The Ethiopia-Kenya Project: HVDC vs AC Transmission

This article was originally written by Eng. Daisy Karimi Muthamia and published on Energy Central. The article has been re-published here with minor changes, with the permission of the original author.

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In 2022 the Kenya Electricity Transmission Company (KETRACO) and Ethiopia Electric Power Company (EEP) interconnected their two power systems through the construction of a 1069 km, 500 kV bipolar High Voltage Direct Current (HVDC) transmission line, known as the Ethiopia-Kenya HVDC project. With a 2000 MW capacity, the transmission project runs from Woloyta in Ethiopia to Suswa in Kenya.

Since completion, the project has supported the interconnection of countries within the Eastern Africa Power Pool - including creating part of the backbone of interconnectors for the region. This forms one of various transmission interconnection projects which are currently under development to integrate the Eastern Africa power market. At a later stage, these interconnectors will see the region connected to the Southern Africa Power Pool through Tanzania.

Typical utility-scale power plants generate alternating current (AC) electricity, and most electrical loads run on AC power. Therefore, most transmission lines are of the AC type. Nevertheless, we often see DC application in connecting off-shore wind generation; in instances where right‐of‐way is constrained; where power is to be transmitted over long distances; and when interconnecting asynchronous grids. In this article we are going to have a brief look at the HVDC technology and how it differs from AC transmission.

One of the limiting factors for HVDC projects is the capital cost of converter stations as compared to the equivalent AC substations. Breakeven studies looking at distance, voltage, power transfer and lifecycle costs show that the capital cost slope for HVDC systems flattens as distance increases - usually with a breakeven point achieved at distances between 500 - 700km for overhead transmission. This is partly due to the way HVDC conductors are configured. AC systems have three separate phases, usually with heavy multiple bundles of conductors. Consequently, the towers must be massive to support the weight. A HVDC line can deliver comparable amounts or even higher amounts of power, using only two sets of conductors as opposed to three, so the towers don’t have to be quite as large,  resulting in a much lower installed cost on the transmission line part.

At a time when grid development should be environmentally sustainable, HVDC transmission offers environmental advantages as it typically has less wayleave requirement compared to an AC transmission line of the same power rating. AC transmission lines also have higher visual impact due to requiring larger support structures.

HVDC transmission provides other advantages over AC transmission such as power flow control which offers stability to the network and opportunities for controlled trading. HVDC systems have highly dynamic components which interact with AC systems contributing to system stability by improving overall dynamic behavior of power systems. For example, the voltage control of VSC HVDC links can be used to damp oscillations at the AC side; and the HVDC link can provide frequency support to other asynchronous zones through adequate control of the power flow through the link.

AC and DC technologies also differ in the components used. Some of the key components in a HVDC system include converter stations, converter valves, converter transformers, AC filters, DC filters, electrodes, and smoothing reactors.

AC power from the sending-end station is first converted into DC with the help of a rectifier. The DC power then flows through the overhead lines and, at the receiving end, is then converted into AC using an inverter. Thus, converters on the terminal ends of a HVDC link carry out the AC to DC and DC to AC conversion.

A converter station includes valve bridges and converter transformers. Modern HVDC converters use a 12-pulse valve bridge - made up of in-series-connected thyristor modules. The valves are installed in valve halls and require cooling with water or air.

The converter transformers have two sets of three phase windings, connected in star and delta. The AC side winding is connected to the AC bus bar and the valve side winding is connected to the valve bridge. The valve side transformer winding is designed to withstand a combination of alternating voltage stress and direct voltage stress from the valve bridge.

HVDC converters generate harmonics which have some adverse effects. AC and DC filters are therefore used to minimize these harmonics. The AC filters are RLC circuits connected between phases and earth; and offer low impedances to high harmonic frequencies. They also provide reactive power required during operation of converters. DC filters divert DC harmonics to ground and are connected between the pole bus and the neutral bus.

Each pole consists of series-connected smoothing reactors which reduce the steepness of voltage-current surges from the DC line and are useful in preventing commutation failures occurring in inverters by reducing the rate of rising of the DC line in the bridge. Thus, the stresses on the converter valves and valve surge diverters are reduced.

The system also includes ground electrodes which are used to connect the system to the earth. Other key components include the AC and DC switchgear.

The Eastern Africa Electricity Highway project is not the first HVDC line in Africa. Other major HVDC transmission line projects in Africa include the Inga - Kolwesi in DRC (580MW, 1700 km, in operation since 1982), Cahora Bassa, connecting Mozambique and South Africa (1920 MW, 1420 km, in operation since 1979), and Caprivi Link, connecting Namibia and Zambia (300 MW, 950 km, in operation since 2010). Other HVDC projects in the pipeline include the EuroAfrica Interconnector a 1000MW HVDC link between Egypt and Cyprus and the 1300km with 16km submarine, 500kV, 3000MW Egypt – Saudi Arabia interconnector.

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