This document is an excerpt of Master Thesis "Static Watt Compensation System Design: Modeling, Control Strategies, and Component Dimensioning" by Carlos José Arias Bandry. Mr. Bandry finished his job in collaboration with Department of Electromagnetic Engineering KTH Royal Institute of Technology And
ABB AB, FACTS in August 2016.

The full text can be downloaded at http://www.diva-portal.se/smash/get/diva2:1039097/FULLTEXT01.pdf

Abstract

Stainless steel is a widely used material, from basic household appliances to construction material in large buildings. Nickel is one of the components required to produce stainless steel, and one of its production sources is nickel pig iron.

Nickel pig iron smelting facilities commonly use submerged-arc furnaces (SAFs) to extract the nickel from the ferronickel ores due to their high-temperature smelting capabilities. However, these furnaces present a highly fluctuating electric load to the power system as a result of the loss of arc on their electrodes being part of their normal operation.

The scope of this project is focused on weak islanded power systems with no interconnection with the local utility, meaning that their installed power generation is the solely responsible for the system stability and power supply.

A power system with significant power consumption drops within short time frames is not able to recover stability when the power generators are incapable of following this demand function, and this is the reason why an active power compensation system is required.

This power compensation system, together with a conventional static var compensator, could provide a good solution to address both the stability and power quality issues on these types of islanded power networks.

An active power compensation device is to be designed in this project, including both its components and its control strategies.

The power system is to be modeled and analyzed both in normal operation and in the presence of transient events. Both a technical and a cost analyses are the base to perform an assessment of the functionality and profitability of the active power compensator, drawing conclusions and recommendation for further development of the system.

Abbreviations

SAF/EAF submerged-arc furnace/electric-arc furnace SWC static watt compensator

SVC static var compensator

NPI nickel pig iron

OLTC on-load tap changer

FACTS flexible AC transmission systems

TCR thyristor-controlled reactor

AC alternating current

FFT fast Fourier transform

BESS battery energy storage system

MRO maintenance, repair and operations

1. Introduction

The current technology surrounding the power systems provides a suitable infrastructure to develop small and medium-sized autonomous power networks in remote areas.

Common examples of these networks are mining facilities, which harvest natural resources, such as coal, natural gas, and nickel among others, occasionally located at inaccessible regions for the power transmission grid.

Islanded power networks with weak interconnections to the transmission grid or local power generation have to constantly deal with frequency fluctuation problems due to partial or full load shedding (due to switching or failure).

In the specific case of mining facilities, it is of particular importance to guarantee the controlled operation of the whole network. A single disturbance or failure could lead to load shedding, resulting in loss of the system power stability followed by a complete blackout of the power system for the mine.

In consequence, a cost-efficient and optimized solution is required to provide a proper active power compensation system capable of mitigating considerable active power swings to retain the stability and power quality of the islanded system.

1.1 Project background

Stainless steel is a widely used material due to its resistance to corrosion and staining and its low maintenance requirement. It is utilized as a material in many applications, ranging from basic household cutlery to power transmission and distribution components, and to the automotive and aircraft industries.

Nickel is one of the components required to produce stainless steel, and two thirds of the world’s nickel production is destined to stainless steel manufacture [1]. Nickel pig iron (NPI) is a low grade ferronickel which offers a cheaper alternative to pure nickel for stainless steel manufacture [2].

The power generation sources found on the already operational NPI smelting facilities within the scope of this project are a mix between diesel and coal, with a significant amount of diesel generation needed to handle the submerged-arc furnaces’ (SAFs’) load variations and frequent load drop caused by arc loss.

Considering the drop in nickel price in the last few years, it is of particular interest to reduce the operating costs of the production plant. Even though the diesel generation has a higher controllability, it also has a higher operating cost. Therefore, solutions to reduce the use of diesel generation considering an increase in generation from the coal- fired power plant are desirable.

The implementation of an active power compensation system could provide additional tools for the power generation mix optimization, resulting in efficient usage of both the diesel and the coal-fired generators. Thereby, the operating costs of the ferronickel plant can be reduced.

1.2 Submerged-arc furnaces

Submerged-arc furnaces (SAFs) are implemented for material purification and reduction processes, being the most important component of a NPI smelting facility.

The most common physical arrangement consists of a circular bath with three vertical electrodes arranged in a triangle, as shown in figure 1 below [3]:

Figure 1. Submerged-arc furnace scheme

The concept behind the SAFs mode of operation is the electric resistance heating. Both the NPI ores and the slag are subjected to high temperatures as a result of an electric current flowing between the electrodes and the furnace hearth [3].

