ORCID Jessica Taytard

Nadia Nathan Orcid

MD Harriet Corvol
email: harriet.corvol@aphp.fr

Blandine Prevost

Guillaume Aubertin

Guillaume Thouvenin

Nadia Nathan

COVID-19 and pediatric pulmonology: feedback from an expert center after the first year of the pandemic

Keywords: COVID-19, SARS-CoV-2, children, lung, immunosuppression, sickle cell disease, cystic fibrosis

The coronavirus disease 2019 (COVID-19) outbreak has evolved with different waves corresponding to subsequent severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) mutations. While the most severe cases have been observed in the elderly and in individuals with underlying comorbidities, severe pediatric and young adult cases have been observed, as well as post-infectious inflammatory syndromes and persistent symptoms leading to long-COVID. This manuscript describes the experience of a pediatric respiratory unit during the first year of the pandemic and reviews the corresponding literature with a special emphasis on children and young people with underlying conditions, such as immunosuppression, sickle cell disease, and cystic fibrosis.

1/ Introduction

Since its inception in December 2019, the coronavirus disease 2019 (COVID- [START_REF] Nathan | Atypical presentation of COVID-19 in young infants[END_REF] outbreak has evolved with different waves, corresponding to subsequent severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) mutations. As of August 25, 2022, at least 600 million people have been infected with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), and 6.4 million individuals have died of this disease worldwide [START_REF]Chiffres clés et évolution de la COVID-19 en France et dans le Monde[END_REF]. The most severe cases have been observed in the elderly and in 10990496, ja, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/ppul.26235 by Cochrane France, Wiley Online Library on [16/11/2022]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License This article is protected by copyright. All rights reserved.

Accepted

Article individuals with underlying comorbidities, such as obesity, diabetes, hypertension, and immunosuppression. Initially thought of as being involved in the carriage and dissemination of the virus, it was quickly established during the first wave that young children were neither very contagious nor symptomatic with respect to COVID-19 [START_REF] Cohen | Assessment of SARS-CoV-2 infection by Reverse transcription-PCR and serology in the Paris area: a cross-sectional study[END_REF]. However, severe pediatric cases and post-infectious inflammatory syndromes have been reported worldwide [START_REF] Harwood | Which children and young people are at higher risk of severe disease and death after hospitalisation with SARS-CoV-2 infection in children and young people: A systematic review and individual patient meta-analysis[END_REF]. This manuscript describes the experience of a pediatric respiratory unit during the first year of the pandemic and reviews the corresponding literature with a special emphasis on children and young people with underlying conditions, such as immunosuppression, sickle cell disease, and cystic fibrosis.

2/ Pediatric epidemiology of SARS-CoV-2

At the beginning of the pandemic, Wu et al. described the first 72,314 cases of COVID-19 in China [START_REF] Wu | Characteristics of and Important Lessons From the Coronavirus Disease 2019 (COVID-19) Outbreak in China: Summary of a Report of 72 314 Cases From the Chinese Center for Disease Control and Prevention[END_REF]. Children and adolescents accounted for only 2% of these cases, and no deaths were reported among children under nine years of age. Since then, data from different countries have confirmed a lower initial incidence of infection in children (2-5%) and its reduced severity in children, although severe cases and deaths have been reported in all countries, including 28 deaths in France between 2020 and 2021 [START_REF]Chiffres clés et évolution de la COVID-19 en France et dans le Monde[END_REF].

In late 2021-early 2022, a resurgence of cases in young adults and children was observed with the delta and omicron variants in France (5)[START_REF] Taytard | More on BNT162b2 Covid-19 Vaccine in Children 5 to 11 Years of Age[END_REF][START_REF] Taytard | SARS-CoV-2 B.1.1.529 (Omicron) Variant Causes an Unprecedented Surge in Children Hospitalizations and Distinct Clinical Presentation Compared to the SARS-CoV-2 B.1.617.2 (Delta) Variant[END_REF]. This has caused the recrudescence of severe COVID-19 forms and hospitalizations in children, particularly in the age group of 0-9 years because of the limited vaccination coverage in this population (7-10).

3/ Clinical symptoms of COVID-19 and risk factors for severity in children

The first pediatric COVID-19 cases were described by authors from Shanghai, This article is protected by copyright. All rights reserved.

Accepted Article

China [START_REF] Dong | Epidemiology of COVID-19 Among Children in China[END_REF]. They reported 2,135 children infected with SARS-CoV-2, with 56% boys and a median age of 7 years. The symptoms were mostly mild (51% of the cases) or moderate (38% with pneumonia and wheezing), and severe in only 6% of the cases with hypoxia and/or respiratory failure. Subsequently, it has been established that during the first wave, the infection is usually mild in children, with non-specific clinical signs, such as cough, fever, rhinitis, headache, and digestive signs (vomiting and diarrhea) [START_REF] Nathan | The Wide Spectrum of COVID-19 Clinical Presentation in Children[END_REF][START_REF] Hoang | COVID-19 in 7780 pediatric patients: A systematic review[END_REF][START_REF] Kalyanaraman | COVID-19 in Children[END_REF][START_REF] Hu | Characteristics of SARS-CoV-2 and COVID-19[END_REF][START_REF] Wang | Epidemiological features of coronavirus disease 2019 in children: a meta-analysis[END_REF].

Very few studies have been conducted on paucisymptomatic ambulatory patients.

Nevertheless, a French study in March 2021 (before vaccination of adolescents) showed that in 2,079 children presenting signs compatible with COVID-19, the PCR positivity rates for SARS-CoV-2 reached 7.3%, with rates increasing with age [START_REF] Eskander | Infection SARS CoV-2 en ambulatoire chez l'enfant[END_REF]. In this study, apart from a loss of taste, no symptoms were predictive of COVID-19 compared with other viral infections. After the first wave, a systematic review of COVID-19 in pediatric patients evaluated 7,780 pediatric cases described in 131 studies from 26 countries from December 2019 to May 2020 [START_REF] Hoang | COVID-19 in 7780 pediatric patients: A systematic review[END_REF]. They concluded that children diagnosed with COVID-19 had an overall excellent prognosis, with only 3.3% of children requiring intensive care unit (ICU) care and seven deaths. More than 35% had underlying medical conditions, with immunosuppression being the most common (30.5%), followed by respiratory (21%) and cardiovascular (13.7%) disorders.

