ZAOTTINI; Cosimo PISANI
Original assignee: Terna SpA
Current assignee: Terna SpA
Priority date: 2021-05-05
Filing date: 2022-05-05
Publication date: 2024-03-13
Also published as: WO2022234497A1; IT202100011459A1

Abstract

System for damping frequency variations and/ or voltage variations in a high voltage electricity transmission network, the system (3) comprising: a high to medium voltage transformer (5); converter cabinets (6), each of which comprises an electronic power converter assembly (7), the input (8) of which is connected to a medium voltage winding of the transformer (5), and devices (9) for measuring voltage and current at said input (8); banks of resistors (10), each of which is connected to the output (11) of a respective electronic power converter assembly (7) and a processing and control unit (12) configured to calculate, as a function of measured voltage and current, the active power (Pi) absorbed by each bank of resistors (10) and the reactive power (Qi) exchanged between each converter cabinet (6) and the transmission network (1) and to control each power electronic converter assembly (7) according to the difference (ΔP) between the active power (Pi) and a reference active power (Pref) and the difference (ΔQ) between the reactive power (Qi) and a reference reactive power (Qref).

Description

"SYSTEM FOR DAMPING MAINS FREQUENCY VARIATIONS AND/OR MAINS

VOLTAGE VARIATIONS IN AN ELECTRICITY TRANSMISSION NETWORK"

Cross-Reference to Related Applications

This Patent Application claims priority from Italian Patent Application No. 102021000011459 filed on May 5, 2021, the entire disclosure of which is incorporated herein by reference.

Technical Field of the Invention

The present invention relates to a system for damping mains frequency variations and/or mains voltage variations in an electricity transmission network.

In particular, the present invention finds advantageous, but not exclusive application in an electricity transmission network powered by electric generators from renewable energy sources, which the following description will explicitly refer to without thereby losing generality.

State of the Art

The de-carbonisation process decided by the Kyoto Protocol involves the progressive replacement of thermal power plants with electric generators from renewable energy sources, which are interfaced to the electricity transmission network through electrical power converters and supply electricity in a typically non-programmable manner. The increasing penetration of non-programmable renewable sources that is currently ongoing leads to a decreasing inertia of an electricity production system.

This inertia is, in fact, linked to the rotating masses of the turbine-alternator rotors of both thermo-electric and hydro-electric generators connected to the electricity transmission network.

The inertia of an electricity production system has a beneficial effect on the electricity transmission network when a fault, e.g. a short circuit, or a disturbance, e.g. an insertion or loss of a large load, occurs somewhere in the same network.

The occurrence of a fault or disturbance in the electricity transmission network generates a mains frequency transient, i.e. a so-called angular pendulum, which, if of a certain magnitude, can trigger automatic disconnection devices to switch on, resulting in the disconnection of part of the load. An angular pendulum is a periodic variation of the mains frequency generally less than 1 Hz. The inertia of rotating machines makes it possible to counteract a mains frequency transient produced by a fault or disturbance event up to half a second after the event, thus contributing to the stability and continuity of the electricity supply service.

Another disadvantage of the increasing use of renewable sources of electricity is related to their non programmability, which does not allow to have high power reserve margins and therefore reduces the ability to regulate the mains voltage, especially in the high voltage nodes of the transmission part of the electricity transmission network.

To mitigate mains voltage variations due to mains faults or disturbances, it is known to connect either a rotating compensator or a static compensator to the electricity transmission network.

A rotating compensator consists of a synchronous generator whose rotor is not connected to a motor to a turbine but is driven by an electronic power converter. The rotating compensator exchanges reactive power with the electricity transmission network and consequently allows the voltage to be regulated at the connection point. However, although this solution contributes to the increase in the inertia of the electricity production system, it does not make it possible to compensate for the variations in mains frequency in a controllable manner.

A static compensator comprises an electronic converter for interfacing with the electricity transmission network. The static compensator allows to modulate the reactive power exchange and thus allows to dampen the variations in mains frequency and regulate the mains voltage. A static compensator may also comprise an electrical energy storage system, typically electrochemical, in order to be able to inject or absorb active power into or from the electricity transmission network in addition to the exchange of reactive power.

However, this solution has multiple disadvantages related to the storage system: a relatively limited service time; an aging proportional to the charge and discharge cycles; and a constant absorption of active power for the supply of auxiliary systems (heating and/or conditioning) that have the purpose of avoiding an early aging of the storage system.

