Pasi Nuutinen

Short-Circuit Protection in a Converter-Fed Low-Voltage Distribution Network

The short-circuit protection of a low-power converter-fed low-voltage dc distribution network is investigated. In a public distribution network, the short-circuit protection is implemented by using fuses and circuit breakers, which may require a current that is over ten times the nominal currents of low-power dc/ac converter units to satisfy the recommendations. In the paper, the operation of the converter in a short circuit is studied, and the control scheme for the short-circuit current control and the fault ride through operation is proposed. The operation of the short-circuit control scheme is verified by measurements. To overcome the problem of high fault current injection, alternative short-circuit protection realizations that are based on a simple low-current compact circuit breaker (but are not approved in standards) are proposed and demonstrated by measurements.

Research Site for Low-Voltage Direct Current Distribution in a Utility Network—Structure, Functions, and Operation

This paper introduces a research site for a low-voltage direct current (LVDC) distribution system. The research site was established to enable practical studies concerning different areas of the LVDC distribution. Because the LVDC distribution is a novel approach to public electricity distribution, the objective is to combine a fully functional LVDC system and a flexible research platform, where various measurements and tests can be conducted. The scope of this paper is to discuss the design, structure, functionalities, and operation of the site. Experiences of use and measurement results are provided to demonstrate the major functionalities and other important system properties. Finally, future research tasks and the development of the site towards the LVDC μGrid are presented.

Implementing a laboratory development platform for an LVDC distribution system

Low-Voltage Direct Current (LVDC) distribution network is a novel approach to the LV distribution. The system greatly differs from the AC solutions used today and therefore, a laboratory platform of the LVDC distribution system has to be implemented. The system is designed to provide testing and measuring platform for the LVDC distribution technology. This paper presents the implementation of the platform. The paper discusses the structure of the platform and presents some results of the studies in which the platform is involved. Also, the flaws of the system and their influence to the measurements and operation are discussed. Also, a real-network platform implementation is in progress and the future developments of the laboratory platform are covered in the end of the paper.

Noise analysis of a power-line communication channel in an LVDC smart grid concept

It has been shown that high-frequency (HF) band power-line communication (PLC) is a suitable data transmission method for the smart grid concept, namely a low-voltage direct current (LVDC) distribution system. In this paper, the PLC channel analysis in the LVDC system is continued with a detailed noise analysis. Noise signals from the terminals of PLC couplers are measured in the time domain. Noise power spectral densities (PSDs) are derived from the measured noise voltage, and the variation in the noise PSDs in the time domain is analyzed. The effects of noise on the performance of the HF band PLC are analyzed theoretically and by communication tests. The noise voltages are measured, and communication tests are carried out both in the LVDC laboratory system and in an LVDC field pilot system. The noise PSD is observed in the frequency band between 100 kHz and 30 MHz, which covers the band applied to the PLC in the channel. The results show that noise (mainly impulsive) in the channel has a major effect on the performance of the PLC.

Observer-Based Output Voltage Control for DC Power Distribution Purposes

Observer-based output voltage control is proposed for single-phase full-bridge inverter, that is used to produce the voltage for a customer in a low-voltage direct current distribution network. To avoid difficulties to realize a feasible-output-voltage-tracking control for sinusoidal references, the control system is realized in a virtual dq-frame. The virtual frame is created using a second-order generalized integrator. In the proposed control system, two different observer structures are applied. Using observer-based control, also provides means to reduce the number of measurements down to one. In this paper, different realizations of an estimator-based control system for a single-phase inverter are investigated. Specifically, it is considered how many measurements are needed and which variables should be measured and which estimator structure is to be applied in order to achieve the best overall performance.

Voltage Oriented Control of a Single-Phase LVDC Distribution Network Inverter

Capacitor voltage oriented PI control is implemented to control the phase voltage of a customer-end single-phase inverter in a low-voltage DC (LVDC) distribution system. Synchronous dq-frame is used to control the phase voltage to avoid the drawbacks of the PI control, such as possible steady-state error. In the paper the controller equations are derived from the system equations and then the controllers are simplified and their performances are studied. In addition to using different controller structures also different synchronization methods are considered. Finally, the measurements are made with two different control system structures, namely, PI control and PI control with capacitor voltage error feedforward, to verify the simulation results. The study reveals that the synchronous frame control is a suitable control method to control the phase voltage of the LVDC customer-end inverter. It also shows that the controller structures can be simplified drastically if the load is resistive.

On Common-Mode and RF EMI in a Low-Voltage DC Distribution Network

This paper addresses common-mode (CM) and radio frequency (RF) electromagnetic interferences (EMI) in a converter-fed low-voltage direct current (LVDC) distribution research network. The LVDC distribution is a novel approach to public electricity distribution, and a research site is set up to enable comprehensive studies concerning the LVDC distribution. Because the customers in the LVDC system are supplied with customer-end inverters (CEI), CM and RF EMI generated by the CEIs have to be analyzed. To this end, common-mode currents in the DC network and the customer-end networks are studied by on-site measurements. The objective is to determine whether there are disturbances that exceed the requirements of the standards or whether the CM current could cause safety issues in the converter-fed user-end network. In the DC network, the effect of the CM current on the feasibility of power line communication (PLC) in a DC network is analyzed. Finally, RF disturbances are measured and discussed.

Galvanic Isolation and Output LC Filter Design for the Low-Voltage DC Customer-End Inverter

In this paper the need for a better design of the galvanic isolation and the output filter inductor in the customer-end inverter (CEI) of the low voltage DC (LVDC) network is discussed. The galvanic isolation can be implemented either with a 50 Hz transformer after the CEI or with an isolated DC-DC converter between the DC network and the CEI. However, the 50 Hz transformer solution adds a significant amount of volume and mass to the system. Based on calorimetric measurements conducted with the current system in which the galvanic isolation is implemented with the former method, the minimum requirements for the isolated DC-DC converter are presented and a comparison is carried out. For the system to be cost efficient the DC-DC converter should result in reduced lifetime cost compared with the transformer solution. In the output filter part a design method for an amorphous core filter inductor based on minimization of the lifetime cost is presented.

Applicability of narrowband power line communication in an LVDC distribution network

It has been shown that high-frequency (HF) band power line communication (PLC) is a feasible data transmission method for a smart grid concept, namely for a low-voltage direct current (LVDC) electricity distribution system. However, because of the signal attenuation, the communication distances in the LVDC grid are limited with the HF band PLC. Hence, the new narrowband (NB) PLC should also be tested for the concept. In this paper, the PLC channel analysis in the LVDC system is carried out for the novel NB PLC technology, that is, the G3-PLC in the frequency band of 150-490 kHz. For the channel analysis, channel gain and noise signals from the terminals of the PLC couplers designed for the NB PLC are measured. The performance of the G3-PLC is analyzed theoretically and by communication tests. The noise voltages are measured, and communication tests are carried out in the LVDC laboratory system and in an LVDC field installation grid. The results show that NB PLC could be suitable for the concept.

On Low-Voltage DC Network Customer-End Inverter Energy Efficiency

In this paper, the results of the energy efficiency and power loss measurements of a 16 kVA customer-end inverter (CEI) unit are analyzed. The CEI is part of the low-voltage dc (LVDC) distribution network. The efficiency of the CEI power conversion significantly affects the energy efficiency of the LVDC distribution network and the electricity supply chain. The efficiency of the CEI and its components is measured by applying calorimetric and electrical input-output methods. The power loss measurement results are analyzed, and the power loss mechanisms are described. The efficiency optimization goals are discussed and set.