In hospitalized children, poorly tolerated fever and fatigue are common [START_REF]COVID-19 symptoms at hospital admission vary with age and sex: results from the ISARIC prospective multinational observational study[END_REF][START_REF] Nathan | The Wide Spectrum of COVID-19 Clinical Presentation in Children[END_REF]. At hospital admission, respiratory symptoms are present in approximately 50% of cases, with more than half being associated with respiratory distress [START_REF]COVID-19 symptoms at hospital admission vary with age and sex: results from the ISARIC prospective multinational observational study[END_REF]. Notably, the severity seems to be greater in infants under one year of age and adolescents [START_REF] Harwood | Which children and young people are at higher risk of severe disease and death after hospitalisation with SARS-CoV-2 infection in children and young people: A systematic review and individual patient meta-analysis[END_REF]. During the first wave, a retrospective cohort study was conducted in Colorado, USA, in 454 children with confirmed SARS-CoV-2 infection, with the aim of evaluating the risk factors for This article is protected by copyright. All rights reserved.

Accepted Article

severe COVID-19 [START_REF] Graff | Risk Factors for Severe COVID-19 in Children[END_REF]. They showed that extremes of age (e.g. 0-3 months or >20 years), preterm birth history, comorbidities (immunosuppression, gastrointestinal condition, diabetes, and asthma), and an elevated CRP were predictors of severe disease in children. At the beginning of the pandemic, we described hemodynamic disorders and transient neurological manifestations in young infants [START_REF] Nathan | Atypical presentation of COVID-19 in young infants[END_REF]. A systematic review of COVID-19 in 63 infants under three months of age reported moderate severity and nonspecific clinical signs; however, one fifth of children required ICU care [START_REF] Mark | Community-Onset Severe Acute Respiratory Syndrome Coronavirus 2 Infection in Young Infants: A Systematic Review[END_REF]. Among hospitalized adolescents, children with underlying conditions, such as asthma, obesity, or sickle cell disease are overrepresented [START_REF] Harwood | Which children and young people are at higher risk of severe disease and death after hospitalisation with SARS-CoV-2 infection in children and young people: A systematic review and individual patient meta-analysis[END_REF][START_REF] Nathan | The Wide Spectrum of COVID-19 Clinical Presentation in Children[END_REF][START_REF] Ouldali | Factors Associated With Severe SARS-CoV-2 Infection[END_REF][START_REF] Zimmermann | Coronavirus Infections in Children Including COVID-19: An Overview of the Epidemiology, Clinical Features, Diagnosis, Treatment and Prevention Options in Children[END_REF][START_REF] Woodruff | Risk Factors for Severe COVID-19 in Children[END_REF][START_REF] Prévost | Risk factors for admission to the pediatric critical care unit among children hospitalized with COVID-19 in France[END_REF]. Rare cases of pulmonary embolism associated with COVID-19 have also been reported in pediatric patients [START_REF] Chima | COVID-19-Associated Pulmonary Embolism in Pediatric Patients[END_REF][START_REF] Odievre | Dramatic improvement after tocilizumab of severe COVID-19 in a child with sickle cell disease and acute chest syndrome[END_REF].

In children, a specific hyperinflammatory syndrome with multiorgan involvement associated with COVID-19 has been described and named as multisystemic inflammatory syndrome in children (C-MIS) or pediatric multisystemic inflammatory syndrome (PIMS) [START_REF] Abrams | Factors linked to severe outcomes in multisystem inflammatory syndrome in children (MIS-C) in the USA: a retrospective surveillance study[END_REF][START_REF] Davies | Intensive care admissions of children with paediatric inflammatory multisystem syndrome temporally associated with SARS-CoV-2 (PIMS-TS) in the UK: a multicentre observational study[END_REF][START_REF] Toubiana | Kawasaki-like multisystem inflammatory syndrome in children during the covid-19 pandemic in Paris, France: prospective observational study[END_REF]. PIMS occurs 2-8 weeks after a SARS-CoV-2 infection that may have been asymptomatic. It shares several similarities with Kawasaki syndrome, which has also been described as possibly secondary to viral infection. PIMS is rare, but potentially serious. At the end of August 2022, 1,070 cases of PIMS related to COVID-19 were reported to the French health authorities (61% boys, median age: 8 years with 32% of cases in children under 5 years of age and 72% under 11 years of age) [START_REF]Situation épidémiologique liée à la COVID-19 chez les 0-17 ans[END_REF]. The main clinical symptoms of PIMS are persistent fever, systemic inflammation, gastrointestinal manifestations (abdominal pain, vomiting, and diarrhea), mucocutaneous abnormalities (skin rashes and conjunctivitis), headache, and cardiac dysfunction [START_REF] Abrams | Factors linked to severe outcomes in multisystem inflammatory syndrome in children (MIS-C) in the USA: a retrospective surveillance study[END_REF][START_REF] Davies | Intensive care admissions of children with paediatric inflammatory multisystem syndrome temporally associated with SARS-CoV-2 (PIMS-TS) in the UK: a multicentre observational study[END_REF][START_REF] Toubiana | Kawasaki-like multisystem inflammatory syndrome in children during the covid-19 pandemic in Paris, France: prospective observational study[END_REF][START_REF] Blondiaux | Cardiac MRI of Children with Multisystem Inflammatory Syndrome (MIS-C) Associated with COVID-19: Case Series[END_REF]. A large American study of 1080 children hospitalized for PIMS

Accepted

Article with a median age of 8 years reported 60% hospitalizations in the ICU for heart failure, coronary dysfunction, hemodynamic shock, or myocarditis; and 18 (2%) deaths [START_REF] Abrams | Factors linked to severe outcomes in multisystem inflammatory syndrome in children (MIS-C) in the USA: a retrospective surveillance study[END_REF]. In France, among the 1070 cases of PIMS, 725 (68%) patients were hospitalized in the ICU and one 9-year-old child died [START_REF]Situation épidémiologique liée à la COVID-19 chez les 0-17 ans[END_REF].