Subject and Summary of the Invention

Aim of the present invention is to realise a system for damping mains frequency variations and/or mains voltage variations in an electricity transmission network, which system is free of the drawbacks described above and, at the same time, is easy and inexpensive to implement.

In accordance with the present invention, there is provided a system for damping mains frequency variations and/or mains voltage variations in an electricity transmission network, according to what is defined in the appended claims.

Brief Description of the Drawings

The present invention will now be described with reference to the attached drawings, which show a non-limiting embodiment thereof, wherein:

- Figure 1 shows a single-line diagram of the system for damping mains frequency variations and/or mains voltage variations, according to the present invention; and

- Figure 2 shows an example of installation of the system of Figure 1.

Detailed Description of Preferred Embodiments of the

Invention In Figure 1, 1 generically denotes an electricity transmission network comprising high voltage busbars 2, and 3 denotes the system of the present invention for damping mains frequency variations and/or mains voltage variations in the electricity transmission network 1. The system 3 comprises a transformer bay 4, which is connectable to the high voltage busbars 2 and comprises a high to medium voltage transformer 5, and a plurality of converter cabinets 6, each of which comprises an electronic power converter assembly 7 of the AC/DC or AC/AC type having an input 8 connected to a respective medium voltage winding of the transformer 5 and voltage and current measuring devices 9 for measuring voltage and current at the input 8. The system 3 comprises a plurality of banks of resistors 10, each of which is connected to the output 11 of the electronic power converter assembly 7 of a respective converter cabinet 6. The transformer 5 comprises a number of medium voltage windings equal to the number of converter cabinets 6. The output 11 of each electronic power converter assembly 7 provides medium voltage electrical power.

In the embodiment example considered, high voltage means a voltage equal to 150 kV and medium voltage means a voltage equal to 3 kV.

The system 3 comprises a processing and control unit 12 configured to calculate, as a function of voltage and current measured in each converter cabinet 6, the instantaneous active power Pi absorbed by each bank of resistors 10 and the instantaneous reactive power Qi exchanged between each converter cabinet 6 and the electricity transmission network 1 and to control each electronic power converter assembly 7 as a function of a difference DR between the instantaneous active power Pi and a reference active power Pref and a difference AQ between the instantaneous reactive power Qi and a reference reactive power Qref.

The voltage and current measuring devices 9 consist of current measuring transformers of known type.

The transformer bay 4 further comprises further voltage and current measuring devices 13 for measuring currents at the input 14 of the transformer 5 and voltages on the medium voltage windings of the transformer 5. The voltage and current measuring devices 13 consist of current measuring transformers of known type.

The reference active power Pref and the reference reactive power Qref are obtained in two alternative manners.

In a first manner, the reference values Pref and Qref are obtained in a centralised manner. The electricity transmission network 1 comprises a plurality of monitoring nodes (not shown) in each of which a Phase Measurement Unit (PMU) is installed, which measures the amplitude, the phase of the nodal voltages, i.e. the phase shift of the node voltages with respect to the phase of a reference node, and the frequency of the nodal voltages. When there is a variation in the phase of the nodal tensions, it means that an angular pendulum is taking place. The measured values of amplitude, phase and frequency of the nodal voltages are sent to a control centre (not shown) of the electricity transmission network operator 1. The operator's control centre processes the measured phase and frequency values of the nodal voltages to detect the presence or absence of an angular pendulum and, if present, to calculate the reference values Pref and Qref as a function of the aforesaid measured values. The reference values Pref and Qref are then sent to the system 3 via a communication system (not shown) of the electricity transmission network operator 1.

In the second manner, the reference values Pref and Qref are calculated locally by the processing and control unit 12. In particular, the processing and control unit 12 is configured to measure frequency variations Dί and phase variations Df of the voltages and currents measured by the voltage and current measuring devices 13 of the transformer bay 4, and to calculate the reference active power Pref and the reference reactive power Qref as a function of frequency variations Dί and/or phase variations Df. The second manner of calculating reference values Pref and Qref is advantageous as it works even in the absence of the operator's communication system.

Each electronic power converter assembly 7 comprises a first power converter stage 15 of AC/DC type, and a second power converter stage 16 of DC/DC or DC/AC type connected downstream of the first power converter stage 15. The converter cabinet 6 comprise a disconnector device 17, e.g. manually operated, for connecting the output of the second power converter stage 16 to the respective bank of resistors 10.