Figure 2. Submerged-arc furnace in operation

There are two main sources of heating in this process: electric arc generated between the electrode tip and the hearth, and the electric current flowing through the charge (nickel ores) itself.

The SAF’s normal mode of operation presents a highly fluctuating load profile, characterized by two important aspects:

First, the nature of the melting process generates a constant variation of the equivalent resistance of the burden to the electric current. This could be a consequence of burden drop or reshaping as well as changes on the resistivity of the material due to impurities of the ore.

Second, the recurrent loss of electric arc results in a power load loss on the system. The arc produced between the electrode and the hearth is surrounded by an unstable

and dynamic melting mass of material, which could extinguish the electric arc at any random moment.

Whenever the arc is lost the charge repositions itself, causing the loss of a major percentage of the load within a few seconds time frame until the arc is formed again and the melting process is able to continue. AC generators are not able to handle this type of fast active power variation with such a steep loading ramp if the SAFs are a significant part of the generator’s load, resulting in tripping of the generation units and loss of system stability.

1.3 Motivation

Major active power swings generated due to the fluctuating nature of the SAFs operation constitute a critical issue for the power stability of the system. Having a load reduction of 80% within a time frame of only seconds can lead to a preventive disconnection of the generators by their protection system to avoid damage or over speeding.

The implementation of an active power compensation system, here referred to as a static watt compensator (SWC), provides mitigation of large power fluctuations, resulting in a more controllable and secure power system operation.

Moreover, the addition of the SWC to the islanded power network of the NPI smelting facility allows fuel optimization, both diesel and coal, resulting in a cost reduction for the ferronickel smelting plant owner.

1.4 Purpose

The SWC’s main purpose is to compensate the power consumption deficit due to the loss of electric arc within the SAFs.

This system also provides load ramping capabilities, not possible to follow by coal- based generation alone, when a SAF is disconnected due to a failure or maintenance. The SWC can also be implemented to ramp up/down smoothly while starting- up/shutting-down the SAFs.

In the simple form studied here, the SWC is a system that wastes energy for few seconds in a high-power resistor, but provides system stability while running steam generators. As a consequence, less diesel is burned (replaced by coal) and the operating cost of the smelting facility is reduced.

1.5 Aim

The aim of this thesis is to design an active power compensation system along with its control strategy for a NPI smelting facility. In addition to that, a requirement set list is to be developed for the component dimensioning of the SWC.

For this design, the system is to be analyzed both in steady state (normal and contingency operation modes) as well as during transient events such as faults. Nevertheless, both the detailed modelling of the system network and the dynamic analysis of the generators are outside the scope of this project.

A detailed description of the system to be studied can be found in chapter 3: study case, and a detailed description of the different scenarios to be modelled can be found in chapter 4: SWC system development.

4. SWC system development

The power system modelling and simulation on this project is to be implemented in the PSCADTM/EMTDCTM software for power system transient simulations.

This development process can be divided into four major stages: study case, SWC components, thyristor-firing circuits, and control scheme.

A significant assumption made when modelling the system, and for the development of the SWC, is the existence of an SVC installed together with the SWC and the SAF on each bus bar. The SVCs are assumed to guarantee the unity power factor, power balance, and the rated voltage value of 33 kV at the furnace bus bars. The specific type of SVC is yet to be defined and it is not part of the scope of this project.

4.1 System model

The power network is conformed three major areas: generation, SAFs, and other loads as it is shown in figure 6 below.

Figure 6. NPI smelting facility power network modeled in PSCAD

4.2 SWC components

With only active power compensation being the main goal of the SWC, the implementation of a predominately-resistive impedance is considered for compensating the drop in active power consumption due to the arc loss.

The SWC concept is based on the Thyristor-Controlled Reactor (TCR) scheme, only considering a resistor in series with the inductor. The principle behind the SWC is firing the thyristors at a phase angle to consume the specific amount of active power required to compensate the SAF active power consumption drop.

A simple RL circuit is analyzed to obtain the expression of the RL-branch current as a function of time. The simple RL circuit analyzed is shown below in figure 8.

Figure 8. Simple RL circuit

At first, the required resistance value for a 13.33 MW consumption per phase was considered for the design with roughly 22 Ω per phase. However, a purely resistive circuit would produce a high harmonic content due to the non-continuities on the power function produced by the thyristor switching. Therefore, an inductance with the lowest value possible (in order to reduce reactive power consumption) was required, considering it being significant enough to smoothen the wave form, but small enough to have a low reactive power consumption.