Another long-term consequence of SARS-CoV-2 infection in adults and children is long-COVID [START_REF] Izquierdo-Pujol | Post COVID-19 Condition in Children and Adolescents: An Emerging Problem[END_REF][START_REF] Lopez-Leon | Long-COVID in children and adolescents: a systematic review and meta-analyses[END_REF]. The definition proposed by the WHO for this condition is that it generally occurs three months after the onset of SARS-CoV-2 infection, with symptoms that last for at least two months and cannot be explained by an alternative diagnosis [START_REF] Soriano | A clinical case definition of post-COVID-19 condition by a Delphi consensus[END_REF]. A systematic review, which evaluated 80,071 children and adolescents from 21 studies, reported a long-COVID prevalence of 25.24% [START_REF] Lopez-Leon | Long-COVID in children and adolescents: a systematic review and meta-analyses[END_REF]. The most prevalent clinical manifestations were mood symptoms (16.5%), fatigue (9.7%), and sleep disorders (8.4%). Moreover, this review showed that children with a history of SARS-CoV-2 infection had a higher risk of persistent dyspnea, anosmia/ageusia, and fever. A large prospective pediatric cohort of children admitted to hospital with confirmed COVID-19 also showed that a quarter of children had persistent symptoms (>5 months) at the time of follow-up with fatigue, sleep disturbance and sensory problems being the most common [START_REF] Osmanov | Risk factors for post-COVID-19 condition in previously hospitalised children using the ISARIC Global follow-up protocol: a prospective cohort study[END_REF]. In this study, almost one in 10 children reported multisystem impacts with at least two categories of persistent symptoms. Children in mid-childhood and adolescents (aged 6-18 years) as well as those with history of allergic diseases (defined here as asthma, allergic rhinitis, eczema or food allergy) were at higher risk of persistent symptoms. Although long-COVID represents a significant public health concern that may increase in the future, there are still no guidelines for its diagnosis and management in children [START_REF] Izquierdo-Pujol | Post COVID-19 Condition in Children and Adolescents: An Emerging Problem[END_REF][START_REF] Lopez-Leon | Long-COVID in children and adolescents: a systematic review and meta-analyses[END_REF]. This article is protected by copyright. All rights reserved.

4/ Focus on atypical respiratory manifestations of COVID-19 in children and young

Accepted

Article people observed in a French expert center during the first year of the pandemic

Approximately 50% of children hospitalized with COVID-19 have respiratory involvement, more than half of which is associated with respiratory distress [START_REF]COVID-19 symptoms at hospital admission vary with age and sex: results from the ISARIC prospective multinational observational study[END_REF][START_REF] Nathan | The Wide Spectrum of COVID-19 Clinical Presentation in Children[END_REF]. An international study conducted between February and October 2020 in 671 children hospitalized with confirmed and symptomatic COVID-19 reported a 13.3% incidence of viral pneumonitis [START_REF] Bourgeois | International Analysis of Electronic Health Records of Children and Youth Hospitalized With COVID-19 Infection in 6 Countries[END_REF]. Similar to adults, some underlying conditions have been shown to be associated with a higher risk of developing severe respiratory diseases, particularly chronic respiratory diseases (asthma, cystic fibrosis (CF), and bronchopulmonary dysplasia), immunosuppression, cancer, sickle cell disease, and neurological disorders [START_REF] Harwood | Which children and young people are at higher risk of severe disease and death after hospitalisation with SARS-CoV-2 infection in children and young people: A systematic review and individual patient meta-analysis[END_REF][START_REF] Nathan | The Wide Spectrum of COVID-19 Clinical Presentation in Children[END_REF][START_REF] Hoang | COVID-19 in 7780 pediatric patients: A systematic review[END_REF][START_REF] Ouldali | Factors Associated With Severe SARS-CoV-2 Infection[END_REF][START_REF] Zimmermann | Coronavirus Infections in Children Including COVID-19: An Overview of the Epidemiology, Clinical Features, Diagnosis, Treatment and Prevention Options in Children[END_REF][START_REF] Odievre | Dramatic improvement after tocilizumab of severe COVID-19 in a child with sickle cell disease and acute chest syndrome[END_REF][START_REF] Moeller | COVID-19 in children with underlying chronic respiratory diseases: survey results from 174 centres[END_REF][START_REF] Shekerdemian | Characteristics and Outcomes of Children With Coronavirus Disease 2019 (COVID-19) Infection Admitted to US and Canadian Pediatric Intensive Care Units[END_REF][START_REF] González-Dambrauskas | Pediatric Critical Care and COVID-19[END_REF][START_REF] Bixler | SARS-CoV-2-Associated Deaths Among Persons Aged <21 Years -United States, February 12-July 31, 2020[END_REF]. Below, we describe some of these particular cases, with a focus on patients hospitalized in our pediatric pulmonology unit during the first year of the pandemic. The description of the cases hospitalized in our unit has been approved by the local ethics committee of our institution, which waived the need for patients' consent (PED_COVID N°20200717191204).

Immunocompromised children

As of yet, there are limited published data describing the outcomes of immunocompromised children infected with SARS-CoV-2 [START_REF] Kotecha | Challenges posed by COVID-19 to children with cancer[END_REF][START_REF] Zamperlini-Netto | COVID-19 after hematopoietic stem cell transplantation: report of two children[END_REF][START_REF] Feldman | The impact of COVID-19 on the pediatric solid organ transplant population[END_REF][START_REF] Cesaro | Recommendations for the management of COVID-19 in patients with haematological malignancies or haematopoietic cell transplantation, from the 2021 European Conference on Infections in Leukaemia (ECIL 9)[END_REF]. In adults, the risk of the worst COVID-19 evolution has frequently led to the postponement of anti-cancer therapies [START_REF] Cesaro | Recommendations for the management of COVID-19 in patients with haematological malignancies or haematopoietic cell transplantation, from the 2021 European Conference on Infections in Leukaemia (ECIL 9)[END_REF][START_REF] Liang | Cancer patients in SARS-CoV-2 infection: a nationwide analysis in China[END_REF]. However, in childhood, most cancers require intensive treatment, which, if postponed, may be life-threatening. We observed an atypical respiratory course of severe SARS-CoV-2 infection in a 16-year-old adolescent, who was immunocompromised when infected with SARS-CoV-2 (45). This teenager was diagnosed with bone Ewing sarcoma, which was initially treated with chemotherapy and surgical resection. Given the partial response to this treatment, high-dose chemotherapy This article is protected by copyright. All rights reserved.

Accepted Article

followed by autologous hematopoietic stem cell transplantation (HSCT) was considered.

HSCT was delayed by two weeks because of SARS-CoV-2 infection (isolated dysgeusia initially). Nevertheless, the patient developed an acute respiratory distress syndrome (ARDS) after the HSCT, which required ICU care with 15 days of invasive ventilation, systemic corticosteroids, convalescent plasma, and tocilizumab. Thoracic computed tomography (CT) revealed alveolar condensations in the lower lobes with diffuse confluent ground-glass opacities. At ICU discharge, the patient remained dyspneic and hypoxic. The CT scan revealed a worsening condition with lesions suggestive of post-COVID-19 organized pneumonia (Figure 1A). The treatment consisted of high doses of corticosteroids for two months and respiratory rehabilitation with favorable outcomes.

