Innovative voltage control technology for rural distribution grids

Innovative voltage control technology for rural distribution grids

Posted by Ernestas Zimkus

In the last few years increasing numbers of connected EV chargers, solar PV (prosumers), heat-pumps started to have significant negative impact on voltage at the AB Energijos skirstymo operatorius (ESO) low voltage electricity network. For example, 3 years ago ESO experienced rapid uptake of new solar PV installations. In 2019 prosumers installed capacity was only 27,5 MW and till 2022-08 this number grown to 258 MW.

Such a rapid increase of generated electricity feed-in and number of high power and consumption equipment (EV chargers, heat-pumps) negatively affects voltage levels in ESO 0,4 kV distribution grid. Changing residential customers’ load curves creates big voltage variations, voltage asymmetries, over-voltages and under-voltages during day and night periods.

Situations arise in certain grid locations, where due to changing load profiles electricity supplied to customers does not meet power quality requirements according to the LST EN 50160 standard. Such grid locations require expensive and time-consuming grid expansions or reconstructions.

Mainly distribution grids in rural areas were designed for small energy consumption without consideration of decentralized generation, EV chargers or heat-pumps.

Furthermore, the electricity network was designed for symmetrical 3 phase high power loads, but today we have single phase 7 kW EV chargers or single-phase heat-pumps connected to 3 phase networks.

During the period of 2020-2022 ESO piloted small power STATCOM equipment in the 0,4 kV rural distribution grid. STATCOM is a fast-acting device capable of providing or absorbing reactive current and thereby regulating the voltage at the point of connection to a power grid. Such technology has considerable potential in rural distribution grids (long 0,4 kV overhead lines) to control and stabilize fast changing voltage levels due to earlier mentioned reasons.  

First tested system was Ecojoule Ecovar STATCOM with 30 kVAr capacity in three phase configuration.

Ecovar is capable of asymmetrically controlling reactive power in each phase individually (10 kVAr per phase). Such functionality provides benefits in long rural distribution lines. It can compensate a single phase under or over voltages caused by single phase loads or single-phase PV generators. In the graph below can be seen the grid voltages and effect on voltage during Ecovar testing.

During the period of 2021-07-20/2021-08-20 with ECOVAR turned OFF we can see noticeable over-voltages (up to 265 V), under-voltages (up to 190 V) and voltage asymmetry in the grid. These parameters are outside of LST EN 50160 requirements for voltage quality. On the other hand, with ECOVAR turned ON voltage levels were within LST EN 50160 acceptable limits.

Second tested system was reactive power inverter from KACO. It‘s a three phase symmetrical inverter with 50 kVAR total capacity. Inverter design optimized for PV plants, where reactive power support is required by utility.

KACO inverter was adapted to be used on low voltage overhead distribution lines. Additionally, an inverter controller was needed to control reactive power depending on grid voltage level. Control algorithm for the inverter was developed by Elseta and implemented on the Elseta WCC Lite RTU controller. Pilot location was with an overhead line of 650 m in length and at the end of line one prosumer with PV, EV charger and heat-pump.

Pilot tests revealed limitations of symmetrical 3 phase reactive power inverter:

  1. During the night customer charges EV with 32 A single phase load, which creates voltage asymmetry. Reactive power inverter boosts grid voltage by injecting symmetrical reactive power to the grid according to the preset Q(U) volt-var curve. All 3 phase voltages are increased by inverter’s injected reactive power;
  2. Symmetrical voltage dip when the inverter is turned off for testing purpose;
  3. Symmetrical voltage increases when inverter injects maximum of 50 kVAr of reactive power to the grid.
  4. Single phase charger stops changing and voltage asymmetry disappears in the grid (KACO stops reactive power injection into the grid).  

STATCOM technology for voltage control in long rural grids is a good alternative for traditional voltage regulators with autotransformers. It has an advantage over regulators to control voltage upstream and downstream the line. The voltage control effectiveness depends on grid line length and conductor inductance.

Reactive power in the grid creates substantial active network losses due to increased current in the line. Graph below illustrates pilot site grid losses depending on the inverter reactive power (- 20 kVAR in the graph means the inverter is injecting reactive power to the grid to boost voltage).

Test results show, that most economical way to use STATCOM technology for grid voltage reduction application during day time (high PV energy injection to the grid). In such configuration STATCOM operates relatively small amount of time (during sunny days only) and total grid losses are acceptable for DSO. It is not economical to use STATCOM technology for constant voltage boosting application, as such application will generate considerable annual active network losses in the grid. From economical point of view STATCOM technology in rural grids should be as a temporary solution to mitigate voltage problems till grid reconstruction.

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