Practical applications of static compensators (III) [Part 5/5: Static var compensators (SVC) for hot rolling mills]

Static var compensators

Static var compensators (SVC for short), also called static var systems (SVS for short) have been around since the 1970s. Description of their topology and operating principle can be found as far back as 1976. Since then, a large number of SVCs have been commissioned around the world, making SVCs a well-proven technology.

They were developed thanks to the technological evolution of thyristor valves in the 1970s to take care of the problems in the electric power system created by fast changing reactive power demand or by highly dynamic loads that conventional solutions like mechanically switched capacitor banks (MSC for short), mechanically switched reactors (MSR for short) or synchronous condensers could not handle.

SVCs can be applied to medium or large installations in a range of segments. They have several high voltage potential applications where their use offers many benefits including equipment or facilities where reactive power and/or power factor fluctuate rapidly or in big steps, installations with solar inverters or wind turbine generators, transmission and distribution lines, electrolysers for hydrogen production, railway electrification systems, and loads with low power factor, to name a few.

Functions

SVCs combine the technical advantages of thyristor switched compensation devices with the cost-effectiveness of mechanically (contactor or circuit breaker) switched compensation devices to form an economical real-time compensator with a single controller without the drawbacks of conventional solutions.

Modern SVCs can take care of several power quality problems, provide ancillary services and support the development of clean energy by combining different control functions in a single device.

Connection

An SVC is a power electronics-based shunt compensation device connected in 3-wire electric power systems in parallel with the equipment generating the power quality problems or that has issues to comply with grid code and energy efficiency requirements. The SVC behaves as a controlled impedance providing any kind of current waveform (in terms of phase, amplitude and frequency) in real time (typical reaction time is under 5 milliseconds and typical overall response time is under 10 milliseconds).

The most common operating voltage range for SVCs is 3 kV up to 36 kV as they are built using high voltage thyristor valves. It is possible to connect them to higher voltages using a suitable step-up transformer.

SVCs for hot rolling mills

Hot rolling mills are some of the facilities in the steel manufacturing process with the highest power consumption. The starting material of hot rolling mills is usually large pieces of metal, like semi-finished casting products, such as ingots, slabs, blooms and billets. These heated metals are passed between rollers to produce sheets or bars of a required cross section and shape.

Cycloconverters (CCV for short) and rectifiers have been used successfully in hot rolling mills for many years for meeting rolling mill drive requirements including speed control accuracy, rapid speed control response and fast load recovery.

Typically, several high power drives are installed in hot rolling mills, including 6 and 12-pulse cycloconverters with synchronous motors and 6 and 12-pulse rectifiers with DC motors. All these motors are used to move the rolls in the stands of the hot rolling mill. Because of the technical characteristics of these type of power electronic drives, there is an important reactive power consumption, as well as harmonics are produced during their operation. A high reactive power demand increases the losses in the electric power system, reduces the power transmission capability of transmission and distribution lines and causes that the voltage profile of the system is outside the permissible values. In addition, power quality disturbances like voltage sags could make the drives disconnect, bringing several economical and technical problems for the process.

Requirements

Background

The fast reactive power demand of the hot rolling mill during its operation causes system voltage fluctuations and voltage drops decreasing production efficiency and lowering the power factor of the installation. The cycloconverters and rectifiers also cause harmful harmonics affecting the operation of the whole installation.

The requirements of the hot rolling mill owner are to stabilise the system voltage under varying load conditions which occur during the operation of the hot rolling mill, mitigate voltage sags, improve the energy efficiency by improving the power factor and mitigate the harmonics in the installation.

System description

The hot rolling mill main substation is fed by a 220/33 kV power transformer. The PCC is considered in the main substation busbar, at 33 kV, where all the drives are connected to through power transformers.

The hot rolling mill has a roughing stand rated 30 MW with two synchronous motors driven by 12-pulse cycloconverters. In addition, the finishing rolling mill consists of 8 stands with a total rated power of 72 MW, some of them with synchronous motors driven by 6-pulse cycloconverters and, the other ones with DC motors?driven by 12-pulse rectifiers.

Solution

Analysis

In order to design the needed solution, it is necessary to do a power quality analysis of the installation. Through this analysis, active and reactive power flows will be evaluated as well as the main power quality problems will be identified and quantified. In addition, it is needed to develop electric models of the installation and the possible solutions to evaluate the performance in different conditions and with different harmonic contents. These models allow the analysis of the voltage stability and the transients of the system.

Proposed solution

Based on the analysis of the measurements, it is possible to dimension a solution that would comply with customer’s requirements. The solution is an SVC installed at 33 kV in parallel with the hot rolling mill with a nominal overall rating of 0 Mvar (inductive) to 80 Mvar (capacitive).

The main purpose of the SVC is to stabilise the 33 kV bus voltage under the varying load conditions and mitigate possible voltage sags which occur during the operation of the mill. The SVC can also improve the power factor of the installation within the requirements of the owner.

The SVC is formed by a thyristor controlled reactor, harmonic filters and a control and protection system. The harmonic filters rated 40 Mvar, 20 Mvar and 20 Mvar are tuned to the 5th, 7th and 11th harmonic orders in order to suppress harmonics generated by the hot rolling mill drives and by the thyristor controlled reactor of the SVC, thereby preventing all these harmonics from flowing into the surrounding parts of the electric power system.

Based on the values monitored, the following functions are proposed for the SVC.

Conclusions

Besides being a source of harmonics, a rolling mill as a load on the electric power system is generally characterised by a high and fluctuating consumption of reactive as well as active power. As a consequence, rolling mills will give rise to voltage fluctuations in the feeding network, the severity of which being dependent on the relationship between the size of the rolling mill and the strength of the network.

Static var compensators can solve all these problems stabilising the system voltage and reducing the harmonic distortion of hot rolling mills. Furthermore, they can improve the power factor of these installations, thereby relieving the electric power system of the reactive power burden and contributing to a considerable decrease of the running costs of hot rolling mills.

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About the author:

Pedro Esteban is a versatile, multicultural and highly accomplished marketing, communications, sales and business development leader who holds since 2002 a broad global experience in sustainable energy transition including renewable energy, energy efficiency and energy storage. Author of over a hundred technical publications, he delivers numerous presentations each year at major international trade shows and conferences. He has been a leading expert at several management positions at General Electric, Alstom Grid and Areva T&D, and he is currently working at Merus Power Plc.