Children with sickle cell disease

Several studies have reported that sickle cell disease is a risk factor for severe COVID-19, particularly in children [START_REF] Hoogenboom | Clinical outcomes of COVID-19 in patients with sickle cell disease and sickle cell trait: A critical appraisal of the literature[END_REF][START_REF] Hoogenboom | Vaincre-la-Mucoviscidose[END_REF]. At the beginning of the pandemic, we described the favorable outcome of a 16-year-old girl with sickle cell disease who developed acute chest syndrome (ACS) with pulmonary embolism secondary to SARS-CoV-2 infection [START_REF] Odievre | Dramatic improvement after tocilizumab of severe COVID-19 in a child with sickle cell disease and acute chest syndrome[END_REF]. This adolescent had a severe form of sickle cell disease with a history of repeated vaso-occlusive crises and bilateral ischemic retinopathy, with no history of ACS. Seven days after the onset of SARS-CoV-2 infection, which initially manifested as an isolated fever, she developed ACS with acute chest pain and ARDS.

CT pulmonary angiography confirmed ACS and showed bilateral pulmonary embolism (Figure 1 B1&B2). She was admitted to the ICU and required four days of noninvasive ventilation, six weeks of anticoagulation, and one pulse of intravenous tocilizumab. The patient subsequently returned to her pre-COVID clinical status. pwCFs were reported to be infected with SARS-CoV-2, 12 of whom were post-lung transplantation [START_REF] Corvol | First Wave of COVID-19 in French Patients with Cystic Fibrosis[END_REF]. The majority of patients had CF-related diabetes (61%) and moderate respiratory disease in the year prior to infection (65% had a pre-infection FEV1 >70%). More recently, this study was updated with a description of 223 French pwCFs diagnosed with COVID-19 during the first year of the pandemic (50). It showed a higher risk of infection in adults and post-transplant individuals. Using objective clinical markers (e.g., respiratory failure and/or death), this study highlighted that the risk factors for severity included low lung function and diabetes in non-transplanted individuals and post-transplant status. An international registry was also set up at the beginning of the pandemic, which described 181 pwCFs (32 post-transplant) with COVID-19 during the first wave [START_REF] Mcclenaghan | The global impact of SARS-CoV-2 in 181 people with cystic fibrosis[END_REF]. The impact of SARS-CoV-2 infection appeared to be similar in pwCF and the general population, with 11 patients admitted to the ICU (7 post-transplant) and seven deaths (3 post-transplant). This international consortium also detailed more precisely the 105 pediatric international COVID-19 cases: 54% were males, with a median age of 10 years, and a median FEV 1 of 94% [START_REF] Bain | Clinical characteristics of SARS-CoV-2 infection in children with cystic fibrosis: An international observational study[END_REF]. Only 24 children were hospitalized, six children required additional oxygen therapy, and two children required noninvasive ventilation. The hospitalized children had significantly worse lung function before infection. One child died six weeks after testing positive for SARS-CoV-2 because of respiratory worsening that was not attributed to COVID-19. We observed similar features in the pediatric CF [START_REF] Thouvenin | The omicron wave modifies the COVID-19 paradigm in children with cystic fibrosis[END_REF].

Young people with cystic fibrosis (pwCF)

It is still difficult to estimate the true susceptibility of pwCF to SARS-CoV-2 infection. Indeed, they are used in barrier gestures, such as wearing masks, frequent hand washing, and social distancing, in order to limit exposure and avoid infection.

Epidemiological studies performed during the first wave of the pandemic have shown that the incidence remained low, with a short-term evolution that appeared relatively favorable in non-transplanted pwCF, but more severe in post-transplant patients.

5/ Radiological signs of COVID-19 in children

When a child is hospitalized for respiratory distress associated with COVID-19, chest radiography should be performed. If the chest radiograph is observed to be abnormal with the presence of serious respiratory clinical signs and/or underlining conditions known to lead to severe COVID-19 (sickle cell disease, obesity, etc.), CT pulmonary angiography should be performed to identify COVID-19 complications, such as ARDS and pulmonary embolism. A meta-analysis of the CT scans of 1747 children with COVID-19 found 63.2% of abnormalities on chest CT, including 39.5% of groundglass opacities and 65.1% of bilateral lung lesions, but also rare abnormalities, such as pulmonary nodules (25.7%), patchy shadows (36.8%), halo signs (24.8%), consolidation (24.1%), air bronchogram signs (11.2%), cord-like shadows (9.7%), crazy-paving pattern (6.1%), and pleural effusion (9.1%) [START_REF] Wang | Computed tomography features of COVID-19 in children: A systematic review and meta-analysis[END_REF]. Compared to adults, ground-glass opacities are less frequently reported, as is the bilaterality of lesions [START_REF] Waller | CT presentations of adult and pediatric SARS-CoV-2 patients: A review of early COVID-19 data[END_REF]. It is also important to note that pulmonary embolism must be considered in adolescents, In PIMS, respiratory involvement is relatively rare or mild (mainly bronchial thickening), whereas extra-respiratory radiological involvement is more common with cardiomegaly, mesenteric inflammation, hepatosplenomegaly, periportal edema, ascites, or thickening of the digestive walls [START_REF] Blondiaux | Cardiac MRI of Children with Multisystem Inflammatory Syndrome (MIS-C) Associated with COVID-19: Case Series[END_REF][START_REF] Ucan | Multisystem inflammatory syndrome in children associated with SARS-CoV[END_REF].

6/ Management of COVID-19 in children

The management of SARS-CoV-2 infections in children is mostly based on adult recommendations and has evolved through various waves of the pandemic. Recent recommendations have been developed by the National Institutes of Health (56). Most children require only symptomatic treatment, particularly fever control with acetaminophen. In cases of severe respiratory diseases, treatment with dexamethasone is discussed in a manner similar to that in adults. Therefore, systematic antibiotic therapy was not recommended. Other therapies, such as paxlovid, remdesivir, tocilizumab, convalescent plasma, and monoclonal antibodies, have been shown to be effective (56).

Prophylactic management is based on barrier gestures and the SARS-CoV-2 vaccine. Indeed, vaccination is recommended in children older than five years, for whom the BNT162b2 COVID-19 vaccine has been deemed safe, immunogenic, and effective (57). However, the vaccination rates in children remain limited in most countries, making it difficult to evaluate their effectiveness on a large scale (58).

7/ Conclusion

The knowledge of COVID-19 in children is evolving through consecutive waves of different SARS-CoV-2 variants. These variants have caused variable pathogenicity and transmissibility, with an increase in incidence in children in the coming months.

  This article is protected by copyright. All rights reserved.AcceptedArticle Since the first wave of COVID-19 in France in March 2020, all 47 CF centers (which follow approximately 7,500 pwCF (48)) have been collaborating to conduct a prospective observational study to describe the clinical manifestations of SARS-CoV-2 infection in French pwCF. During the first wave (March to June 2020), 31 French

  This article is protected by copyright. All rights reserved.AcceptedArticle center host of our pediatric pulmonology unit. Most children with CF who were diagnosed as infected by SARS-CoV-2 during the first COVID wave remained asymptomatic; the diagnosis was confirmed by a systematic assessment of SARS-CoV-2 serology or by SARS-CoV-2 RT-PCR performed in case of contact

  This article is protected by copyright. All rights reserved.AcceptedArticle especially in cases of thromboembolic risk factors or suggestive biological markers[START_REF] Chima | COVID-19-Associated Pulmonary Embolism in Pediatric Patients[END_REF].

