Abstract: With the acceleration of the urbanization process, in the
reconstruction of old cities and the construction of new areas, GIS is used to
reduce land occupation, and it must also adapt to the fire protection
requirements of cables entering the city, cables going to the ground, oil-free
and fire prevention and flame retardancy. Some substations The use of new 110kV
transformers is an unavoidable choice. In addition to the existing 110kV
oil-immersed transformer, new transformers can be selected...
1. Selection of 110kV transformer
With the acceleration of urbanization, in the reconstruction of old cities
and the construction of new areas, GIS is used to reduce land occupation, and to
adapt to the fire protection requirements of cables entering the city, cables
going to the ground, oil-free and fire-proof and flame-retardant. Some
substations adopt The new 110kV transformer is an unavoidable choice. In
addition to the existing 110kV oil-immersed transformer, the new transformers
that can be selected include:
1. Dry-type transformer. It is mainly a resin cast type product. It is
currently commonly used in 35kV power distribution systems. There are already
single-phase transformer group products in operation at the 110kV level. The
main problem is that the design field strength along the surface of the
three-phase transformer coil is too high and long-term operation. There are
still some problems with the insulation reliability and adaptability to the
environment. The IEC standard is being revised. The three tests of environment,
climate and combustion proposed in the new standard to be adopted are important
methods for assessing the insulation performance of dry-type transformers.
2. Gas insulated transformer. Mainly refers to SF6 gas insulated
transformers. In the future, new gas insulating media with better environmental
performance will be developed. The sealed transformer shell is filled with a gas
insulating medium of a certain pressure, and the gas is circulated and
dissipated under the action of the air pump. It is required that the body and
the radiator have good pressure-bearing sealing performance and keep the annual
air leakage rate at a relatively high level. low level. There are currently
imported products in operation in China. Some manufacturers are also actively
introducing technology to achieve localization as soon as possible. IEC already
has corresponding technical standards.
3. Liquid-immersed transformer with high temperature resistance and
insulation. The solid insulating materials used include conventional paper
insulation or high-temperature-resistant insulating materials, which are filled
with high-ignition-point liquid insulating media such as silicone oil and beta
oil. They are divided into different combinations such as hybrid, semi-hybrid,
and composite. For general The distribution transformer used can reduce its
size, increase its overload capacity, and improve its fire and flame retardant
properties. It is a choice between dry-type transformers and conventional
oil-immersed transformers. Because of its moderate price and good environmental
protection performance, it has certain room for development. For 110kV
transformers, it is also an optional product. Semi-hybrid and hybrid
high-temperature resistant liquid-immersed transformers are already in use in
China. Corresponding IEC standards are also being formulated. There are problems
such as oil chromatography and gas protection, and the load loss is relatively
high.
4. Cable transformers. Some large foreign companies have launched practical
products. Some domestic transformer manufacturers and universities are
developing similar products. It is expected that the voltage level of 110kV can
better solve the problem of oil-free and fireproof transformers. Flame retardant
properties. However, the concept of main longitudinal insulation and
conventional winding wave process theory have been fundamentally changed when
used in cable transformers. Its insulation structure design and testing
technology need to be studied and discussed.
2. Transformer’s ability to withstand short circuit
Because of the relatively large voltage and relatively low impedance of
power or distribution transformers used in urban power grids, their ability to
withstand short circuits is also very important. IEC standards and national
standards have been revised recently, and the requirements for the test of the
transformer's ability to withstand short circuits have been increased, from
three tests in the past to nine tests, and the voltage value of the repeated
insulation withstand test after the short circuit test has also been changed to
100%. Type tests such as lightning impact should also be conducted after short
circuit. The test assessment is more stringent. This puts forward higher
requirements for transformers used in urban power grids, which is undoubtedly
very beneficial to advance their operational reliability. It is worth
considering whether a transformer that has passed the short-circuit withstand
test according to the old standard can pass the test requirements of the new
standard. Of course, the implementation of standards still needs a process.
However, from the date of implementation of the standard, tests should be
carried out according to the new standard, otherwise it will violate the
provisions of the mandatory standard.
3. Energy saving and consumption reduction
The issue of energy saving and consumption reduction of urban power grid
transformers has always received widespread attention. Since the 1970s, the
country has implemented a policy of phasing out high-energy-consuming products.
