Carry out Watch Keeping and Machinery Operations
Training Objective: 1.1 Operate the distribution system
Task Number: 1.1.3
With the aid of a block diagram, describe the operation of the high voltage filters. List the
trips associated with the HV filter
The switching operation of the drive converters and other non-linear devices introduces
considerable harmonics into the HV distribution system.
Linear/ non-linear loads
Loads in AC electrical systems can be designated as linear or non-linear depending on how they
draw current from the supply.
Linear loads (transformers, motors, capacitors)
 Current and voltage waveforms are sinusoidal
 Current is proportional to voltage (ohms law applies)
 Impedance is constant
Non-linear loads (VSDs, UPS, electronic equipment)
 Impedance changes with applied voltage
 Current is not proportional to voltage (ohms law does not apply)
 Waveform of current drawn by non-linear load is not sinusoidal (non-linear loads draw in
currents in abrupt short pulses)
 These pulses distort the current waveforms & generate harmonics
Electronic devices used in switching draw current differently to other loads. Instead of the load
having a constant impedance drawing current in proportion to the supply voltage, electronic
devices change their impedance by switching on and off near the peak of the voltage waveform.
Switching loads on and off during part of the waveform results in short, abrupt, non-sinusoidal
current pulses during a controlled portion of the incoming peak voltage waveform. This pulsating
current can introduce reflective currents (harmonics) back into the power distribution system.
Harmonics are defined as sinusoidal waveforms that operate at a frequency which is a multiple of
the fundamental frequency (60Hz).
The design of the advanced induction motors (AIM) and converters themselves helps to reduce
harmonics.
Harmonics that are a multiple of 3 are automatically cancelled.
Passive harmonic filters are employed to provide added Total Harmonic Distortion (THD) removal.
Excessive harmonic distortion of current and voltage can cause heating in the windings of
transformers, generators and induction motors which presents a fire risk. Furthermore, inductive
heating of current carrying conductors such as breakers, conductors, fuses etc are a symptom of
harmonics. They can also cause unpredictable electronic equipment operation.
Symptoms of harmonic distortion in electrical system:
 Circuit breakers tripping
 Equipment failure/ malfunction
 Conductor failure
 Fluorescent lights flickering
 Interference on communication systems
 Fuses blowing
 Transformer failures
Methods to reduce the effects of harmonic distortion include:
 Active or passive filters
 Increasing the number of pulses in power converters
 Use of phase shifting transformers (star-star/ star-delta)
 Installing generators with a large sub-transient reactance
The main harmonics are the 5th, 7th, 11th and 13th with smaller magnitudes in harmonics of higher
orders. Each filter is designed to absorb a large proportion of the main harmonics generated by
the propulsion motor converters and to provide 6.6 MVAr of power factor correction at 60Hz (i.e.
capacitive).
There is a harmonic filter for each section of the HV system that supplies a propulsion motor
converter i.e. HVSB1, HVSB3 and HVSB4. These filters are designed to absorb the harmonics
generated by the propulsion motor converters and prevent disturbance to the HVSB busbar
voltage.
A maximum of 2 filters can be connected in parallel at any one time to prevent leading VARs on the
connected generation capacity if propulsion is lost.
Configurations as follows:
 Minimum of 1 filter for 2 motors, maximum of 2 filters on any one island
 Minimum of 2 filters for 4 motors, maximum of 2 filters on any one island
 Maximum of one filter if only 1 DG connected
The internal components of the filters consist as follows:
 Capacitors
o Each of the Capacitors is a 3-phase Capacitor which, when combined with a
Reactor, will form a 3-phase Passive Harmonic Filter.
o The HV Passive Filter consists of 3 limbs – 5th Notch Circuit, 5th C-type Circuit and
11th Notch Circuit.
o 33 capacitors spread across 3 limbs which make up the filter (limb1, limb2, limb3)
o Capacitors in all 3 limbs are connected in 2 parallel star group configurations.
o Discharge resistors in parallel with harmonic reactors allow fast discharge.
o Neutral current is monitored between 2 star arrangements using a current
transformer.
 Reactors
o 3 reactors
o Tappings on the core windings allow for fine tuning which enables the resonant
frequency of the filter to be adjusted
 Pre-charge resistors
o Limits inrush current to capacitors, minimising voltage disturbance on the HV
busbars when the filter is energised
o One per phase connected in parallel to the pre-charge VCB
 Damping resistors
o 3 Damping resistors
o Combine with reactors and capacitors to form a passive harmonic filter
o Damping resistor dissipates harmonic currents as heat
o Damping resistors are metal elements mounted in midel oil cooling tanks
o Wild heat transferred to chilled water system via heat exchanger
 Harmonic protection relays
o Each limb of the filter has its own harmonic protection relay
o Relays provide:
 Overvoltage protection
 Thermal overcurrent protection
 Capacitor neutral unbalance
 Line current unbalance protection
 Earth fault protection
 Fundamental frequency overcurrent protection
 RMS overcurrent/ undercurrent protection
 Capacitor bank discharge time
 Discharge reactors
o Two fitted within harmonic filter assembly
o Damping reactors provide protection for capacitor banks from outrush currents.
o Connected to VCB and outgoing busbar
o Function is to quickly discharge the filter system if the mains power is disconnected
 Cooling system
o 2 systems – chilled water/ air and chilled water/ midel
o 3 440v cooling fans – temperature within reactor is monitored by RTD
o 440v midel pump
 VCB
o rated to 1250A, provides O/C and earth fault protection
Harmonic filters work by the use of a tuned filter to provide a low impedance ‘sink’ for the harmonic
current at a particular frequency. They comprise, in simple terms, a series inductance and
capacitance whose impedance is zero at the tuned frequency e.g. 5th, 11th, etc. They work by the
harmonic current flowing preferentially into the filter rather than the generators, minimising the
voltage drop in the generator and, hence, the voltage THD.
Several banks of filters (limbs) are connected in parallel to provide harmonic filtration across the
orders encountered in the HV system.
Band pass filter (LC filter)
 Reactor connected in series with capacitor
 Gives minimum impedance at resonant frequency and blocks signals of other frequencies
 Used to filter lower order harmonics (normally up to 7th)
High-Pass Filter (5th & 11th Notch)
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Resistor added in parallel with the reactor in the band-pass filter = high-pass filter
Commonly used for filtering higher order harmonics
Better filtering characteristics for higher frequencies
Increased losses at fundamental frequency
C-Type Filter (5th Notch)
 Modified high-pass filter for low tuning and heavy damping
 Eliminates losses at fundamental frequency
Trips Associated with HVHFs:
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Reactor high temperature 125°C
Resistor high temperature 150°C
Critical shutdown – Midel pump stop
Capacitor C2L overpressure alarm
Midel temperature trip 65°C
Loss of chilled water cooling
Peak repetitive overvoltage protection
Thermal overcurrent protection.
Capacitor neutral unbalance protection using a dedicated CT.
Line current unbalance protection
Earth fault protection
Fundamental frequency overcurrent protection
RMS overcurrent protection
Undercurrent protection
Capacitor bank discharge timer to inhibit the breaker from closing onto partially charged
capacitors.