Advantageously, the electronic power converter assembly 7 comprises a bank of capacitors 18 connected between the output of the power converter stage 15 and the input of the power converter stage 16. Specifically, the output of the power converter stage 15 is connected to the input of power converter stage 16 via a DC-bus 19. The bank of capacitors 18 is connected in parallel to the DC-bus 19. The power converter stage 15 comprises a three-stage AFE (Active Front End) type impressed voltage converter with neutral point clamping topology.

In accordance with a first embodiment, the power converter stage 16 is of DC/DC type and comprises a Chopper converter for modulating the current circulating on the bank of resistor 10 and thus for modulating the instantaneous active power Pi absorbed by the bank of resistors 10. This embodiment makes it possible to obtain a converter cabinet 6 having relatively small costs and dimensions.

In accordance with a second embodiment, the power converter stage 16 is of DC/AC type and comprises an impressed current inverter for modulating the current circulating on the bank of resistors and thus for modulating the instantaneous active power Pi absorbed by the bank of resistors 10. This embodiment allows standard components to be used and thus easily obtainable on the market.

The processing and control unit 12 is configured to control each electronic power converter assembly 7 as follows. The power converter stage 15 is controlled so as to regulate the DC voltage at its output, i.e. the voltage on the DC-bus 19, according to the reactive power difference AQ, and in particular so as to minimise the reactive power difference AQ, and the power converter stage 16 is controlled so as to regulate the current circulating in the bank of resistors 10 according to the active power difference DR, and in particular so as to minimise the reactive power difference DR.

The power converter stage 15 thus enables to modulate the reactive power exchange between the relative converter cabinet 6 and the electricity transmission network 1. The modulation of the exchanged reactive power contributes, in a controllable way, to the damping of voltage variations in the electricity transmission network 1. The bank of capacitors 18 allows the voltage on the DC-bus 19 to be kept stable during the regulation of the reactive power. The power converter stage 16, on the other hand, allows the active power absorbed by the relative bank of resistors 10 to be modulated. The modulation of the active power absorbed contributes, in a controllable way, to the damping of the frequency variations in the electricity transmission network 1.

By acting on the disconnector device 17 to disconnect the bank of resistors 10 from the electronic power converter assembly 7, the circulation of current is interrupted in the bank of resistors 10 and therefore only a modulation of the exchanged reactive power can be carried out. In this case, only the power converter stage 15 is powered and controlled.

The plurality of converter cabinets 6 defines a modular structure that allows the damping capacity of the system 3 to be dimensioned in the active-reactive power plane using commercial components for the electronic power converter assembly 7.

Again with reference to Figure 1, the system 3 also comprises a plurality of auxiliary services (not shown) of known type, such as control panels, lighting systems, uninterruptible power supplies, fans, etc., and a number of medium voltage to low voltage transformers, indicated with 20, connected to as many medium voltage windings of the transformer 5, to supply the auxiliary services through switches and/or disconnectors, indicated overall with 21. In the embodiment example considered, low voltage means 400 V.

With reference to Figure 2, the system 3 comprises one or more housing cabinets 22 for the banks of resistors 10. In particular, each bank of resistors 10 comprises a plurality of resistive elements 23, which are arranged neatly inside a relative housing cabinet 22.

The resistive elements 23 of each bank of resistors 10 are made of a metal having a low coefficient of variation of resistivity with temperature, e.g. austenitic stainless steel. Each bank of resistors 10 is dimensioned to operate at medium voltage and absorb an active power of a few MW and therefore dissipates a lot of heat.

In order to reduce the overall floor space, the resistive elements 23 of each block of resistors 10 are stacked on top of each other within the relative housing cabinet 22 and the latter comprises a forced ventilation system 24. In particular, the forced ventilation system 24 comprises an air inlet 25 for the entry of fresh air into the housing cabinet 22, an outlet duct 26 for the exit of hot air from the housing cabinet 22, and a fan 27 arranged in the housing cabinet 22 so as to create a flow of air circulating from the air inlet 25 to the outlet duct 28 lapping the surface of the resistive elements 23. The forced ventilation system 24 is powered through the transformers 20 (Figure 1).

