5.2 Power quality analysis
5.2.2 Effect of the power quality issues of the MV network voltage 103
The European Standard EN 50160 on power quality regulates the voltage characteristics of the electricity supplied in public distribution networks. For the LVDC network we may state that
· Variations in the MV network power frequency produce a corresponding variation in the sixth-harmonic component frequency in the DC network; in particular, for the LVDC network with a passive thyristor/diode rectifier at the
network front end. Variations in the MV network power frequency have no effect on the customer-end LVAC power frequency.
· Variation in the voltage magnitude has an impact on the network DC voltage level at the rectifier, in the case of the passive thyristor/diode rectifier at the network front end. With the active rectifier DC voltage is regulated to the reference value. The customer-end LVAC voltage quality in both cases is not affected. Voltage magnitude variations have no effect on the magnitude of the customer-end LVAC voltage.
· Supply voltage dips may reduce the DC voltage level at the rectifier. Capacitive energy storages located at the network front-end rectifier allow ride-through during severe voltage dips. Supply voltage dips have no effect on the customer-end LVAC voltage quality. If the DC voltage level is reduced below the CEI limit, the voltage magnitude of the LVAC will be reduced by 10 %, being still within the standard limits.
· Short interruptions of supply voltage affect the DC voltage level at the rectifier.
Depending on the network load, capacitive energy storages located at the network front-end rectifier can enable ride-through during short interruptions.
Short interruptions (less than one second), have no effect on the customer-end LVAC voltage quality. During longer interruptions, the network is powered down and the customer electricity supply is interrupted.
· Power frequency overvoltages will temporarily increase the voltage level of the DC network. The customer electricity supply is interrupted for safety reasons if the overvoltage exceeds the threshold settings of the CEI.
· Transient overvoltages appear as a transient voltage on the DC network.
Depending on the voltage level, they could cause an interruption in the customer electricity supply.
· A supply voltage unbalance (up to 3 %) causes variation in the DC network voltage. A supply voltage unbalance has no effect on the customer-end LVAC voltage quality.
· A harmonic voltage causes harmonics to the DC network voltage. A harmonic voltage has no effect on the customer-end LVAC voltage quality.
· An interharmonic voltage causes harmonics to the DC network voltage. It has no effect on the customer-end LVAC voltage quality.
Common to all of the above-listed events in the MV network is the fact that most of them affect the DC network voltage. This sets a design requirement for the CEI to withstand harmonic variations in the DC network voltage without causing LVAC power quality issues.
Supply voltage unbalance
Because of a supply voltage unbalance, multiples of the second-harmonic components of the main frequency are injected by the rectifier bridge to the DC network. The DC voltage has a corresponding ripple. Figure 5.5 demonstrates a case of a 3 % unbalance
on the MV network. In this case, the harmonic content of the DC network becomes richer than in the case of a balanced MV network (Figure 5.3 and Figure 5.4).
Figure 5.5. DC network voltages and currents on the terminals of the rectifier and the inverter.
5.2.3 Harmonic measurement on the research platform
In this section, the measurement results of the harmonic content in the DC distribution system on an actual network distribution system are be presented. Variation and propagation of the harmonic content are shown and discussed.
A residential load is not balanced neither constant in nature. The current harmonic content varies as a result of the connection and disconnection of appliances, as described in subsection 2.2.1. These changes in the load produce changes to the harmonic currents in the DC network. From the weekly measurement data, the case of the lowest sixth-harmonic current was chosen to represent the network in light load conditions. Figure 5.6 and Figure 5.7 present the corresponding measurements of the harmonic currents at the CEIs.
In Figure 5.6, the CEI 1 load is mostly a single-phase non-linear load, and the phase C current spectrum clearly represents the harmonic content of the single-phase diode bridge input current. Because of the three-phase unbalance of the CEI load, the DC current content contains 2nd– and 4th-order harmonics because of the 3rd and 5th harmonic currents. The 6th harmonic is a sum of the rectifier injected harmonic and the CEI-injected harmonic produced by the 5th and 7th load current harmonics. The CEI 3 load harmonic content is naturally different; it is also sinusoidal and single-phase non-linear, and further, the three-phase diode bridge input current harmonic content is noticeable. Because of the richer harmonic content of the load, the CEI input DC current harmonic content is also richer.
Idc (line)
From the weekly measurement data, the case of the highest sixth-harmonic current was chosen to represent the network during a high load. Figure 5.8 and Figure 5.9 present the corresponding snapshot of the harmonic currents at the CEIs. The DC network DC current amplitude is much higher than in the previous measurements in light load conditions. The 2nd, 4th and 6th harmonics are still the most significant harmonic components in the DC network current. Moreover, as presented, the measurements of the 6th harmonic component of the DC current on the CEI allowed identification of the light-load and high-load conditions on the LVDC network.
From the weekly measurement data, the case of the highest second-harmonic current was chosen to represent the CEI during an unbalanced load. Figure 5.10 and Figure 5.11 present the corresponding snapshot of the harmonic currents at the CEIs. The measurements showed that the 2nd harmonic current, which is injected by the CEI as a result of the three-phase unbalance, can be significant in amplitude.
Figure 5.6. Values of current harmonics for the CEI 1, light-load condition snapshot.
Figure 5.7. Values of current harmonics for the CEI 3, light-load condition
Figure 5.8. Values of current harmonics for the CEI 1, high-load condition snapshot.
Figure 5.9. Values of current harmonics for the CEI 3, high-load condition snapshot.
Figure 5.10. Values of current harmonics for the CEI 1, unbalanced load conditions.
Figure 5.11. Values of current harmonics for the CEI 3, unbalanced load conditions.
DC1 2