  Vaccination of children older than five years for whom the BNT162b2 COVID-19 10990496, ja, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/ppul.26235 by Cochrane France, Wiley Online Library on [16/11/2022]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons LicenseThis article is protected by copyright. All rights reserved.AcceptedArticle vaccine has been shown to be safe and effective is also likely to modify the epidemiology and the immediate and post-infectious manifestations of COVID-19 in children.

  10990496, ja, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/ppul.26235 by Cochrane France, Wiley Online Library on [16/11/2022]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License

  10990496, ja, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/ppul.26235 by Cochrane France, Wiley Online Library on [16/11/2022]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License

  10990496, ja, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/ppul.26235 by Cochrane France, Wiley Online Library on [16/11/2022]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License

This article is protected by copyright. All rights reserved.

  10990496, ja, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/ppul.26235 by Cochrane France, Wiley Online Library on [16/11/2022]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License

  10990496, ja, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/ppul.26235 by Cochrane France, Wiley Online Library on [16/11/2022]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License

  10990496, ja, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/ppul.26235 by Cochrane France, Wiley Online Library on [16/11/2022]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License

  10990496, ja, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/ppul.26235 by Cochrane France, Wiley Online Library on [16/11/2022]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License

  10990496, ja, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/ppul.26235 by Cochrane France, Wiley Online Library on [16/11/2022]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License

Acknowledgements

The authors would like to thank the medical and nursing staff of the Pediatric Pulmonology Department at Trousseau Hospital, Assistance Publique-Hôpitaux de Paris (AP-HP), Sorbonne Université, Paris, France.

10990496, ja, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/ppul.26235 by Cochrane France, Wiley Online Library on [16/11/2022]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License This article is protected by copyright. All rights reserved. Accepted Article Respiratory Syndrome Coronavirus 2 Variant Delta Infects All 6 Siblings but Spares Comirnaty (BNT162b2, BioNTech/Pfizer)-Vaccinated Parents. J Infect Dis. 10990496, ja, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/ppul.26235 by Cochrane France, Wiley Online Library on [16/11/2022]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License

This article is protected by copyright. All rights reserved. Accepted Article Mitchell WB, et al. Clinical outcomes of COVID-19 in patients with sickle cell disease and sickle cell trait: A critical appraisal of the literature. Blood Rev. 2022;53:100911. 10990496, ja, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/ppul.26235 by Cochrane France, Wiley Online Library on [16/11/2022]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License This article is protected by copyright. All rights reserved.

Data Availability Statement

Data sharing is not applicable to this article as no new data were created or analyzed in this study.


J D Páez

F Morel

S Bacha

P Dworakowski

Fault blocking capability in the DC-MMC with reduced number of submodules

The capability of DC-DC converters to block DC faults is an important issue for the development of HVDC grids. To include such feature, converters are generally oversized in terms of components by adding more semiconductors than needed in normal operation. This paper proposes to use a main switch instead of adding sub-modules in the topology to provide fault blocking capability. A converter control method is proposed to open the switch at zero current; thus, no breaking capability is required. An analysis on the impact on the converter design and a comparison with the classical solution are done. The operation of the proposed solution is verified through transient simulations. In the analysis, the requirements in terms of opening time for the switch were determined to be around 1 ms to 6 ms, which is feasible with a fast disconnector. However, the required control to keep the current on the switch at zero amps adds constraints to the lower arms of the topology in terms of installed capacitance and current withstanding.

Introduction

DC-DC converters are needed to enable the development of future HVDC grids. These structures allow the interconnection of DC systems with different characteristics like voltage rating or line configuration [START_REF] Gómez | Req irements for interconnection of inks with converters[END_REF]. While enabling the interconnection of DC systems, DC-DC converters can also provide several functionalities to the system, like power flow control, DC voltage regulation and fault blocking capability (FBC) [START_REF] Páez | Overview of -DC converters dedicated to dc grids[END_REF]. This last characteristic makes that DC-DC converters can play a role in the protection strategy of DC networks [START_REF] Páez | t dy of the im act of DC-converters on the rotection strategy of grids[END_REF].

The FBC can be understood as the capability of the converter to prevent the apparition of anormal voltages or currents in one of the DC systems being interconnected by the converter when a fault appears in the second DC system. Anormal voltages and currents are defined as values that could lead to stop the operation of the network or to damage some of its components. This feature is achieved by liming the contribution of the healthy system to the fault current. The FBC can be achieved by a DC-DC converter integrated with an external protection device like a DC circuit breaker (DCCB), or by a DC-DC converter designed to provide the functionality by controlling the contribution of the heathy system to the fault current or being capable of interrupting this current.

Several DC-DC topologies adapted to HVDC applications have been proposed in literature. The common trend are modular multilevel converters that use chains of sub-modules (SMs) [START_REF] Páez | Overview of -DC converters dedicated to dc grids[END_REF]. The SMs can be of different types, the half-bridge (HB) and the full-bridge (FB) being the most common. Two main categories of DC-DC converters are identified: isolated and non-isolated circuits. Non-isolated topologies seem to be more competitive in term of losses and component count [4], but they do not provide inherent FBC as the isolated circuits do. The non-isolated converters generally need some modifications in the structure to provide the FBC, like the use of FB-SMs instead of HB-SMs.

Among the non-isolated topologies, the DC-MMC has been identified as an interesting solution [4]- [8]. In this circuit, the FBC is provided by adding more SMs compared to those needed for normal operation [9]. This leads to an increase on the losses and costs, which can reduce the interest of the topology. This paper proposes an alternative solution to include the FBC.

The solution proposed in this paper relies in the use of a main high voltage switch (HV-Sw) instead of SMs to include the FBC against faults on the low-voltage side. A converter control method is proposed during faults to operate the HV-Sw at zero current. The aim is to reduce the constraints of the switch in terms of breaking capability.

The paper is organized in three sections. In the first section the DC-MMC topology is introduced and the constraints in terms of number of SMs to provide FBC are presented. The second section presents the proposed solution and the proposed control during faults. Finally, the third section presents the validation of the solution in simulation as well as a sensitivity analysis on the converter sizing and the constraints to the proposed HV-Sw. A comparison of the converter losses in the proposed solution and the classical DC-MMC is also done.