As a result of the iterative design process performed in MATLAB, the selected SWC per-phase impedance contains a resistance value of 20 Ω together with an inductance value of 0.01 H. The PSCAD model of the SWC is presented below in figure 9.

Figure 9. SWC model in PSCAD

The thyristors utilized in the SWC model are the typical component configuration provided within PSCAD, and each thyristor possesses its own firing circuit to be presented in the section below.

4.3 Thyristor-firing circuits

Each thyristor gate receives a triggering pulse from its dedicated circuit, and the PSCAD per-phase implementation is shown below in picture 10.

Figure 10. Thyristor-firing circuit for phase a in PSCAD

Given the fact that each phase requires two thyristors, both the positive and negative half of the AC current, there are two firing circuits associated to each phase. The algorithm carried out for each circuit is presented below in figure 11.

Figure 11. Thyristor-firing circuit algorithm

The voltage on each phase is measured and a zero-crossing detector sends a pulse whenever the signal crosses zero. Three possible outputs are produced by the zero detectors: 1 when the signal crosses zero towards the positive half of the cycle, 0 when no zero crossing is detected, and -1 when the signal crosses zero towards the negative half of the cycle.

In order to implement the exact same algorithm for both the positive and negative cycles, the voltage measurement is multiplied by a gain of -1 in the firing circuit of the negative cycle thyristor.

This output signal is sent to an AND logic gate together with a “fire” signal (0 as default when no compensation is required, and 1 to send the current pulse to the thyristor’s gate for compensation). When both signals are 1 simultaneously, the firing pulse is sent

to the thyristor gate after a specific delay which is related to the amount of power consumption required for compensation.

The “fire” signal is just a true/false (1/0) logic signal, so it is combined to the time step (Delta-T) to provide the right signal dimension for the AND gate to compare both input signals over time.

Details regarding the origin and values of both the “fire” signal and the delay are covered in the next section 4.4 “Control scheme” of this document.

4.4 Control scheme

Based on the active power compensation requirement being the three-phase active power drop of the SAF, the control scheme of the SWC has being developed with three- phase measured and controlled variables.

The main purpose of the SWC is to follow the power consumption of the SAF, compare it against a preset value, and provide the deficit of power consumption to reach that value if needed. A block diagram of the basic SWC control system is shown in figure 12.

Figure 12. SWC block diagram

First, the power consumption of the SAF is subtracted from the preset power value (the 40 MW rated value of the SAFs), resulting in the reference value required for the SWC power consumption.

Second, the actual SWC power consumption is subtracted from this SWC power reference value in order to calculate the error between the current SWC power consumption and the required value.

Subsequently, this error is fed into a PI controller, which in turn outputs a new consumption power value for the SWC.

Afterwards, a lookup table created from the RL circuit analysis in MATLAB is implemented to calculate the delay (associated to the phase angle) required to fire the thyristors at the right moment in order to obtain the newly requested SWC power consumption.

Each SAF has been assumed to have its own SWC sharing the same bus bar, resulting in two SWCs for the study case of this project. The implementation of this control scheme per SAF in PSCAD is shown in pictures 13 and 14 below, presenting the scheme of the SWC connected to BusSAF1.

Figure 13. SWC control scheme in PSCAD Figure 14. SWC control scheme in PSCAD (cont.)

There are several extra components in the PSCAD implementation that are not represented in the block diagram of the control scheme. These components along with the full explanation of the control scheme in PSCAD are to be detailed next, assuming that the main control philosophy has been reviewed before in the block diagram along with its explanation.

At first, a comparator block is used to guarantee that only positive values or zero power is to be consumed by the SWC, setting any possible negative value to 0 MW.

Then the SWC power consumption signal has three different switches on its control line related to alternative modes of operation of the SWC: start-up, shut-down, and trip. All three switches share the same principle, to shift the SWC power signal required from the normal mode to a different, operator-controlled, value or set of values.

The start-up sequence involves the ramping up of power consumption by the SWC, with the proper ramp function, for the steam power generators to be able to follow the demand increase up to the operation point of the SAF. This ramping-up sequence is an input from the NPI smelting facility operator, and it should take into consideration the specific power consumption increase function of each SAF so it shapes its function accordingly. Likewise, the shut-down sequence provides the same level of controllability to the NPI smelting facility operator, but associated to the ramping-down capabilities of the SWC.

