HVDC TRANSMISSION SYSTEM PERFORMANCE
FROM EMC PERSPECTIVE
HVDC technology deals with efficient transportation of hundreds of million watts of
electrical power through conversion from ac to dc at the source end and dc to ac at the
load end. Irrespective of system configuration, the converter generates significant amount
of power harmonics and feeds onto the transmission system. System of these kind
couples unwanted electrical noise into co-located telephone circuits. This paper analyses
the generation and coupling mechanism of power harmonics from HVDC to
telecommunication systems and describes a case study involving Under Ground
telephone cables running parallel to HVDC transmission lines.
With the advent of High voltage / High power solid-state control rectifiers (thyristors,
GTOs), and micro electronic devices, it has become possible to design and develop
HVDC (High Voltage Direct Current) transmission system as a viable alternative to the
widely known 50Hz, HT (High Tension) power transmission system, to achieve low
transmission loss, high transient stability and better inter-state and inter-national power
synchronization and linking. HVDC transmission system adopts line-frequency phase-
controlled rectification and inversion technology to control the dc power output.
From analysis point of view, the transmission lines may be considered as a simple two
wires electrical network, forming a large loop with earth or solid conductor as current
return path. A system of this nature feeds considerable amount of harmonic voltage at dc
output side and injects harmonic current into the ac input side. It is not uncommon to run
power and signal lines in parallel for considerable distance due to logistic reasons, like
terrain constraints, maintenance accessibility etc., Under this circumstance, it is likely
that the harmonics in dc side may couple unwanted electrical noise into the co-located
underground or open wire telephone systems. This may lead to serious consequences like
voice degradation and errors in telecommunication signalling. In extreme cases, it may
lead to break-down of telephone terminal equipments. To analyze the EMI
(Electro Magnetic Interference) phenomenon, a site survey was conducted for HVDC
transmission system operating in three phase, six- pulse converter configuration. The
observation suggests that the preference of HVDC transmission system over ac
transmission system should always consider the likelihood of unwanted power harmonic
induction (cross-talk) from power lines to co-located communication lines. Unless
adequate care is taken to suppress the power harmonics to an acceptable limit at the
source end, the HVDC system may turn-out to be a potential source of EMI.
ANALYSIS OF HVDC TRANSMISSION SYSTEM INTERFERENCE
A typical one-line diagram of an HVDC transmission system for inter connecting two ac
systems by an HVDC transmission line is shown in the Figure 1. Power flow can be
controlled by varying the conduction angle of line-frequency. Assuming that the power
flow to be from system 'A' to 'B', the system input ac voltage in the range of 69- 230KV
is transformed up to the transmission level and then rectified by means of the converter
terminal 'A' and transmitted over the HVDC transmission line. At the receiving end, the
dc voltage is inverted by means of the converter terminal 'B' and the voltage is
transformed down to match the ac voltage of system 'B'. The power received over the
HVDC transmission line is then transmitted over ac transmission and distribution lines.
HVDC converters generally fall in any one of the following categories:
Star-Star 6-pulse mono-pole rectification
Star-Star, Delta-Star mono-pole 12-pulse
Star-Star, Delta-Star bi-pole 12-pulse rectification.
The converter terminal given in Figure.1 consists of a positive pole and negative pole.
Each pole consists of two 6-pulse, line frequency bridge converter connected through Y-
Y and D-Y transformers to yield a 12-pulse converter arrangement on the ac side of the
converter. The filters are required to reduce the current harmonics generated by the
converters from entering the ac system.