Distribution transformers have gone through 2-3 major upgrades. Currently,
various types of dry-type and oil-immersed transformers are close to the most
advanced in the world. s level. However, there is still a certain gap between
the requirements of high efficiency and energy saving. At present, due to the
urgent requirements for resource and environmental protection, the country is
formulating a series of mandatory energy-saving standards for electrical
products and guiding the certification of energy-saving labels for
high-efficiency and energy-saving electrical products, supplemented by certain
policy incentives. Distribution transformers are no exception. When working on
energy conservation and consumption reduction, attention should be paid to:
1. Energy saving and consumption reduction of transformers must be based on
the premise of not sacrificing product operation reliability. In any case, we
must adhere to the idea of reliability first, taking into account the
technical performance of the product, further reduce losses, improve product
performance indicators, and avoid the decline in dynamic stability and
over-excitation capabilities of upgraded products that have occurred in the
past. And the overload capacity has also been reduced.
2. To save energy and reduce consumption of transformers, we must adhere to
the policy of key breakthroughs. First, solve the energy saving and consumption
reduction of large-volume and wide-ranging products, and promote the energy
saving of similar products. For example, does the energy saving of distribution
transformers need to be mandatory for the entire series, or should a general
series of products be determined through investigation, such as a standard for
limiting losses for products in the small capacity segment. Learn from developed
countries abroad and attach great importance to the standardization, versatility
and interchangeability requirements of small-capacity distribution transformers,
so as to drive the energy-saving and consumption-reducing design of other
products.
3. The energy saving and consumption reduction of transformers must take
into account the joint effects of reducing line losses, reducing voltage
fluctuations, and improving power supply quality in low-voltage distribution
systems. The energy-saving work of distribution transformers must not be viewed
in isolation. The standard should provide correct guidance for the
miniaturization of distribution transformers, dense distribution points,
limiting the power supply radius, reducing low-voltage distribution network line
losses, and improving power supply quality. In addition to the needs of
high-power centralized power supply loads, architectural design departments
should be encouraged to rationally configure transformers according to local
conditions. , make a comprehensive comparison between the economy and
rationality of long-term operation and the one-time investment in increasing the
distribution device, and change the abnormal situation that the capacity of a
single distribution transformer is getting larger and larger. [Date: 2008-07-26]
Source: Author: Unknown [Font: Large, Medium, Small] 4. The copper-iron loss
ratio problem, that is, the load and no-load loss ratio of the transformer, is
determined by the load rate of the transformer. Generally speaking, when the
load rate is low, the copper-to-iron loss ratio should be increased; on the
contrary, when the load rate is high, the copper-to-iron loss ratio should be
reduced, but this is all based on the continuous improvement of the material
properties of silicon steel sheets. The common practice in the past was to
develop two series, one series suitable for urban networks with high load rate;
one series suitable for rural networks with low load rate. Can we grasp the
different characteristics of typical capacities of urban and rural power grid
transformers in the same series and select transformers with different
copper-to-iron consumption ratios? That is to say, the transformers used in
rural power grids generally have small capacity and low load factor due to
scattered residential areas, so transformers with a large copper-to-iron
consumption ratio can be used. However, transformers used in urban power grids
generally have more concentrated residential areas and higher load factors, so
they should be selected with slightly larger capacity. The copper-iron
consumption of transformers is relatively small. That is to say, there is no
need to insist on a uniform copper-iron consumption ratio for low-end and
high-end musical instruments in the series. For the evaluation of transformer
losses, in addition to active losses, that is, the sum of no-load and load
losses, reactive losses should also be considered. In this regard, the core
multi-level seaming process currently used can reduce the no-load current to a
lower level. The no-load current of the wound core transformer is also very low,
which has certain advantages. The impedance voltage generally changes little and
has little impact on reactive power loss, so it can be ignored. For
transformers, active power loss accounts for a large proportion and is the main
aspect affecting transformer losses.
5. When launching a new energy-saving transformer series, we must consider
the material consumption issues in the design of products with different
capacities and specifications. While saving energy, we must not excessively
increase material consumption. The material consumption per unit capacity is
considered comprehensively as an indicator to evaluate the technological
advancement and economy of a series of products. And we should not simply pursue
material performance index requirements and reduce excessive dependence on
foreign material suppliers.
4. Fully sealed transformer
In order to reduce the workload of operation and maintenance of
distribution transformers, a considerable number of transformer factories have
changed the oil tanks from flat tube radiator oil tanks or fin radiator oil
tanks to corrugated radiator oil tanks or expansion fin radiator oil tanks,
eliminating the oil pillow. . The corrugated fuel tank is available in two
types, with or without air cushion. The Transformer Research Institute has
formulated corresponding technical standards. In the actual production process,
attention should be paid to the coordination of the throughput of the corrugated
oil tank and the expansion of the oil. Excessive positive pressure or vacuum
cannot be generated in the tank, otherwise the operation will be easily damaged
under repeated positive pressure or vacuum. Seal of medium transformer. The
throughput of the fuel tank has a great relationship with the elasticity of the
steel plate of the corrugated sheet or expansion sheet radiator used. If the
expected throughput cannot be achieved, an appropriate oil cup or small oil
pillow should be used to partially compensate for the excessive drop in liquid
level. For fully sealed transformers with air cushions, attention should also be
paid to the sealing condition of the top structure of the oil tank to adapt to
the test of alternating hot and cold conditions during long-term operation. It
truly functions as a complete seal.