These sequences provide the facility operator with the capability of starting-up and shutting-down the SWCs as desired in order to follow the SAF ramping power variations accordingly.

The trip signal represents the possibility of shutting down the SWC as a result of an external signal, a protection system trip signal for example. For the simulation purposes, the SWC should consume 0 MW when this trip signal is activated.

Subsequently, the SWC power consumption signal goes through a hard-limiter block in order to set the SWC power consumption values between 0 and 40 MW, with an output signal “Pval” which is the required active power consumption level from the SWC.

Following the three-phase context of the control scheme, this “Pval” signal is divided by 3 in order to obtain the per-phase required power consumption value, since the lookup table made in MATLAB is based on per-phase values.

After calculating the per-phase power consumption value required, the signal flows into two different paths; one is to obtain the associated delay, and the other one is to obtain the “fire” signal, both being essential signals for the thyristor-firing circuits.

Another comparator block is implemented on the “fire” signal path, so only “Pval” values greater than 0 MW will output a logic true (1 as an integer). This is to avoid sending currents to the thyristor gate to obtain 0 MW power consumption values on the SWC, thus unnecessarily wearing out the thyristor valve components.

This configuration is followed by another comparator block, which compares the current power consumption of the SAF (PSAF) and the power value of reference to be compensated (Pref).

If PSAF is greater than Pref minus 5 MW, then the SWC should not compensate, leaving the SAF to operate in normal condition on its own. This condition was calculated taking into consideration the ± 5 MW maximum fluctuation required by the NPI smelting facility owner, and it should be adjusted accordingly in the case of a different fluctuation tolerance requirement.

Regarding the delay path, “Pval” is directly fed into the lookup table, which is a characteristic of time as a function of power conformed by 179001 pairs of data produced from the analysis in MATLAB. This function returns the specific time (in milliseconds) at which the thyristor should be fired in order to obtain the specific power consumption value “Pval”.

A hard-limiter block is used then to set the output values between 1 and 15 MW per phase, providing a minimum three-phase power consumption value of the SWC of 3 MW when the thyristors are fired.

Finally, the delay is divided by 1000 to provide the delay in seconds to the thyristor-firing circuits.

4.4.1 Operator controls

In order to simulate the input signals to be provided from the facility operator in real life, a set of control interfaces have been implemented in the PSCAD model.

The set of online controls implemented are identical for both SAFs, and an example of the controls for the SWC1 is presented below in figure 15.

Figure 15. Online controls for SWC1

These online controls allow the adjustment of certain values in real-time while the simulation is being conducted. Three switches and three sliders have been employed as online controls to provide real-time input signals in the following sections of the control scheme:

Power setting

A slider allows the user to modify the target power value to which the SWC is compensating at any given time.

Start-up sequence

A switch allows the user to select the normal condition of the control scheme after the start-up sequence of the SAF has been completed.

In addition to that, a slider provides the user the capability of ramping up the power consumption of the SWC according to the SAF power consumption increase.

The aim of these controls is to emulate the adapting power consumption increase of the SWC in response to the SAF behavior, or according to pre-defined starting-up protocols implemented in the NPI smelting facility.

These input signals are manually adjusted while simulating the system for simulation purposes, but they should be defined by the facility operator for real operation (either with an automated algorithm, a manual procedure, or both).

Shut-down sequence

These online controls share the same principle as those associated with the start-up sequence.

A switch allows the user to trigger the shutting-down sequence while the SWC is working on normal condition of the control scheme after the SAF has power consumption has reached 0 MW (either by the SAF being shut down or being disconnected due to a failure or maintenance).

In addition to that, a slider provides the user the capability of ramping down the power consumption of the SWC according to the SAF power consumption decrease.

In the case of a sudden power consumption drop on the SAF, the SWC allows the operator to ramp down the power consumption with the proper power-drop steps per minute, so the steam generators are able to follow the variations on the demand and the system remains stable.

Trip signal

A switch allows the user to emulate a trip signal during the normal condition of the control scheme, switching the SWC power consumption to a preset value (0 MW for the simulation purposes).

This input signal allows the facility operator to send a signal to the SWC in the presence of a given event (a system failure or an emergency shut-down for example), setting a specific value or set of values for the SWC to be followed.

Again, these input signals are manually adjusted while simulating the system for simulation purposes, but they should be defined by the facility operator for real operation (either with an automated algorithm, a manual procedure, or both).

The full simulation canvas file can be found in the Appendix C: “PSCAD simulation file canvas” of this document.