The DC Modular Multilevel Converter

The DC-MMC (also known as M2DC) (Fig. 1) is a non-isolated converter formed by several legs connected in parallel to the HV DC system. Each leg is formed by two arms made by a stack of SMs and an inductor. The legs have a middle point which interconnects both arms. On this middle point a filter interconnects the LV side DC system. The simplest filter is an inductance. In this paper a threephase DC-MMC is studied.

The operation principle of the circuit is to use the SM stacks as controllable voltage sources by acting on the insertion and bypass of the SM capacitors. AC and DC voltages are produced on the SM stacks, and thanks to the voltage drop that is generated on the converter inductors, AC and DC currents are controlled. The DC currents control the power flow between DC systems while the AC currents are used to balance the energy in the converter, i.e. to control the voltage on the SM capacitors, by exchanging energy between upper arms and lower arms for instance. The balanced operation between the legs makes that the AC currents circulate between arms and legs but not into the DC ports. In normal operation, each arm generates a maximum voltage described by Eq. (1) and Eq. (2) [9] which leads to the number of SMs per arm that would be required for normal operation which is shown on Eq.

(3) and Eq. (4), where 𝑉 𝑆𝑀 is the nominal voltage of one SM.

𝑣 𝑢 𝑚𝑎𝑥 = 𝑉 𝐻 -𝑉 𝐿 + min(𝑉 𝐿 , 𝑉 𝐻 -𝑉 𝐿 )

(1) 𝑣 𝑙 𝑚𝑎𝑥 = 𝑉 𝐿 + min(𝑉 𝐿 , 𝑉 𝐻 -𝑉 𝐿 )

(2)

𝑁 𝑆𝑀 𝑢 𝑛𝑜𝑟𝑚𝑎𝑙 = ceil ( 𝑣 𝑢 𝑚𝑎𝑥 𝑉 𝑆𝑀 ) (3)
𝑁 𝑆𝑀 𝑙 𝑛𝑜𝑟𝑚𝑎𝑙 = ceil ( 𝑣 𝑙 𝑚𝑎𝑥 𝑉 𝑆𝑀 ) (4)
For HV side faults, to provide the FBC, the upper arms should generate a voltage equal to -𝑉 𝐿 . For LV side faults, they should withstand 𝑉 𝐻 . Thus, the number of SMs required in upper arms to provide FBC are defined by Eq. ( 5) and Eq. ( 6) [9].

𝑁 𝑆𝑀 𝐹𝐵 𝑢 ≥ ceil ( 𝑉 𝐿 𝑉 𝑆𝑀 ) (5)
𝑁 𝑆𝑀 𝐻𝐵 𝑢 ≥ ceil ( 𝑉 𝐻 𝑉 𝑆𝑀 ) -𝑁 𝑆𝑀 𝐹𝐵 𝑢 (6)
Comparing Eq. ( 3) and Eq. ( 4) with Eq. ( 5) and Eq. ( 6) it is seen how FBC leads to oversize the upper arms for transformation ratios less than 2 (defining the transformation ratio as the ratio between the HV and LV DC voltages, i.e . 𝑛 𝑑𝑐 = 𝑉 𝐻 /𝑉 𝐿 ). This means that, that the upper arms require FB-SMs and more HB-SMs compared to the normal operation requirements. For the lower arm the number and kind of SMs to provide FBC is the same that for normal operation. Therefore, The DC-MMC has to be oversized on the upper arms to provide FBC [9]. This oversizing is presented in Fig. 2. As an example, from Fig. 2, for 𝑛 𝑑𝑐 = 1.2 (𝑉 𝐿 = 533 kV) the required number of SMs for an operation in normal conditions is 107 per upper arm, all of them are HB-SMs. To include the FBC against HV side faults, the number of SMs per upper arm increases to 267 and must be of FB-SM type (Eq. ( 5)). To include the FBC against LV side faults, in addition to these SMs, 53 HB-SMs must be added (Eq. ( 6)). Thus, in total to include FBC for both types of faults the number of SMs per upper arm increased from 107 HB-SMs to 320 SMs (267 FBs+53 HBs). Considering that there are three upper arms, the oversizing is considerable. From Fig. 2, it is also observed that for 𝑛 𝑑𝑐 > 2 there is no oversizing to include FBC.

The use of a HV-Sw to avoid adding HB-SMs

To provide FBC without adding HB-SMs that are not necessary in normal operation on the upper arms, it is proposed to use an external switch HV-Sw between the HV side and the upper arms as shown in Fig. 3 [10]. This switch replaces the added HB-SMs on the upper arms to provide FBC against LV side faults. For HV side faults, the use of FB-SMs is still required as the proposed solution does not rely on a DCCB. During normal operation the switch is closed, and it is open only when there is a fault on the LV side.

Since breaking a DC current requires a circuit breaker, which is a costly solution, it is proposed to open the HV-Sw at zero current. To achieve this, a control method of the converter during the fault is proposed. With this approach it is expected that the requirements of breaking capability for the HV-Sw would be decreased and then a cost-effective solution like a disconnector could be used.

Reduction of number of SMs and losses

Fig. 4 shows the number of SMs (FBs and HBs) required on each upper arm in function of the transformation ratio for the classical DC-MMC as well as those needed with the proposed solution. It is observed how with the proposed solution, the number of HB-SMs needed in the topology to provide FBC is reduced. In the figure, the savings in SMs per upper arm are presented. For 𝑛 𝑑𝑐 = 1.5 the solution gives the maximum reduction on SMs. Since the conduction losses in a mechanical disconnector are lower than in the semiconductors of the removed SMs and because the current seen by the HV-Sw is only the DC current, contrary to the classical solution where the added SMs in the arms see the DC current and the circulating AC currents, it is expected that the proposed solution leads to an overall reduction on the conduction losses.

In addition, because the HV-Sw is kept closed during normal operation contrary to the classical solution where the added HB SMs are switching permanently to keep balanced the energy in the SM capacitors, it is expected that the proposed solution leads as well to a reduction on switching losses In the section of validation of the proposed solution, the losses are quantified.

Proposed control

In order to open the HV-Sw at zero current it is proposed to use the lower arms to control the DC current on HV side after a LV side fault. The proposed control scheme is based on the following steps:

1. LV side fault detection by a measurement of the voltage drop, the rise of the DC current (di/dt) or by overcurrent. 2. Blocking of all SMs in the upper arms by turning OFF all IGBTs in the SMs. 3. Control of the lower arm SMs to generate the highest possible DC voltage by inserting all the SM capacitors. 4. When the current on each of the upper arms is zero, the corresponding lower arm in the same leg is controlled to maintain the current at zero amps. 5. When the current in all upper arms is zero (the DC current on the HV side is also zero), the HV-Sw is opened at zero current. The opening time depends on the HV-Sw technology. 6. When the HV-Sw is totally opened, (which can be verified by the voltage at its terminals), all the SMs in the lower arms are blocked. The proposed control relies in the energy stored on the lower arms SMs to control the current. Since the lower arms still operate during the fault, the output inductances play an important role to decrease the fault current circulating by the arms.