5. Components and materials
In recent years, with the development of reform and opening up, many
foreign advanced technologies have been introduced, and the components and
materials of transformers are no exception. For example, components such as
bushings, tap-changers, radiators, transposed wires, paper insulation materials,
insulating oil, etc., many foreign manufacturers have set up factories in China
to produce and sell them. It has greatly promoted the improvement of the quality
of transformer components and materials. In addition to traditional structural
forms, some new structural component manufacturing technologies have been
introduced, such as plug-in cable terminal sleeves, which have been supplied in
series from 10kV to 110kV, etc., which undoubtedly provides us with more
choices. Even some small components, such as thermostats, gas relays, oil flow
meters, oil level gauges, oil pumps, fans, pressure relief valves, etc., have
also been introduced. Overall, the internal and external quality level of the
transformer is improved. But it is undeniable that there are also some small
domestic factories that produce low-quality components and materials, such as
ash powder and insulating paper with high dielectric loss, self-adhesive
transposition wires with flowing self-adhesive glue, and recycled oil instead of
new insulating oil. wait. In the current fierce price competition in the market,
the profit margin of manufacturing plants is very small. Some transformer plants
have low management levels. In order to achieve profitability, they do not
hesitate to sacrifice the long-term operational reliability and service life of
the transformers. This leads to a situation where good and bad are not
distinguished and the fake is confused with the real. Transformers made of some
low-quality components and materials can still pass factory routine tests, and
problems will not be immediately exposed during short-term operation. Once
exposed, there will be symptoms of frequent similar accidents, posing a
potential threat to the safe operation of the power system. We must remain
vigilant about this. Pay attention to component and material standards and
strengthen factory inspection.
6. Single-phase power supply and 20kV high-voltage distribution network
power supply
Currently, there are pilot projects for single-phase power supply and 20kV
distribution network power supply in China. Single-phase power supply is more
advantageous for power supply in villa communities and resorts where residents
are scattered. The 20kV high-voltage distribution network power supply is
beneficial to cities in improving power grid planning, power supply nodes, and
expanding power supply range. It should be said that these two pilot projects
are not too difficult for the transformer manufacturing industry. In particular,
single-phase three-wire power supply single-phase transformers adopt a rolled
core structure, which has great advantages in energy saving and noise reduction.
It is recommended to continue to expand the pilot scope in the construction of
new areas.
7. 110kV transformer partial discharge test
The new IEC standards and national standards stipulate that 110kV
oil-immersed transformers should add partial discharge measurement requirements
to the induction withstand voltage test and make this a routine test item for
the transformer before leaving the factory, one by one. This is beneficial to
controlling the manufacturing quality of transformer products and improving
product design and process levels. At present, 110kV transformer factories are
actively carrying out this test that has never been done in the past. It is
believed that transformer products that pass partial discharge measurement will
have higher operational reliability. The power supply department should also
actively create conditions to add requirements for partial discharge measurement
of 110kV transformers in the handover acceptance test to control the
installation quality of the transformer.
8. New power distribution devices for urban power grids
Among electrical products for urban power supply, many new distribution
transformer products have been introduced in recent years, such as box-type
substations and combined transformers, commonly known as European box
transformers and American box transformers. In fact, the American box
transformer is just a simple combination of transformers. It also does not have
the function of high and low voltage remote operation of box-type substations.
But it is combined with the ring network switch and cable branch box. It can
form a "building block" power supply network that supplies power to residential
areas, and has mature operating experience abroad. Some domestic power supply
departments and manufacturers have developed many different types of simple
combination box-type substation products to meet different needs. They call this
product "Chinese-style box substation". It has its own characteristics and can
adapt to the needs of different regions in the country. In addition, underground
or semi-underground transformers, box transformers, etc. have also been
introduced to solve the power supply needs of street lights and other equipment.
Dry-type transformers are also available in cast, wound and open-ventilated
types. The introduction and development of new power supply equipment meets the
needs of urban and rural power grid construction for diversified selection of
electrical products. However, the current situation is that the speed of
standard formulation often cannot keep up with equipment manufacturing and
technology introduction. Standards for new products should be formulated as soon
as possible to clarify their Performance and special requirements of the power
grid.
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