The resistive elements 23 of each bank of resistors 10 are circuitally connected to each other through a system of horizontal busbars 28 arranged next to the housing cabinets 22. As regards to the system of horizontal busbars 28, two vertical uprights of a frame supporting the horizontal busbars of the system of horizontal busbars 28 are visible in Figure 2.

The main advantage of the system 3 described above is to contribute in a controllable way to the damping of the variations of mains frequency and, independently, to the damping of the variations of mains voltage, thanks to the electronic power converter assembly 7 controlled according to the active power difference DR and according to the reactive power difference AQ. In particular, the power converter stage 15 allows to contribute in a controllable way to the damping of the variations of mains voltage, while the other power converter stage 16 allows to contribute in a controllable way to the damping of the variations of mains frequency.

Furthermore, the system 3 is easily configurable, by acting on the disconnecting device 2, to contribute only to the damping of the variations of the mains voltage.

Finally, the system 3 has a modular structure that allows dimensioning the damping capacity of the variations of mains frequency and variations of mains voltage while using commercial-grade components.

Claims

C LA IM S

1. A system for damping mains frequency variations and/or mains voltage variations in an electricity transmission network (1) comprising high voltage busbars (2), the system (3) comprising: a transformer bay (4), which is connectable to the high voltage busbars (2) and comprises a high to medium voltage transformer (5); a plurality of converter cabinets (6), each of which comprises electronic power converter means (7) of the AC/DC or AC/AC type having an input (8) connected to a respective medium voltage winding of the high to medium voltage transformer (5) and comprises first voltage and current measuring means (9) for measuring voltage and current at said input (8); a plurality of banks of resistors (10), each bank of resistors (10) being connected to an output (11) of the electronic power converter means (7) of a respective converter cabinet (6); and processing and control means (12) configured to calculate, as a function of voltage and current measured in each converter cabinet (6), the instantaneous active power (Pi) absorbed by each bank of resistors (10) and the instantaneous reactive power (Qi) exchanged between each converter cabinet (6) and the electricity transmission network (1), and to control said electronic power converter means (7) of each converter cabinet (6) according to a difference (DR) between the instantaneous active power (Pi) and a reference active power (Pref) and a difference (AQ) between the instantaneous reactive power (Qi) and a reference reactive power (Qref).

2. System according to claim 1, wherein said transformer bay (4) comprises second voltage and current measuring means (13) for measuring currents at the input (14) of the high to medium voltage transformer (5) and voltages on the medium voltage windings; said processing and control means (12) being configured to measure frequency variations (Af) and phase variations (Df) of the voltages and currents measured by the second voltage and current measuring means (13) and to calculate said reference active power (Pref) and reference reactive power (Qref) as a function of frequency variations (Af) and/or phase variations (Df).

3. System according to claim 1 or 2, wherein said electronic power converter means (7) comprise a first power converter stage (15) of AC/DC type, a second power converter stage (16) of DC/DC or DC/AC type connected downstream of the first power converter stage (15); each converter cabinet (6) comprising a disconnector device (17) for connecting an output of the second power converter stage (16) to the respective bank of resistors (10).

4. System according to claim 3, wherein said electronic power converter means (7) comprise a bank of capacitors (18) connected between the output of the first power converter stage (15) and the input of the second power converter stage

5. System according to claim 3 or 4, wherein said processing and control means (12) are configured to control the first power converter stage (15) so as to regulate the DC voltage at the output of the first power converter stage

(15) according to said difference (Dz)) between the instantaneous reactive power (Qi) and a reference reactive power (Qref) and to control the second power converter stage

(16) in order to regulate the current circulating in said bank of resistors (10) according to said difference (DR) between the instantaneous active power (Pi) and a reference active power (Pref).

6. System according to any one of claims 3 to 5, wherein said first power converter stage (15) comprises an impressed voltage converter of AFE type.

7. System according to any one of claims 3 to 6, wherein said second power converter stage (16) is of DC/DC type and comprises a Chopper converter for modulating the current circulating on the bank of resistors (10). 8. System according to any one of claims 3 to 6, wherein said second power converter stage (16) is of DC/AC type and comprises an impressed current inverter for modulating the current circulating on the bank of resistors (10).

9. System according to any one of claims 1 to 8, and comprising at least one housing cabinet (22) for at least

16 one bank of resistors (10) of said plurality of banks of resistors (10); each bank of resistors (10) comprising a plurality of resistive elements (23) and the housing cabinet (22) comprising a forced ventilation system (24) for cooling the resistive elements (23).