In the following section the proposed control approach is validated, and a sensitivity analysis on the output inductor and SM capacitors is done.

Validation of the proposed solution in simulation

Control validation

To validate the proposed solution, simulations are done in Matlab/Simulink using the SimPowerSystems Toolbox. A case study based on the values of Table I is proposed. To model the circuit, an average model including the blocked state is used per arm as shown in Fig. 5. These models were proposed in [11], [12]. In these models, the SM stack is replaced by equivalent voltage and current sources, and all SM capacitors by an equivalent capacitor 𝐶𝑒𝑞. IGBTs and diodes are added into the model to simulate the blocked state according to the blocking signal of the arm 𝐵𝑙𝑘.

The value of the equivalent capacitor depends on the number of SMs on the arm and the SM capacitance:

𝐶𝑒𝑞 = 𝐶 𝑆𝑀 𝑁 𝑆𝑀 (7)
The voltage at the terminals on this capacitor represents the sum of the voltages of all the SMs in the arm.

The blocking signal is a boolean and is calculated by the controller of the converter. The modulation index 𝑚 is also used on the models. This variable represents the percentage of SMs being inserted at a given moment. It is calculated by the controller of the converter. The details of converter control during normal operation are omitted here for simplicity, but the reader can find more details about the control the converter in [5]. The focus of the paper is the proposed algorithm after the fault. In the simulation, a LV side fault is done at the converter terminals at t=0.8 s, at nominal power, then the different stages of the proposed control are executed. It is assumed that there is no delay between the detection of the fault and the start of the control algorithm. The fault detection is done by undervoltage and overcurrent measurements at the converter terminals. The controller time step is 40 µs. Fig. 6 presents the simulation results. Only the variables related to one of the converter legs are presented for clarity (except for the voltage on the equivalent arm capacitance and the lower arms currents that are presented for the three legs). The figure highlights the different control steps described in the previous section. A delay of 1.5 ms is assumed on the opening of the HV-Sw, between the trigger signal and the effective switch opening. From the simulation results it is seen that effectively the opening of the HV-Sw (t=801.76 ms) is done at zero current. The control scheme achieves to maintain the 𝐼 𝐻 current on the switch at zero amps during all the opening. Once the switch is totally open, it is seen that the HV voltage is withstood by the HV-Sw and not by the FB-SMs on the upper arm.

According to the simulation results, during the transient, the high-side current 𝐼 𝐻 did not reached unacceptable values. Thus, the FBC is demonstrated.

In the figure it is also observed that, to keep controlled the HV current at zero during the opening of the HV-Sw, the lower arm should generate a voltage which feeds the LV fault. Thus, the SM capacitors are discharged into the LV side fault and the fault current increases. This is evidenced by the decrease of the capacitor voltage and the increase on the lower arm currents. This phenomenon poses two constraints to the proposed solution. From one side, if the capacitor voltages are discharged below the required voltage to control the upper arm currents, the control is lost and it is not anymore possible to regulate the HV current. From the other side, the current increase on the lower arms poses a constraint in the sizing of the semiconductor current rating. If the HV-Sw opening is too long, any of both situations will appear: the lower arm SM capacitors will be discharged below the control limit or the currents on the lower arms will reach unacceptable values.

Sensitivity analysis

An analysis of the maximum time to open the HV-Sw at zero current was done in function of the sizing of the converter. Two parameters were varied: the lower arm SM capacitance and the filter inductances Lo. The results are presented on Fig. 7. The maximum time before losing the control is presented as well as the current on the lower arms at that moment. For example, for a design with Lo= 250 mH and Csml= 10 mF the maximal time is around 3.5 ms and the lower arms must withstand a current of 6 kA.

The times presented on Fig. 7 are calculated as the time between the fault and when the current 𝐼 𝐻 cannot be longer controlled. Thus, this time represents the maximum time to open the switch including the fault detection, processing, triggering, and switch opening.

Power losses analysis

In this section, the losses of the proposed solution are compared with the losses of the classical DC-MMC. Only the semiconductor losses are considered. To calculate the conduction and switching losses, a semi-analytic detailed model of the converter is built. In this model the equivalent current source and equivalent capacitor used in the average models (Fig. 5) are replaced by a set of equations that represent the behavior of the individual SMs. The details of this modelling technique can be found in [14]. In these simulations, it assumed that the ac voltages are the same for a converter with or without FBC (even if a converter with FBC has additional SMs and then can generate different ac voltages).

From the simulation, the arm currents are extracted as well as the control signals of each SM. With this data, it is possible to establish which semiconductor is conducting at each time and to detect the switching actions (bypass or insertion of a SM). Then it is possible to establish the RMS and average currents per device as well as the switched current.

The parameters 𝑅 𝑂𝑁 , 𝑉 𝑜 , 𝑎 𝑜𝑛,𝑜𝑓𝑓,𝑟𝑒𝑐 , 𝑏 𝑜𝑛,𝑜𝑓𝑓,𝑟𝑒𝑐 and 𝑐 𝑜𝑛,𝑜𝑓𝑓,𝑟𝑒𝑐 are obtained from the device datasheet.

In this paper a 3.3 kV / 1500 A power module is assumed (FZ1500R33HL3).

For the simulation, a low-level controller was added to control the switching of each SM. The choice was a Nearest Level control modulation with a Balancing Control Algorithm (BCA) based in a tolerance band as described in [15]. The tolerance band was set to 180 V. The control of the FB-SMs in the structure does not consider a negative insertion.

The sim ation is done d ring a ong eriod of time (9 seconds) to average the "random" switching phenomena caused by the BCA. The data of one SM on the upper arms and one SM on the lower arms was recorded to do the post-processing. The losses are calculated for different scenarios. As base values, the parameters of Table I are used, Table II presents those parameters which are changed in comparison with the previous case. IGBT Losses parameters 𝑅 𝑂𝑁 = 0.94 mΩ , 𝑉 𝑜 = 1.5 𝑉, 𝑎 𝑜𝑛 = 0.5435, 𝑏 𝑜𝑛 = 0.9946𝑥10 -3 and 𝑐 𝑜𝑛 = 5.7705𝑥10 -7 𝑎 𝑜𝑓𝑓 = 0.2929, 𝑏 𝑜𝑓𝑓 = 1.7079𝑥10 -3 and 𝑐 𝑜𝑓𝑓 = 0.4486𝑥10 -7

Diode Losses parameters 𝑅 𝑂𝑁 = 0.43 mΩ , 𝑉 𝑜 = 1.2 𝑉, 𝑎 𝑟𝑒𝑐 = 0.5706, 𝑏 𝑟𝑒𝑐 = 2.8654𝑥10 -3 and 𝑐 𝑟𝑒𝑐 = -7.2667𝑥10 -7 Fig. 8 shows the power losses of the proposed solution compared with the classical DC-MMC. It is verified that the proposed scheme reduces the converter power losses. The reduction is mainly on the switching losses. The reduction on conduction losses is lower and increases with the transformation ratio 𝑛 𝐷𝐶 . The conduction losses are still penalized by the high amount of FB SMs required on the upper arms to provide FBC against HV side faults.

  a. Three phase DC-MMC b. DC-MMC currents (the DC currents of only one leg are showed) Fig. 1: The DC-MMC and its operation principle with DC and AC circulating currents

Fig. 2 :

 2 Fig. 2: Number of SMs required on each upper arm for normal operation and to provide FBC. A DC system voltage of 𝑉 𝐻 = 640 𝑘𝑉 and SM voltage of 𝑉 𝑆𝑀 = 2 𝑘𝑉 were assumed as an example.

Fig. 3 :

 3 Fig. 3: DC-MMC with external HV-Sw to provide FBC against LV faults.

Fig. 4 :

 4 Fig. 4: Number of SMs per upper arm on the classical DC-MMC, in the proposed solution and the economy of HB-SMs for the proposed solution. A DC system voltage of 𝑉 𝐻 = 640 𝑘𝑉 and SM voltage of 𝑉 𝑆𝑀 = 2 𝑘𝑉 were assumed as an example.

Fig. 5 :

 5 Fig. 5: Simulation models of an arm used to model the DC-MMC.

Fig. 6 :

 6 Fig. 6: Simulation results highlighting the control steps (1 to 6). The LV side fault is done at t=800ms

Fig. 7

 7 Fig. 7 shows that the technology of the HV-Sw must be capable of opening in some milliseconds, which can be done with a fast disconnector [13]. The increase on the SM capacitors can increase the available time to open the switch, but as a consequence the lower arms should withstand more current. Increasing the filter inductance can help to decrease the current rating and provide more time to open the HV-Sw.

Fig. 7 :

 7 Fig. 7: Maximal time before losing the control at zero amps on the HV-Sw and current on the lower arms at that time in function of the lower arm SM capacitance and output filter inductance.

Fig. 8 :

 8 Fig. 8: Semiconductor losses comparison between the proposed solution and the classical DC-MMC.

Table I : Circuit parameters for DC fault simulations

 I

	SM capacitance (sized to have an acceptable SM voltage ripple during normal operation at nominal power)	2.8 mF (Upper arms) and 10 mF (Lower arms)
	Number of SMs per arm	313 FBs (Upper arms) and 400 HBs (Lower arms)
	SM nominal voltage	2 kV
	Arm inductance	25 mH
	Filter inductance	250 mH
	Parameter	Value
	DC voltage	640 kV (HV side) and 500 kV (LV side)
	Nominal Power	700 MW
	Operating frequency	150 Hz

  The conduction losses are estimated by Eq. (8) where 𝑉 𝑜 represents the saturation voltage if the switch is an IGBT or the threshold voltage if it is a diode, 𝑅 𝑂𝑁 represents the device equivalent resistance in the ON state, 𝐼 𝑅𝑀𝑆 the device RMS current, 𝐼 𝐴𝑉𝐺 the device average current.The switching losses are calculated with Eq. (9), where 𝑎 𝑜𝑛,𝑜𝑓𝑓,𝑟𝑒𝑐 , 𝑏 𝑜𝑛,𝑜𝑓𝑓,𝑟𝑒𝑐 and 𝑐 𝑜𝑛,𝑜𝑓𝑓,𝑟𝑒𝑐 are coefficients that approximate the energy loss at each switching of the device by a polynomial regression in function of the switched current 𝐼 𝑠𝑤 . The power losses are calculated by adding all the energy losses and dividing by the elapsed time.

			𝑃 𝑐𝑜𝑛𝑑𝑢𝑐𝑡𝑖𝑜𝑛 = 𝐼 𝑅𝑀𝑆 2 𝑅 𝑂𝑁 + 𝐼 𝐴𝑉𝐺 * 𝑉 𝑜	(8)
	𝑃 𝑠𝑤𝑖𝑡𝑐ℎ𝑖𝑛𝑔 =	1 𝑇 𝑡𝑜𝑡𝑎𝑙	∑(𝑎 𝑜𝑛,𝑜𝑓𝑓,𝑟𝑒𝑐 + 𝑏 𝑜𝑛,𝑜𝑓𝑓,𝑟𝑒𝑐 * 𝑖 𝑠𝑤𝑖𝑡𝑐ℎ𝑒𝑑 + 𝑐 𝑜𝑛,𝑜𝑓𝑓,𝑟𝑒𝑐 * 𝑖 𝑠𝑤𝑖𝑡𝑐ℎ𝑒𝑑 2	)

Table II : Parameters for simulations for the calculation of power losses

 II

	Parameter			Value
	LV side DC voltages	533 kV	457 kV	427 kV	400 kV	356 kV
	Ndc	1.2	1.4	1.5	1.6	1.8
	Number of SMs (FBs) per upper arm	334	286	267	250	223

Acknowledgements

This work was supported by a grant overseen by the French National Research Agency (ANR) as part of the "Investissements d'Avenir" Program ANE-ITE-002-01.

Conclusions

An alternative method for providing FBC on the DC-MMC is proposed in this paper. The method is based on the use of an external switch. With the proposed solution, a decrease in the number of installed SMs is achieved and as well as a decrease in power losses. A control method was proposed to operate the switch at zero current. This avoids the need of breaking capability on the switch. Thus, it can be implemented with a fast disconnector. The operation of the switch and the control method was verified in simulation. A sensitive analysis on the circuit parameters was done to analyse the maximum opening time of the switch. It is seen that the HV-Sw must operate in some milliseconds and that the lower arms must withstand the fault current. The opening time of the HV-Sw is related to the maximal current that the semiconductors in the lower arms must be able to switch and to the minimal SM capacitance in lower arms. Thus, the proposed solution reduces the number of SM in the upper arms but, according to the HW-Sw performance, can lead to oversize the lower arm SMs. Finally, the proposed method can provide FBC against LV side faults but for HV side faults the DC-MMC is still oversized by the need of FB-SMs. Conference on AC and DC Power Transmission (ACDC 2019), 2019, pp. 1-6.

[4] J. D. Páez